Sensory interaction in the superior colliculus of freely moving rat indicated by evoked potentials.

Sensory monomodal and bimodal interaction was compared in the anterior medial superior colliculus (CS) of freely moving Long-Evans rats with chronically implanted electrodes using pairs of click and flash stimuli separated by intervals of 100 ms. The amplitudes and peak times of first and second evoked potentials were statistically evaluated and compared with the uninfluenced control values of visually and acoustically evoked potentials (VEP and AEP) on the back-ground of relatively constant relaxed wakefulness. Heteromodal interactions were characterized by only very small and in most cases insignificant changes, compared with very striking depressions of component amplitudes of the second EP in monomodal paired stimulation. Significant differences of AEP and VEP amplitude and peak time changes in superficial and in deep layers of the CS indicated that the sensory interaction is different, corresponding to the functional structure of CS layers. The amplitude of the second negativity N32 in AEP 100 ms after flash is significantly influenced in the superficial layers, but not in the deeper ones. The VEP peak times are prolonged after click only in the deeper layers.