Medical School · Year 2 · Neuroscience · includes a quiz and discussion video

Lecture 7: Cerebral Cortex and Higher Functions

Unit 2.5: Neuroscience

Learning Objectives

By the end of this lecture, students will be able to:

1. Describe the cytoarchitecture and columnar organization of the cerebral cortex
2. Explain the functions and clinical syndromes associated with each cerebral lobe
3. Describe language processing, localize language areas, and classify aphasia syndromes
4. Explain the neural basis of spatial attention and hemispatial neglect
5. Describe the memory systems and their neural substrates including the hippocampus
6. Explain executive functions, consciousness, and their assessment in clinical practice

1. Cortical Organization

The cerebral cortex represents the pinnacle of neural evolution, a thin sheet of neurons just a few millimeters thick that underlies all conscious experience, voluntary action, and higher cognitive function. Understanding its organization provides the foundation for localizing lesions and predicting functional deficits from cortical damage.

The neocortex, comprising the vast majority of human cortex, exhibits a six-layer organization that varies in relative thickness depending on the cortical region's function. Layer I, the molecular layer, contains few cell bodies but abundant dendrites and axon terminals where extensive synaptic processing occurs. Layers II and III, the external granular and pyramidal layers, contain small pyramidal neurons that give rise to corticocortical connections linking different cortical areas. Layer IV, the internal granular layer, receives thalamic input and is particularly prominent in sensory cortices. Layer V contains large pyramidal neurons, including the giant Betz cells of the motor cortex, that project to subcortical targets including the spinal cord. Layer VI projects reciprocally to thalamus.

Cortical types vary in their laminar organization. Granular or koniocortex, found in primary sensory areas, exhibits a prominent layer IV adapted for receiving abundant thalamic input. Agranular cortex, characteristic of primary motor cortex, has minimal layer IV but a prominent layer V for generating output. Allocortex, found in evolutionarily older structures like the hippocampus and olfactory cortex, has only three layers rather than six.

Brodmann areas divide the cortex based on cytoarchitectural differences, providing a standardized map for localizing cortical functions. Key areas include area 4 for primary motor cortex, areas 3, 1, and 2 for primary somatosensory cortex, area 17 for primary visual cortex, and areas 41 and 42 for primary auditory cortex. Functionally related Brodmann areas often act together, as in language processing involving areas 44 and 45 for speech production and area 22 for comprehension.

<image>A comprehensive cortical organization illustration. Panel A shows a cross-section of neocortex with six layers labeled and characterized: Layer I (molecular, few cells, many synapses), Layers II-III (small pyramidal, corticocortical connections), Layer IV (granular, thalamic input), Layer V (large pyramidal including Betz cells, subcortical projections), Layer VI (multiform, corticothalamic). Panel B compares cortical types: granular/koniocortex (prominent layer IV, sensory cortex), agranular cortex (minimal layer IV, thick layer V, motor cortex), and allocortex (three layers, hippocampus). Panel C displays a lateral view of the cerebral hemisphere with major Brodmann areas color-coded and numbered: 4 (primary motor), 3,1,2 (somatosensory), 17 (visual), 41/42 (auditory), 44/45 (Broca's), 22 (Wernicke's), 6 (premotor), 8 (frontal eye field). Panel D illustrates columnar organization: vertical columns 0.5-1mm wide where cells share similar response properties, exemplified by ocular dominance columns in V1.</image>

2. Frontal Lobe

The frontal lobe, the largest of the four cerebral lobes, occupies everything anterior to the central sulcus and superior to the lateral sulcus. It encompasses the entire motor hierarchy from motor planning through execution, as well as the prefrontal cortex that governs executive function and personality.

The primary motor cortex in the precentral gyrus executes voluntary movement through somatotopically organized outputs to spinal motor neurons. The premotor cortex, lateral to primary motor cortex in Brodmann area 6, plans movements based on external sensory cues and controls proximal and trunk musculature. The supplementary motor area on the medial surface plans internally generated movement sequences, particularly learned motor programs. Lesions here initially cause mutism that typically recovers over time. The frontal eye fields in area 8 generate voluntary saccadic eye movements toward contralateral visual targets.

