Cognitive Neuroscience and Functional Neuroanatomy

Clinical Neuroanatomy and Aphasias

• Broca's Aphasia (Expressive Aphasia):

  • Historical Case Study: Patient known as "Tan" (Louis Victor Leborgne), who was incapable of producing any speech output other than the single repeated syllable "tan".

  • Post-Mortem Findings: Brain examination following the patient's death revealed localized structural damage to the left inferior frontal cortex, likely caused by a stroke.

  • Functional & Clinical Deficits:

  • Word Retrieval Deficits: Words are inaccessible or cannot be brought up during speech generation.

  • Expressive Speech Impairment: Patients understand spoken questions completely but cannot answer in full sentences.

  • Writing Deficits: Complete inability to write words down.

  • Preserved Capabilities: Patients retain the ability to copy or imitate existing written words and maintain intact comprehension of incoming auditory language.

• Wernicke's Aphasia (Receptive Aphasia):

  • Historical Case Study: Identified in patients presenting with severe deficits in language comprehension and speech perception.

  • Post-Mortem Findings: Autopsy examination demonstrated focal brain damage in the posterior superior temporal region (Wernicke's area).

  • Functional & Clinical Deficits:

  • Comprehension Deficits: Patients do not understand what is being asked of them and are unable to process spoken or written language.

  • Expressive Output Features: Uncontrolled babbling and production of non-sensical, jargon-filled speech output. Patients are unable or unwilling to answer basic questions meaningfully.

  • Literacy Loss: Complete inability to read and write.

  • Echolalic Features: Patients frequently echo or imitate the exact questions asked by the examiner without understanding their semantic meaning.

• Functional Mapping Summary:

  • Language processing requires distinct cerebral regions for verbal comprehension versus verbal motor output.

  • Broca's area (left inferior frontal cortex) governs speech output, sentence construction, and writing production.

  • Wernicke's area (superior temporal region) governs auditory language comprehension, semantic processing, and literacy decoding.

Historical Debates, Cortical Stimulation, and Functional Localization

• The Great Brain Debate & Localization Theories:

  • Phrenology & Early Localization: Proposed that distinct regions of the cerebral cortex serve dedicated, isolated psychological functions. Early claims asserted that individual differences in personality traits manifest directly as variations in cortical tissue size and corresponding bumps on the skull.

  • Equipotentiality & Brain Holism: Alternative perspective asserting that the entire brain operates as a unified organ around behavior, with cortical regions capable of being co-opted for alternative functions when needed.

  • Example: In an individual born blind, cortical areas normally dedicated to visual processing are reallocated and co-opted for tactile and auditory functions.

  • Modern Theoretical Synthesis: Cortical regions cannot be rigidly partitioned into isolated "senses" or fixed functions; however, specific cognitive functions maintain discrete anatomical core locations within network systems.

  • Methodological Inclusion Criteria: Early cognitive neuroscience experiments explicitly excluded left-handed individuals from study panels because left-handed subjects display atypical brain lateralization and functional organization ("weird test subjects").

• Wilder Penfield and Intraoperative Cortical Mapping:

  • Historical Context: Work conducted by neurosurgeon Wilder Penfield ($1891\text{--}1976$) utilizing direct electrical stimulation during surgery.

  • Experimental Methodology:

  • Penfield performed brain surgeries on conscious, awake patients under local anesthesia.

  • Cortical tissue was directly stimulated using electrical current ("electrify the cortex") to map functional boundaries prior to tissue resection.

  • Patients were questioned in real time regarding their subjective experiences, sensations, or motor responses during artificial stimulation.

  • Key Neurophysiological Discoveries:

  • Absence of Nociceptors: The brain tissue itself lacks pain receptors, meaning patients felt no direct physical sensation of stimulation on the cortical surface.

  • Evoked Responses: Stimulating specific motor or sensory neurons directly triggered involuntary motor movements or speech disruptions.

  • Subjective Realization of Agency:

    • Examiner prompt: "raise ur arm"

    • Patient response: "I raised my arm" $\rightarrow$ Corrective subjective realization: "I didn't, you did" (demonstrating the distinction between voluntary intentional movement and artificially evoked involuntary motor output).

  • Penfield demonstrated that direct stimulation evokes an internal "sense of brain" activity and conscious perceptual experience rather than a feeling of physical local contact.

Anatomical Views, Planes, and Directional Terminology

• Three Common Coordinate Planes of the Brain:

  • Coronal Slice: Cut in the frontal plane, yielding cross-sectional slices viewed head-on (analogous to slicing a loaf of bread).

