Brain Structure and Function: Methods, Structures, and Lateralization

Methods for Studying Brain Function

• Early approaches focused on brain damage to infer function: observe deficits after stroke, lesions, or traumatic brain injury to determine which structures support which behaviors (e.g., language, memory).

• Modern techniques allow us to study the brain in action:

  • PET scans and fMRI (example provided): measure brain activity via blood flow changes.

  • fMRI example: participants rest (baseline) vs perform a language task; active areas light up during task.

  • Distinction between MRI (anatomy) and fMRI (function) is often illustrated with Homer Simpson as a playful cue.

• Practical points about brain imaging:

  • fMRI shows functional activation patterns by detecting blood flow changes, not direct neuronal firing.

  • Diagram conventions: brains are shown in sagittal orientation, with the front of the brain on the left and the back on the right.

• Overall takeaway: structure and function are intertwined; early damage studies seeded our understanding, while imaging reveals function in living, intact brains.

Brain Planes and Orientation

• Three common planes used to orient brain diagrams:

  • Coronal plane: slices from front to back (forehead toward back of head).

  • Horizontal plane: slices from top to bottom (top of head down to neck).

  • Sagittal plane: slices left to right (between the ears).

• Most diagrams depict a sagittal view, showing the two hemispheres: left hemisphere and right hemisphere.

• Notation about orientation:

  • Front of the brain is on the left side of the diagram; back is on the right.

• Quick orientation aid: the brain is a 3D structure with two hemispheres; the corpus callosum connects them functionally.

Major Components of the CNS (Four Principal Brain Regions)

• Central nervous system components mentioned as major pieces (spinal cord included for completeness, but focus remains on brain):

  • Brain Stem: connects spinal cord to the brain; vital for survival functions.

  • Cerebellum: movement control, balance, and coordination; also linked to other cognitive processes.

  • Subcortical Structures (Limbic System): below the cortex; involved in drives, emotions, memory, and basic regulation.

  • Cerebral Cortex: outer layer; higher-order cognitive processes.

• Spinal Cord is noted as part of the CNS, though not the focus of the subsequent detailed discussion.

The Brain Stem

• Medulla (lower brain stem):

  • Vital life functions controlled here (breathing, heart rate, blood pressure).

• Pons: the bridge between cerebellum and the rest of the brain; involved in consciousness level, sleep, arousal patterns; partial role in facial expressions.

• Midbrain: located above the pons; involved in reflexes, movement, and other functions; includes components that mediate sensory reflexes.

• Reticular Formation: core within the brainstem; crucial for arousal, wakefulness, attention.

• Overall role: supports essential, automatic and arousal-related processes that underlie all other brain activity.

The Cerebellum

• Literally means "little brain"; evolutionarily ancient structure.

• Primary role: motor coordination and balance; crucial for involuntary motor control.

• Additional notes:

  • Damage can produce deficits beyond motor control, including perception, language, and other cognitive processes.

• Location: at the back of the head; connected to the brainstem via the pons (the bridge).

The Limbic System (Subcortical Structures)

• General role: emotion, motivation, memory, and basic drives; emotionally salient and evolutionarily ancient.

• Core components and their roles:

  • Thalamus: sensory relay station; routes sensory information to cortical areas; two thalamic discs, one in each hemisphere.

  • Basal Ganglia: important for voluntary movement control.

  • Amygdala: processes emotional states (fear, disgust, aggression).

  • Hippocampus: critical for memory formation, especially short-term/episodic memory; location around the thalamus; named for its seahorse shape.

  • Hypothalamus: located under the thalamus; regulates temperature, hunger, hormone release, autonomic nervous system (ANS) functions; modulates arousal and has a role in reward/pleasure.

  • Nucleus Accumbens: central to reward, pleasure, and social inclusion; part of the reward circuitry.

  • Pituitary Gland: controls growth hormone and other hormone release (not a primary focus here).

• Spatial relationships (in several diagrams): limbic structures are nestled inside the cortex; thalamus is central with hippocampus surrounding it; amygdala sits at the ends of the hippocampus; hypothalamus sits just beneath the thalamus.

• Conceptual takeaway: limbic system integrates emotion, motivation, memory, and rewards, supporting learning and goal-directed behavior.

The Cerebral Cortex and Its Lobes

• Four major lobes (with approximate functions):

  • Frontal Lobe: higher cognition, planning, decision-making; contains motor cortex (fine voluntary movement) near its posterior edge.

  • Parietal Lobe: processing somatosensory information; spatial orientation and relations.

  • Occipital Lobe: visual processing; visual cortex.

  • Temporal Lobe: processing auditory information; memory-related functions; auditory cortex located here.

• Specific cortical areas (six key regions):

  • Motor Cortex: located at the back of the frontal lobe; controls fine voluntary movements.

  • Somatosensory Cortex: located in the parietal lobe; processes sense of touch.

  • Broca's Area: frontal lobe; language production and articulation.

  • Wernicke's Area: temporal lobe; language comprehension.

  • Auditory Cortex: in the temporal lobe; processes auditory information.

  • Visual Cortex: in the occipital lobe; processes visual information.

• Association cortex: mention that there are additional associative regions; not the focus here.

