Week 3: Introduction to the Visual Brain

This lecture delves into the visual pathway, tracing how reflected light from an object, like an apple, is processed to form a robust perceptual representation in the brain. It highlights the ability of this pathway to identify an object's location, perceive its color and shape, and link sensory experience with top-down cues such as memories or imagined tastes. The lecture also promises to demystify complex anatomical names using quick Greek and Latin lessons.

Building on the previous week's discussion of eye anatomy, the lecture explains how axons of retinal ganglion cells exit the eye as the optic nerve. These optic nerves meet at the optic chiasm, where a partial crossing of fibers occurs. The term 'chiasm' is derived from the Greek word for 'crossing'. After this reorganization, the bundles of axons become known as optic tracts, with 90% of fibers projecting to the lateral geniculate nucleus, a key visual area of the thalamus.

The visual system provides exceptional spatial acuity, enabling accurate object localization. This is reflected in the spatial organizing principle throughout the ascending visual pathway, based on a stimulus's position within the visual field. The lecture divides the visual field into right and left hemifields, emphasizing their contralateral organization: the left brain hemisphere represents the right hemifield, and vice-versa. It clarifies that this applies to visual hemifields, not individual eyeballs.

The spatial arrangement of retinal ganglion cells is maintained in the optic nerve, with fibers representing areas like the macula, nasal, temporal, superior, and inferior retina. This segregation is vital for the partial crossing of fibers at the optic chiasm. For instance, an object in the right visual hemifield projects to the nasal portion of the right retina and the temporal portion of the left retina. At the chiasm, axons from the right nasal retina cross over to join axons from the left temporal retina, ensuring all neurons representing the right visual field exit via the left optic tract.

Optic tract axons terminate in the lateral geniculate nucleus, the visual center of the thalamus. The name 'geniculate' describes its bent shape, and 'lateral' indicates its position towards the outside of the head, distinct from the medial geniculate nucleus involved in audition. The thalamus acts as a crucial relay station before information reaches the cortex, with different nuclei targeting specific cortical areas. The LGN is the last stop before visual information arrives in the visual cortex.

The lateral geniculate nucleus is a multi-layered structure with distinct functional and structural characteristics. Although projections from a given hemifield combine at the optic chiasm and travel to the contralateral thalamus, the projections from the two eyes remain separate at the LGN level. Ganglion cell axons from the left and right eyes target different layers of the LGN, seen as alternating bands, indicating that while visual fields are integrated, individual eye input is still distinct.