The visual system of the minnow Phoxinus phoxinus with special reference to its relationship in colour change and behaviour

Abstract

The anatomy of the retina was studied and counts wore made of the retinal elements. It was found that the rode single and double cones decrease in number centrally and the triple and quadruple cones increase centrally. the visual acuity calculated from the counts do not agree with the acuity figures given by other workers. The dorsal or ventral ports of the retina were removed surgically or destroyed by high intensity light. The ehromatie behaviour of these fish led to the conclusion that the whole of the retina is important for normal chromatic adaptation to white or black baekgmands.The anatomy of the optic tract, diiasma, geniculate complex, and optic tectum are described. The fibres from the optic tractwere traced into the brain. The ability of the fish to adapt chromati-cally after cutting the optic tract and/or the ablation of the optic tectun indicated that the fibres which are important in backgroundadaptation enter the geniculate complex from the retina and from thererun to the tectum. In the tectum the final interpretation of thebackground occurs.The region where the fibres involved in chromatic adaptation pass out of the tectum was identified and the fibres were traced to the medullary centre. This is described. Encephalograms were recored from bipolar electrodes in the optic tectum. The surface ECG amplitude appears to be correlated with the retinal input. Recordings from electrodes implanted at different depths showed frequency changes associated with the tint of the background. A possible hypothesis for the mechanism of the central nervous control of colour change is proposed. The pattern of the locomotory behaviour of normal, blind, and tectal damaged fish in conditions of limited confinement are described and the role of the optic tectum in the control of general and motor behaviour are discussed. It is experimentally demostrated that the optic tectum plays an important role in the control of the mauthner cells of the medulla, and the relationship between the tectum and the Mauthner cells is discussed.