Sources and References

Primary textbook — Banich, M. T. (2003). Cognitive Neuroscience and Neuropsychology. Houghton Mifflin.

Supplementary texts — Gazzaniga, M. S., Ivry, R. B., & Mangun, G. R. (2014). Cognitive Neuroscience: The Biology of the Mind, 4th Ed. W.W. Norton.

Assigned articles:

• Eling, P. & Finger, S. (2015). Franz Joseph Gall on greatness in the fine arts: A collaboration of multiple cortical faculties of mind. Cortex, 71, 102–115.
• Sandrone, S., Bacigaluppi, M., Bhatt, D. L., Bhatt, M. R., Bhatt, D. L., Bhatt, D. L., … & Bhatt, D. L. (2014). Weighing brain activity with the balance: Angelo Mosso’s original manuscripts come to light. Brain, 137, 621–633.
• Miller, M. B., Sinnott-Armstrong, W., Young, L., King, D., Paggi, A., Fabri, M., Polonara, G., & Gazzaniga, M. S. (2010). Abnormal moral reasoning in complete and partial callosotomy patients. Neuropsychologia, 48, 2215–2220.
• Whitwell, R. L., Buckingham, G., Enns, J. T., Chouinard, P. A., & Goodale, M. A. (2016). Rapid decrement in the effects of the Ponzo display dissociates action and perception. Psychonomic Bulletin & Review, 1–7.
• Konkle, T. & Caramazza, A. (2013). Tripartite organization of the ventral stream by animacy and object size. Journal of Neuroscience, 33, 10235–10242.
• Saj, A., et al. (2014). Functional neuro-anatomy of egocentric versus allocentric space representation. Neurophysiologie Clinique / Clinical Neurophysiology, 44, 33–40.
• Hayden, B. Y., Pearson, J. M., & Platt, M. L. (2011). Neuronal basis of sequential foraging decisions in a patchy environment. Nature Neuroscience, 14, 933–939.
• Saj, A., Fuhrman, O., Vuilleumier, P., & Boroditsky, L. (2014). Patients with left spatial neglect also neglect the “left side” of time. Psychological Science, 25, 207–214.
• Oflaz, S., et al. (in press). Memory functioning in dissociative identity disorder. Journal of Psychiatric Research, 1–7.
• Decety, J., Smith, K. E., Norman, G. J., & Halpern, J. (2014). A social neuroscience perspective on clinical empathy. Social Neuroscience, 9, 36–49.
• Hummer, T. A., et al. (2014). Prefrontal cortex volume and executive cognition. Brain and Cognition, 88, 26–34.
• Scheibel, R. S. (2017). Functional magnetic resonance imaging of cognitive control following traumatic brain injury. Frontiers in Neurology, doi:10.3389/fneur.2017.00352.
• Nguyen, L., et al. (in press). Cognitive reserve and aging. Journal of Clinical and Experimental Neuropsychology, 1–14.
• Menon, V., et al. (2011). Salience network dysfunction in schizophrenia. Biological Psychiatry, 70, 1127–1133.
• Zhang, J., et al. (2013). Language and aphasia. Clinical Neurology and Neurosurgery, 115, 2230–2233.

Chapter 1: History of Neuropsychology

The Ancient Roots of Brain–Behaviour Inquiry

The question of where the mind resides is among the oldest in human intellectual history. Ancient Egyptian medical papyri, including the Edwin Smith Surgical Papyrus (ca. 1700 BCE, likely copied from a text written around 3000 BCE), contain some of the earliest known descriptions of brain injuries and their behavioural consequences. Egyptian physicians documented cases of skull fractures associated with contralateral motor deficits, suggesting an early, albeit rudimentary, awareness that the brain controlled bodily functions. Paradoxically, Egyptians also discarded the brain during mummification, assigning the heart primacy as the seat of consciousness—a view that persisted for millennia.

The cardiocentric hypothesis, championed most influentially by Aristotle (384–322 BCE), held that the heart was the organ of sensation, thought, and emotion, while the brain merely served to cool the blood. In contrast, Hippocrates (ca. 460–370 BCE) and later Galen (129–ca. 216 CE) argued for the cephalocentric hypothesis, locating mental functions within the brain. Galen’s extensive anatomical work on gladiators and animals provided compelling evidence: damage to the brain disrupted cognition and sensation, while the heart could be exposed without such effects. Galen’s ventricular doctrine—the idea that mental faculties resided in the fluid-filled ventricles of the brain—dominated medical thinking for over a thousand years throughout the medieval period.

The Rise of Localization: Phrenology and Its Critics

The modern era of neuropsychology begins in earnest with Franz Joseph Gall (1758–1828), a Viennese physician who proposed that the brain was the organ of the mind and, crucially, that different mental faculties were localized in distinct cortical regions. Gall’s system, later branded phrenology by his collaborator Johann Spurzheim, asserted that the size of a cortical region reflected the strength of the corresponding faculty and that this size could be inferred from the shape of the overlying skull. Eling and Finger (2015) examined Gall’s ideas about the fine arts in detail, revealing that Gall proposed that greatness in artistic endeavour required the collaboration of multiple cortical faculties—including perception of relationships, colour sense, “constructiveness,” and locality. While Gall’s cranial measurements were scientifically unfounded, his core insight that the cortex was functionally differentiated proved remarkably prescient and laid the groundwork for all subsequent localization research.

Pierre Flourens (1794–1867) challenged Gall through systematic ablation experiments in animals. By removing portions of the cerebrum, cerebellum, and brainstem in pigeons and other animals, Flourens argued for equipotentiality—the idea that all cortical areas contribute equally to higher functions and that the brain operates as an undifferentiated whole. While his conclusions overstated cortical homogeneity, Flourens correctly identified the cerebellum’s role in motor coordination and established the experimental ablation method as a cornerstone of neuropsychological methodology.

Broca, Wernicke, and the Language Debate

The localizationist position received its strongest empirical support from clinical case studies in the 1860s and 1870s. In 1861, the French surgeon Paul Broca (1824–1880) presented the case of his patient “Tan” (Louis Leborgne), who had lost the ability to produce speech but retained comprehension. Post-mortem examination revealed a lesion in the left inferior frontal gyrus (now called Broca’s area). Broca’s subsequent cases reinforced the link between left frontal damage and expressive language deficits, leading to his famous declaration that “nous parlons avec l’hemisphere gauche” (we speak with the left hemisphere). This was the first clear demonstration of both functional localization and hemispheric lateralization.

In 1874, the German neurologist Carl Wernicke (1848–1905) described patients with a complementary deficit: fluent but meaningless speech coupled with severely impaired comprehension. The responsible lesion lay in the posterior portion of the left superior temporal gyrus (now Wernicke’s area). Wernicke went further by proposing a connectionist model: language production depended on Broca’s area, language comprehension on Wernicke’s area, and the connection between them—the arcuate fasciculus—enabled repetition. Damage to this white matter tract produced conduction aphasia, characterized by impaired repetition with preserved comprehension and fluent (though paraphasic) speech. Wernicke’s model was the first disconnection account in neuropsychology and anticipated modern network-based approaches to brain function.

The Diagram Makers and the Holistic Response

The late 19th century saw a proliferation of diagram makers—neurologists like Ludwig Lichtheim who extended Wernicke’s connectionist logic to predict additional aphasia subtypes from specific patterns of disconnection. While these models were remarkably successful in classifying language disorders, they were criticized by John Hughlings Jackson (1835–1911), who argued that brain functions were organized hierarchically rather than in discrete centres. Jackson proposed that higher cortical areas re-represented and modulated the functions of lower areas, and that brain damage produced both negative symptoms (loss of function) and positive symptoms (release of lower-level functions from higher-level inhibition). His concept of dissolution—the reversal of evolutionary development—influenced subsequent thinking about recovery and compensation after brain injury.

In the early 20th century, holistic perspectives gained prominence through the work of Karl Lashley (1890–1958), who performed systematic cortical lesion studies in rats learning mazes. Lashley formulated two principles: mass action (the severity of learning impairment is proportional to the amount of cortex destroyed, regardless of location) and equipotentiality (all cortical areas contribute equally to complex behaviours). While later research showed these principles apply only to certain types of learning in certain species, Lashley’s rigorous experimental approach and his challenge to simplistic localizationism were invaluable.

