Studies of touch intensity discrimination in monkeys have identified a component of the somatosensory-evoked cortical potential, N1, generated within 50 ms of the stimulus, that predicts their behavioral performance. This study employed multiple-electrode arrays with relatively high spatial resolution (0.1 or 0.2 mm spacing) to record laminar profiles of somatosensory-evoked potentials (SEPs), multiple unit activity (MUA) and current source-densities (CSDs) at several sites across the postcentral gyri of two awake monkeys. This high resolution laminar analysis strongly supports our earlier hypothesis based upon low resolution data that N1 is generated by synaptic excitation targeted specifically at the most superficial cortical layers I/II: (1) The excitatory nature of N1 was indicated by elevated MUA which was maximal in layer III and extended down to subcortical white matter where fiber activity exceeded prestimulus levels; (2) In addition to CSD analysis, the observation that N1 was maximally negative within 0.10 mm of the border between layers I and II verified the superficial site of N1 synaptic excitation regardless of conductivity boundaries near the pial surface. A review of the anatomical literature finds that the most likely inputs responsible for N1 activation are the "backward" cortico-cortical projections from secondary somatosensory areas to SI which in area 1 are the major source of sensory-related input that specifically terminates in layers I/II. We suggest, therefore, that backward projections are involved in the conscious process of touch sensation as it is signaled by N1.
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.
The response of primary somatosensory cortex (SI) to cutaneous stimulation of the hand was studied in one unanesthetized monkey to examine more closely the previously reported dependence of a behaviorally relevant late component of the evoked potential, N1, upon two behavioral states of arousal, slow-wave sleep (SWS) and inactive wakefulness. Simultaneous recordings of the somatosensory-evoked potentials (SEP), multiple unit activities (MUA) and current source-densities (CSD) at 12 depths through area 1 were collected during both states and analyzed to identify the cortical events that generate the SEP. As previously reported, the N1 component of the SEP was diminished during sleep. Although the earlier components of the SEP were evoked during both states, the excitatory events that characterize the awake N1 were replaced during SWS by an interval of strong inhibition which was terminated by a burst of cortical MUA. CSD analysis of the SWS inhibitory interval revealed large current sources through layer III above the depth of maximal MUA inhibition indicating that an outward conductance became activated on the apical trunks of pyramidal neurons during SWS. These data support the proposed involvement of events underlying N1 in the coding of touch experience.
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