  • Axial Slice (also termed Transverse or Bird's-Eye View): Cut horizontally across the brain, producing top-down sections viewed lying flat.

  • Sagittal Slice (Profile View): Cut longitudinally along the sagittal plane from side to side (left to right), revealing medial or lateral profile views.

• Anatomical Directional Axis Terms:

  • Anterior vs. Posterior: Towards the front end versus towards the back end (interchangeable with Rostral vs. Caudal in lower animals).

  • Rostral vs. Caudal: Towards the nose/beak versus towards the tail/posterior end.

  • Dorsal vs. Ventral: Towards the back/top (dorsal) versus towards the belly/bottom (ventral).

  • Superior vs. Inferior: Towards the top of the head versus towards the foot/base.

  • Medial vs. Lateral: Towards the structural midline versus towards the outer sides.

Vascular Supply of the Brain

• Metabolic Requirements & Oxygen Dependency:

  • The brain utilizes approximately $20\%$ of the total cardiac blood output at rest.

  • Nervous tissue possesses no internal storage capacity for glucose or oxygen.

  • Any acute disruption in cerebral blood supply causes loss of consciousness within seconds and irreversible tissue necrosis or cellular death within minutes.

• Major Arterial Systems and Structural Branches:

  • Internal Carotid Artery System (Anterior Circulation):

  • Anterior Cerebral Artery (ACA): Supplies the medial surfaces of the frontal and parietal lobes.

    • Key Branches: Callosomarginal artery, Pericallosal artery, Frontopolar artery, and Medial orbitofrontal artery.

  • Middle Cerebral Artery (MCA): Passes laterally through the Sylvian fissure to supply the vast lateral surfaces of the frontal, parietal, and temporal lobes.

    • Superficial (Cortical) Branches: Run along the lateral sulcus supplying primary motor, somatosensory, and language cortices.

    • Deep (Lenticulostriate) Branches: Penetrate deeply to supply the basal ganglia and internal capsule.

  • Vertebrobasilar System (Posterior Circulation):

  • Vertebral Arteries: Ascend along the ventral surface of the brainstem and merge at the pons to form the single Basilar Artery.

  • Posterior Cerebral Artery (PCA): Terminal branch supplying the occipital cortex, inferior temporal lobe, and deep diencephalic structures.

  • Cerebellar Arteries: Superior cerebellar artery, Anterior inferior cerebellar artery (AICA), and Posterior inferior cerebellar artery (PICA).

  • Anterior Spinal Artery: Descends along the anterior median fissure of the spinal cord.

  • Anterior Choroidal Artery: Supplies the choroid plexus, hippocampus, and adjacent subcortical structures.

  • Circle of Willis: A ring-like arterial anastomosis located at the base of the brain connecting the internal carotid and vertebrobasilar systems, providing collateral circulation in case of focal arterial blockage.

Neurohistology: Gray Matter, White Matter, and Myelination

• Gray Matter:

  • Composed primarily of neuronal cell bodies (soma), dendrites, unmyelinated axon terminals, synapses, and neuroglia.

  • Forms the outer cerebral cortex mantle, the cerebellar cortex, and deep subcortical nuclei.

• White Matter:

  • Composed primarily of myelinated axon bundles forming interconnecting nerve fiber tracts.

  • Located deep to the cerebral cortex surrounding subcortical nuclei.

• Myelination:

  • Axons are encased in a protective lipid myelin sheath produced by oligodendrocytes in the central nervous system.

  • Myelin functions as an electrical insulator that increases action potential conduction velocity via saltatory conduction.

Gross Anatomy of the Cerebral Cortex

• Surface Topography Terms:

  • Sulcus / Fissure: Structural grooves, furrows, or indentations on the cortical surface (terms are largely interchangeable, though "fissure" typically denotes a deeper primary divide).

  • Gyrus: Raised cortical folds, ridges, or convolutions bounded by sulci.

• Major Anatomical Boundaries and Landmarks:

  • Longitudinal Fissure: Deep midline fissure separating the left and right cerebral hemispheres.

  • Central Sulcus (Central Fissure of Rolando): Divides the frontal lobe anteriorly from the parietal lobe posteriorly.

  • Lateral Fissure (Sylvian Fissure): Deep lateral groove separating the temporal lobe inferiorly from the frontal and parietal lobes superiorly.

  • Parieto-Occipital Sulcus: Medial structural boundary separating the parietal lobe from the occipital lobe.

