Evidence for visual function mediated by anomalous projection in goldfish

Abstract

IN goldfish, there is a complete decussation of the optic nerves and only contralateral projection to the optic tectum is present. If one tectum is removed surgically, a few months later, the contralateral eye projects to the remaining ipsilateral tectum, superimposes the normal projection in a mirror-image fashion and retains its normal polarity¹. The method of mapping visuotectal projection involved presynaptic recordings of the optic fibre terminals in the tectum. It is not known, however, whether or not the anomalous projection is able to mediate visual behaviour. To determine whether the regenerated optic nerve fibres have formed functional connections in the tectum, post-synaptic recordings and (or) behavioural tests are required. We report here a preliminary finding that the anomalous ipsilateral visuotectal projection, after removal of the contralateral tectum, may mediate the detection of a visual stimulus, with a threshold not significantly different from the normal projection. Sharma¹ has observed that in animals with complete removal of the right tectum “after removal of the right eye in these animals, the behaviour mediated by the left eye was apparently normal”, and here we supplement those observations with psychophysical data.