The pigmentary effector system - IX. The receptor fields of the teleostean visual response

Abstract

Both among vertebrates and among Crustacea one commonly meets with two co-existent modes of chromatic response to photic stimulation. One is the dispersion (“expansion”) of melanophores and certain other chromatophores under the local (primary) influence of light on the skin. The other is aggregation (“contraction”) of melanophores and of certain other chromatophores when light reflected from the surroundings impinges on the organs of vision, in contradistinction to dispersion (“expansion”) when only overhead illumination strikes the eye. Though the primary (local) response is usually subordinate to and is more or less overruled by the secondary or visual response, the relative importance of the two components varies within wide limits. In particular species either may be negligible in comparison with the other. When, as more commonly, both contribute significantly to the observed result, a blinded animal is necessarily more pale in darkness than in light. Probably this fact influenced all the earlier investigators who, including the senior author (1924), paid little attention to the otherwise paradoxical fact that animals kept on a “black background” (i. e. under conditions of overhead illumination in light absorbing surroundings) are much darker than animals kept in similar conditions with no light at all. Subsequent analysis of the normal course of colour change, both in vertebrates and in Crustaces, has shown that this is also true of species which have no appreciable primary response, and that the difference generally exceeds the limits of variation consistent with the co-existence of a detectable primary response. It is therefore clear that the difference between the “white background” response and the “black background” response is not due to intensity alone.