• Quick mapping reminder: frontal lobe → plans and actions; motor cortex → movement; parietal → touch and space; occipital → vision; temporal → hearing and memory.

Language Areas and Aphasia

• Visual-to-language pathway (briefly described):

  • Visual stimulus (written word) is seen by the eyes and travels to the visual cortex in the occipital lobe.

  • Visual information is transmitted to the angular gyrus, which translates the visual representation into an auditory code.

  • From there, information goes to Wernicke's area for comprehension.

  • Language production then involves Broca's area (motor control of speech muscles).

• Aphasia definitions and examples:

  • Broca's Aphasia (production/expressive aphasia):

  • Comprehension relatively intact; difficulty producing speech; may understand but struggle to articulate (e.g., may say "store cereal" instead of full sentence).

  • Wernicke's Aphasia (receptive/comprehension aphasia):

  • Fluent, well-formed speech, but semantic content is often nonsensical or meaningless; difficulty understanding language.

• Important distinction:

  • Broca's area damage → production deficits; Wernicke's area damage → comprehension deficits.

• Practical note: language processing involves multiple brain areas beyond these two, and real tasks recruit widespread networks.

The Somatosensory Cortex and the Homunculus

• Somatosensory cortex: located in the parietal lobe; processes touch sensations.

• Homunculus concept: body parts are represented in proportion to tactile sensitivity, not actual size.

  • High-sensitivity areas (face, lips, tongue, hands, fingertips) take up more cortical space.

  • Less sensitive areas (e.g., knee) take up less space.

• This somatotopic map illustrates how perception is physically organized in the brain.

Hemispheric Specialization and Interhemispheric Communication

• Corpus Callosum: thick band of neural fibers that connects the left and right hemispheres, enabling communication between them.

• Hemispheric specialization (lateralization):

  • Left hemisphere tends to be stronger in language-related tasks (e.g., speech production and comprehension).

  • Right hemisphere tends to be stronger in facial recognition, spatial and perceptual tasks.

• Contralateral processing: information from one side of the body or visual field is primarily processed by the opposite hemisphere.

  • Left visual field → processed by the right hemisphere; right visual field → processed by the left hemisphere.

  • Left hand controlled by the right hemisphere; right hand controlled by the left hemisphere.

• Important concept for understanding split-brain research (below).

Split-Brain and Corpus Callosum Disconnect

• Split-brain procedure (corpus callosum severed):

  • Used in severe epilepsy when other treatments fail; aim is to reduce seizures by disconnecting the two hemispheres.

  • Result: two hemispheres operate independently with no direct communication; still part of the same brain but with limited interhemispheric transfer.

• Implications discussed in class demonstrations:

  • The study of split-brain patients provides insight into lateralization of functions and how each hemisphere contributes to perception and action.

  • Real-world consequences include how tasks may be performed differently when the hemispheres cannot communicate.

• Note on ethical and practical considerations (implicit in discussion): such interventions are considered only in extreme cases where benefits outweigh significant risks and are subject to ethical review and informed consent.

Connections and Takeaways

• Brain functions arise from distributed networks across multiple structures; no single area does only one job.

• Imaging and lesion studies complement each other: structure-function mapping improves with both approaches.

• Basic brain organization hints at evolutionary development: limbic system and brainstem provide foundational processes; cerebral cortex enables higher-order cognition.

• When learning about brain areas, use diagrams to map planes, lobes, and connections, and relate them to functions such as movement, sensation, language, emotion, and memory.

• Practical implications include understanding deficits from brain injury, planning for rehabilitation, and appreciating how different brain networks collaborate during simple tasks like reading a word aloud.

Notes on Terminology and Concepts to Remember

• CNS: central nervous system; consists of the brain and spinal cord.

• Brain Stem: medulla, pons, midbrain; basic life-sustaining functions and arousal.

• Reticular Formation: arousal and attention mechanisms within the brainstem.

• Cerebellum: motor coordination and balance; influences some cognitive functions.

• Limbic System: thalamus, hypothalamus, hippocampus, amygdala, basal ganglia, nucleus accumbens; emotion, memory, drives, reward.

• Thalamus: sensory relay station to cortex.

• Basal Ganglia: voluntary movement control.

• Hippocampus: memory formation (especially short-term memory).

• Amygdala: emotion processing (fear, aggression).

• Hypothalamus: temperature regulation, hunger, hormone regulation; ANS interactions; reward.

• Nucleus Accumbens: reward and reinforcement.

• Broca's Area: language production and articulation.

• Wernicke's Area: language comprehension.

• Angular Gyrus: transforms visual representations into auditory code for language.

• Somatosensory Cortex: tactile processing; somatotopic (homunculus).

• Motor Cortex: voluntary motor control; located at the boundary of frontal and central sulci.

• Visual Cortex: optical processing in the occipital lobe.

• Auditory Cortex: processing in the temporal lobe.

• Planes: Coronal, Horizontal, Sagittal (for brain slicing and orientation).

• Contralateral Processing: information from one side is processed in the opposite hemisphere.

• Split-Brain: severed corpus callosum; hemispheric independence; a key research tool for understanding lateralization.