Unit study of exteroceptive inputs to claustrocortex in awake, sitting, squirrel monkey

Abstract

This exploratory study is concerned with the question of exteroceptive inputs to the cortex of the insula overlying the claustrum (claustrocortex of Brockhaus). Experiments were performed on awake, sitting squirrel monkeys with a chronically fixed stereotaxic device for exploring the brain with microelectrodes while testing unit responses to gustatory, somatic, auditory, and photic stimulation. Observations were made on more than 1800 units located in the claustrocortex, the frontal and temporal opercula, the claustrum and elsewhere. In the claustrocortex approximately one-fifth of 500 units responded to some form of sensory stimulation. Differences in the location of units responding, respectively, to somatic, auditory, and gustatory stimulation were found to be statistically significant. Units responding to gustatory stimulation were found in the rostral part of the claustrocortex and the adjacent cortex of the frontal operculum. Because of problems of control in the awake, sitting animal, it could not be conclusively proved that the responses were due to activation of taste receptors. Other units in the same cortical areas were activated by mechanical stimulation of the oral cavity. About 9% of units in the claustrocortex were specifically activated by somatic stimulation, with latencies ranging from 30 to 300 msec. Only 3 units appeared to be solely responsive to blunt and sharp pressure; the remainder could also be activated by tactile stimulation. Receptive fields were usually large and bilateral. More than 10% of units in the claustrocortex responded to auditory stimulation. Two main types of units could be identified on the basis of latencies, response rates, and duration of discharge. For one type, latencies were as short as 7 msec. A few units were found which responded to tones within a narrow range of frequencies. One unit in the frontal operculum responded only to the vocalization of another monkey. Only two units of the claustrocortex responded to photic stimulation. A new type of unit was encountered which was activated by approaching objects. Such ‘approach-type’ units were found in the claustrocortex and frontal operculum. The results are discussed in the light of existing knowledge about the insula.