  • Preoccipital Notch: Inferolateral landmark marking the anterior boundary of the occipital lobe.

• Functional Roles of the Four Cerebral Lobes:

  • Frontal Lobe: Positioned anterior to the central sulcus. Responsible for motor movement planning and execution, working memory, executive function, decision-making, and inhibition of inappropriate behavior. Houses the Precentral Gyrus (Primary Motor Cortex).

  • Parietal Lobe: Positioned posterior to the central sulcus and superior to the Sylvian fissure. Responsible for processing somatosensory inputs and body sensations. Houses the Postcentral Gyrus (Primary Somatosensory Cortex).

  • Temporal Lobe: Positioned inferior to the Sylvian fissure. Responsible for auditory perception, language comprehension, and advanced visual object recognition. Houses the Superior Temporal Gyrus (Primary Auditory Cortex).

  • Occipital Lobe: Positioned posterior to the parieto-occipital sulcus. Dedicated exclusively to primary and secondary visual processing.

Cerebral Interhemispheric Connections and Commissures

• Corpus Callosum:

  • Massive white matter structure composed of hundreds of millions of myelinated axons.

  • Serves as the principal interhemispheric bridge connecting corresponding cortical regions of the left and right cerebral hemispheres.

• Minor Commissures of the Brain:

  • Anterior Commissure: Crosses the midline anterior to the thalamus, interconnecting temporal lobes, amygdalae, and olfactory tracts.

  • Posterior Commissure: Crosses superior to the cerebral aqueduct, mediating pupillary light reflexes.

  • Hippocampal Commissure (Commissure of the Fornix): Connects left and right hippocampal formations across the midline.

  • Habenular Commissure: Located anterior to the pineal gland, connecting the habenular nuclei.

  • Collicular Commissures: Interconnect the superior and inferior colliculi across the tectum.

  • Massa Intermedia (Interthalamic Adhesion): Variable tissue bridge connecting left and right thalamic bodies across the third ventricle.

  • Optic Chiasm: X-shaped structure where nasal retinal fibers decussate to the contralateral optic tract.

Functional Mapping, Contralateral Control, and Cortical Homunculi

• Principle of Contralateral Control:

  • Each cerebral hemisphere controls motor output to and processes sensory input from the opposite (contralateral) side of the body.

• Cortical Representation & Homunculi:

  • Primary Motor Cortex (Precentral Gyrus): Topographically mapped motor output strip.

  • Primary Somatosensory Cortex (Postcentral Gyrus): Topographically mapped sensory input strip.

  • Spatial Layout along the Cortical Strip:

  • Medial surface / Superior boundary: Toes, Foot, Ankle, Knee, Hip, Trunk.

  • Dorsolateral surface: Neck, Arm, Elbow, Wrist, Hand, Fingers, Thumb, Eye, Nose.

  • Inferolateral surface: Face, Lips, Teeth, Gums, Jaw, Tongue, Swallowing.

  • Genital representation: Located on the medial somatosensory strip adjacent to the foot and toes.

  • Proportional Distortion: Body parts requiring precise motor control or possessing high tactile receptor density (hands, thumbs, lips, tongue) occupy disproportionately large cortical areas compared to the trunk or legs.

• Association Areas Across Evolution:

  • Cortical areas not dedicated exclusively to primary motor or primary sensory processing.

  • Responsible for complex multi-modal integration, cognitive reasoning, and abstract thought.

  • Evolutionary Expansion: Low association area proportion in rodents (rat); moderate expansion in carnivores (cat) and non-human primates (chimpanzee); massive proportional expansion in humans.

Comparative Neuroanatomy and Encephalization Quotient

• Encephalization Quotient (EQ) & Allometric Scaling:

  • Species vary dramatically in absolute brain size (e.g., Mouse, Macaque, Cat, Dog, Gorilla, Dolphin, Elephant, Human).

  • Absolute brain weight alone does not correlate directly with cognitive capacity; brain mass must be evaluated relative to total body weight.

  • Encephalization Quotient (EQ): Quantitative ratio comparing actual measured brain weight to expected brain weight for a mammal of equivalent body weight.

  • Mathematical Formula:     $EQ = \frac{\text{measured brain wt}}{0.12 \times (\text{body wt})^{2/3}}$

  • Specific Measured EQ Values:

  • Homo sapiens: $EQ = 7.5$

  • Monkey: $EQ = 4.0$

  • Raccoon: $EQ = 1.3$

  • Dog: $EQ = 1.0$

  • Mole: $EQ = 0.5$

  • Rhinoceros: $EQ = 0.5$

  • Allometric Scaling Principles:

  • Smaller animals display higher relative brain-to-body mass ratios, yet require a baseline neuron count for somatic maintenance.

