Behavioural responses to electrical and visual stimulation of the toad tectum.

Slow potential shifts in brain structures have been recorded and correlated with motivational state in several species. Previous studies have also found that application of an electrical current to the surface of brain tissue generates such slow potential shifts. The present study was conducted to examine if imposed dc shifts to the brain influenced motivation in the toad (Bufo bufo). Toads (B. bufo) had stimulating electrodes implanted on the surface of each optic tectum. After 1 day of recovery combined dc stimuli and a prey-like visual stimulus were presented to the animal. A current-dependent increase in prey-catching activities occurred with dc currents from 0.1 to 500 micro A and in avoidance behaviours from 50 to 500 micro A. There is also evidence of additivity of dc and visually induced negativity increasing some behaviours. The dc current was applied in order to start a movement of ions through the brain structure but more specifically through radial glia. The resulting flux of ions is thought to be responsible for the recorded slow potential shift associated with motivation and these experiments hopefully shed further light on the possible neuromodulatory role played by radial glia through the spatial buffering of potassium and the associated slow potential shifts.

Neuronal responses are differentially affected by the polarity of tectal DC stimulation in the toad Bufo bufo.

Male toads were tested behaviourally for their prey catching responses to worm-like stimuli before being prepared for visual unit and slow potential shift (SPS) recording from the optic tectum. The neuronal responses of toads to a prey-like visual stimulus reflected their motivational tendency prior to operations. One second of DC stimulation to the tectum was followed by an SPS of reversed polarity during which time a visual prey-like stimulus was presented. A negative SPS following positive DC stimulation was associated with enhanced neuronal responses to a visual stimulus. The positive SPS that followed negative stimulation was associated with a decline in neural responses below background when a visual stimulus was additionally given. The SPS was largely a result of DC stimulation that interacted with the motivational tendency to produce enhanced neuronal responses, while the potential was negative and vice versa.