The prefrontal cortex comprises the anterior frontal lobe and underlies executive functions, the cognitive processes that enable goal-directed behavior. The dorsolateral prefrontal cortex supports working memory, the ability to hold information "online" while manipulating it, as well as planning, reasoning, and cognitive flexibility. The ventromedial prefrontal cortex integrates emotion with decision-making, weighing costs and benefits to guide choices. The orbitofrontal cortex mediates social behavior, impulse control, and reward processing. The anterior cingulate cortex monitors for conflict and errors, signaling when cognitive control adjustments are needed.

Frontal lobe syndromes produce characteristic changes depending on the region affected. Dorsolateral lesions impair planning, abstract reasoning, and working memory, with patients unable to shift cognitive sets and showing perseveration. Orbitofrontal lesions produce disinhibition, impulsivity, and socially inappropriate behavior; the famous case of Phineas Gage exemplified this syndrome. Medial frontal lesions cause apathy and reduced spontaneous activity, progressing in severe cases to akinetic mutism where patients lie motionless despite intact motor pathways.

<image>A frontal lobe illustration with multiple panels. Panel A shows a lateral and medial view of the frontal lobe with regions color-coded: primary motor cortex (precentral gyrus), premotor cortex (lateral area 6), supplementary motor area (medial area 6), prefrontal cortex (anterior), frontal eye field (area 8), Broca's area (44/45). Panel B depicts prefrontal cortex subdivisions: dorsolateral (working memory, planning), ventromedial (decision-making, emotion), orbitofrontal (social behavior, impulse control), anterior cingulate (conflict monitoring). Panel C illustrates executive functions as a flowchart: goal setting, planning, execution, monitoring, with corresponding prefrontal regions. Panel D shows frontal lobe syndromes: dorsolateral (perseveration, poor planning), orbitofrontal (disinhibition, social dysfunction with Phineas Gage case reference), medial (apathy, akinetic mutism).</image>

3. Parietal and Temporal Lobes

The parietal lobe, bounded by the central sulcus anteriorly, the parieto-occipital sulcus posteriorly, and the lateral sulcus inferiorly, processes somatosensory information and integrates multiple sensory modalities for spatial awareness and action guidance.

Primary somatosensory cortex in the postcentral gyrus contains the sensory homunculus, receiving tactile, proprioceptive, and nociceptive input from the contralateral body via VPL thalamus. The posterior parietal cortex, comprising the superior and inferior parietal lobules, performs higher-order sensory integration. The superior parietal lobule (area 5 and 7) participates in spatial orientation and visuomotor coordination for reaching movements. The inferior parietal lobule includes the supramarginal gyrus (phonological processing) and angular gyrus (language, reading, mathematical operations). The intraparietal sulcus contains areas specialized for eye movements, reaching, grasping, and numerical processing. Lesions of the dominant angular gyrus produce Gerstmann syndrome: finger agnosia, right-left confusion, agraphia, and acalculia. Bilateral posterior parietal lesions cause Balint syndrome: simultanagnosia, optic ataxia, and ocular apraxia.

The temporal lobe, inferior to the lateral sulcus and anterior to the occipito-temporal junction, processes auditory information, language comprehension, and memory formation. Primary auditory cortex in Heschl's gyrus (areas 41 and 42) receives tonotopically organized input from the medial geniculate nucleus. Wernicke's area in the posterior superior temporal gyrus (area 22) of the dominant hemisphere processes language comprehension; lesions cause fluent but meaningless speech with poor comprehension.

The temporal lobe also contributes to the ventral visual stream, the "what" pathway for object identification. Inferior temporal cortex recognizes complex visual stimuli. The fusiform face area specializes in face recognition; its damage causes prosopagnosia, the inability to recognize familiar faces despite intact visual perception. The medial temporal lobe, including the hippocampus and entorhinal cortex, is essential for forming new declarative memories. Bilateral hippocampal damage causes profound anterograde amnesia, as demonstrated in the famous patient HM.

