visual pathway

Meaning: Rods and cones in the retina transduce light. Signals pass to bipolar cells, then to ganglion cells, whose axons form the optic nerve. At the optic chiasm, fibers from the nasal half of each retina cross to the other side, while temporal fibers stay on the same side. Fibers then run in the optic tract to the lateral geniculate nucleus (LGN) of the thalamus, and on to the primary visual cortex (V1) in the occipital lobe. From V1, the ventral stream to the temporal lobe supports "what" (object identity), and the dorsal stream to the parietal lobe supports "where/how" (location and motion).

On the MCAT: A passage describes a lesion and asks which part of the visual field is lost. Cutting one optic nerve affects vision in that eye. Damage after the chiasm, such as the left optic tract or left occipital cortex, removes the right visual field of both eyes. Damage at the chiasm itself removes the outer parts of both visual fields. Questions may also ask about parallel processing, in which separate cells handle color, form, and motion.

Common confusion: Students assume the left eye feeds the left hemisphere. The split is by visual field, not by eye: the left visual field lands on the right half of each retina and is processed in the right hemisphere. Also, the lens inverts the image on the retina, and the brain does not need to "flip it back."