Lecture 2: Neuroanatomy

What this lecture covers

This lecture gives a rapid refresher on basic neuroanatomy (neurons, the four major brain components, and key subcortical structures) ahead of an in-class brain dissection, then moves into cortical organization: what makes a patch of cortex a distinct "area," using visual motion area MT as the worked example. It follows directly from the previous lecture's demo on why motion perception matters.

By the end, you'll be able to name the major subcortical structures and what damage to each produces, explain what a retinotopic map is, and describe the three kinds of evidence (function, connectivity, and cell architecture) used to argue that a patch of cortex is a distinct area.

Key ideas

• Brain stem: the most primitive part of the brain, essential for life-sustaining functions like breathing and consciousness, but not involved in higher cognition.
• Thalamus: often described as a relay station where most sensory information (except smell) makes a stop before reaching cortex, but its far greater number of backward connections suggests it also actively gates information flow.
• Hippocampus and amygdala: the hippocampus supports long-term episodic memory and navigation, illustrated by patient H.M.; the amygdala supports recognizing and experiencing emotions, especially fear, illustrated by patient S.M.
• Retinotopic mapping: nearby neurons in visual cortex respond to nearby locations in the visual field, producing a spatial map of the retina on the cortical surface.
• Criteria for a cortical area: a region counts as distinct if it differs from its neighbors in function, has its own connectivity fingerprint, and (for some regions) looks different under a microscope.
• Area MT: a small visual region shown, through single-neuron recording, fMRI, motion after-effects, electrical stimulation, and patient studies, to be selectively involved in processing the direction of visual motion.
• Akinetopsia: a rare deficit, following damage near area MT, in which a patient loses the ability to perceive motion, causing real difficulty crossing streets or pouring liquids.

Walkthrough

Brain basics (5:18)

Kanwisher reviews that the brain has about 100 billion neurons, runs on roughly 20 watts, and can be divided into four major components: brain stem, cerebellum, subcortical limbic structures, and cortex plus white matter. She notes the lecture is intentionally fast because an in-class dissection is coming up and mostly reveals subcortical structures, since different patches of cortex look alike to the naked eye.

Subcortical structures (8:18)

She covers the brain stem (basic survival functions), cerebellum (motor coordination, with an open debate over its role in cognition), thalamus (sensory relay and gating, illustrated with the LGN as the visual system's thalamic relay), hippocampus (episodic memory and navigation, via patient H.M. and Lonnie Sue Johnson), and amygdala (fear and emotion recognition, via patient S.M., who lost her amygdala and cannot experience or recognize fear).

White matter and connectivity (19:23)

The lecture explains that white matter, the myelinated axons connecting distant brain regions, makes up 45% of the human brain, and that each cortical patch has a distinctive "connectivity fingerprint" that helps identify it as a separate area, even though wiring diagrams across the whole cortex remain largely unknown.

Retinotopic maps in primary sensory cortex (22:27)

Using the classic concept of a receptive field (the part of the visual world that makes a given neuron fire), Kanwisher explains that neighboring neurons in visual cortex have neighboring receptive fields, producing a map of visual space. She shows historical deoxyglucose data from a monkey and modern fMRI data from a human, both showing a bullseye or letter-shaped stimulus reproduced as a spatial pattern of activity in visual cortex. She notes touch and auditory cortex have their own analogous maps of body location and sound frequency.

What makes a cortical area distinct (31:34)

She introduces three criteria for calling a patch of cortex a separate "area": distinct function, distinct connectivity, and (for some regions) distinct cell architecture. Primary sensory regions meet these criteria easily; the interesting scientific challenge is applying them to the rest of cortex.

Area MT as a case study (32:34)

Kanwisher builds the case that visual area MT is a distinct cortical area. Single-neuron recordings in monkeys show direction-selective firing and a systematic map of motion direction across the region. In humans, fMRI shows stronger response to moving than stationary dots, a motion after-effect demo suggests direction-tuned neuron pools, and electrical stimulation of a small patch of monkey MT biases perceived motion direction, providing causal evidence. A patient with damage near MT who cannot perceive motion (akinetopsia) offers further causal evidence. Finally, cytochrome oxidase staining shows MT is also metabolically and structurally distinct from its neighbors, satisfying all three criteria for a cortical area.

Before you watch

• Watch Lecture 1 first for the course's framing and the motion-perception demo that opens this lecture.
• Basic terms like neuron, axon, dendrite, and myelin, typically covered in an introductory neuroscience course, are assumed known.

Check your understanding

1. What is a connectivity fingerprint, and why does it help identify a cortical area as distinct?
2. Why does the existence of backward connections from cortex to thalamus complicate the simple "relay station" view of the thalamus?
3. What does a retinotopic map show about the relationship between the retina and visual cortex?
4. Name the three kinds of evidence used to establish that area MT is a distinct visual area, and give one experimental result for each.
5. What does akinetopsia reveal about the causal role of area MT in motion perception?