The Modern Synthesis

The resolution of the localization–holism debate came not through the victory of one side but through a synthesis recognizing that the brain operates through distributed networks anchored to specialized regions. Alexander Luria (1902–1977), the Soviet neuropsychologist, proposed that complex mental functions are accomplished by functional systems—constellations of brain areas each contributing a specific processing component. Damage to any node in the system disrupts the overall function but in a manner characteristic of that node’s contribution. Luria’s approach, developed through decades of clinical work with brain-injured soldiers during and after World War II, remains foundational to modern clinical neuropsychology.

The late 20th century brought the cognitive neuroscience revolution, integrating neuropsychological case studies with functional neuroimaging, computational modelling, and cognitive theory. The field moved from asking “where is function X located?” to asking “how do distributed neural networks implement cognitive processes?” This shift reflects the recognition that brain–behaviour relationships are best understood at the level of circuits and systems rather than isolated regions.

Chapter 2: Methods in Neuropsychology

The Logic of Lesion Studies

The lesion method remains the oldest and most direct approach to understanding brain–behaviour relationships. The fundamental logic is straightforward: if damage to brain region X impairs cognitive function Y while leaving function Z intact, then region X is necessary for function Y but not for function Z. This reasoning yields single dissociations, which are informative but potentially misleading. A single dissociation between tasks could reflect differences in task difficulty rather than truly separate neural substrates.

The gold standard in lesion-based inference is the double dissociation: Patient A has damage to region X and is impaired on task Y but not task Z, while Patient B has damage to region W and shows the reverse pattern—impaired on task Z but not task Y. Double dissociations provide strong evidence that the two tasks depend on genuinely distinct neural systems, as the pattern cannot be explained by differential task difficulty.

Historical Foundations of Neuroimaging: Angelo Mosso’s Balance

The intellectual origins of modern functional neuroimaging extend back to the late 19th century. As Sandrone et al. (2014) documented, the Italian physiologist Angelo Mosso (1846–1910) constructed a “human circulation balance” in the late 1870s—essentially a precisely calibrated seesaw on which a subject lay horizontally. Mosso hypothesized that cognitive effort would increase blood flow to the brain, causing a measurable tilt of the balance toward the head. His original manuscripts, rediscovered by Sandrone and colleagues, confirmed that Mosso observed shifts consistent with increased cerebral blood flow during mental arithmetic and emotional stimulation. Although his measurements lacked the spatial precision of modern imaging, Mosso’s core insight—that neural activity is coupled to local changes in blood flow—is the very principle that underlies functional magnetic resonance imaging (fMRI) and positron emission tomography (PET) today. Mosso’s work thus represents a direct conceptual forerunner of the haemodynamic neuroimaging methods that now dominate cognitive neuroscience.

Structural Neuroimaging

Computed tomography (CT) uses X-rays to produce cross-sectional images of the brain and is particularly useful for detecting acute haemorrhage, fractures, and large structural lesions. Magnetic resonance imaging (MRI) exploits the magnetic properties of hydrogen atoms in water molecules to generate high-resolution images of brain anatomy without ionizing radiation. Different MRI sequences (T1-weighted, T2-weighted, FLAIR) highlight different tissue contrasts, enabling the detection of tumours, demyelination, and subtle cortical atrophy.

Diffusion tensor imaging (DTI), a specialized MRI technique, measures the directional diffusion of water molecules along white matter tracts, providing in vivo maps of structural connectivity. Fractional anisotropy (FA), a DTI-derived metric, indexes the coherence of white matter fibres: higher FA values indicate more organized tracts. DTI has been critical for understanding disconnection syndromes and for mapping pathways such as the arcuate fasciculus, the superior longitudinal fasciculus, and the corpus callosum.

Functional Neuroimaging

Functional MRI (fMRI)

Functional MRI measures the blood-oxygen-level-dependent (BOLD) signal, which reflects local changes in the ratio of oxygenated to deoxygenated haemoglobin. When neurons in a brain region become more active, local blood flow increases, delivering more oxygenated haemoglobin than the neurons consume. Because oxygenated and deoxygenated haemoglobin have different magnetic properties, this creates a detectable signal change. The BOLD signal is an indirect measure of neural activity with a temporal resolution of approximately 1–2 seconds (limited by the sluggishness of the haemodynamic response) and a spatial resolution on the order of 2–3 mm.

Common fMRI experimental designs include:

Positron Emission Tomography (PET)

PET requires injection of a radioactively labelled tracer (e.g., \(^{18}\)F-fluorodeoxyglucose for metabolic imaging, or \(^{15}\)O-water for blood flow). Detectors surrounding the head register pairs of gamma rays emitted when positrons from the tracer annihilate with electrons. PET provides direct measures of glucose metabolism, blood flow, and neurotransmitter receptor binding, but its spatial resolution (4–8 mm) and temporal resolution (30–60 seconds) are inferior to fMRI. Its unique advantage lies in neurochemical specificity: PET can quantify dopamine receptor density, serotonin transporter availability, and amyloid plaque burden—measurements that fMRI cannot provide.

Electroencephalography (EEG) and Event-Related Potentials (ERPs)

Electroencephalography records electrical potentials generated by synchronous postsynaptic activity of cortical pyramidal neurons via electrodes placed on the scalp. EEG’s temporal resolution is excellent (on the order of milliseconds), enabling the study of rapid cognitive processes, but its spatial resolution is poor due to volume conduction through the skull and scalp. Event-related potentials (ERPs) are obtained by time-locking EEG recordings to repeated stimulus presentations and averaging across trials to extract signal from noise. Well-studied ERP components include:

• P100/N170: Early visual processing; the N170 is particularly sensitive to faces.
• N200: Conflict detection and response inhibition.
• P300: Context updating and working memory; amplitude reflects the degree of attentional resource allocation.
• N400: Semantic processing; amplitude increases for semantically unexpected words.

Magnetoencephalography (MEG)

Magnetoencephalography detects the tiny magnetic fields generated by neuronal currents using arrays of superconducting quantum interference devices (SQUIDs). Because magnetic fields are less distorted by the skull than electrical potentials, MEG offers better spatial localization than EEG while retaining millisecond temporal resolution.

Brain Stimulation Methods

Transcranial Magnetic Stimulation (TMS)

Transcranial magnetic stimulation uses a coil placed over the scalp to generate a rapidly changing magnetic field that induces electrical currents in underlying cortical tissue. A single TMS pulse can transiently disrupt or facilitate processing in the targeted region, creating a temporary “virtual lesion.” Repetitive TMS (rTMS) delivers trains of pulses and can produce longer-lasting effects: low-frequency rTMS (1 Hz) generally decreases cortical excitability, while high-frequency stimulation (5–20 Hz) increases it. TMS provides causal evidence about brain–behaviour relationships in healthy participants, complementing the correlational evidence from neuroimaging.

Transcranial Direct Current Stimulation (tDCS)

tDCS delivers a weak constant current (typically 1–2 mA) through scalp electrodes. Anodal stimulation generally increases cortical excitability beneath the electrode, while cathodal stimulation decreases it. Although tDCS lacks the spatial precision of TMS, it is inexpensive, portable, and well-tolerated, making it valuable for both research and clinical rehabilitation.

Neuropsychological Assessment

Clinical neuropsychological assessment employs standardized batteries of tests to characterize a patient’s cognitive profile. Core domains assessed typically include:

• Intellectual functioning: Wechsler Adult Intelligence Scale (WAIS)
• Attention and processing speed: Trail Making Test, Digit Span
• Memory: Wechsler Memory Scale, California Verbal Learning Test, Rey Complex Figure
• Executive functions: Wisconsin Card Sorting Test, Stroop Test, Tower of London
• Language: Boston Naming Test, Token Test
• Visuospatial skills: Block Design, Judgment of Line Orientation

The interpretation of neuropsychological test results relies on comparison to normative data adjusted for age, education, and sometimes sex and ethnicity. A critical distinction is between premorbid functioning (estimated from demographic variables and reading ability) and current performance. The discrepancy between these estimates helps determine whether cognitive decline has occurred.