  • Jerison (1973) comparative analysis plotted log brain weight ($\text{g}$) against log body weight ($\text{kg}$), defining distinct developmental polygons for Birds ($B$), Mammals ($M$), Reptiles ($R$), Amphibians ($A$), Fish, and Sharks/Rays ($S$).

• Vertebrate Brain Divisions & Evolutionary Trends:

  • Five Primary Divisions of the Vertebrate Brain:

  1. Telencephalon: Olfactory bulb, Cerebral hemispheres, Limbic system, Striatum.

  2. Diencephalon: Thalamus, Hypothalamus.

  3. Mesencephalon: Optic tectum, Tegmentum.

  4. Rhombencephalon: Cerebellum, Brain Stem.

  5. Spinal Cord.

  • Telencephalic Expansion: Progressive evolutionary scaling demonstrates a vast increase in forebrain (telencephalon) volume relative to midbrain and hindbrain structures from jawless fish (lamprey), cartilaginous fish (shark), bony fish, amphibians, reptiles, and birds up to mammals.

• Artificial Selection Experiments on Brain Size:

  • Experimental Study in Guppies (Poecilia reticulata): Multi-generational artificial selection ($F0 \rightarrow F1 \rightarrow F2$) for divergent relative brain size generated distinct Large Brain and Small Brain lines.

  • Cognitive & Biological Trade-Offs:

  • Females selected for large brains achieved significantly higher correct choices in spatial learning and discrimination tasks.

  • Evolutionary Cost: Large-brained females produced significantly fewer offspring in their first parturition (litter size reduction), illustrating energetic trade-offs between expensive neural tissue and gut/reproductive investment.

Anatomy and Physiology of the Spinal Cord and Reflex Arcs

• Spinal Cord Internal Microanatomy:

  • Gray Matter (Central butterfly shape containing cell bodies and interneurons):

  • Dorsal Horn: Posterior gray matter region receiving incoming sensory (afferent) inputs.

  • Ventral Horn: Anterior gray matter region containing somatic motor (efferent) neuron cell bodies.

  • White Matter (Peripheral columns containing myelinated axon tracts):

  • Dorsal Columns: Ascending sensory pathways carrying fine touch and proprioception.

  • Ventral Columns: Descending motor pathways and ascending sensory tracts.

• Spinal Roots and Ganglia:

  • Dorsal Root: Carries afferent sensory fibers into the dorsal horn. Contains the Dorsal-Root Ganglion (swelling housing pseudounipolar sensory neuron cell bodies).

  • Ventral Root: Carries efferent motor fibers away from the ventral horn to target skeletal muscles.

  • Spinal Nerve: Formed by the union of dorsal and ventral roots outside the spinal cord.

• Reflex Arc Neural Circuitry:

  • Sensory Receptors in Skin: Detect cutaneous mechanical, thermal, or painful stimuli.

  • Afferent Neuron: Propagates action potentials through the dorsal-root ganglion into the dorsal horn.

  • Interneuron: Synapses within spinal gray matter, integrating information and stimulating motor pathways locally without requiring prior cerebral processing.

  • Efferent Neuron: Propagates motor output out through the ventral root.

  • Effector (Muscle): Contracts automatically to perform an immediate involuntary motor reflex.

Diencephalic Structures: Thalamus and Hypothalamus

• Thalamus:

  • Paired symmetrical subcortical structure serving as the central relay station for all ascending sensory pathways (except olfaction) en route to the cerebral cortex.

  • Thalamic Subnuclei and Connectivity:

  • Anterior Nucleus: Receives inputs from mammillary bodies; projects to cingulate cortex (limbic memory/emotion circuit).

  • Ventral Anterior (VA) Nucleus: Receives inputs from Globus Pallidus and Substantia Nigra; projects to motor planning cortices.

  • Ventrolateral (VL) Nucleus: Receives inputs from Cerebellum; projects directly to Primary Motor Cortex.

  • Ventral Posterolateral (VPL) Nucleus: Relays somatosensory signals from the body to the Primary Somatosensory Cortex.

  • Ventral Posteromedial (VPM) Nucleus: Relays somatosensory signals from the head/face (trigeminal system) to Primary Somatosensory Cortex.

  • Lateral Geniculate Nucleus (LGN): Relays visual signals from retinal ganglion cells to Primary Visual Cortex (occipital lobe).

