Premed · Premed · Introductory Psychology
Lecture 4: The Brain: Structure and Function
Introductory Psychology
Learning Objectives
By the end of this lecture, students will be able to:
- Describe the major methods used to study the brain
- Identify the structures and functions of the hindbrain, midbrain, and forebrain
- Explain the organization of the cerebral cortex and the functions of each lobe
- Discuss lateralization and the split-brain research findings
- Describe the concept of neuroplasticity and its implications
Lecture Content
I. Methods of Studying the Brain
Scientists have developed numerous techniques for investigating the brain. Lesion studies examine changes in behavior following damage to specific brain areas, whether caused naturally by strokes, tumors, or injuries, or produced experimentally in animal research. The celebrated case of Phineas Gage, who in 1848 survived an iron rod passing through his prefrontal cortex, provided early evidence that this region plays a role in personality and decision-making. Electrical stimulation has also been instrumental: Wilder Penfield mapped cortical functions by stimulating the cortex during surgery, and transcranial magnetic stimulation (TMS) now allows researchers to non-invasively stimulate the brain through the skull.
The electroencephalogram (EEG) records electrical activity across the scalp using electrodes. It offers excellent temporal resolution, capturing changes on the scale of milliseconds, but provides poor spatial resolution. EEG is widely used to study sleep stages, seizures, and event-related potentials. Among neuroimaging techniques, CT scans use X-rays to produce structural images of the brain, while MRI uses magnetic fields to generate far more detailed structural images. Functional MRI (fMRI) measures blood flow to detect which brain areas are active during a task, offering good spatial resolution with moderate temporal resolution. PET scans track radioactive glucose to reveal metabolic activity, and magnetoencephalography (MEG) measures the magnetic fields generated by neural activity.
<image>A comparison panel of six brain imaging techniques. Each panel shows: the technique name, a simplified diagram of the equipment, a sample brain image produced by the technique, and a brief note on spatial and temporal resolution. Panel A: EEG with electrode cap and wavy signal output. Panel B: CT scan with X-ray cross-section. Panel C: Structural MRI with detailed grayscale brain slice. Panel D: fMRI with color-coded activation map overlaid on brain. Panel E: PET scan with color-coded metabolic activity. Panel F: A table comparing all techniques on resolution, invasiveness, and typical use cases.</image>
II. The Hindbrain (Brainstem and Cerebellum)
The hindbrain is the oldest part of the brain in evolutionary terms. The medulla oblongata controls vital autonomic functions including breathing, heart rate, blood pressure, and swallowing; damage to this structure is often fatal. The pons bridges signals between the cerebellum and the rest of the brain and is involved in sleep, arousal, and facial expressions. The reticular formation is a diffuse network of neurons running through the brainstem that controls arousal, attention, and sleep-wake transitions; damage to it can result in coma or a persistent vegetative state.
The cerebellum, meaning "little brain," is located at the base of the brain behind the brainstem. It coordinates voluntary movement, balance, posture, and motor learning, and it also contributes to procedural memory and some cognitive functions such as timing and attention. Damage to the cerebellum causes ataxia, a condition characterized by uncoordinated, clumsy movements.
III. The Midbrain
The midbrain sits between the hindbrain and forebrain. Its upper portion, the tectum, includes the superior colliculus, which processes visual reflexes, and the inferior colliculus, which handles auditory reflexes. Within the tegmentum lies the substantia nigra, a structure that produces dopamine; its degeneration is the hallmark of Parkinson's disease. The ventral tegmental area (VTA), also located in the midbrain, is a key component of the brain's reward circuitry and releases dopamine in response to pleasurable stimuli.
IV. The Forebrain
The thalamus functions as the brain's sensory relay station, routing incoming sensory information to the appropriate cortical areas. All senses except smell pass through the thalamus. Just below it, the hypothalamus is a small but critically important structure that regulates hunger, thirst, body temperature, sexual behavior, and circadian rhythms. It also controls the pituitary gland, linking the nervous and endocrine systems, and contains reward and punishment centers.
The limbic system is a collection of structures centrally involved in emotion, memory, and motivation. The amygdala, an almond-shaped nucleus, processes emotions — particularly fear and aggression — and plays a key role in emotional memory by tagging experiences as emotionally significant. Damage to the amygdala can produce Kluver-Bucy syndrome, characterized by reduced fear and altered sexual behavior. The hippocampus, shaped like a seahorse, is critical for forming new explicit (declarative) memories and for converting short-term memories into long-term ones through consolidation. Bilateral hippocampal removal in patient H.M. (Henry Molaison) produced severe anterograde amnesia, the inability to form new memories. Research on London taxi drivers has shown that extensive spatial navigation experience is associated with a larger posterior hippocampus, illustrating the brain's capacity for structural change. The cingulate cortex participates in emotion regulation, conflict monitoring, and decision-making, while the basal ganglia — a cluster of nuclei — are involved in voluntary motor control, procedural learning, and habit formation. Dysfunction of the basal ganglia is linked to Parkinson's and Huntington's diseases.
