Electrophysiology of the frontal granular cortex. I. Patterns of focal field potentials evoked by stimulations of dorsomedial thalamus in conscious monkey

Focal field potentials and unit discharges of impulses evoked by low frequency (2–10/sec) stimulations of dorsomedial thalamus (DM) were simultaneously recorded with microelectrodes in the middle and inferior frontal gyri of the dorsal frontal granular cortex (FC) in the normal unanesthetized monkey. The depth profiles and sequences of responses of the FC evoked by the stimulations of DM were observed to be generally similar to the augmenting type of responses reported previously by others in the sensorimotor cortex of encephale isoleor paralyzed cat. Stimulations of DM evoked short latency (3.2–4.0 msec) negative waves at a depth of 0.6–1.7 mm and correlated positive waves on the surface of the FC. Additional waves of alternating polarity succeeded the primary waves during augmenting sequences according to variations of stimulus strength and frequency. Unit discharges recorded with the same electrode simultaneously with field potentials revealed that a brief burst of 2–4 impulses occurred in association with each of the short latency deep negative waves evoked by repetitive stimuli. The latency of the earliest of the spikes of a burst was in the range of 6.5–9 msec. Following the burst of spikes, there was a long period of absence of spikes in each response. Surface corticograms of FC and nearby precentral cortex revealed that the evoked responses were highly pronounced and restricted to the frontal granular cortex and did not spread diffusely to the agranular cortex. Only a prolonged (40–60 sec) stimulation caused a synchronizing tendency in the corticogram of the agranular area. Furthermore, the evoked potentials in the corticogram of FC were presently only during quiet wakeful states, and became highly disorganized and almost indecipherable during states of synchronizing EEG and decreased consciousness as in natural slow-wave sleep or under Nembutal anesthesia. From these results, it is concluded that the dorsomedial thalamus is organized more like a specific thalamic nucleus to the frontal association cortex, and less like an integral part of the diffuse projection system.