Chapter 3: Neuroanatomy

Overview of Brain Organization

The human brain weighs approximately 1.3–1.4 kg and contains roughly 86 billion neurons and an equal or greater number of glial cells. It is organized into three major divisions derived from embryological development:

The Cerebral Cortex

The cerebral cortex is a sheet of neural tissue approximately 2–4 mm thick, folded into gyri (ridges) and sulci (grooves) to increase surface area. The cortex is divided into four lobes by major sulci:

Frontal Lobe

The frontal lobe extends from the central sulcus anteriorly to the frontal pole. It contains the primary motor cortex (precentral gyrus, Brodmann area 4), premotor cortex (area 6), supplementary motor area (SMA), and the vast prefrontal cortex (PFC). The PFC is further subdivided into:

• Dorsolateral prefrontal cortex (DLPFC): Working memory, planning, cognitive flexibility, and abstract reasoning.
• Ventromedial prefrontal cortex (vmPFC): Decision-making, emotion regulation, and representation of reward value.
• Orbitofrontal cortex (OFC): Evaluation of reinforcers, impulse control, and social behaviour. Damage produces disinhibition, poor judgment, and personality change, as dramatically illustrated by the case of Phineas Gage (1848).
• Anterior cingulate cortex (ACC): Conflict monitoring, error detection, and motivational control.

Parietal Lobe

The parietal lobe lies posterior to the central sulcus and superior to the lateral fissure. It contains the primary somatosensory cortex (postcentral gyrus, areas 1, 2, 3) and the posterior parietal cortex (PPC), which is subdivided into the superior parietal lobule (SPL, area 7, involved in visuomotor integration and spatial attention) and the inferior parietal lobule (IPL, areas 39 and 40, important for spatial awareness, number processing, and multimodal integration). The intraparietal sulcus (IPS) contains regions critical for reaching, grasping, and numerical cognition.

Temporal Lobe

The temporal lobe lies below the lateral fissure. It contains the primary auditory cortex (Heschl’s gyrus, areas 41 and 42), Wernicke’s area (posterior superior temporal gyrus), and extensive association cortex involved in auditory processing, language comprehension, semantic memory, and object recognition. The medial temporal lobe houses the hippocampal formation (hippocampus proper, dentate gyrus, subiculum) and the surrounding parahippocampal, perirhinal, and entorhinal cortices—structures essential for episodic memory formation. The fusiform gyrus on the ventral surface plays a critical role in face and object recognition.

Occipital Lobe

The occipital lobe occupies the posterior pole of the brain and is devoted primarily to visual processing. The primary visual cortex (V1, area 17) receives input from the lateral geniculate nucleus of the thalamus and contains a retinotopic map of the visual field. Surrounding extrastriate areas (V2, V3, V4, V5/MT) extract progressively more complex visual features, from orientation and colour to motion and form.

Subcortical Structures

Thalamus

The thalamus is a paired, egg-shaped structure at the centre of the brain that serves as the principal relay station for sensory information (except olfaction) en route to the cortex. Different thalamic nuclei project to specific cortical regions: the lateral geniculate nucleus (LGN) relays visual information to V1, the medial geniculate nucleus (MGN) relays auditory information to Heschl’s gyrus, and the ventral posterolateral (VPL) and ventral posteromedial (VPM) nuclei relay somatosensory information to S1. The pulvinar nucleus is involved in attentional modulation, and the mediodorsal nucleus connects extensively with the prefrontal cortex.

Hypothalamus

The hypothalamus, despite its small size (roughly 4 g), regulates homeostasis, endocrine function (via the pituitary gland), circadian rhythms (via the suprachiasmatic nucleus), hunger, thirst, body temperature, and motivated behaviours including aggression and sexual behaviour.

Basal Ganglia

The basal ganglia are a set of subcortical nuclei including the caudate nucleus, putamen (together forming the striatum), globus pallidus, subthalamic nucleus, and substantia nigra. They participate in motor control, action selection, habit formation, and reward processing through parallel cortico-striato-thalamo-cortical loops. The direct pathway facilitates desired movements, while the indirect pathway suppresses competing movements. Degeneration of dopaminergic neurons in the substantia nigra pars compacta disrupts this balance, producing the motor symptoms of Parkinson’s disease (bradykinesia, rigidity, tremor). Hyperkinetic disorders such as Huntington’s disease result from degeneration of striatal neurons in the indirect pathway.

Limbic System

The limbic system is a loosely defined set of structures involved in emotion, motivation, and memory, including the amygdala, hippocampus, cingulate gyrus, septum, and mammillary bodies. The amygdala is particularly important for processing emotional stimuli, fear conditioning, and modulating memory consolidation for emotionally significant events.

White Matter Pathways

Cortical regions communicate via three types of white matter tracts:

The corpus callosum, the largest white matter structure in the brain, contains approximately 200 million axons connecting the two cerebral hemispheres. Its anterior portion (genu) connects prefrontal regions, the body connects motor and somatosensory regions, and the posterior portion (splenium) connects parietal, temporal, and occipital regions. Sectioning the corpus callosum produces the split-brain syndrome, revealing the independent processing capabilities of each hemisphere.

Chapter 4: Hemispheric Specialisation

Lateralization of Function

Although the two cerebral hemispheres appear grossly symmetrical, they differ substantially in their functional specializations. Hemispheric lateralization refers to the differential contribution of the left and right hemispheres to particular cognitive functions. The most dramatic demonstration of lateralization comes from research with split-brain patients, but converging evidence from lesion studies, neuroimaging, and behavioural paradigms in healthy individuals confirms that the two hemispheres are specialized for different types of processing.

Left Hemisphere Specializations

The left hemisphere is dominant for language in approximately 95% of right-handed individuals and 70% of left-handed individuals. Its specializations include:

• Speech production and comprehension (Broca’s and Wernicke’s areas)
• Reading and writing
• Analytical and sequential processing
• Mathematical computation (particularly exact arithmetic)
• Fine motor control of the contralateral (right) hand
• Logical reasoning and categorical classification

Right Hemisphere Specializations

The right hemisphere contributes critically to:

• Visuospatial processing: Spatial perception, mental rotation, navigation
• Prosody: The emotional intonation and rhythm of speech
• Holistic and configural processing: Face recognition, gestalt perception
• Emotional processing: Particularly the perception and expression of negative emotions (though this is debated)
• Attentional control: The right hemisphere plays a disproportionate role in spatial attention, as evidenced by the preponderance of left-sided hemispatial neglect following right hemisphere damage
• Belief attribution and theory of mind: The right temporoparietal junction (rTPJ) is critical for understanding others’ mental states

Split-Brain Research

The most compelling evidence for hemispheric specialization comes from studies of patients who underwent corpus callosotomy—surgical severing of the corpus callosum to control intractable epilepsy. Roger Sperry and Michael Gazzaniga, beginning in the 1960s, developed ingenious testing paradigms exploiting the fact that, after callosotomy, sensory information presented to one visual field or one hand is available only to the contralateral hemisphere.

Key findings from split-brain research include:

• When an object is presented to the left visual field (right hemisphere), the patient cannot verbally name it (because the language-dominant left hemisphere has no access to the information) but can select it by touch with the left hand.
• The right hemisphere demonstrates superior performance on visuospatial tasks, pattern recognition, and emotional processing.
• The left hemisphere engages in confabulation, constructing plausible verbal explanations for actions initiated by the right hemisphere, revealing its role as an “interpreter.”

Moral Reasoning in Callosotomy Patients

Miller et al. (2010) provided a striking demonstration of the consequences of hemispheric disconnection for higher-order cognition. They tested six callosotomy patients (three complete, three partial anterior resections) and 22 healthy controls on moral reasoning scenarios that pitted an agent’s belief against the action’s outcome. Normal subjects typically incorporate the agent’s belief (intention) into their moral judgments—judging attempted harm as morally wrong even when no harm results. However, all six callosotomy patients based their judgments primarily on outcomes rather than beliefs: they judged actions as permissible when no harm occurred, regardless of malicious intent, and judged actions as wrong when harm occurred, regardless of innocent belief. Only 31% of patients’ judgments reflected agents’ beliefs compared to 72% in controls.

Handedness and Lateralization

Approximately 90% of humans are right-handed, a strong population-level bias not observed in other primates. Handedness is correlated with, but not perfectly predictive of, language lateralization. The Wada test (intracarotid sodium amobarbital injection) was historically used to determine language dominance before epilepsy surgery; fMRI laterality indices have largely replaced it. Factors influencing handedness include genetics (multiple genes of small effect), prenatal environment (intrauterine position, hormonal exposure), and possibly early experience.

