Involvement of cortical neurones in conditioned and unconditioned startle reflex

Abstract

Postsynaptic potentials in the sensorimotor cortex of unanaesthetized rabbits were recorded simultaneously with electromyogram of the unconditioned startle reflex or the ‘local conditioned startle reflex’. The startle reflex was produced by a loud click in naive animals. The ‘local conditioned startle reflex’ was evoked by a click of a moderate intensity after a conditioning procedure (pairing of the formerly neutral click with direct cortical and hypothalamic stimulation). The latency of the startle reflex and the ‘local conditioned startle reflex’ was from 12 to 17 ms. Postsynaptic potentials or spike discharges after less than 7 ms latency were found in about 20% of neurones in the sensorimotor cortex during both the startle reflex and the ‘local conditioned startle reflex’. Stimulation of subcortical auditory structures evoked EMG responses after 4–8 ms latency. About 25% of sensorimotor cortical neurones responded with postsynaptic potentials and spike discharges within 4 ms after the stimulation of the colliculus inferior. The data support an idea of multiple level organization of the startle reflex and suggest that a pathway for the startle reflex and the ‘local conditioned startle reflex’ may pass through the sensorimotor cortex.