<image>A parietal and temporal lobe illustration. Panel A shows parietal lobe anatomy: primary somatosensory cortex (postcentral gyrus with homunculus), superior parietal lobule (spatial orientation, reaching), inferior parietal lobule (supramarginal and angular gyri), intraparietal sulcus. Dorsal visual stream ("where/how" pathway) is indicated flowing from occipital to parietal cortex. Panel B depicts temporal lobe: primary auditory cortex (Heschl's gyrus), auditory association cortex, Wernicke's area (posterior superior temporal gyrus), inferior temporal cortex, fusiform face area, medial temporal structures (hippocampus, entorhinal cortex). Ventral visual stream ("what" pathway) flows from occipital to temporal cortex. Panel C shows clinical syndromes: Gerstmann syndrome (finger agnosia, right-left confusion, agraphia, acalculia from dominant angular gyrus lesion), prosopagnosia (face blindness from fusiform lesion), anterograde amnesia (bilateral hippocampal damage, with HM case referenced).</image>

4. Occipital Lobe and Visual Processing

The occipital lobe occupies the posterior pole of the cerebral hemisphere, devoted entirely to visual processing from primary reception through complex analysis via a hierarchy of specialized areas.

Primary visual cortex, V1 or striate cortex in Brodmann area 17, lines the banks of the calcarine sulcus on the medial occipital surface. It maintains precise retinotopic organization, with the fovea represented at the posterior pole and occupying disproportionately large cortical territory. The upper visual field projects to cortex below the calcarine sulcus, while the lower field projects above it. V1 neurons respond to oriented edges and lines, extracting basic features from the visual scene that will be combined into objects and scenes by higher areas.

Beyond V1, visual information flows through a hierarchy of extrastriate areas. V2 surrounds V1 and continues basic feature processing. V4, in ventral occipitotemporal cortex, specializes in color processing; lesions cause cerebral achromatopsia, color blindness from cortical rather than retinal pathology. V5, also called area MT, processes motion; lesions cause akinetopsia, the inability to perceive motion with the world appearing as a series of static frames.

Visual processing divides into dorsal and ventral streams with distinct functions. The dorsal stream flows from V1 through posterior parietal cortex, processing spatial location and motion to guide action, often called the "where" or "how" pathway. The ventral stream flows from V1 through inferior temporal cortex, processing object identity and often called the "what" pathway. This division explains why lesions can dissociate spatial processing from object recognition.

Occipital lobe lesions produce characteristic visual deficits. Unilateral V1 lesions cause contralateral homonymous hemianopia, often with macular sparing because the posterior pole receives dual vascular supply. Bilateral V1 destruction causes cortical blindness, the complete loss of conscious vision despite intact eyes and subcortical visual pathways. Some cortically blind patients exhibit blindsight, the ability to respond to visual stimuli without conscious awareness, through preserved subcortical pathways. Anton syndrome describes cortical blindness with denial of visual loss, where patients confabulate rather than acknowledge their deficit.

<image>An occipital lobe and visual processing illustration. Panel A shows medial occipital anatomy: V1 along calcarine sulcus with retinotopic organization (fovea at pole, upper field below calcarine, lower field above). V2 surrounding V1, with V4 and V5/MT indicated. Panel B depicts the visual processing hierarchy: V1 (edges, orientation), V2 (continued processing), V4 (color), V5/MT (motion). Panel C illustrates dorsal versus ventral streams: dorsal ("where/how") from V1 through posterior parietal cortex (spatial location, action guidance), ventral ("what") from V1 through inferior temporal cortex (object identity, faces). Panel D shows clinical correlations: hemianopia visual field diagram from unilateral V1 lesion, cortical blindness from bilateral V1, akinetopsia description (motion blindness from V5), Anton syndrome (blindness with denial).</image>

5. Language

Language represents one of the most lateralized cognitive functions, with critical areas residing in the left hemisphere in approximately 95% of right-handed and 70% of left-handed individuals. Understanding language organization enables precise localization of lesions and classification of aphasia syndromes.

Broca's area in the posterior inferior frontal gyrus (Brodmann areas 44 and 45) governs speech production, including the motor programming of speech and syntax. Its proximity to primary motor cortex for face and tongue facilitates articulation. Wernicke's area in the posterior superior temporal gyrus (area 22) processes language comprehension, analyzing the phonological and semantic content of heard speech. The arcuate fasciculus, a white matter tract, connects these regions, enabling the translation of comprehended language into spoken output. The angular gyrus participates in reading and writing, converting visual word forms into linguistic representations.

Aphasia, acquired language impairment from brain damage, classifies based on three key features: fluency, comprehension, and repetition. Broca's aphasia from frontal lesions produces nonfluent, effortful speech with simplified grammar but relatively preserved comprehension; repetition is impaired. Wernicke's aphasia from temporal lesions produces fluent but meaningless speech with poor comprehension; repetition is impaired. Conduction aphasia from arcuate fasciculus damage preserves fluency and comprehension but impairs repetition, particularly of multisyllabic words and phrases. Global aphasia from large middle cerebral artery strokes affecting both Broca's and Wernicke's areas produces nonfluent speech with poor comprehension and repetition.