Anatomical Asymmetries

Structural brain asymmetries correlate with functional lateralization. The planum temporale, a region of the superior temporal lobe encompassing part of Wernicke’s area, is larger on the left in approximately 65% of brains. The lateral (Sylvian) fissure is longer and more horizontal on the left. The right frontal lobe tends to be wider and protrude further anteriorly (right frontal petalia), while the left occipital lobe shows a corresponding posterior petalia. These asymmetries are evident in utero and in fossil endocasts of Homo erectus, suggesting ancient evolutionary origins.

Chapter 5: Visual Perception — The Duplex Model

From Retina to Cortex

Visual processing begins at the retina, where approximately 126 million photoreceptors (120 million rods for scotopic vision; 6 million cones for photopic, colour vision) transduce light into neural signals. Retinal ganglion cells, whose axons form the optic nerve, are classified into:

• Magnocellular (M) cells: Large receptive fields, high temporal resolution, achromatic, sensitive to motion and low spatial frequency.
• Parvocellular (P) cells: Small receptive fields, high spatial resolution, chromatic, sensitive to form and fine detail.

These streams remain segregated through the lateral geniculate nucleus (LGN) of the thalamus (magnocellular layers 1–2; parvocellular layers 3–6) before converging in the primary visual cortex (V1).

The Two Visual Systems Hypothesis

The most influential framework for understanding visual cortex organization is the duplex model (or two visual systems hypothesis) proposed by Melvyn Goodale and A. David Milner in 1992. Building on earlier work by Ungerleider and Mishkin (1982), who distinguished a ventral “what” pathway from a dorsal “where” pathway, Goodale and Milner reconceptualized the distinction in terms of the purpose of visual processing:

The most compelling evidence came from the patient D.F., who had bilateral damage to the lateral occipital complex (ventral stream) following carbon monoxide poisoning. D.F. was profoundly unable to recognize objects, judge their orientation, or discriminate simple shapes (visual form agnosia). Yet when asked to post a card through an oriented slot or to reach out and grasp objects, she accurately scaled her hand aperture and oriented her wrist appropriately—demonstrating intact visuomotor processing despite devastated visual perception.

The reverse dissociation is observed in patients with optic ataxia, typically resulting from damage to the superior parietal lobule (dorsal stream). These patients can recognize and describe objects but misreach, failing to adjust their grasp or trajectory to target properties.

Evidence from Visual Illusions

Whitwell et al. (2016) contributed to the two-streams debate by examining how the Ponzo illusion (a size-contrast illusion created by converging lines suggesting depth) affects perception and grasping. They found that while perceptual estimates of target size remained persistently biased by the illusion across repeated trials, the influence of the illusion on grip aperture during grasping decreased rapidly after only a few trials. This rapid decrement in the illusion’s effect on action, but not on perception, provides further evidence that the dorsal (action) stream can calibrate its motor parameters through error-based feedback mechanisms that are independent of the conscious perceptual representation constructed by the ventral stream. The sensorimotor system effectively “learns” the true size of the object through haptic feedback upon grasping, rapidly correcting the illusion-induced bias, while the perceptual system continues to be fooled.

Subdivisions Within the Dorsal Stream

More recent work has subdivided the dorsal stream into a dorso-dorsal pathway (projecting to the superior parietal lobule, involved in online motor control) and a ventro-dorsal pathway (projecting to the inferior parietal lobule, involved in action understanding and spatial awareness). This refinement accounts for findings that some parietal functions—such as understanding tool use or recognizing actions—seem more “perceptual” than the strict vision-for-action framework predicts.

Chapter 6: Object Recognition

The Challenge of Object Recognition

Recognizing objects is computationally remarkable: the visual system must identify a particular object despite enormous variation in retinal input caused by changes in viewpoint, distance, lighting, occlusion, and context. Neuropsychological research has identified multiple levels at which this process can break down, providing insights into the stages and neural substrates of object recognition.

Agnosia

Agnosia (from the Greek for “without knowledge”) refers to a failure of recognition that cannot be attributed to elementary sensory deficits, language impairment, or general intellectual decline. The term was coined by Sigmund Freud in 1891. Agnosias are classified by the sensory modality affected (visual, auditory, tactile) and by the level of processing disrupted.

Apperceptive Agnosia

Apperceptive agnosia involves a failure to form an adequate perceptual representation of the object. Patients cannot copy drawings, match objects by shape, or distinguish objects from non-objects. It results from diffuse damage to early visual processing areas, often following carbon monoxide poisoning or bilateral posterior cortical lesions. Patient D.F. is a classic example.

Associative Agnosia

Associative agnosia involves a failure to access meaning from an intact perceptual representation. Patients can copy drawings accurately, match objects by shape, and describe visual features, but they cannot name the object, describe its function, or associate it with stored semantic knowledge. This is sometimes described as a “percept stripped of its meaning.” It typically results from bilateral or left-hemisphere damage to the inferotemporal or temporo-occipital cortex.

Face Recognition and Prosopagnosia

Face recognition represents a particularly demanding form of object recognition because faces are structurally similar stimuli that must be distinguished at the individual level. The fusiform face area (FFA), located on the fusiform gyrus of the inferotemporal cortex, shows selective activation for faces compared to other object categories. The occipital face area (OFA) processes early structural descriptions of faces, while the superior temporal sulcus (STS) processes changeable aspects of faces such as gaze direction and expression.

Prosopagnosia is the selective inability to recognize faces. Acquired prosopagnosia results from damage (typically bilateral or right-hemisphere) to the fusiform and/or occipital face areas. Patients may fail to recognize family members or their own face in a mirror while retaining the ability to recognize objects, read text, and identify people by voice or gait. Developmental (congenital) prosopagnosia occurs without apparent brain damage and may affect up to 2–3% of the population.

Neural Organization of Object Representations

Category-Specific Deficits

Clinical evidence for the neural organization of object categories comes from patients with category-specific recognition deficits. Some patients show disproportionate difficulty recognizing living things (animals, plants) compared to nonliving artefacts (tools, vehicles), while others show the reverse pattern. Living-thing deficits are associated with anterior and medial temporal damage, while artefact deficits are associated with more lateral and posterior temporal–parietal damage. These dissociations have been explained by theories emphasizing:

• Differential feature weighting: Living things are distinguished primarily by visual/perceptual features (colour, texture), while artefacts are distinguished by functional features (what they do, how they are used).
• Domain-specific modules: Evolutionary pressures may have created specialized neural systems for biologically significant categories (conspecifics, predators, food sources).
• Correlated feature structure: Living things share many correlated features, making individual items harder to distinguish when representations are degraded.

Chapter 7: Spatial Perception

Spatial Cognition and the Parietal Lobe

Spatial perception encompasses the ability to perceive the locations of objects, their spatial relationships to each other and to the observer, and to navigate through the environment. The posterior parietal cortex (PPC) is the primary cortical substrate for spatial processing, integrating visual, auditory, somatosensory, and vestibular inputs into coherent spatial representations.

Egocentric and Allocentric Reference Frames

Spatial representations are constructed in multiple reference frames:

• Egocentric (body-centred): Locations are coded relative to the observer’s body (eye, head, hand, or trunk). These representations are essential for visually guided action—reaching toward a cup requires knowing its position relative to your hand.
• Allocentric (world-centred): Locations are coded relative to external landmarks or to other objects. These representations support navigation, spatial memory, and scene recognition.

The parietal cortex primarily constructs egocentric representations, while the hippocampus and medial temporal lobe construct allocentric representations. Damage to parietal regions disrupts egocentric spatial processing (e.g., optic ataxia, constructional apraxia), while hippocampal damage disrupts allocentric spatial memory and navigation.

Vestibular Contributions to Spatial Perception

Saj et al. (2014, Neurophysiologie Clinique) investigated the neural basis of egocentric versus allocentric spatial representations, with particular attention to the role of vestibular input. The vestibular system provides crucial information about head position and body orientation relative to gravity, which anchors spatial representations. Right-hemisphere regions, particularly the temporo-parietal junction and posterior insula (the parieto-insular vestibular cortex), are critical for integrating vestibular signals with visual and somatosensory information. Damage to these regions disrupts the sense of spatial orientation, contributing to disorders such as hemispatial neglect and tilted perception of the vertical.

