Influence of stationary and moving textured backgrounds on the response of visual neurons in toads (Bufo bufo L.)

Previous studies have shown that configurational prey recognition in common toads is performed by feature-analyzing functional units consisting of assemblies of connected neurons such as retinal (classes R2, R3), tectal [classes T5(1), T5(2), T5(3)], and pretectal (class TH3) cells. In the present paper, effects of textured backgrounds on the response of these neurons to a configurational moving stimulus have been tested quantitatively. (1) In all investigated neurons, neither the overall activation nor the respective stimulus-response relationships were significantly influenced by a stationary black/white-textured background as far as black stimulus objects are concerned. (2) The neuronal activity in response to a moving object (signal) could be inhibited (masked) if a black/white-textured background (noise) was moving simultaneously at the same speed. The strength (I) of this 'surround inhibition' (signal masking by the background) was different in the various classes of neurons, i.e. strongest for T5(2) and weakest for R3: IT5(2) greater than IT5(1) greater than IT5(3) greater than IR2 greater than ITH3 greater than IR3. These inhibitory effects were not correlated with the size of the neuronal excitatory receptive field (ERF), since T4 neurons (ERF = 180 degrees) in this context displayed response properties similar to T5(2) neurons (ERF less than 30 degrees). (3) It is suggested that the signal (prey)-masking effect of a moving textured background is brought about by pretecto (TH3)-tectal [T5(1), T5(2)] inhibitory connectivity which allows toads: (a) to select prey from nonprey; (b) to discriminate between prey and a textured background, and (c) to determine the origin of moving retinal images caused either by object movement or by self-induced motion.