EVIDENCE FOR SYNCHRONOUS ACTIVATION OF NEURONS LOCATED IN DIFFERENT LAYERS OF PRIMARY SOMATOSENSORY CORTEX

Although many studies have examined the columnar organization of prima ry somatosensory (SI) cortex; the functional relationship among neuron s in different layers remains unclear. To understand how activity is c oordinated among different cortical layers, the present investigation tested the hypothesis that the initial part of a peripheral stimulus p roduces a serial pattern of laminar activation in SI cortex. Extracell ular discharges of 334 histologically recovered neurons were recorded from the medial bank of the coronal sulcus in nine anesthetized cats d uring electrical or cutaneous stimulation of the distal forelimb. Mean responses during the initial 50-msec period following stimulus onset were largest in layers IIIb or IV for both types of stimulation, but l aminar differences in the magnitude of onset responses were not statis tically significant. Among 175 neurons with responses exceeding 0.5 sp ikes per stimulus, electrical stimulation consistently produced shorte r response latencies than mechanical indentation in the extragranular (II, ma, V, VI), but not in the middle (IIIb, IV), cortical layers. Th e average minimum latencies for different cortical layers ranged from 7.4 to 10.1 msec for responses to electrical stimulation and from 10.3 to 11.6 msec for responses to mechanical indentations, but these lami nar differences were not statistically significant. In some experiment s, neurons in different layers of a cortical column were recorded simu ltaneously with dual-electrode assemblies; among 37 neuron pairs in wh ich both neurons responded with more than 0.5 spikes per stimulus, res ponse latencies were similar, even though the neurons were separated b y several hundred microns. Cross-correlation analysis of the onset res ponses for neurons recorded simultaneously from different layers also indicated that many cells throughout a cortical column were activated nearly simultaneously by the initial phase of a peripheral stimulus. R esults from the present study are compared with previous reports exami ning laminar patterns of activation.