Spatial Disorders

Constructional Apraxia

Constructional apraxia is the inability to copy drawings, assemble objects, or construct spatial arrangements despite adequate motor strength and comprehension. It is more common and more severe following right parietal damage. Patients with left parietal lesions tend to produce simplified drawings that retain overall spatial configuration, whereas patients with right parietal lesions produce drawings with intact detail but disorganized spatial relationships.

Topographical Disorientation

Topographical disorientation is the inability to navigate in familiar or novel environments. Subtypes include:

• Egocentric disorientation (parietal damage): Inability to represent locations relative to the self.
• Heading disorientation (posterior cingulate/retrosplenial damage): Inability to derive directional information from landmarks.
• Landmark agnosia (lingual gyrus/parahippocampal damage): Inability to recognize salient environmental features.
• Anterograde disorientation (hippocampal damage): Inability to form new spatial memories in novel environments.

Balint’s Syndrome

Balint’s syndrome, resulting from bilateral parietal–occipital damage, comprises three core deficits: simultanagnosia (inability to perceive more than one object at a time), optic ataxia (misreaching under visual guidance), and ocular motor apraxia (inability to voluntarily direct gaze). Balint’s syndrome reveals the parietal lobe’s critical role in binding spatial attention to multiple objects and guiding actions toward visual targets.

Chapter 8: Attention

What Is Attention?

Attention is the set of processes by which the brain selects relevant information for enhanced processing while suppressing irrelevant information. William James famously wrote in 1890: “Everyone knows what attention is. It is the taking possession by the mind, in clear and vivid form, of one out of what seem several simultaneously possible objects or trains of thought.” Neuropsychological research has revealed that attention is not a unitary process but comprises multiple dissociable components.

Types of Attention

Selective Attention

Selective attention is the ability to focus on task-relevant stimuli while ignoring distractors. In the visual domain, selective attention can be directed to spatial locations (spatial attention), visual features (feature-based attention), or entire objects (object-based attention). The classic cocktail party effect—the ability to follow one conversation in a noisy room—illustrates auditory selective attention.

Sustained Attention (Vigilance)

Sustained attention is the ability to maintain focus over prolonged periods. Performance on vigilance tasks typically declines over time (the vigilance decrement), a phenomenon with important implications for tasks requiring continuous monitoring (e.g., air traffic control, medical screening). Sustained attention depends critically on right-lateralized fronto-parietal networks and ascending noradrenergic systems originating in the locus coeruleus.

Divided Attention

Divided attention is the ability to process multiple sources of information simultaneously. Dual-task performance depends on the degree of overlap between the two tasks’ processing demands: tasks that rely on different sensory modalities and different response systems can be performed concurrently with less interference than tasks competing for the same processing resources.

Neural Networks of Attention

Michael Posner and colleagues proposed an influential tripartite model of attention comprising three functionally distinct but interacting networks:

Anterior Cingulate Cortex and Foraging Decisions

Corbetta and Shulman’s Two-Network Model

Corbetta and Shulman (2002) proposed that attention is governed by two distinct but interacting cortical networks:

• Dorsal attention network (DAN): Comprising the frontal eye fields and the intraparietal sulcus, the DAN mediates top-down, goal-directed allocation of attention. It is active when attention is voluntarily directed to expected locations or features.
• Ventral attention network (VAN): Comprising the temporoparietal junction and the ventral frontal cortex (especially the right hemisphere), the VAN detects behaviourally relevant stimuli that are unexpected—a “circuit breaker” that interrupts ongoing top-down processing to redirect attention toward salient events.

The interaction between these networks explains many clinical phenomena: damage to the VAN (particularly in the right hemisphere) produces hemispatial neglect, while damage to the DAN impairs voluntary spatial attention.

Chapter 9: Unilateral Neglect

Definition and Clinical Presentation

Unilateral neglect (also called hemispatial neglect or hemi-inattention) is a devastating attentional disorder in which patients fail to attend to, respond to, or orient toward stimuli on the side of space contralateral to their brain lesion. Most commonly, neglect follows right hemisphere damage (particularly involving the right temporoparietal junction, inferior parietal lobule, superior temporal gyrus, and/or inferior frontal gyrus) and manifests as neglect of the left side of space. Left-hemisphere damage can also produce right-sided neglect, but it is less frequent and typically less severe, reflecting the right hemisphere’s dominant role in spatial attention.

Neglect is remarkably pervasive in daily life. Patients may:

• Eat food from only the right side of their plate
• Shave or apply makeup only to the right side of their face
• Fail to dress the left side of their body
• Collide with objects on their left when navigating
• Read only the right portion of words or sentences (e.g., reading “cowboy” as “boy”)
• Draw only the right half of objects when copying

Distinguishing Neglect from Hemianopia

Neglect must be distinguished from homonymous hemianopia, a loss of vision in one half of the visual field due to damage to primary visual pathways. While both conditions can cause patients to miss stimuli on one side, they differ fundamentally: hemianopia is a sensory deficit (the patient cannot see stimuli in the affected field), whereas neglect is an attentional deficit (the patient can see but does not attend to stimuli in the affected space). Patients with hemianopia typically compensate by turning their head or eyes toward the blind field, whereas neglect patients often fail to explore the affected side at all. Furthermore, neglect extends beyond vision to affect tactile, auditory, and even imagined space.

Varieties of Neglect

Neglect manifests across multiple spatial reference frames:

• Personal neglect: Neglect of the contralesional side of the body (failing to groom or dress the left side).
• Peripersonal neglect: Neglect of space within arm’s reach (the space of object manipulation).
• Extrapersonal neglect: Neglect of far space (beyond arm’s reach).

Dissociations between these forms indicate that spatial attention is organized separately for different sectors of space.

Representational Neglect

In a landmark study, Bisiach and Luzzatti (1978) asked neglect patients to imagine standing at one end of the Piazza del Duomo in Milan and describe the buildings. Patients described only buildings on the right side of their imagined viewpoint. When asked to imagine standing at the opposite end, they described the previously neglected buildings (now on their imagined right) and neglected the previously described ones. This demonstrated that neglect is not merely a perceptual deficit but extends to internal mental representations.

Extinction

Extinction is a milder form of attentional bias in which patients can detect single stimuli on either side of space but fail to report the contralesional stimulus when stimuli are presented simultaneously on both sides. Extinction is thought to reflect competition for limited attentional resources, in which the ipsilesional stimulus “wins” when presented concurrently.

Neglect and Temporal Representation

Neural Basis of Neglect

The neural basis of neglect is best understood within the framework of Corbetta and Shulman’s two-network model:

• Damage to the ventral attention network (right TPJ, right ventral frontal cortex) disrupts the ability to reorient attention toward unexpected stimuli, particularly on the left.
• Secondary dysfunction of the dorsal attention network (intraparietal sulcus, frontal eye fields), even without direct structural damage, contributes to the spatial bias. Functional imaging shows that the intact left DAN becomes hyperactive after right hemisphere damage, creating an imbalanced competition that biases attention rightward.

White matter disconnection also plays a critical role. Damage to the superior longitudinal fasciculus (connecting frontal and parietal attention regions) and the inferior fronto-occipital fasciculus is associated with more severe and persistent neglect than cortical damage alone.

Rehabilitation of Neglect

Rehabilitation approaches include:

• Prism adaptation: Wearing prisms that shift the visual field rightward induces a leftward aftereffect that can temporarily reduce neglect symptoms.
• Visual scanning training: Systematic practice directing attention and gaze toward the neglected side.
• Vestibular stimulation: Caloric (cold water) vestibular stimulation of the left ear can temporarily reduce left neglect, possibly by activating right-hemisphere vestibular networks.
• Limb activation: Actively moving the contralesional limb in contralesional space may recruit attention to the neglected side.

Chapter 10: Memory

Memory Systems

Memory is not a single entity but comprises multiple systems that differ in the type of information stored, the duration of storage, and the neural substrates involved.

The Hippocampal Memory System

The hippocampus is the lynchpin of the declarative memory system. Its critical role was first demonstrated through the case of patient H.M. (Henry Molaison, 1926–2008), who underwent bilateral medial temporal lobe resection in 1953 to treat intractable epilepsy. The surgery, performed by William Beecher Scoville, removed the hippocampus, amygdala, and surrounding cortex bilaterally. H.M. was left with profound anterograde amnesia—the inability to form new episodic and semantic memories—coupled with some retrograde amnesia for events in the years preceding surgery. His working memory, procedural learning, and remote memories from childhood remained intact.

