Introduction to the Bases of Colour Vision

The Sensorineural Basis of the Colour Experience

Our discussion will follow the mechanisms from the retina, through the visual pathways, and on to the cortex.

The Retina's Role in Colour Vision
The retina contains two types of photoreceptors: rods and cones. Rods, which are more numerous than cones, converge at high rates on the retina's bipolar cells. As a result, they have poor acuity but are well suited for the role of mediating our perception in low illuminance (i.e., scotopic) conditions. Rods are also colour-blind. The cones which are sensitive to colour information are responsible for acuity and vision in daylight (i.e., photopic) conditions. Research indicates that the cones feed into three neural channels for colour processing, each composed of opponent pairs: red-green, yellow-blue, and black-white. The first two mediate the perception of hue and form, the latter is responsible for brightness and darkness (i.e., luminance).

Cones synapse directly onto bipolar cells. These in turn synapse onto ganglion cells which form the optic nerve.

Two types of cells that cross the receptor/bipolar/ganglion path perpendicularly: amacrine and horizontal cells. There are two types of horizontal cells, C-units and L-units. The C-units are colour-opponent cells, one type responding to red-green, the other to blue-yellow. The L-units, in contrast, do not respond to colours in an opponent fashion, but code only for brightness, thus contributing to the so-called luminance channel.

The Geniculostriate Visual Pathways
The geniculostriate is the primary visual pathway for the processing of form and colour. It extends from the retina through the lateral geniculate nucleus (LGN) of the thalamus to the primary visual cortex (i.e., V1). The ganglion cells, which carry information from the retina to the LGN are of two types, each named for the LGN layer to which they project. The parvocellular ganglion cells are colour-sensitive and project to the four parvocellular layers of the dLGN. The magnocellular ganglion cells are achromatic and project to the two magnocellular layers of the LGN.

The slow-conducting parvocellular pathway mediates our perception of colour and acuity; the fast-conducting magnocellular pathway is responsible for our perception of stimulus change (including motion) and is largely colour-blind.

The Cortical Processing of Colour
Beyond the LGN the dendritic connections, called optic radiations, extend to higher visual cortical areas. These radiations synapse at V1 in the occipital lobe at the posterior of the brain. From V1 nerve fibres carry information to many other cortical areas including the "extrastriate" areas V2, V3, V4, and V5 (the latter also known as the medial temporal or MT cortex).

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.

Beyond 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 and form.

The global analysis of colour appears to occur in cortical area V4. Unlike cells in V2 and V1, cells in V4 respond only to one narrow band of wavelengths. In V4 there is a direct correlation between a perceived colour and wavelength, made possible by a global analysis of information from neighbouring cells. Damage to V4 can impair or even eliminate the ability to see or even imagine 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
Despite all that vision science has learned about colour vision, it is clear that many interesting mysteries remain. This is exemplified by Zeki's case history of a young girl who suffered from carbon monoxide poisoning from a devastating fire in Boston in 1942 at the Cocoanut Grove nightclub, which killed 491 people. Although she escaped, she inhaled a great deal of smoke and carbon monoxide, and initially thereafter experienced total blindness. After several days, however, she reported seeing colours--and only colours--of objects she claimed she could now see rather than a complete achromatopsia that might be expected from extensive damage to V4. Remarkably, the carbon monoxide seemed to have selectively impaired all visual functioning but left her the ability to see colour. (See also module on Acquired Colour Deficiencies in this tutorial.) The figure below is an artist's conception of the visual experience of this young girl.

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).