VISUAL PERFORMANCE OF THE TOAD (BUFO-BUFO) AT LOW-LIGHT LEVELS - RETINAL GANGLION-CELL RESPONSES AND PREY-CATCHING ACCURACY

The accuracy of toad snapping towards moving worm dummies under variou s levels of dim illumination (from absolute threshold to ''moonlight'' ) was video-recorded and related to spike responses of retinal ganglio n cells exposed to equivalent stimuli. Some toads (at ca. 16-degrees-C ) successfully snapped at dummies that produced only one photoisomeriz ation per 50 rods per second in the retina, in good agreement with thr esholds of sensitive retinal ganglion cells. One factor underlying suc h high sensitivity is extensive temporal summation by the ganglion cel ls. This, however, is inevitably accompanied by very long response lat encies (around 3 s near threshold), whereby the information reaching t he brain shows the dummy in a position where it was several seconds ea rlier. Indeed, as the light was dimmed, snaps were displaced successiv ely further to the rear of the dummy, finally missing it. The results in weak but clearly supra-threshold illumination indicate that snaps w ere aimed at the advancing head as seen by the brain, but landed furth er backwards in proportion to the retinal latency. Near absolute thres hold, however, accuracy was ''too good'', suggesting that the animal h ad recourse to a neural representation of the regularly moving dummies to correct for the slowness of vision.