  • Medial Geniculate Nucleus (MGN): Relays auditory signals from ascending brainstem pathways to Primary Auditory Cortex (temporal lobe).

  • Pulvinar Nucleus: Large posterior association nucleus modulating visual attention.

  • Laterodorsal & Lateroposterior Nuclei: Integrated with parietal association and limbic cortices.

  • Dorsomedial Nucleus: Processes limbic and olfactory inputs; projects to Prefrontal Cortex.

  • Intralaminar Nuclei: Modulate cortical arousal, pain awareness, and alertness.

• Hypothalamus:

  • Located ventral to the thalamus along the base of the third ventricle.

  • Master regulator of neuroendocrine function, visceral autonomic activity, and internal bodily homeostasis (temperature, hunger, thirst, circadian cycles).

  • Coordinates with the Pituitary Gland and Adrenal Glands to modulate hormonal release and sympathetic/parasympathetic balance.

Subcortical Structures: Limbic System and Basal Ganglia

• The Limbic System:

  • Functional Roles: Relates the organism to its environment based on current bodily needs, present context, and prior experiences; essential for emotional regulation, motivation, learning, memory, and autonomic responses.

  • Anatomical Components:

  • Cingulate Gyrus: Superior cortical fold above corpus callosum; mediates emotional and cognitive regulation.

  • Subcallosal Gyrus: Inferior medial cortical strip.

  • Hippocampus: Medial temporal lobe structure composed of 3-layered archicortex; critical for memory encoding, learning, and spatial navigation. (Named for its structural resemblance to a seahorse, Hippocampus).

  • Amygdala: Anterior temporal nuclear group involved in emotional processing, fear conditioning, and threat detection.

  • Anterior Thalamic Nuclei & Mammillary Bodies: Core nodes of the Papez circuit.

  • Fornix: Major output white matter bundle connecting hippocampus to mammillary bodies.

  • Entorhinal Cortex: Primary cortical gateway into the hippocampus.

• The Basal Ganglia:

  • Subcortical nuclear collection that modulates motor output and action selection via reciprocal feedback loops with frontal cortical areas.

  • Anatomical Structures:

  • Striatum: Composed of the Caudate Nucleus and Putamen.

  • Globus Pallidus: Divided into Globus Pallidus Externa (GPe) and Globus Pallidus Interna (GPi).

  • Subthalamic Nucleus (STN).

  • Substantia Nigra: Divided into Pars Compacta (SNc) (dopaminergic supply) and Pars Reticularis (SNr).

  • Pathways & Clinical Disorders:

  • Direct Pathway: Facilitates motor movement via thalamic dis-inhibition.

  • Indirect Pathway: Suppresses unwanted motor movement via STN excitation of GPi/SNr.

  • Parkinson's Disease: Progressive degeneration of dopaminergic neurons in Substantia Nigra Pars Compacta, leading to hypokinetic symptoms (tremor, rigidity, bradykinesia).

  • Huntington's Disease: Degeneration of striatal GABAergic medium spiny neurons, causing hyperkinetic symptoms (chorea, involuntary spastic movements).

Autonomic Nervous System: Sympathetic and Parasympathetic Divisions

• Overview: Controlled centrally by the hypothalamus to regulate smooth muscle, cardiac tissue, and glandular activity.

• Sympathetic Division ("Fight or Flight"):

  • Pupils: Dilates pupil (mydriasis).

  • Tear Glands: Stimulates tear gland secretion.

  • Salivation & Sweating: Inhibits salivation; increases sweating.

  • Heart: Accelerates heart rate and contraction force.

  • Bronchi: Dilates bronchi (increases airway volume for rapid respiration).

  • Digestive System: Decreases digestive activity in stomach, pancreas, and intestines.

  • Adrenal Gland: Stimulates secretion of adrenaline (epinephrine) and noradrenaline into systemic circulation.

  • Bladder: Inhibits bladder wall contraction (relaxes detrusor muscle).

  • Genitals: Inhibits blood flow to genital organs.

• Parasympathetic Division ("Rest and Digest"):

  • Pupils: Constricts pupil (miosis).

  • Tear Glands: Inhibits tear glands.

  • Salivation: Increases salivation.

  • Heart: Slows heart rate.

  • Bronchi: Constricts bronchi (reduces airway volume, breathe less rapidly).

  • Digestive System: Increases digestive activity in stomach, pancreas, and intestines.

  • Bladder: Stimulates bladder wall contraction (promotes micturition).

  • Genitals: Stimulates blood flow to genital organs (induces erection).