The effects of pretectal lesions on neuronal, sustained potential shift and electroencephalographic responses of the toad tectum to presentation of a visual stimulus

Abstract

1. 1. Extracellular recordings of visual unit activity, sustained potential shifts and EEGs were made simultaneously from the tectum of both immobilised and freely moving toads ( Bufo bufo). Configurational moving stimuli were presented and the bioelectric responses monitored from the region of the same neuron, both before and after a surgical lesion of the caudal dorsal thalamus and the pretectum (TP region). These lesions caused a loss of configurational selectivity in the response of T5(2) units recorded in immobilised animals and a loss of selectivity in prey-catching behavior in the freely-moving specimens. Visual neurons in general showed a more persistent response after pretectal lesions, and T5(2) neurons also showed an enhanced firing rate. 2. 2. TP lesions caused reductions in the amplitude of the sustained potential shift and EEG responses of the tectum to visual stimuli, moving in the visual field of the contralateral eye, and also reduced the amplitude of the ipsilateral pretectal EEG response. The telencephalic EEG response was eliminated altogether. EEG responses of the tectum in freely-moving animals, though smaller in amplitude, were more lasting and of higher frequency after lesion formation. The results are considered to support earlier contentions that there are specific, inhibitory neuronal connections from pretectum to tectum which contribute to the configurational selectivity of the visual system of toads. Non-specific SPS and EEG responses do not appear to contribute directly to the “disinhibited” prey-catching behavior of toads or the loss of configurational selectivity of visual neurons, resulting from pretectal lesions.