19.10: Vision
Vision is the result of light being detected and transduced into neural signals by the retina of the eye. This information is then further analyzed and interpreted by the brain. First, light enters the front of the eye and is focused by the cornea and lens onto the retina—a thin sheet of neural tissue lining the back of the eye. Because of refraction through the convex lens of the eye, images are projected onto the retina upside-down and reversed.
Light is absorbed by the rod and cone photoreceptor cells at the back of the retina, causing a decrease in their rate of neurotransmitter release. In addition to detecting photons of light, color information is also encoded here, since different types of cones respond maximally to different wavelengths of light.
The photoreceptors then send visual information to bipolar cells near the middle of the retina, which is followed by projection to ganglion cells at the front of the retina. Horizontal and amacrine cells mediate lateral interactions between these cell types, integrating information from multiple photoreceptors. This integration aids in the initial processing of visual information, such as detecting simple features, like edges.
Along with glial cells, the axons of the retinal ganglion cells make up the optic nerve, which transmits visual information to the brain. The optic nerve partially crosses at the base of the brain. Thus, each side of the brain receives input from both eyes, enabling depth perception.
Most optic nerve fibers synapse in the lateral geniculate nucleus in the thalamus of the brain, where different characteristics, such as color and motion, are processed in parallel. The thalamus then sends information to the primary visual cortex (V1) at the back of the brain. Cells in V1 respond to more complex visual characteristics, such as specific orientations and directions of movement. V1 contains a well-defined map of the visual field, with a relatively large area devoted to processing information from the fovea of the retina—a central region that has the highest density of photoreceptors.
Visual information is sent from V1 to adjacent areas of the cerebral cortex for even higher-level processing, such as identifying an object or face and determining the spatial location of visual stimuli.
