What do the color blind see?

This apparently straightforward question is not straightforwardly answerable, but it remains irresistibly intriguing because of its theoretical connection to the nature of the neural representation of color and the physiological basis of color deficiency. Red-green color blindness (dichromacy) was originally conceived as a loss of one cone type, but in fact usually originates instead from a pigment swap in which red-or green-sensitive cones both get the same visual pigment. Rare individuals with one dichromatic eye and one normal eye can report meaningfully on the range of sensations generated by the dichromatic eye; remarkably, though, the reports of these observers are never consistent with the predictions for a simple pigment swap (according to which a match for the pigments is a perceptual match). Since the molecular genetic rationale for a pigment swap is clear, there must be additional differences between dichromatic and trichromatic visual systems in their postreceptoral neural organization. One possibility with some observational support is that the colors seen are the ones that are most likely given the limited information available from the dichromatic eye. This could happen if the postreceptoral representation of color is adaptively adjusted in accordance with the history of photoreceptor signals during development.
Anomalous trichromats have reduced red-green discrimination because a visual pigment of intermediate spectral sensitivity replaces the normal red-or green-sensitive pigment, and a similar process may operate in these individuals. Their judgments of pure yellow agree well with those of normal trichromats for typical natural colors. The simple pigment swap model can easily be modified to accommodate this, but predicts that their impressions of red and green stimuli will perceptually regress toward yellow. A sufficiently sophisticated recalibration process, however, might restore the anomalous observer's red and green to normal vividness. We test this point using experiments on color discrimination, color contrast and color aftereffects. Results suggest that the visual system of anomalous observers has a postrceptoral increase in chromatic gain that allows it to compensate partially for the impoverishment of the chromatic signals at the photoreceptor level.

Speaker: Don MacLeod, UC San Diego