Cerebral cortical somatosensory evoked responses, multiple unit activity and current source-densities: their interrelationships and significance to somatic sensation as revealed by stimulation of the awake monkey's hand.

In awake monkeys, electrical pulse stimuli which mimic touch stimulation were delivered to the thenar eminence while electrophysiologic recordings were made from surface to depth in postcentral gyrus. Cortical depth profiles of somatosensory evoked responses (SEPs), multiple unit activity (MUA) and current source-densities (CSDs) were analyzed to gain insight into the neural process underlying the SEP and somatic sensation. The following was found: The thenar stimulus evoked four main SEP components which were seen over wide regions of postcentral gyrus: P1 at 12 ms, the primary evoked response; P1a, near 20 ms; N1, near 50 ms; and P2, around 120 ms after the stimulus. MUA was observed during the P1, P1a and N1 temporal intervals whose vigor changed as a function of the respective SEP component's amplitude. CSD analysis showed that during P1 a current sink and source appeared within the middle and superficial cortical layers, respectively. During P1a, a sink just above that of P1 and a superficial source became evident. During N1, a large superficial sink and one or two deep sources appeared. Evoked MUA during P1 and P1a was most prominent at the level of their current sinks while MUA during N1 appeared at the level of the current sources, in general. When stimulation was moved from the thenar portion of the hand to a region which most closely matched the receptive field of the cortical recording site, P1 and P1a increased amplitude while N1 both increased in amplitude and decreased in peak latency. Also, MUA activity during the early temporal intervals become more vigorous. These changes were similar to those observed with thenar stimuli as the recording site approached the thenar cortical representation. Over repeated trials at a single stimulus intensity, the spontaneous changes in SEP amplitude were found to be directly correlated with MUA and CSD measures within the same temporal interval. In contrast, SEP, MUA or CSD measures within the early temporal intervals (i.e., P1 or P1a) were uncorrelated or less frequently, inversely correlated with the same measures in the late temporal interval (i.e. N1). The multiple measures complemented one another which led to a descriptive model of the neural process underlying the evoked cortical response in postcentral gyrus of awake monkey.