H.M.’s case established several foundational principles:

1. The hippocampus is necessary for forming new declarative memories but not for short-term/working memory (the distinction between STM and LTM is neuroanatomically real).
2. Memory is not a unitary function: H.M. could acquire new motor skills (mirror tracing) despite having no conscious memory of the training sessions.
3. Retrograde amnesia follows a temporal gradient: Remote memories are relatively preserved, suggesting that memories gradually become independent of the hippocampus through consolidation.

Memory Consolidation

The standard consolidation theory proposes that new memories initially depend on the hippocampus but are gradually transferred to neocortical networks over weeks to years through a process of repeated reactivation (particularly during sleep). The multiple trace theory offers an alternative: the hippocampus remains involved in vivid episodic memories regardless of their age, while semantic memories can become hippocampus-independent. Evidence from patients with extensive hippocampal damage who show flat (temporally ungraded) retrograde amnesia for episodic memories supports the multiple trace account.

Other Forms of Amnesia

Korsakoff’s syndrome, resulting from thiamine deficiency (typically due to chronic alcoholism), causes damage to the mammillary bodies and mediodorsal thalamus, producing severe anterograde and retrograde amnesia along with confabulation. Transient global amnesia involves a sudden, temporary episode of anterograde amnesia lasting hours, possibly related to transient ischaemia or spreading depression in medial temporal structures.

Memory and Dissociative Identity Disorder

Chapter 11: Emotion

Neural Basis of Emotion

Emotional processing involves a distributed network of cortical and subcortical structures. While the concept of a unitary “limbic system” as the brain’s emotional centre has been challenged, several structures remain central to emotional processing.

The Amygdala

The amygdala, an almond-shaped structure in the medial temporal lobe, is the most extensively studied brain region in emotional neuroscience. It comprises multiple nuclei, including the lateral nucleus (primary input; receives sensory information from thalamus and cortex), basal nucleus (integrates information and projects to striatum and cortex), and central nucleus (primary output; orchestrates autonomic, endocrine, and behavioural fear responses).

The amygdala’s functions include:

• Fear conditioning: Learning to associate neutral stimuli with threat. LeDoux’s work demonstrated a “low road” (thalamus to amygdala, fast but crude) and a “high road” (thalamus to cortex to amygdala, slower but more detailed) for fear processing.
• Emotional modulation of memory: The amygdala enhances consolidation of emotionally arousing memories by modulating hippocampal processing, explaining why emotional events are often remembered more vividly.
• Social evaluation: Processing the emotional significance of faces (especially fearful expressions) and social stimuli. Patients with bilateral amygdala damage (e.g., patient S.M. with Urbach-Wiethe disease) show impaired fear recognition, impaired fear conditioning, and abnormal social judgment but relatively preserved recognition of other emotions.
• Threat detection: Rapid, automatic processing of potentially threatening stimuli, even outside conscious awareness.

Prefrontal Cortex and Emotion Regulation

The ventromedial prefrontal cortex (vmPFC) and orbitofrontal cortex (OFC) play critical roles in:

• Somatic marker hypothesis (Damasio): The vmPFC links knowledge about a situation with the autonomic/somatic states previously associated with that situation, generating “gut feelings” that guide decision-making. Patients with vmPFC damage (like Phineas Gage and patient E.V.R.) show impaired decision-making on the Iowa Gambling Task despite intact intellect.
• Extinction of conditioned fear: The vmPFC suppresses amygdala-mediated fear responses when a previously threatening stimulus is no longer dangerous.
• Emotional regulation: Top-down modulation of amygdala activity during cognitive reappraisal and emotion regulation.

Insula

The insula, buried within the lateral fissure, is increasingly recognized as critical for interoception (awareness of internal bodily states), disgust processing, empathy, and emotional awareness. The anterior insula is a key node in the salience network, detecting behaviourally relevant stimuli and coordinating appropriate emotional and cognitive responses.

Empathy and Its Neural Substrates

Lateralization of Emotion

Two competing hypotheses address hemispheric asymmetries in emotional processing:

• Right hemisphere hypothesis: The right hemisphere is dominant for all emotional processing, both positive and negative. Supported by evidence that right hemisphere damage more severely impairs emotional prosody, facial emotion recognition, and emotional memory.
• Valence hypothesis: The left hemisphere is specialized for positive emotions and approach motivation, while the right hemisphere is specialized for negative emotions and withdrawal motivation. Supported by EEG studies showing left frontal activation during positive emotional states and right frontal activation during negative states.

Current evidence suggests a nuanced view: the right hemisphere may be dominant for emotional perception (recognizing emotions in others), while the valence asymmetry applies more to emotional experience and motivated behaviour.

Chapter 12: Executive Functions

What Are Executive Functions?

Executive functions are the higher-order cognitive processes that enable goal-directed behaviour, particularly in novel or complex situations. They include planning, cognitive flexibility, inhibitory control, working memory updating, problem-solving, and monitoring of ongoing behaviour. The prefrontal cortex (PFC) is the primary neural substrate for executive functions, though it operates in concert with subcortical structures (basal ganglia, thalamus) and posterior cortical regions.

Components of Executive Function

Miyake et al. (2000) proposed that executive function comprises three core components that are correlated but separable:

1. Inhibition: The ability to suppress prepotent or automatic responses. Assessed by tasks such as the Stroop task (naming the ink colour of a colour word, e.g., the word “RED” printed in blue), go/no-go tasks, and the stop-signal task. Critically depends on the right inferior frontal gyrus and pre-supplementary motor area.

2. Updating: The ability to monitor and revise the contents of working memory. Assessed by tasks such as the n-back task and running memory span. Depends on the dorsolateral prefrontal cortex and basal ganglia.

3. Shifting (cognitive flexibility): The ability to switch between mental sets or task rules. Assessed by the Wisconsin Card Sorting Test (WCST) and task-switching paradigms. Depends on the lateral prefrontal cortex and posterior parietal cortex.

Prefrontal Cortex Organization

The PFC is not functionally homogeneous. Different regions contribute distinct executive processes:

Prefrontal Cortex Structure and Executive Performance

Dysexecutive Syndrome

Damage to the prefrontal cortex produces a constellation of deficits collectively termed dysexecutive syndrome:

• Perseveration: Inability to shift responses when task rules change.
• Utilization behaviour: Compulsive grasping and use of objects placed within reach, reflecting failure of inhibitory control.
• Environmental dependency: Behaviour driven by environmental cues rather than internal goals.
• Confabulation: Production of false memories or narratives without deliberate intent to deceive, reflecting failure of monitoring and source memory.
• Impaired planning and sequencing: Difficulty organizing multi-step actions to achieve goals.
• Reduced fluency: Decreased generation of words (verbal fluency) or designs (design fluency).

The frontal lobe paradox refers to the observation that patients with extensive frontal damage may perform normally on structured neuropsychological tests (where the examiner provides goals, rules, and monitoring) yet show devastating impairments in unstructured real-world settings that require self-initiated goal-setting and self-monitoring.

Chapter 13: The Default Mode Network

Discovery of the Default Mode Network

The default mode network (DMN) was discovered through a convergence of PET and fMRI studies showing that a consistent set of brain regions are more active during rest than during externally directed cognitive tasks. Marcus Raichle and colleagues coined the term in 2001, building on earlier observations by Shulman et al. (1997) that certain regions systematically decreased activity (“deactivated”) during attention-demanding tasks.

Core Regions

The DMN comprises:

• Medial prefrontal cortex (mPFC): Self-referential processing, social cognition, mental simulation.
• Posterior cingulate cortex (PCC) / precuneus: Autobiographical memory retrieval, spatial navigation, integration of self-relevant information.
• Angular gyrus / inferior parietal lobule: Semantic processing, episodic recollection, attention to internal mentation.
• Medial temporal lobe (hippocampus, parahippocampal cortex): Episodic memory, scene construction, future imagination.
• Lateral temporal cortex: Semantic memory, narrative comprehension.

Functions of the Default Mode Network

Rather than representing mere “neural noise” during rest, the DMN supports several important cognitive functions:

Mind Wandering and Spontaneous Thought

The DMN is most active during mind wandering—spontaneous, internally generated thought that is decoupled from the external environment. Mind wandering serves constructive functions including planning, creative problem-solving, and consolidation of self-relevant information, though it can also impair performance on external tasks.