Transcortical aphasias spare repetition because the perisylvian language core remains intact while surrounding watershed zones are damaged. Transcortical motor aphasia resembles Broca's but with intact repetition. Transcortical sensory aphasia resembles Wernicke's but with intact repetition. These patterns arise from watershed infarcts surrounding the perisylvian language network. Anomic aphasia presents with word-finding difficulty in the context of otherwise fluent, comprehensible speech with intact repetition; it can result from various lesion locations or represent the recovery phase of other aphasias.

<image>A language illustration with multiple panels. Panel A shows language area anatomy on a lateral left hemisphere: Broca's area (areas 44, 45 in inferior frontal gyrus), Wernicke's area (area 22 in posterior superior temporal gyrus), arcuate fasciculus (white matter tract connecting them, shown coursing through parietal white matter), angular gyrus (reading, writing). Panel B presents an aphasia classification flowchart: fluency first branch (nonfluent vs fluent), then comprehension (intact vs impaired), then repetition (impaired vs intact), leading to diagnosis (Broca's, transcortical motor, Wernicke's, transcortical sensory, conduction, anomic, global). Panel C depicts the perisylvian language network showing how transcortical aphasias involve watershed zones while sparing the core. Panel D shows language lateralization: left hemisphere dominant in 95% of right-handers and 70% of left-handers, with right hemisphere contributing prosody (emotional tone) and pragmatics.</image>

6. Attention and Hemispatial Neglect

Attention determines which of countless sensory inputs reach conscious awareness and receive cognitive processing. Multiple neural networks govern different aspects of attention, with the right hemisphere playing a dominant role in spatial attention that explains the clinical phenomenon of hemispatial neglect.

Three attention networks serve distinct functions. The alerting network, involving the locus coeruleus and right frontal cortex, maintains vigilance and sustained attention over time through noradrenergic modulation. The orienting network, engaging posterior parietal cortex, frontal eye fields, and superior colliculus, directs attention to specific spatial locations or sensory features. The executive attention network, centered on the anterior cingulate and lateral prefrontal cortex, resolves conflict between competing stimuli and monitors for errors.

Spatial attention shows striking hemispheric asymmetry. The right hemisphere can direct attention to both left and right space, while the left hemisphere attends primarily to right space. This asymmetry means that right hemisphere damage leaves only left hemisphere attention to right space, while nothing compensates for the lost ability to attend left. Consequently, right parietal lesions commonly produce left hemispatial neglect, while left parietal lesions rarely cause right neglect because the intact right hemisphere can compensate.

Hemispatial neglect manifests as failure to detect, respond to, or orient toward stimuli in contralateral space despite intact primary sensory function. Patients may eat food from only the right side of their plate, shave or apply makeup to only the right side of their face, and collide with obstacles on their left. On drawing tasks, they omit left-sided details from objects. On line bisection, they mark the midpoint significantly rightward. Extinction, a milder manifestation, appears when bilateral simultaneous stimulation reveals that the patient fails to detect the left stimulus while detecting either stimulus presented alone. Importantly, neglect is not simply a sensory deficit: patients can detect left-sided stimuli when attention is explicitly directed there, demonstrating that the problem lies in spontaneous attentional allocation.

<image>A comprehensive attention and neglect illustration. Panel A shows the three attention networks on a brain diagram: alerting network (locus coeruleus, right frontal - sustained attention), orienting network (posterior parietal, FEF, superior colliculus - directing attention), executive network (anterior cingulate, lateral prefrontal - conflict resolution). Panel B illustrates spatial attention asymmetry: right hemisphere shown attending to both hemifields, left hemisphere attending primarily to right; explanation of why right parietal lesions cause left neglect (no compensation) while left lesions don't cause right neglect (right hemisphere compensates). Panel C shows neglect manifestations: patient drawing showing omitted left-sided details, clock with all numbers on right side, line bisection with mark shifted rightward, eating from only right side of plate. Panel D illustrates extinction testing: single left stimulus detected, single right stimulus detected, bilateral stimulation results in left stimulus missed.</image>

7. Memory Systems

Memory encompasses multiple distinct systems mediated by different brain structures, each serving particular types of information and timeframes. This dissociation explains why patients with specific lesions show selective memory impairments while other memory functions remain intact.

