The Geniculostriate Visual Pathways
The Cortical Processing of Colour
The figure at left shows a cross-section of the striate cortex in the occipital lobe and the adjacent visual association areas.
Visual processing at the cortical level is initiated in V1, the primary visual area. V1 is functionally specialized to analyse orientation, ocular dominance, and colour information from specific retinal locations. The cells in V1 are organized in an array of hypercolumns, each of which corresponds to a point on the retina. As shown in the figure below, each column in the hypercolumn responds to a particular orientation; adjacent columns manage information from adjacent retinal locations. Of the many different types of cells in V1, blobs and interblobs are most important to the perception of colour. These cells, named after their blob-like appearance in the hypercolumns, receive input from the parvo cells of the LGN, and continue the processing of colour information.
Blobs are composed of many colour-opponent cells, and are functionally similar to colour-opponent cells in the dLGN and horizontal C-units. They are activated and inhibited by a colour and its opposite (red vs. green, blue vs. yellow). For example, the cell will fire at a specific rate for only one specific (range of) wavelength (above spontaneous rate is excitatory; below is inhibitory). It should be noted, however, that colour perception is a global perception of all the wavelengths of light that comprise a visual scene. V1 wavelength-specific responses of one point in space do not give any information about its perceived colour.
Interblobs are situated between blob cells in the hypercolumns, and function to relay information about form. Research seems to suggest that only the red-green trichromatic channel is involved in the perception of form, but not the blue-yellow channel. For this function, the red-green information is passed along to the inferotemporal cortex in the parietal lobe. In other words, the red-green channel serves the dual function of coding for both form and colour. The complex processing of colour and form that begins in the retina and extends to cortical areas is depicted schematically below.
V1, there are two general streams of information processing: one for
motion and location, and
the other for colour and form. These
are known as the ventral and dorsal streams, respectively.
Because of the functions they serve,
are also called the "where" and "what" paths.
The ventral stream terminates
in the temporal lobe; the dorsal stream in the parietal lobe. It is
the ventral stream that is most involved in the perception of colour
Because it integrates information from other cells, it is thought that V4 achieves colour constancy. Colour constancy is the perception that an object's colour remains constant despite changes in lighting level and other conditions. For example, an orange still looks orange in fluorescent, daylight, or tungsten lighting. Therefore the two trichromatic channels, red-green and blue-yellow, have reached their "colour analysis destination." The black-white channel, in contrast, codes for brightness in the magnocellular pathway and synapses in the MT cortex.
Final Thoughts on the
Cortical Processing of Colour
It is almost impossible for someone not so afflicted to imagine the experience of colour in the absence of form. The figure below, however, may suggest something of how visual perception was changed for the young woman who survived the tragic Cocoanut Grove fire. NOTE FIGURE CAPTION: An artist's impression of how a scene with colour but not form (left panel) compares to the more usual condition where the two properties are perceived as part of an integrated whole (right panel).