Self-Referential Processing

The DMN, particularly the medial prefrontal cortex, activates during tasks requiring evaluation of information in relation to the self—judgments about one’s own personality traits, preferences, and emotional states. This has led to proposals that the DMN is the neural substrate of the narrative self—the ongoing inner story we construct about who we are.

Episodic Memory and Future Simulation

There is substantial overlap between the DMN and the brain network activated during episodic memory retrieval and episodic future thinking (imagining personal future events). The DMN may support a general-purpose simulation or scene construction mechanism that enables both remembering past experiences and projecting oneself into hypothetical future scenarios.

Social Cognition

The DMN overlaps extensively with the mentalizing network (also called the theory of mind network)—the set of regions active when inferring others’ mental states, beliefs, and intentions. This overlap suggests that understanding other minds and understanding one’s own mind rely on fundamentally similar neural machinery.

The DMN and Cognitive Control

A critical feature of normal brain function is the anticorrelation between the DMN and task-positive networks (such as the dorsal attention network and the central executive network). When external task demands increase, the DMN deactivates, freeing resources for goal-directed processing. When external demands diminish, the DMN reactivates.

DMN Dysfunction in Neuropsychiatric Disorders

Abnormal DMN function has been implicated in numerous neuropsychiatric conditions:

Chapter 14: Aging and the Brain

Structural Changes in the Aging Brain

Normal aging is associated with progressive changes in brain structure:

• Grey matter volume decline: Beginning in the third decade of life, with the prefrontal cortex and hippocampus showing the most pronounced shrinkage. Sensory cortices (V1, A1) are relatively preserved.
• White matter deterioration: Decline in white matter volume and integrity (reduced fractional anisotropy on DTI), particularly in prefrontal regions and long-range association tracts. This follows an anterior-to-posterior gradient, with frontal tracts declining earliest and most severely.
• Ventricular enlargement: The ventricles enlarge progressively with age, reflecting overall brain tissue loss.
• Neurotransmitter changes: Dopamine levels decline approximately 10% per decade from early adulthood, with reductions in D1 and D2 receptor density particularly in the striatum and prefrontal cortex. This dopaminergic decline contributes to slowed processing speed, reduced working memory capacity, and impaired reward learning.

Cognitive Changes in Normal Aging

Not all cognitive abilities decline equally with age:

Theories of Cognitive Aging

Processing Speed Theory (Salthouse)

Timothy Salthouse proposed that age-related cognitive decline is primarily driven by a slowing of information processing speed. According to this view, slower processing causes two problems: (1) relevant operations cannot be completed in available time (limited time mechanism), and (2) the products of earlier processing have decayed by the time later processing is complete (simultaneity mechanism). Processing speed, as measured by simple tasks like digit–symbol substitution, statistically accounts for a large proportion of age-related variance in higher-order cognitive tasks.

Frontal Lobe Hypothesis

The frontal lobe hypothesis of aging posits that because the prefrontal cortex undergoes the most pronounced age-related decline, executive functions should show the greatest age-related impairment. The age-related decline in working memory, inhibitory control, and multitasking—functions dependent on the PFC—supports this hypothesis. However, the hippocampus also declines substantially, and memory deficits in aging cannot be fully explained by frontal lobe dysfunction alone.

Compensation and Reserve

Neural compensation refers to the recruitment of additional brain regions or networks to maintain cognitive performance despite age-related structural decline. The HAROLD model (Hemispheric Asymmetry Reduction in Older Adults, Cabeza, 2002) describes the finding that older adults show more bilateral prefrontal activation during cognitive tasks that produce lateralized activation in younger adults. This bilateral recruitment may reflect compensation for declining neural efficiency.

Cognitive reserve refers to the brain’s ability to tolerate pathology before clinical symptoms emerge. Individuals with higher education, occupational complexity, and intellectual engagement throughout life can sustain more brain damage before reaching the threshold for cognitive impairment. Reserve may operate through both brain reserve (larger brain/more neurons providing a structural buffer) and cognitive reserve (more efficient or flexible neural networks).

The PASA and STAC Models

The PASA model (Posterior-Anterior Shift in Aging, Davis et al., 2008) describes a shift from posterior (occipital/parietal) to anterior (frontal) activation during visual and memory tasks in older adults, suggesting frontal regions compensate for declining posterior function.

The STAC model (Scaffolding Theory of Aging and Cognition, Park & Reuter-Lorenz, 2009) provides an integrative framework: the aging brain continually builds “scaffolds”—compensatory neural circuits, often involving frontal cortex—that partially offset the effects of neural decline on cognition. The degree to which scaffolding is successful determines the trajectory of cognitive aging for each individual.

Chapter 15: Dementias

Definition and Overview

Dementia is a clinical syndrome characterized by progressive decline in cognitive function severe enough to interfere with daily activities, occurring in the setting of clear consciousness (distinguishing it from delirium). Dementia is not a single disease but a common endpoint of many different neuropathological processes.

Alzheimer’s Disease

Alzheimer’s disease (AD) is the most common cause of dementia, accounting for 60–80% of cases. It is characterized neuropathologically by two hallmark lesions:

• Amyloid plaques: Extracellular deposits of beta-amyloid (A\(\beta\)) protein, a cleavage product of amyloid precursor protein (APP). The amyloid cascade hypothesis proposes that abnormal accumulation of A\(\beta\) (particularly the A\(\beta\)42 isoform) triggers a pathological cascade leading to neuronal death.
• Neurofibrillary tangles: Intracellular aggregates of hyperphosphorylated tau protein, which normally stabilizes microtubules. When tau becomes hyperphosphorylated, it detaches from microtubules and forms paired helical filaments that disrupt intracellular transport and eventually kill the neuron.

Clinical Presentation

AD typically begins with episodic memory impairment (difficulty learning new information, repeatedly asking the same questions) reflecting early involvement of the entorhinal cortex and hippocampus. As the disease progresses, additional cognitive domains are affected:

• Semantic memory: Word-finding difficulty, impaired object naming
• Visuospatial function: Getting lost in familiar environments, difficulty with construction tasks
• Executive function: Poor judgment, impaired planning, loss of insight
• Language: Progressive anomia evolving to non-fluent speech
• Praxis: Difficulty performing learned motor sequences (dressing, using utensils)

Braak Staging

The spread of tau pathology in AD follows a predictable pattern described by Braak staging:

• Stages I–II (Transentorhinal): Tau tangles confined to the entorhinal cortex; clinically asymptomatic or minimal symptoms.
• Stages III–IV (Limbic): Tangles spread to the hippocampus and surrounding limbic structures; mild cognitive impairment.
• Stages V–VI (Neocortical): Tangles spread throughout the neocortex; frank dementia.

Risk Factors

The greatest risk factor for AD is age: prevalence doubles approximately every 5 years after age 65, reaching 30–50% by age 85. The APOE \(\varepsilon\)4 allele is the strongest genetic risk factor for late-onset AD, increasing risk 3-fold (one copy) to 12-fold (two copies). Other risk factors include cardiovascular disease, diabetes, traumatic brain injury, low educational attainment, and social isolation.

Frontotemporal Dementia

Frontotemporal dementia (FTD) is a group of neurodegenerative disorders primarily affecting the frontal and temporal lobes. It is the second most common cause of early-onset dementia (before age 65). Three clinical variants are recognized:

• Behavioural variant FTD (bvFTD): Characterized by personality change, disinhibition, apathy, loss of empathy, compulsive/ritualistic behaviours, and dietary changes (craving sweets). Atrophy is typically bilateral but may be right-predominant in the orbitofrontal and anterior temporal regions.
• Semantic dementia (semantic variant primary progressive aphasia): Progressive loss of word meaning and object knowledge, with fluent but “empty” speech. Atrophy predominantly affects the left anterior temporal lobe.
• Progressive non-fluent aphasia (non-fluent variant PPA): Effortful, halting speech with grammatical errors (agrammatism), impaired articulation, and preserved comprehension of single words. Atrophy affects the left inferior frontal region and insula.

Vascular Dementia

Vascular dementia results from cerebrovascular disease—strokes (large or small), chronic ischaemia, or haemorrhage. It may present as sudden stepwise deterioration following strokes or as insidious decline due to cumulative small-vessel disease. Subcortical vascular disease produces a characteristic profile of slowed processing speed, executive dysfunction, and gait disturbance, with relatively preserved recognition memory.

