Location

In the posterior pole of the occipital cortex (the occipital cortex is responsible for processing visual stimuli).

It is the simplest, earliest cortical visual area. It is highly specialized for processing information about static and moving objects and is excellent in pattern recognition.
The functionally defined primary visual cortex is approximately equivalent to the anatomically defined striate cortex.
The primary visual cortex is divided into six functionally distinct layers, labeled 1 through 6. Layer 4, which receives most visual input from the lateral geniculate nuceus (LGN), is further divided into 4 layers, labelled 4A, 4B, 4Cα, and 4Cβ. Sublamina 4Cα receives most magnocelluar input from the LGN, while layer 4Cβ receives input from parvocellulat pathways.
The average number of neurons in the adult human primary visual cortex, in each hemisphere, has been estimated at around 140 million.


Function

V1 has a very well-defined map of the spatial information in vision. It involves neuronal processing of:
-Spatial frequency
-Orientation
-Motion
-Direction
-Speed (thus temporal frequency)
-Other spatiotemporal features.

The tuning properties of V1 neurons (what the neurons respond to) differ greatly over time.
Early in time (40 ms and further) individual V1 neurons have strong tuning to a small set of stimuli. That is, the neuronal responses can discriminate small changes in visual orientations, spatial frequencies and colors.
Furthermore, individual V1 neurons have ocular dominance, namely tuning to one of the two eyes. In V1, and primary sensory cortex in general, neurons with similar tuning properties tend to cluster together as cortical columns.

Current consensus seems to be that early responses of V1 neurons consists of tiled sets of selective spatiotemporal filters. In the spatial domain, the functioning of V1 can be thought of as similar to many spatially local, complex Fourier transforms, or more accurately, Gabor transforms.

Later in time (after 100 ms), neurons in V1 are also sensitive to the more global organisation of the scene.
These response properties probably stem from recurrent processing (the influence of higher-tier cortical areas on lower-tier cortical areas) and lateral connections from pyramidal neurons. Evidence shows that feedback originating in higher-level areas such as V4, IT, or MT, with bigger and more complex receptive fields, can modify and shape V1 responses, accounting for contextual or extra-classical receptive field effects.

The visual information relayed to V1 is not coded in terms of spatial (or optical) imagery but rather as the local contrast.
Example: image comprising half side black and half side white, the divide line between black and white has strongest local contrast and is encoded, while few neurons code the brightness information (black or white per se). As information is further relayed to subsequent visual areas, it is coded as increasingly non-local frequency/phase signals.

The upper bank of the calcarine sulcus responds strongly to the lower half of visual field (below the center), and the lower bank of the calcarine to the upper half of visual field. In concept, this retinotopic mapping is a transformation of the visual image from retina to V1. The correspondence between a given location in V1 and in the subjective visual field is very precise: even the blind spots are mapped into V1. In terms of evolution, this correspondence is very basic and found in most animals that possess a V1. In human and animals with a fovea in the retina, a large portion of V1 is mapped to the small, central portion of visual field, a phenomenon known as cortical magnification. Perhaps for the purpose of accurate spatial encoding, neurons in V1 have the smallest receptive field size of any visual cortex microscopic regions.