The visual equivalent of a sorting office?

The primary visual cortex (V1) or striate cortex is an important area in which partially processed information from the retina and LGN is separated and packaged up for more elaborate analysis in the specialised visual areas of the extrastriate cortex. But V1 is more than just a neural version of a post office sorting department. The response properties of most neurons in V1 are very different from those of neurons in the preceding area. New response features, such as sensitivity to lines and bars of different orientations and movements are created, along with a specialisation of some neurons to an existing visual feature such as colour. Moreover, the functional organisation of V1 into repeating columns and modules seems to be a standard pattern in all cortical visual areas, and this pattern of organisation is an efficient way of mapping a multi-dimensional stimulus, such as vision, on to an irregularly shaped piece of two-dimensional cortex.

Visual information passes to the cortex from the LGN through the optic radiation. In the monkey, the first cortical visual area (V1) consists of a folded plate of cells about 2mm thick, with a surface area of a few square inches. This is a much larger and more complex structure than the LGN, for example, the LGN is composed of 1.5 million cells, whereas V1 is composed of around 200 million. V1 lies posteriorly in the occipital lobe and can be recognised by its characteristic appearance.