Visual sensitivity and spatial summation in goldfish with compressed retinotectal projections or total tectal ablation

Abstract

When a goldfish optic nerve is crushed and allowed to re-innervate a half-ablated lobe of the optic tectum, electrophysiological mapping shows that nearly the whole visual field eventually comes to be represented on the half tectum that remains2,7,12,1L In such a 'compressed' retinotectal projection, a given region of the tectal surface should receive input from a larger than normal region of visual space. There is, therefore, the opportunity for greater convergence of optic afferents onto tectal neurons, possibly enlarging their receptive fields. Among the conditions necessary for this to occur is that the retina and tectum do not alter their complement of neurons after tectal surgery in such a way as to redress the mismatch of retina and tectum. There is evidence that tectal neurons do not proliferate any faster than normal after partial tectal ablation 14 and that the number of retinal ganglion cells remains about the same after optic nerve crushL Increased convergence might also be expected to result from optic nerve crush and regeneration, because abnormally large numbers of optic axons are found in the stratum opticum of the tectum some months after its reinnervation has taken place 8. The visual consequences of optic nerve regeneration and tectal compression are most likely to be seen in measures of spatial vision, and in keeping with this expectation, there have been findings of reduced visual acuity in fishZ,l~, 16. In the present experiments, a related aspect of vision, the limit of spatial summation (Ricco area) was determined by measuring thresholds behaviorally in goldfish with rearranged retinotectal systems. Goldfish of standard length (5-6 cm) were maintained under a 12-h light 12-h dark lighting regime at 20-23 °C. Psychophysical data were obtained from 5 fish with unilateral, half tectal ablations (LCT fish), two with complete tectal ablation (NT fish), and two normals of the same batch. The surgical procedures were carried out under anaesthesia with tricaine methanesulfonate. A flap of bone overlying the optic rectum was relfected and the meninges removed. To prepare the compressed retinotectal projection, the caudal half of the left tectal lobe was removed by aspiration, and the right optic nerve was crushed in the orbit. Tectumless fish were prepared by aspirating both tectal lobes down to the