The effect of reticular stimulation on spontaneous and evoked activity in the cat visual cortex.

The mesencephalic reticular formation (MRF) of cats anesthetized with N2O was stimulated electrically, and the effects of this stimulation on activity in the striate cortex were studied. The variations of intra- and extracellularly recorded unit activity and the changes in the extracellular potassium concentration were investigated. At all levels of analysis the prevailing effect of MRF stimulation was facilitation. Half of the cells reacted with brief bursts of activity to reticular stimuli. A decrease of resting activity was rare. The cells activated by MRF stimulation had in common: (1) to show a high degree of excitatory convergence from extrinsic and intrinsic afferents, (2) to possess often corticofugal axons, and (3) to have preferentially complex receptive fields. In the large majority of cortical cells MRF stimulation facilitated responses evoked by stimulation of the optic radiation or by light stimuli. This facilitation could lead to a loss of orientation and direction selectivity. Reticular activation further led to a large increase of the extracellular potassium concentration, whereas stimulation of specific afferents led to a decrease. It is concluded that these phenomena are not merely a consequence of altered thalamic transmission, but are caused by a projection system which is organized in parallel to the specific projection and exerts a direct control over cortical excitability. The mechanism for this control appears to be a slight and rather unselective depolarization of most neurons. If disinhibitory processes are involved at all, their role is much less prominent than at the thalamic level. The functional implications of such an unselective but powerful modulation of cortical excitability are discussed in respect to corollary reticular activation as it occurs with rapid eye movements.