The central nervous control of colour change in the minnow (Phoxinus phoxinus L.). II. Tectal ablations in normal fish.

Except for a few observations by von Frisch (1911), Dijkgraaf (1949), and Wichers (cited by Healey, 1957), no work has been done on the central nervous control of colour change, anterior to the medulla oblongata. Von Frisch performed a series of experiments in which he stimulated various parts of the minnow brain. After negative results from the cerebellum and the optic tectum he concluded that there might possibly be a darkening centre in the diencephalon. Further evidence for a possible diencephalic centre was provided by Wichers who found that the minnow could change colour after removal of both the optic tectum and the eye on the same side. Because of the complete chiasma this would suggest that the paths from the retina avoid the optic tectum. Dijkgraaf, however, found that complete tectal removal from a fish which had darkened as a result of bilateral optic nerve section resulted in pronounced paling. From this work of Dijkgraaf it would appear that the optic tectum is involved in chromatic adaptation. The experiments to be described here are concerned with the colour of minnows following optic nerve section and tectal ablation and with the role of the optic tectum and the fibre pathways involved in colour change.