Dynamic response properties of visual neurons and context-dependent surround effects on receptive fields in the tectum of the salamander Plethodon shermani.

Neuronal responses to complex prey-like stimuli and rectangles were investigated in the tectum of the salamander Plethodon shermani using extracellular single-cell recording. Cricket dummies differing in size, contrast or movement pattern or a rectangle were moved singly through the excitatory receptive field of a neuron. Paired presentations were performed, in which a reference stimulus was moved inside and the different cricket dummies or the rectangle outside the excitatory receptive field. Visual object recognition involves much more complex spatial and temporal processing than previously assumed in amphibians. This concerns significant changes in absolute number of spikes, temporal discharge pattern, and receptive field size. At single presentation of stimuli, the number of discharges was significantly changed compared with the reference stimulus, and in the majority of neurons the temporal pattern of discharges was changed in addition. At paired presentation of stimuli, neurons mainly revealed a significant decrease in average spike number and a reduction of excitatory receptive field size to presentation of the reference stimulus inside the excitatory receptive field, when a large-sized cricket stimulus or the rectangle was located outside the excitatory receptive field. This inhibition was significantly greater for the large-sized cricket stimulus than for the rectangle, and indicates the biological relevance of the prey-like stimulus in object selection. The response properties of tectal neurons at single or paired presentation of stimuli indicate that tectal neurons integrate information across a much larger part of visual space than covered by the excitatory receptive field. The spike number of a tectal neuron and the spatio-temporal extent of its excitatory receptive field are not fixed but depend on the context, i.e. the stimulus type and combination. This dynamic processing corresponds with the selection of the stimuli in the visual orienting behavior of Plethodon investigated in a previous study, and we assume that tectal processing is modulated by top down processes as well as feedback circuitries.