SI neuron response variability is stimulus tuned and NMDA receptor dependent

Skin brushing stimuli were used to evoke spike discharge activity in single skin mechanoreceptive afferents (sMRAs) and anterior parietal cortical (SI ) neurons of anesthetized monkeys (Macaca fascicularis). In the initial exp eriments 10-50 presentations of each of 8 different stimulus velocities wer e delivered to the linear skin path from which maximal spike discharge acti vity could be evoked. Mean rate of spike firing evoked by each velocity (MF R) was computed for the time period during which spike discharge activity e xceeded background, and an across-presentations estimate of mean firing rat e ((MFR) over bar) was generated for each velocity. The magnitude of the tr ial-by-trial variation in the response (estimated as CV; where CV = standar d deviation in MFR/(MFR) over bar) was determined for each unit at each vel ocity. (MFR) over bar for both sMRAs and SI neurons ((MRA) over bar(sMRA) a nd (MFR) over bar(SI) respectively) increased monotonically with velocity o ver the range 1-100 cm/s. At all velocities the average estimate of intertr ial response variation for SI neurons (<(CV)over bar>(SI)) was substantiall y larger than the corresponding average for sMRAs (<(CV)over bar>(sMRA)). W hereas <(CV)over bar>(sMRA) increased monotonically over the range 1-100 cm /s, <(CV)over bar>(SI) decreased progressively with velocity over the range 1-10 cm/s, and then increased with velocity over the range 10-100 cm/s. Th e position of the skin brushing stimulus in the receptive field (RF) was va ried in the second series of experiments. It was found that the magnitude o f CVSI varied systematically with stimulus position in the RF: that is, CVS I was lowest for a particular velocity and direction of stimulus motion whe n the skin brushing stimulus traversed the RF center, and CVSI increased pr ogressively as the distance between the stimulus path and the RF center inc reased. In the third series of experiments, either phencylidine (PCP; 100-5 00 mu g/kg) or ketamine (KET; 0.5-7.5 mg/kg) was administered intravenously (iv) to assess the effect of block of N-methyl-D-aspartate (NMDA) receptor s on SI neuron intertrial response variation. The effects of PCP on both CV SI and (MFR) over bar(SI) were transient, typically with full recovery occu rring in 1-2 h after drug injection. The effects of KET on CVSI and (MFR) o ver bar(SI) were similar to those of PCP, but were shorter in duration (15- 30 min). PCP and KET administration consistently was accompanied by a reduc tion of CVSI. The magnitude of the reduction of CVSI by PCP or KET was asso ciated with the magnitude of CVSI before drug administration: that is, the larger the predrug CVSI the larger the reduction in CVSI caused by PCP or K ET. PCP and KET exerted variable effects on SI neuron mean firing rate that could differ greatly from one neuron to the next. The results are interpre ted to indicate that SI neuron intertrial response variation is I) stimulus tuned (intertrial response variation is lowest when the skin stimulus move s at 10 cm/s and traverses the neuron's RF center) and 2) NMDA receptor dep endent (intertrial response variation is least when NMDA receptor activity contributes minimally to the response, and increases as the contribution of NMDA receptors to the response increases.