A TAXONOMY OF TRITANOPIAS

Our vision has a number of anomalous features when detection or discrimination is mediated only by signals originating in the short-wavelength receptors. A summary list of these 'anomalies of the blue mechanism' is given and two groups are provisionally distinguished, firstly a group that directly or indirectly reflect the basic insensitivity of the short-wavelength mechanism and secondly a group of interrelated phenomena that are observed during light and dark adaptation to coloured fields. Some of the former group probably arise from properties of the short-wavelength receptors themselves; the second group almost certainly arise from interactions between long- and short -wavelength signals. Under conditions of tritanopic viewing, short-wavelength stimuli often look bluish. To explain this paradox, it is argued that the brain always seeks to reduce discrepancies between sensory inputs and that in this case it chooses to eliminate the discrepancy at that wavelength at which the residual opponent signal from the dichromatic retina is strongest and least easily suppressed. This wavelength, close to 470nm, looks blue, not green, under conditions of trichromatic observation.