Memory stages include encoding, consolidation, storage, and retrieval. Sensory memory, lasting only milliseconds, briefly holds iconic (visual) or echoic (auditory) representations. Working memory, lasting seconds to minutes, actively maintains and manipulates information in conscious awareness; it depends on prefrontal cortex and permits ongoing cognitive operations like mental arithmetic. Long-term memory stores information for extended periods, potentially a lifetime, through structural changes at synapses.

Long-term memory divides into declarative and nondeclarative systems. Declarative or explicit memory, consciously accessible, includes episodic memory for personal events and experiences, and semantic memory for facts and general knowledge. The hippocampus is essential for forming new declarative memories but not for storing remote ones, which reside in distributed cortical networks. Nondeclarative or implicit memory encompasses skills and habits that influence behavior without conscious recollection. Procedural memory for motor skills depends on basal ganglia and cerebellum. Priming effects, where prior exposure facilitates subsequent processing, involve cortical perceptual areas. Classical conditioning engages the amygdala for emotional associations and cerebellum for motor associations.

The hippocampus and associated medial temporal structures are critical for episodic memory formation. Patient HM, who underwent bilateral medial temporal resection for epilepsy, developed profound anterograde amnesia, unable to form new declarative memories, while remote memories and procedural learning remained intact. The Papez circuit connects the hippocampus through the fornix to the mammillary bodies, then via the mammillothalamic tract to the anterior thalamus, which projects to the cingulate gyrus, completing the loop back to the hippocampus. Lesions anywhere in this circuit impair memory. Korsakoff syndrome, from thiamine deficiency typically in alcoholism, damages the mammillary bodies and produces both anterograde and retrograde amnesia with confabulation.

<image>A comprehensive memory systems illustration. Panel A shows memory stages as a flowchart: sensory memory (milliseconds), working memory (seconds-minutes, prefrontal cortex), long-term memory (days-years, distributed). Panel B depicts the memory taxonomy tree: declarative/explicit (episodic - personal events, hippocampus; semantic - facts, temporal cortex) and nondeclarative/implicit (procedural - skills, basal ganglia/cerebellum; priming - perceptual facilitation, cortex; conditioning - associations, amygdala/cerebellum). Panel C illustrates the hippocampus and Papez circuit: hippocampus to fornix to mammillary bodies to mammillothalamic tract to anterior thalamus to cingulate gyrus back to hippocampus. Panel D shows clinical correlations: HM case (bilateral hippocampal resection, anterograde amnesia, intact procedural memory), Korsakoff syndrome (mammillary body damage, amnesia, confabulation).</image>

8. Clinical Syndromes

Focal cortical lesions produce characteristic syndromes that depend on the affected region's specialized function. Recognizing these patterns enables lesion localization and guides diagnostic workup.

Agnosias represent the inability to recognize stimuli despite intact primary sensory processing. Visual agnosia, from bilateral occipitotemporal lesions, prevents object recognition despite adequate vision; patients can describe visual features but cannot identify what they see. Prosopagnosia specifically impairs face recognition while sparing object recognition; patients may fail to recognize close family members but know them immediately by voice. Auditory agnosia prevents recognition of sounds despite normal hearing. Anosognosia, unawareness of one's own deficit, commonly accompanies right parietal lesions causing neglect; patients deny being paralyzed or blind.

Apraxias represent the inability to perform skilled learned movements despite intact motor function, sensation, and comprehension. Ideomotor apraxia, typically from left parietal lesions, impairs the ability to pantomime tool use or gesture on command while the same movements may occur spontaneously or when holding the actual tool. Ideational apraxia impairs the ability to sequence actions in their proper order to accomplish a goal. Constructional apraxia, usually from right parietal lesions, impairs the ability to copy drawings or assemble objects.

Disconnection syndromes arise from white matter lesions interrupting communication between cortical areas. Split-brain syndrome following corpus callosum sectioning demonstrates hemispheric specialization: objects presented to the left visual field cannot be named because visual information reaches only the non-language right hemisphere. Alexia without agraphia, from left occipital and splenium lesions, prevents reading while sparing writing because visual information cannot reach language areas. Callosal apraxia causes ideomotor apraxia of the left hand because motor commands from the language-dominant left hemisphere cannot reach right hemisphere motor areas.

