The Visual Cortex

Much of the primate cortex is devoted to visual processing. In the macaque monkey at least 50% of the neocortex appears to be directly involved in vision, with over twenty distinct areas. Some of the areas concerned are quite well understood, others are still a complete mystery.

The first five visual areas are labelled in the picture. Nearly all visual information reaches the cortex via V1, the largest and most important visual cortical area. Because of its stripey appearance this area is also known as striate cortex, amongst other things. Other areas of visual cortex are known as extrastriate visual cortex; the more important areas are V2, V3, V4 and MT (also known as .....V5!).

[ V1 | V2 | V3 | V4 | MT ]
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V1

In primates nearly all visual information enters the cortex via area V1. This area is located in the occipital lobe at the back of the brain. It is also known as:

- primary visual cortex
- area 17
- striate cortex.

This region, which appears stripey (striated) when the cell bodies are stained and the area examined under a microscope, represents about 15% of the whole neocortical surface in the macaque monkey, though it is probably only about 5% of the neocortex in man. It is the most complex region of the cortex with at least 6 identifiable layers (layer 1 is close to the cortical surface, layer 6 adjoins the white matter below) even though it is only about 0.5mm thick in the monkey.

Projections from the LGN arrive in layer 4, and there are extensive internal connections between layers, as well as projections onward to other visual cortical areas.

Click here to find out more about:
  • The response properties of individual cells in V1
  • Architecture of V1 (how the cells are organised into groups)
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    V1 Cells

    Cells in V1 have elongated receptive fields, and consequently respond best to elongated stimuli, namely bars and edges. Hubel and Wiesel classified these cells according to the complexity of their response, dividing them into two groups called... "simple" and "complex"!

    Simple cells

    Simple cell receptive fields contain sub-regions that exert an excitatory influence on the cell's response (light grey in the picture), and sub-regions that exert an inhibitory influence (dark grey in the picture). The blue lines in the picture are time traces that plot the onset and offset of stimulation. The black vertical lines below them indicate individual nerve impulses. The most effective stimulus for this particular receptive field (left) is one that puts a lot of light in the excitatory region, and only a little in the inhibitory region. It must have the right orientation, the right position, and the right size. Stimuli that are non-optimal in terms of position (middle left), or orientation (middle right), or size (right) are less effective. Simple cell receptive fields could be 'built' in the cortex by collecting responses from LGN cells whose receptive fields fall along a line across the retina, but the exact wiring is still the subject of debate.

    Complex Cells

    Complex cells are the most numerous in V1 (perhaps making up three-quarters of the population). Like Simple cells, they respond only to appropriately oriented stimuli, but unlike Simple cells, they are not fussy about the position of the stimulus, as along as it falls somewhere inside the receptive field (left and middle-left examples above). Many complex cells are also direction-selective, in the sense that they respond only when the stimulus moves in one direction and not when it moves in the opposite direction - see the middle-right and right examples above.

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    V1 Architecture

    Hubel and Wiesel were the first to discover that cells in V1 are arranged in a beautifully precise and orderly fashion. Orientation tuning is a prominent feature of individual cell responses, as described elsewhere. Hubel and Wiesel found that as one advances deeper into the cortex through successive layers perpendicular to the surface, all cells that have orientation tuning prefer the same orientation. On the other hand, moving across the surface of the cortex, orientation tuning mostly changes in an orderly fashion (as shown by the small lines in the picture). Hubel and Wiesel used the term "orientation columns" to describe this arrangement, but they are really slabs rather than columns.

    Another major determinant of cell response is eye-of-origin. Most cortical cells can be driven by stimuli presented in either eye, but they generally prefer (ie. respond more to) one eye or the other - a property called 'ocular dominance'. Some cells prefer the right eye, and others prefer the left eye. Hubel and Wiesel discovered that ocular dominance is organised in a similar way to orientation preference - dominance is unchanging vertically but alternates as one moves horizontally across the cortex (the yellow and green 'ocular dominance columns' in the picture).

    When V1 is stained for cytochrome oxidase, an enzyme involved in metabolism, distinct blobs show up which are most clearly seen in layers 2 and 3. The blobs are arranged in rows that line up above the centres of the ocular dominance columns in Layer 4, thus the blobs are about 0.5mm apart. They are also about 0.25mm wide. Livingstone and Hubel recorded from cells inside the blob regions, and found that they showed no orientation preference, but instead were concentric, and over half of them responded to colour variation (so-called 'double-opponent' colour cells). The blobs appear to project to V2, particularly to the thin stripes of this region. These results have led to speculation that blobs form part of a colour analysis 'stream' in the visual system that passes through V1 blobs, V2 thin stripes, and on to V4.

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    V2

    Area V2 has a long common border with V1. It receives a somewhat patchy input from V1 and has a rather disorderly topographic organisation. Interestingly the mapping of the visual field onto V2 is the mirror-image of the V1 map.

    Like V1 the distribution of cytochrome oxidase in V2 is of great interest. Staining for this metabolic enzyme reveals a pattern of alternating thick and thin cytochrome oxidase stripes, each separated by a thin interstripe region.

    The thick stripes (drawn in yellow)are clearly a part of the magnocellular pathway. They receive input from layer 4B of V1 and they project to V3 and MT.

    The thin stripes (red), and the interstripes (grey), receive input from the blobs and interblob regions of layers 2 & 3 of V1. They then project to V4, thus are a part of the parvocellular pathway.

    Although it may be fashionable to talk of the M and P pathways as if they were completely separate this is clearly not the case. For example, there are connections between the thin stripes and the thick stripes of V2 and there is also a projection from V4 back to the thick stripes. There are also direct connections between MT and V4 and V3.
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    V3

    This area (coloured blue in the picture) receives inputs from the thick stripes in V2, and from layer 4B in V1.Only the lower part of the visual field is represented in V3 (a corresponding area representing the upper part is no longer considered part of V3, but has been given a different name, ventral posterior, or VP).

    Properties of cells in V3 offer few clues as to its function. Most are selective for orientation, and many are also tuned to motion and to depth. Relatively few are colour sensitive.
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    V4

    This area was originally discovered by Semir Zeki. It receives input mainly from the thin and interstripe regions of V2, but also has connections from V1 and V3.

    Although the area contains many cells that are colour selective, indicating a role in colour analysis, cells are also found with complex spatial and orientation tuning, suggesting that the area is also important for spatial vision.
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    MT

    The middle temporal area is sometimes called V5. It receives connections from layers 4B and 6 in V1, and from thick stripes in V2. These connections are ultimately linked with the M pathway from the LGN.

    Most cells in MT are tuned to motion, and the area can be divided into direction and 'axis of motion' columns. The connections from V2 are puzzling, because few cells there are motion selective. Nevertheless, many agree that MT is intimately linked with the analysis of image motion.
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    Created by George Mather, University of Sussex (georgem@biols.susx.ac.uk)Some of the images and text used in these pages were originally developed at the Department of Psychology, York University, as part of the GRASP project.