Evoked single cortical unit activity in the somatic sensory areas

Abstract

1. 1. Evoked activity in single cortical units was isolated in the somatosensory areas of cat and monkey under barbiturate anesthesia or chloralose anesthesia combined with d-tubocurarine or B-erythroidin. Peripheral mechanical or nerve stimulation was used. 2. 2. The spikes were predominantly initially negative when recorded with 4–12 μ electrodes. Larger positive-negative and positive spikes were recorded with 0.5–3 μ electrodes together with smaller initially negative spikes. 3. 3. The latency for the first spike of some units was variable in successive records and varied greatly between units, especially under light anesthesia. Other units showed extraordinary consistency of firing. 4. 4. Some units fired on the commencing limb of the positive primary response which was usually reversed in sign when the electrode tip was 0.5–1.0 mm. deep. The earliest evoked activity at depths of 220–350 μ occurred several milliseconds later than the earliest activity at greater depths in the cortex. 5. 5. High frequency repetitive discharges of the same cortical unit were observed following a single peripheral stimulus. However, this can occur as early in the sensory system as the cuneate nuclear relays. Because injury by the electrode can cause an increased number of spikes per discharge, multiple discharges require a careful evaluation. 6. 6. In some units, increases in strength of nerve stimulation or alterations in site of peripheral stimulation, which reduced the latency for the first spike, lead to an increase in rate of firing and number of spikes per discharge. 7. 7. Cells lying within a few micra of one another often fire with different latencies, number of spikes per high frequency discharge, interspike intervals and may be preferentially activated from different afferent sources. 8. 8. The receptive field of some somesthetic association area units included all four limbs, but the timing of unit firing may significantly differ with the site of stimulation. 9. 9. Absolute and relative unresponsive phases are distinguished in single cortical unit responses to paired shocks to the same nerve. Alteration in strength of the test shock may alter duration and degree of these phases. A later supernormal phase is also distinguished. The response to repetitive peripheral stimulation was studied. 10. 10. Forms of inhibitory and occlusive interaction are distinguished following stimuli to two nerves. Interaction may occur with nearly synchronous volleys in peripheral nerves. 11. 11. The significance of single cortical unit recording is discussed as related to spread of activity in the cortex, the mechanism of the primary response, population sampling and transmission of information in the sensory system.