<image>A clinical syndromes illustration. Panel A shows agnosias: visual agnosia (bilateral occipitotemporal lesion, patient describing object features without recognition), prosopagnosia (fusiform lesion, inability to recognize faces but intact object recognition), anosognosia (right parietal, denial of deficit). Panel B depicts apraxias: ideomotor (left parietal, cannot pantomime but can use actual tool), ideational (impaired action sequencing), constructional (right parietal, impaired copying/drawing). Panel C illustrates disconnection syndromes: split-brain (corpus callosum sectioning, left visual field objects cannot be named), alexia without agraphia (left occipital + splenium, can write but not read), callosal apraxia (left hand ideomotor apraxia from hemispheric disconnection). Panel D shows testing methods for each syndrome type.</image>

9. Consciousness

Consciousness, the state of awareness of self and environment, depends on the integrated function of the ascending reticular activating system, thalamic relays, and widespread cortical areas. Understanding its neural basis enables evaluation of patients with altered consciousness.

The ascending reticular activating system comprises brainstem nuclei that modulate cortical arousal through distinct neurotransmitter systems. The locus coeruleus provides noradrenergic activation promoting alertness. Raphe nuclei provide serotonergic modulation of sleep-wake cycles. Pedunculopontine and laterodorsal tegmental nuclei provide cholinergic activation. The ventral tegmental area provides dopaminergic drive. The tuberomammillary nucleus provides histaminergic activation. These systems project to the thalamus, which relays activating signals to the cortex. Consciousness requires both arousal from brainstem ARAS and awareness from intact cortical function.

Levels of consciousness form a continuum from full alertness to coma. Alert patients are fully awake and responsive to their environment. Lethargy describes drowsiness with easy arousal. Obtundation involves reduced alertness requiring repeated stimulation for arousal. Stupor permits arousal only with vigorous stimulation, and the patient returns to unresponsiveness when stimulation ceases. Coma represents complete unarousability without awareness, indicating profound brainstem or bilateral hemispheric dysfunction.

The Glasgow Coma Scale standardizes assessment through three components: eye opening (spontaneous = 4, to voice = 3, to pain = 2, none = 1), verbal response (oriented = 5, confused = 4, inappropriate words = 3, incomprehensible = 2, none = 1), and motor response (obeys commands = 6, localizes pain = 5, withdraws = 4, abnormal flexion = 3, extension = 2, none = 1). Scores range from 3 to 15, with lower scores indicating deeper impairment. Coma causes include structural lesions (stroke, tumor, trauma, herniation), metabolic derangements (hypoglycemia, uremia, hepatic encephalopathy), toxic exposures (drugs, alcohol), infections (meningitis, encephalitis), and seizures.

<image>A consciousness illustration. Panel A shows the ARAS and its components: brainstem nuclei (locus coeruleus - NE, raphe - serotonin, PPN/LDT - ACh, VTA - dopamine, TMN - histamine) projecting to thalamus then diffusely to cortex. Arousal (brainstem) and awareness (cortex) components are distinguished. Panel B displays the consciousness continuum: alert (full awareness), lethargic (drowsy, easily aroused), obtunded (reduced alertness, needs repeated stimulation), stuporous (vigorous stimulation required), comatose (unarousable). Panel C shows Glasgow Coma Scale scoring with each category and point values. Panel D lists coma causes by category: structural (stroke, tumor, trauma, herniation), metabolic (glucose, uremia, hepatic), toxic (drugs, alcohol), infectious (meningitis, encephalitis), seizure (postictal, status).</image>

10. Executive Functions and Assessment

Executive functions encompass the cognitive capacities that enable goal-directed behavior, including working memory, attention control, cognitive flexibility, planning, and inhibition. Primarily mediated by prefrontal cortex and its connections, these functions develop throughout childhood and adolescence and are vulnerable to various pathological conditions.

Working memory maintains information online while performing mental operations on it, distinct from simple short-term memory that merely holds information briefly. The phonological loop handles verbal information through subvocal rehearsal. The visuospatial sketchpad maintains and manipulates visual and spatial information. The central executive coordinates these systems and allocates attention. Dorsolateral prefrontal cortex is critical for working memory; lesions impair tasks requiring information maintenance and manipulation.

