Trichromatic processing

To get some sense of what is going on during the trichromatic processing of visual information collected during phototransduction within rod and cone cells, we need to imagine the light-receiving layer within the retina of each eye as being composed of around 120 million points. Each point is the location of a rod or cone within a mosaic of cells stretching to the boundary of the retina in every direction. Each point also corresponds with a different location within the overall image of the world outside that is projected onto the retina by the lens as light enters the eyes through the cornea, and pupil.

Every rod and cone cell within the human eye is an independent photo-sensitive neuron that responds independently to incoming photons of light. In the first instance, each one carries out its task of responding to the stream of different wavelengths of light without reference to the receptors around it. We must imagine all these neurons as being capable of functioning simultaneously and being able to fire several times a second. Each time a cell responds, phototransduction takes place, which is a chemical response to light that produces electrical impulses ready for transmission.

What precisely happens depends on lighting conditions.

When light levels are low, each photosensitive rod cell may be hit by a handful of photons of a given wavelength and in some cases, this is enough to trigger the chemical response. But at very low levels of lighting, the response of cone cells is very limited. The outcome is a typical night scene that appears blue-purple-black with very little detail.

When light levels are high, each cone cell may be bombarded with tens of thousands of photons of a given wavelength within the briefest fraction of a second, producing a powerful chemical response. In these conditions, rod cells tend to be overwhelmed by the sheer quantity of light but can still provide useful information in peripheral vision. The outcome is a world rich in colour, full of detail and with contrasting highlight-detail and deep well-defined areas of shadow.

The trichromatic process involves the sampling of this mosaic of rod and cone photoreceptors by bipolar and horizontal cells. Individual photoreceptors are connected in small groups to bipolar cells that receive their electrical output. The bipolar cells compare the response of receptors with the response of neighbours, whilst horizontal cells help to aggregate the result and encode it into three independent data channels.

The comparison process involves what is termed centre-surround antagonism. This refers to the way bipolar neurons organize their receptive fields. A close-up view reveals that centre-surround antagonism relies on small groups of cells being arranged around a centre point where one rod or cone synapses on the dendrites of a bipolar cell. Around this centre are other photoreceptors that also synapse onto other dendrites of the same bipolar cell. The signal received from the centre is compared with a summation of the signals from neighbours to establish to what extent they agree or disagree. This process goes on in real-time as bipolar cells receive successions of signals and horizontal cells modulate the information to improve fidelity. The scale of this enterprise as it takes place across the surfaces of the retina in each eye and in real-time is extraordinary.

The role of horizontal cells in both trichromatic and centre-surround antagonism is best conceived as signal conditioning and particularly with globally adjusting visual information ready for opponent processing. However, ongoing research suggests that horizontal cells may also be involved in an early stage of signal interpretation and so contribute towards things such as the detection of edges and movement.

Trichromatic colour vision is governed by the fact that there are three distinct types of cone cells within the retina, each tuned to respond to a different band of wavelengths of light. As light floods in, each type of cone outputs a signal if it picks up the presence of photons with wavelengths it recognises. Every identifiable point on the retina contains all three types of cone cells and these are tightly packed into a random mosaic pattern across the entire surface. The result is a photosensitive film containing millions of receptors capable of responding to all wavelengths across the visible spectrum.

General descriptions of trichromatic vision often suggest that the three types of cone cells in the human retina are responsive to wavelengths corresponding to red, green and blue. It is more accurate to say that the peak sensitivity of these L, M and S cones types respond with biases towards different regions of the visible spectrum and have a loose correspondence with red, green and blue:

L cones: Respond to long wavelengths (peak sensitivity around 560 – 580 nanometres) so with a region of sensitivity that includes red, orange, green and yellow but with a peak bias between red and yellow.
M cones: Respond to medium wavelengths (peak sensitivity around 530 – 545 nanometres) so with a region of sensitivity that includes orange, green, yellow and cyan but with a peak bias between yellow and green.
S cones: Respond to short wavelengths (peak sensitivity around 420 – 440 nanometres) so with a region of sensitivity that includes cyan, blue and violet but with a peak bias between blue and violet.
Rods: Rod cells are most sensitive to wavelengths around 498 nanometres, so with a peak sensitivity towards green-blue, and are insensitive to wavelengths longer than about 640 nanometres (red).

This arrangement suggests that:

At any specific moment, all three types of cone cells at any specific location on the retina may fire multiple times per second in response to streams of photons that have constantly changing wavelengths.
The assessment of the distribution of wavelengths by every individual cone depends on both its range of sensitivity and the wavelength at which its response peaks.
Centre-surround comparisons check for consistencies and variations within the responses of cone groupings at every location.
As soon as a clean and noise-free consensus is achieved, the data specific to each cone type is output on one of three separate data channels.
Once the composite of trichromatic and centre-surround encoding of colour information is complete it is sent onward for opponent processing within the retina.
Recent research suggests that the three output channels do not correspond directly with the L, M and S cone types. There are indications that two channels contain chromatic information whilst the other contains achromatic data.