Primary visual cortex
The primary visual cortex (usually called V1) is the most well-studied visual area in the brain. It is also the simplest and first cortical visual area. Situated around the calcarine sulcus in the occipital lobe, it is the most caudal structure in the brain. It receives signals relayed from the retina via the lateral geniculate nucleus of the thalamus. Thus, primary visual cortex is only one synapse away from the retina.
The functionally defined primary visual cortex is equivalent to the anatomically defined striate cortex. The name "striate cortex" is derived from the highly visible striations (stripes) that are made up of the prominent somas of neurons in layer 4 of the gray matter. The primary visual cortex is also anatomically equivalent to Brodmann area 17, or BA17. Brodmann areas are based on a histological map of the human brain created by Korbinian Brodmann.
David Hubel and Torsten Wiesel were the first scientists to show that primary visual cortex shows tuning for oriented lines. Supposedly they were presenting visual stimuli on slides to a cat while recording neural signals. They were unable to find any stimulus that would make the cell fire until they presented a slide which had a crack on it. Although they won the Nobel Prize in medicine for later work, it is this discovery for which they are most famous today.
During the 1970s, there was a major debate about the function of primary visual cortex - whether it was a feature detector or a local band-limited Fourier filter. Most scientists today accept the latter hypothesis.
During the 1980s, there was a major debate about the relationship between functional specialization within V1, the magnocellular and parvocellular pathways of the retina and lateral geniculate nucleus, and the dorsal stream and ventral stream. Most scientists today believe that evidence for this hypothesis is weak.
Most research on the primary visual cortex involves recording action potentials from electrodes with the brain of cats, ferrets, or monkeys. Other scientists have studied primary visual cortex by recording intrinsic optical signals and fMRI signals from v1.
One recent discovery about v1 is that metabolic signals measured by fMRI show very large attentional modulation. This result strongly contrasts with macaque physiology research showing very small changes (or no changes) in firing associated with attentional modulation.
Other current work on v1 seeks to fully characterize its tuning properties, and to use it as a model area for the canonical cortical circuit.
Lesions to primary visual cortex usually lead to a scotoma, or hole in the visual field. Interestingly, patients with scotomas are often able to make use of visual information presented to their scotomas, despite being unable to consciously perceive it. This phenomenon, called blindsight, is widely studied by scientists interested in the neural correlate of consciousness.
De Valois and De Valois. Spatial Vision. (1990). Oxford University Press.