Effect of a local focus of enhanced excitability in the mesencephalic reticular formation and sensomotor cortex on superior collicular function.

Abstract

1. Foci of enhanced excitability created in the sensomotor cortex and mesencephalic reticular formation by application of strychnine have a similar action on spontaneous and evoked unit activity in the superior colliculus. This action is manifested as a change in the spatiotemporal characteristics of the response to the photic stimulus and transformation of a phasic response into a prolonged excitatory response without inhibitory pauses. After a certain time the phasic character of the response was restored. 2. Similarity in the character of action of foci of enhanced excitability in the sensomotor cortex and mesencephalic reticular formation is interpreted either as the result of mediation of the cortical influence through the brainstem RF or as a result of convergence of direct corticofugal and reticular inputs into the superior colliculus on the same interneurons. As a result of this the end result of cortical and reticular influences may be identical. 3. The absence of inhibitory phases in the response of superior collicular neurons to a photic stimulus during the first period of action of foci of enhanced excitability in the sensomotor cortex and mesencephalic reticular formation may be the result of the blocking effect on activity of inhibitory interneurons participating in the formation of inhibitory pauses. 4. The presence of correlation of enhanced neuronal activity in structures of the superior colliculus with the appearance of a motor response of the forelimb to an isolated photic stimulus after creation of foci of enhanced excitability in the sensomotor cortex and reticular formation is evidence of the formation of functional relations between the visual and motor analyzers and the possible role of the mesencephalic visual center in this process. 5. Enhanced reactivity of superior collicular neurons to photic stimuli is examined from the standpoint of their increased readiness for involvement in the mechanism of the orienting reaction or of the visually controlled motor act.