Primary+Visual+Cortex

The **primary visual cortex** is the best-studied visual area in the [|brain]. In all mammals studied, it is located 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 name "striate cortex" is derived from the [|line of Gennari], a distinctive stripe visible to the naked eye that represents [|myelinated] [|axons] from the [|lateral geniculate body] terminating in layer 4 of the [|gray matter]. 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 nucleus] (LGN), is further divided into 4 layers, labelled 4A, 4B, 4Cα, and 4Cβ. Sublamina 4Cα receives most [|magnocellular] input from the LGN, while layer 4Cβ receives input from [|parvocellular pathways]. The occipital cortex where the visual cortex resides is the smallest of the four cortexes of the human brain, which also includes the [|temporal cortex], [|parietal cortex], and [|frontal cortex]. The average number of neurons in the adult human primary visual cortex, in each hemisphere, has been estimated at around 140 million.[|[][|13][|]]

Function
V1 has a very well-defined map of the spatial information in vision. For example, in humans, 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].[|[][|14][|]] Perhaps for the purpose of accurate spatial encoding, neurons in V1 have the smallest receptive field size of any visual cortex microscopic regions. 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 in human and animals with [|binocular vision] 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]. [|David Hubel] and [|Torsten Wiesel] proposed the classic ice-cube organization model of cortical columns for two tuning properties: [|ocular dominance] and orientation. However, this model cannot accommodate the color, spatial frequency and many other features to which neurons are tuned[//[|citation needed]//]. The exact organization of all these cortical columns within V1 remains a hot topic of current research. The mathematical modeling of this function has been compared to [|Gabor transforms]. Later in time (after 100 ms), neurons in V1 are also sensitive to the more global organisation of the scene (Lamme & Roelfsema, 2000).[|[][|15][|]] These response properties probably stem from recurrent [|feedback] processing (the influence of higher-tier cortical areas on lower-tier cortical areas) and lateral connections from [|pyramidal neurons] (Hupe et al. 1998). While feedforward connections are mainly driving, feedback connections are mostly modulatory in their effects (Angelucci et al., 2003; Hupe et al., 2001). 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 (Guo et al., 2007; Huang et al., 2007; Sillito et al., 2006). The visual information relayed to V1 is not coded in terms of spatial (or optical) imagery but rather are better described as [|edge detection]. As an example, for an image comprising half side black and half side white, the divide line between black and white has strongest local contrast (that is, edge detection) 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. Note that, at these early stages of cortical visual processing, spatial location of visual information is well preserved amid the local contrast encoding (edge detection).