Intensity-dependent changes of visually evoked potential components in the superior colliculus of freely moving rats

Amplitudes and peak times of averaged visually evoked potentials (VEP) recorded in the superficial layers of the anterior medial superior colliculus were measured in 7 freely moving rats. The amplitudes of P21-N29 decreased significantly to 47% (p = 0.01), the amplitudes of components N41-P54 and P54-N64 to 29 and 32% (p = 0.01) when the flash intensity was lowered six times, whereas components N29-P36 and P74-N85 showed no significant decrease. During exploratory behaviour all the components were significantly lower than during relaxed wakefulness, except N29-P36 and P74-N85. The course of their intensity-dependent decrease was different. Peak times of both early negative components N29 and N41 increased exponentially with decreasing flash intensity during relaxed wakefulness. The peak times of later negative components N64 and N85 increased only with the lowest intensity. During exploratory behaviour peak time changes were evidently smaller. The data suggest that the different changes of components reflect the activity of different neuron types.