Lewy Body Dementia

Dementia with Lewy bodies (DLB) is characterized by fluctuating cognition, recurrent visual hallucinations (often vivid and detailed), and parkinsonism. Lewy bodies are intracellular inclusions of misfolded alpha-synuclein protein. DLB shares pathological features with Parkinson’s disease dementia (PDD), and the two are distinguished primarily by the timing of motor versus cognitive symptom onset (the “one-year rule”).

Chapter 16: Schizophrenia

Clinical Features

Schizophrenia is a severe psychiatric disorder affecting approximately 1% of the global population, with typical onset in late adolescence or early adulthood. Its clinical features are traditionally classified into three clusters:

Positive Symptoms

Positive symptoms represent an excess or distortion of normal function:

• Hallucinations: Most commonly auditory (hearing voices), but visual, tactile, and olfactory hallucinations also occur. Auditory hallucinations have been associated with aberrant activation of Heschl’s gyrus and superior temporal cortex.
• Delusions: Fixed false beliefs resistant to contradictory evidence. Common types include persecutory (beliefs about being watched or targeted), referential (beliefs that neutral events carry personal significance), and grandiose delusions.
• Thought disorder: Disorganized speech reflecting disorganized thinking—tangential, loosely associated, or incoherent.

Negative Symptoms

Negative symptoms represent a diminution of normal function:

• Flat affect: Reduced emotional expression
• Alogia: Poverty of speech
• Avolition: Reduced motivation and goal-directed activity
• Anhedonia: Reduced capacity to experience pleasure
• Social withdrawal

Negative symptoms are often more treatment-resistant than positive symptoms and more strongly associated with poor functional outcomes.

Cognitive Deficits

Cognitive impairment is now recognized as a core feature of schizophrenia, not merely a side effect of medication or psychosis. Deficits are observed in:

• Working memory: Impaired maintenance and manipulation of information, associated with DLPFC dysfunction.
• Executive function: Impaired planning, cognitive flexibility, and inhibitory control.
• Attention: Impaired sustained attention and prepulse inhibition.
• Processing speed: Generalized slowing.
• Episodic memory: Impaired encoding and retrieval.
• Social cognition: Impaired theory of mind, emotion recognition, and social perception.

Neural Basis of Schizophrenia

The Dopamine Hypothesis

The dopamine hypothesis remains the most influential neurochemical model of schizophrenia. In its revised form, it proposes:

• Mesolimbic dopamine hyperactivity: Excessive dopamine signalling in the ventral striatum and limbic system drives positive symptoms. All effective antipsychotic medications block D2 dopamine receptors, and their clinical potency correlates with D2 binding affinity.
• Mesocortical dopamine hypofunction: Reduced dopamine activity in the prefrontal cortex (particularly D1 receptor stimulation) contributes to negative symptoms and cognitive deficits.

Structural Brain Changes

• Ventricular enlargement: Enlarged lateral ventricles, reflecting generalized brain tissue loss.
• Grey matter reduction: Progressive loss of grey matter, particularly in prefrontal, temporal, and parietal cortex. Longitudinal studies show that grey matter loss may precede psychosis onset.
• Hippocampal volume reduction: Consistent finding, associated with memory deficits.
• White matter abnormalities: Reduced fractional anisotropy in multiple tracts, suggesting disconnectivity.

Network Dysfunction: The Salience Network Model

Glutamate Hypothesis

The glutamate hypothesis proposes that hypofunction of NMDA-type glutamate receptors on cortical interneurons leads to reduced GABAergic inhibition, resulting in cortical disinhibition and downstream dopaminergic dysregulation. Support comes from the observation that NMDA receptor antagonists (ketamine, PCP) produce a schizophrenia-like syndrome including positive symptoms, negative symptoms, and cognitive deficits—more closely mimicking the full disorder than dopaminergic agents.

Neurodevelopmental Model

Schizophrenia is increasingly understood as a neurodevelopmental disorder with origins in prenatal or early postnatal brain development. Risk factors include:

• Genetic vulnerability: Heritability is approximately 80%; polygenic risk involving hundreds of common variants of small effect plus rare copy number variants (e.g., 22q11.2 deletion).
• Prenatal insults: Maternal infection, malnutrition, obstetric complications.
• Abnormal pruning: Excessive synaptic pruning during adolescence, particularly in the prefrontal cortex, may precipitate psychosis onset. Recent genetic evidence implicating complement component C4 in schizophrenia risk suggests that overactive synaptic pruning mechanisms may contribute to pathology.

Chapter 17: Language

Language and the Brain

Language is among the most lateralized of cognitive functions, with the left hemisphere dominant for language in the vast majority of individuals. The neural architecture supporting language involves a distributed network of cortical regions connected by white matter pathways.

Classical Language Areas

Broca’s Area

Broca’s area occupies the posterior portion of the left inferior frontal gyrus (Brodmann areas 44 and 45). It is critical for:

• Speech production: Articulatory planning and the motor programming of speech output.
• Syntactic processing: Comprehending and producing grammatically complex sentences.
• Verbal working memory: Maintaining phonological information through subvocal rehearsal.
• Action comprehension: More recent evidence implicates Broca’s area in understanding actions and gestures, suggesting a broader role in hierarchical sequential processing.

Wernicke’s Area

Wernicke’s area encompasses the posterior portion of the left superior temporal gyrus (Brodmann area 22) and adjacent regions. It is essential for:

• Speech comprehension: Mapping acoustic speech signals onto lexical representations.
• Semantic processing: Accessing word meanings.
• Phonological processing: Processing the sound structure of words.

The Arcuate Fasciculus

The arcuate fasciculus, a major white matter tract, connects Broca’s and Wernicke’s areas, enabling the transfer of linguistic information between anterior (production) and posterior (comprehension) language regions. Damage to the arcuate fasciculus produces conduction aphasia.

The Dual-Stream Model of Language

Modern models have moved beyond the classical Broca–Wernicke–Lichtheim framework to propose a dual-stream architecture for language processing (Hickok & Poeppel, 2007):

Aphasia Types

Aphasia is an acquired language disorder resulting from brain damage, most commonly stroke. The major aphasia types are:

Language Disorders After Subcortical Damage

Reading and Writing Disorders

Alexia

Alexia (acquired reading impairment) takes several forms:

• Pure alexia (alexia without agraphia): Patients can write but cannot read, even their own writing. Results from damage to the left visual cortex and splenium of the corpus callosum, disconnecting the intact right visual cortex from left-hemisphere language areas. Patients often read letter-by-letter, with reading time proportional to word length.
• Surface alexia: Difficulty reading irregularly spelled words (e.g., “yacht,” “colonel”) while regular words and nonwords are read normally. Reflects damage to the lexical (whole-word) reading route.
• Phonological alexia: Difficulty reading nonwords (e.g., “bint,” “slig”) while real words are read relatively normally. Reflects damage to the sublexical (grapheme-to-phoneme conversion) reading route.
• Deep alexia: Impaired nonword reading combined with semantic errors (e.g., reading “symphony” as “orchestra”). The most severe acquired reading impairment.

Agraphia

Agraphia (acquired writing impairment) parallels the alexia subtypes:

• Phonological agraphia: Difficulty spelling nonwords.
• Surface agraphia: Difficulty spelling irregular words, with phonologically plausible errors (e.g., writing “sure” as “shor”).
• Deep agraphia: Semantic errors in spelling.

These dissociations support dual-route models of reading and writing, which propose a lexical route (direct mapping between whole-word orthographic and phonological representations) and a sublexical route (rule-based conversion between graphemes and phonemes).

Language Recovery After Brain Damage

Recovery from aphasia depends on lesion size, location, and the patient’s age and premorbid language abilities. Functional neuroimaging studies of aphasia recovery reveal two patterns:

• Perilesional reorganization: Reactivation and expansion of left-hemisphere language areas surrounding the lesion. This pattern is generally associated with better recovery.
• Right hemisphere recruitment: Activation of right-hemisphere homologues of left-hemisphere language areas. The contribution of right hemisphere involvement to recovery is debated—it may reflect genuine compensatory processing or maladaptive competition with left-hemisphere networks.

Constraint-induced language therapy (CILT), inspired by constraint-induced movement therapy for motor recovery, requires patients to communicate using speech (rather than compensatory strategies like gestures) in intensive, structured practice. Evidence suggests that this approach can promote reorganization of left perilesional language networks and improve language function even in chronic aphasia.