<image>A midsagittal (side) view of the brain with key structures labeled and color-coded by region. Panel A: Hindbrain structures (medulla, pons, cerebellum) in blue. Panel B: Midbrain structures (tectum, tegmentum) in green. Panel C: Forebrain structures — thalamus, hypothalamus in yellow; limbic structures (amygdala, hippocampus) in orange; cerebral cortex in pink. Arrows indicate major connections between regions. An inset shows a magnified view of the limbic system structures.</image>
V. The Cerebral Cortex
The cerebral cortex is the thin (2-4 mm) outer layer of the cerebrum, containing approximately 20 billion neurons. It is highly folded — ridges are called gyri, shallow grooves are sulci, and deep grooves are fissures — a design that dramatically increases surface area. The cortex is divided into two hemispheres connected by the corpus callosum, a massive bundle of roughly 200 million nerve fibers. Each hemisphere contains four lobes.
The frontal lobe, at the front of the brain, houses the motor cortex along the precentral gyrus, which controls voluntary movements in a contralateral arrangement: the left motor cortex controls the right side of the body and vice versa. The prefrontal cortex, located at the very front, governs executive functions — planning, decision-making, impulse control, working memory, and aspects of personality. Broca's area, typically in the left hemisphere, is responsible for speech production; damage produces Broca's aphasia, in which speech is effortful and nonfluent but comprehension remains intact.
The parietal lobe, at the top-rear of the brain, contains the somatosensory cortex along the postcentral gyrus, which processes touch, pressure, temperature, and pain. This lobe is also involved in spatial awareness, attention, and body position. Damage can cause hemispatial neglect, a condition in which the patient ignores one side of space, usually the left.
The temporal lobe, along the sides of the brain, contains the auditory cortex for processing sound and Wernicke's area (typically in the left hemisphere) for language comprehension. Damage to Wernicke's area produces fluent but meaningless speech accompanied by poor comprehension. The temporal lobe also contributes to object recognition via the ventral stream, often called the "what" pathway.
The occipital lobe, at the back of the brain, houses the primary visual cortex, which processes visual information. Damage here causes cortical blindness, in which the eyes function normally but the brain cannot interpret visual input.
VI. Lateralization and Split-Brain Research
The two hemispheres of the brain exhibit some degree of functional specialization, a phenomenon known as lateralization. In most right-handed individuals, the left hemisphere is dominant for language, logic, analytical processing, and detail-oriented tasks, while the right hemisphere excels at spatial processing, face recognition, emotional expression, holistic processing, and music. It is important to note, however, that both hemispheres contribute to most tasks; lateralization is a matter of degree rather than an absolute division.
Split-brain research, conducted primarily by Roger Sperry and Michael Gazzaniga, examined patients who had undergone corpus callosotomy — surgical severing of the corpus callosum — to treat severe epilepsy. With the two hemispheres unable to communicate directly, fascinating effects emerged. When an object was flashed in the left visual field, it was processed by the right hemisphere, and the patient could not verbally name it but could pick it up with the left hand. When the same object was flashed in the right visual field, it was processed by the left hemisphere, and the patient could name it easily. These findings powerfully demonstrated that language is typically lateralized to the left hemisphere. The popular notion of "left-brain" and "right-brain" personality types, however, is an oversimplification not supported by the scientific evidence.
VII. Neuroplasticity
Neuroplasticity refers to the brain's remarkable ability to reorganize itself and form new neural connections throughout life. Developmental plasticity involves massive reorganization during critical periods in childhood, while experience-dependent plasticity occurs as learning and experience strengthen or create new connections at any age. After brain damage, other areas may take over lost functions — a process called recovery of function — which tends to be more effective in younger brains. Constraint-induced movement therapy, for example, can promote recovery by forcing use of an impaired limb.
Evidence for neuroplasticity is abundant. Musicians develop larger cortical areas devoted to the hand they use for their instrument. Braille readers show enhanced somatosensory cortex representation for their reading finger. Neurogenesis, the growth of new neurons, has been documented in the hippocampus and possibly other areas throughout life. And phantom limb pain, in which amputees perceive pain in a missing limb, reflects cortical reorganization after the loss of sensory input.
<image>A top-down view of the brain showing the four lobes of the cerebral cortex on both hemispheres, color-coded: frontal lobe in blue, parietal lobe in yellow, temporal lobe in green, occipital lobe in red. Key functional areas are marked with pins: motor cortex, somatosensory cortex, Broca's area, Wernicke's area, auditory cortex, visual cortex, and prefrontal cortex. A callout shows the corpus callosum connecting the two hemispheres. A homunculus figure along the motor and somatosensory strips illustrates how body parts are mapped, with disproportionately large hands and face.</image>
