INFORMATION-PROCESSING AND NEUROMODULATION IN THE VISUAL-SYSTEM OF FROGS AND TOADS

The retinal input to the midbrain optic tectum and pretectal thalamus of frogs and toads synapses with intrinsic neurons in the surface layers. Processing of visual information occurs from surface to a depth involving intratectal connections, probably organized in a columnar fashion, and also connections derived from pretectal thalamus. The latter are involved in determining the specificity of responses of some tectal (T5(2)) units for 'prey'-like stimuli. These T5(2) units also act as pre-motor neurons as they project to more posterior brainstem (bulbar) regions responsible for prey-catching behaviour. Novel stimuli presented to toads induce an increase in synchrony and amplitude of the tectal electroencephalogram (EEG) and a surface negative sustained potential shift (SPS). Phase locking of tectal unit activity with the EEG oscillations provides evidence that the EEG may contribute to modulation of neural responsiveness. The sps representing extracellular potassium fluxes and depolarization of radially distributed glia may also represent a mechanism whereby the efficacy of synaptic transmission may be altered during different states of behavioural responsiveness.