Electrophysiology of neural units in goldfish optic tectum

Axons of retinal ganglion cells showed responses not previously emphasized: (a) many tonic units discharged oscillations, 2--12 spikes per burst, interburst intervals 20--300 msec; (b) phasic units showed concentric or flanking ON and OFF fields, response frequency depended on balance of retinal excitation and inhibition; (c) directional sensitivity was maximal for retinal stimuli moving in naso-temporal direction; (d) in anterior tectum deep afferent layer (DAL) provides for deep electrical sink, fibers of DAL have small fields, mostly in front of fish; (e) color-opponent units are prevalent in the superficial terminal layers, color is spatially and temporally represented. Tectal cell responses were distinguished by large visual fields, spontaneity, multiple spikes and long latencies to optic nerve stimulation, failure to follow above 60 per sec, plasticity of response. Tectal neurons of three classes included (a) cells of one type in upper layers were inhibited in ongoing activity by visual input, receptive fields exceeded 100 degrees, were often oblong, responses did not habituate; (b) cells of second type were excited by visual stimuli, became unresponsive (habituated) or responsive only to stimuli in different position or direction (newness cells); lability precluded field mapping and dishabituation was produced by change in background, extraneous stimulation, and spontaneous firing; (c) pyriform cells in periventricular layer were abundant, difficult to isolate electrically, discharged spontaneously in bursts at intervals of several seconds and responded to visual input by interruption of firing. Some tectal cells responded to non-visual stimuli as well.