Cognitive flexibility, the ability to shift between mental sets or adapt to changing rules, depends on prefrontal cortex and its dopaminergic modulation. Patients with prefrontal lesions show perseveration, continuing to apply rules that are no longer appropriate despite feedback. The Wisconsin Card Sorting Test detects such deficits: patients must discover sorting rules through feedback and shift when rules change. Planning requires formulating a sequence of actions to achieve a goal; the Tower of London test assesses this capacity by requiring patients to rearrange colored balls to match a target configuration in the minimum number of moves.

Inhibition, the suppression of prepotent or automatic responses, enables context-appropriate behavior. The Stroop test measures inhibition by requiring patients to name the ink color of color words printed in incongruent colors, such as the word "red" printed in blue ink. Response inhibition deficits produce impulsivity and socially inappropriate behavior. The orbitofrontal cortex is particularly important for inhibition; patients with orbitofrontal damage may blurt out inappropriate comments or act on immediate impulses despite knowing better.

Clinical assessment of executive function includes bedside tests and formal neuropsychological evaluation. Luria hand sequences test motor sequencing. Verbal fluency tests ask patients to generate words beginning with a letter or belonging to a category in limited time. Trail-making tests require alternating between numbers and letters. Clock drawing tests assess planning and visuospatial function. Formal neuropsychological assessment provides detailed characterization of deficits for diagnosis, treatment planning, and monitoring.

<image>An executive function assessment illustration. Panel A depicts working memory components: phonological loop (verbal rehearsal, shown as speech bubble recycling), visuospatial sketchpad (spatial manipulation, shown as mental rotation), central executive (coordinating, allocating attention). DLPFC involvement is indicated. Panel B shows cognitive flexibility tests: Wisconsin Card Sorting Test (matching cards by shifting rules), Trail Making Test B (alternating numbers and letters). Perseveration from frontal lesions is illustrated. Panel C demonstrates inhibition tests: Stroop test (naming ink color while ignoring word), Go/No-Go task, with OFC role indicated. Panel D shows bedside assessment: Luria sequences (fist-palm-side), verbal fluency (words beginning with F), clock drawing (planning, spatial), with examples of abnormal performance indicating frontal dysfunction.</image>

Summary

The cerebral cortex has six layers, with granular cortex (prominent layer IV) in sensory areas and agranular cortex (minimal layer IV, prominent layer V) in motor cortex. Brodmann areas map cytoarchitectural regions to functions.

The frontal lobe contains primary motor cortex (precentral gyrus), premotor and SMA (motor planning), and prefrontal cortex (executive function). Prefrontal subdivisions include dorsolateral (working memory, planning), ventromedial (decision-making), orbitofrontal (impulse control, social behavior), and anterior cingulate (conflict monitoring). Frontal syndromes include dorsolateral (perseveration), orbitofrontal (disinhibition), and medial (apathy).

The parietal lobe contains primary somatosensory cortex (postcentral gyrus) and posterior parietal cortex (spatial processing, attention). The dorsal visual stream ("where/how") flows through parietal cortex. Syndromes include Gerstmann (angular gyrus) and Balint (bilateral posterior parietal).

The temporal lobe contains primary auditory cortex (Heschl's gyrus), Wernicke's area (language comprehension), inferior temporal cortex (ventral visual stream, "what"), and medial temporal structures (hippocampus for memory).

The occipital lobe contains V1 (primary visual, retinotopic), V4 (color), V5/MT (motion). Cortical blindness, blindsight, and Anton syndrome result from occipital damage.

Language is left-hemisphere dominant. Broca's (frontal) produces nonfluent aphasia with intact comprehension. Wernicke's (temporal) produces fluent aphasia with impaired comprehension. The arcuate fasciculus connects them; its damage causes conduction aphasia.

Attention networks include alerting (vigilance), orienting (directing attention), and executive (conflict resolution). Right hemisphere dominance for spatial attention explains why right parietal lesions cause left hemispatial neglect.

Memory systems include declarative/explicit (episodic and semantic, hippocampus-dependent) and nondeclarative/implicit (procedural, priming, conditioning). The Papez circuit links hippocampus, mammillary bodies, and anterior thalamus.

Consciousness requires ARAS arousal plus cortical awareness. The Glasgow Coma Scale assesses eye opening, verbal, and motor responses (3-15).

Key Terms

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