1. Primary somatosensory cortical (SI) neurons exhibit characteristic activity before the initiation of movements. This premovement activity (PMA) may result from centrally generated as well as from peripheral inputs. We examined PMA for 55 SI neurons (10, 13, 28, and 4 in areas 3a, 3b, 1, and 2, respectively) with activity that was entrained to vibrotactile stimulation (i.e., was temporally correlated with the stimulus). We sought to determine whether the temporal characteristics of vibration-entrained discharges would change throughout the reaction time period, and, if they did, whether these changes might be accounted for by central inputs. 2. Monkeys made wrist flexions and extensions in response to sinusoidal vibration (27, 57, or 127 Hz) of their palms. Vibration remained on until the animal moved at least 5 degrees from the initial hold position. Mean firing rate (MFR), a measure of the level of activity, was derived from the number of spikes per vibratory cycle. The correlation between the vibration and the neuronal firing was described by the mean phase (MP) of the vibratory cycle at which spikes occurred. The degree of entrainment was quantified as synchronicity (Synch), a statistical parameter that could change from 0 for no entrainment to 1 for responses at a constant phase. 3. Premovement MFR increases (activation) and decreases (suppression) were observed. Moreover, two changes in MFR often were observed for the same neuron (2-event PMA). Many MFR shifts, especially the first in the two-event PMA, preceded electromyographic (EMG) onset. The pre-EMG MFR shifts more often had the same sign both for flexion and extension movements rather than having opposite signs. However, with equal frequency, post-EMG PMA events had the same or opposite sign for different movement directions. We suggest that the pre-EMG PMA has an origin different from movement-related peripheral reafference. 4. Premovement activation was accompanied by shifts of MP corresponding to earlier responses to the ongoing vibratory stimulus and by decreases of response Synch. Premovement suppression was not associated with consistent shifts of MP and Synch. We suggest that during premovement activation, asynchronous (uncorrelated with vibration) signals are integrated with the vibratory input. These asynchronous signals may make neurons more likely to discharge and to do so earlier with respect to the vibratory stimulus. The asynchronous component may also disrupt the vibration-entrained activity pattern.(ABSTRACT TRUNCATED AT 400 WORDS)

This study was designed to investigate how activity patterns of primary motor cortical (MI) neurons change when monkeys perform the same movements guided by somatosensory and/or visual cues. Two adult male rhesus monkeys were trained to make wrist extensions and flexions after holding a steady position during an instructed delay period lasting 0.5-2.0 s. Monkeys held against a 0.07 Nm load that opposed flexion movements. Wrist movements were guided by vibratory cues (VIB-trials), visual cues (VIS-trials), or both in combination (COM-trials). Extracellular recordings of 188 MI neurons were made during all three paradigms. Individual neurons were counted twice, once for each movement direction, yielding 376 cases. All neurons had significant task-related activity (TRA) changes relative to delay period activity during at least one of the three paradigms. TRA was analyzed to determine if it was different as a function of the sensory cue(s) that initiated movement and that specified movement endpoints. Cases were grouped by whether the TRA changes were greater in VIB- or VIS-trials; this defined their "bias". One hundred and eighteen cases (31.4%) had greater TRA changes in VIB-trials (Vb-neurons), whereas 185 (49.2%) showed greater TRA changes in VIS-trials (Vs-neurons). The remaining 73 cases (19.4%) had similar TRA changes in VIB- and VIS-trials (Nb-neurons). For Vb- and Vs-neurons, earlier TRA onsets and greater TRA changes were observed in the trials for which these neurons were biased. During the COM-trials, the TRA was intermediate. During the trials for which the activity was not biased, the TRA was the least. For Nb-neurons, no significant TRA differences were observed across paradigms. TRA changes of MI neurons may represent movement planning-related inputs from other central, presumably cortical, sources as well as contribute to motor outflow from the cortex. These data suggest that Vb- and Vs-neurons are affected differently by somatosensory- and visually related central inputs, resulting in different TRAs, even for essentially identical movements. Such differences may depend not only on the type of sensory information that initiates movement but also whether that information specifies movement endpoints or might interfere with movement monitoring.

The present study was to investigate whether neuronal activity in primary somatosensory cortical areas (SI) differs when monkeys expect go-cues of different sensory modalities. Two monkeys made wrist extensions and flexions after steadily holding wrist at a center position. Movements were guided by increases in vibration to the monkey's palm (VIB), visual targets (VIS), or both in combination (COM). Neuronal activity recorded in SI during the early and late phases (i.e., the first and last 250 ms) of the instructed delay periods (IDP) were analyzed. Of 406 neurons recorded during all three paradigms, 263 (64.8%) showed significant changes in firing rates (FR) between the early and late IDP phases during either VIB or VIS trials and were selected for further analyses. The selected neurons were classified as VIB- or VIS-biased, depending on the paradigm (VIB or VIS) in which the greater FR changes occurred. Both increases and decreases in FRs were observed during the analyzed epochs. Most VIB-biased neurons showed the biggest FR changes during VIB trials and the least during VIS trials. Conversely, most VIS-biased neurons had the biggest FR changes during VIS trials and the least during VIB trials. For both VIB- and VIS-biased neurons, however, the FR changes were intermediate during COM trials. These results suggest that SI neurons play an important role in initiating/executing wrist movements. Neurons involved in wrist movements showed biases to the modality of cueing signals. Most SI neurons were biased to only one sensory modality. The expectation-related FR changes suggest different involvement by SI in movement initiation when tasks are guided by vibratory and visual signals.

Vibration-related neurons in monkey primary somatosensory cortex (SI) discharge rhythmically when vibratory stimuli are presented. It remains unclear how functional information carried by vibratory inputs is coded in rhythmic neuronal activity. In the present study, we compared neuronal activity during wrist movements in response to two sets of cues. In the first, movements were guided by vibratory cue only (VIB trials). In the second, movements were guided by simultaneous presentation of both vibratory and visual cues (COM trials). SI neurons were recorded extracellularly during both wrist extensions and flexions. Neuronal activity during the instructed delay period (IDP) and the early reaction time period (RTP) were analyzed. A total of 96 cases from 48 neurons (each neuron contributed two cases, one each for extension and flexion) showed significant vibration entrainment during the early RTPs, as determined by circular statistics (Rayleigh test). Of these, 50 cases had cutaneous (CUTA) and 46 had deep (DEEP) receptive fields. The CUTA neurons showed lower firing rates during the IDPs and greater firing rate changes during the early RTPs when compared with the DEEP neurons. The CUTA neurons also demonstrated decreases in activity entrainment during VIB trials when compared with COM trials. For the DEEP neurons, the difference of entrainment between VIB and COM trials was not statistically significant. The results suggest that somatic vibratory input is coded by both the firing rate and the activity entrainment of the CUTA neurons in SI. The results also suggest that when vibratory inputs are required for successful task completion, the activity of the CUTA neurons increases but the entrainment degrades. The DEEP neurons may be tuned before movement initiation for processing information encoded by proprioceptive afferents.

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