Influence of pretectal lesions on tectal responses to visual stimulation in anurans: field potential, single neuron...

Investigating the fine structure of the anuran's optic tectum (OT), Székely and Lázár suggested that a focal tectal excitation would spread across the tectal network if there were no control by thalamic pretectal (TP) inhibitory influences. Disconnecting OT from TP by lancet lesions in toads, we show that visual prey-catching is hyperexcited and stimulus discrimination nearly abolished. Functional recovery exists. Micro-administration of the axon sparing excitotoxins kainic acid (KA) or ibotenic acid (IBO) confirm that the TP region is actually involved. Recordings from prey-selective tectal neurons in immobilized frogs reveal a KA-induced impairment of stimulus discrimination and an increase in spontaneous firing. Following TP-lesions, in freely moving toads a correlation is observed between any moving stimulus, enhanced neuron firing, and pray-catching. Tectal field potentials evoked by diffuse light on- and off-stimuli before and after administration of the conduction-blocking drug procaine (PRC) suggest that populations of tectal neurons are affected by pretectal inhibitory influences. Considering previous work on physiologically identified pretecto-tectal projection cells, three different kinds of pretecto-tectal influences are discussed. A working hypothesis suggests a loop by which striatal influences are controlling TP, thus gating and tuning the visual information processing in OT.