SI neuron response variability is stimulus tuned and NMDA receptor dependent.
ABSTRACT 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 experiments 10-50 presentations of each of 8 different stimulus velocities were delivered to the linear skin path from which maximal spike discharge activity could be evoked. Mean rate of spike firing evoked by each velocity (MFR) was computed for the time period during which spike discharge activity exceeded background, and an across-presentations estimate of mean firing rate (MFR) was generated for each velocity. The magnitude of the trial-by-trial variation in the response (estimated as CV; where CV = standard deviation in MFR/MFR) was determined for each unit at each velocity. MFR for both sMRAs and SI neurons (MFRsMRA and MFRSI, respectively) increased monotonically with velocity over the range 1-100 cm/s. At all velocities the average estimate of intertrial response variation for SI neurons (CVSI) was substantially larger than the corresponding average for sMRAs (CVsMRA). Whereas CVsMRA increased monotonically over the range 1-100 cm/s, CVSI decreased progressively with velocity over the range 1-10 cm/s, and then increased with velocity over the range 10-100 cm/s. The position of the skin brushing stimulus in the receptive field (RF) was varied in the second series of experiments. It was found that the magnitude of CVSI varied systematically with stimulus position in the RF: that is, CVSI was lowest for a particular velocity and direction of stimulus motion when the skin brushing stimulus traversed the RF center, and CVSI increased progressively as the distance between the stimulus path and the RF center increased. In the third series of experiments, either phencylidine (PCP; 100-500 microg/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) receptors on SI neuron intertrial response variation. The effects of PCP on both CVSI and MFRSI were transient, typically with full recovery occurring in 1-2 h after drug injection. The effects of KET on CVSI and MFRSI were similar to those of PCP, but were shorter in duration (15-30 min). PCP and KET administration consistently was accompanied by a reduction of CVSI. The magnitude of the reduction of CVSI by PCP or KET was associated 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 KET. 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 interpreted to indicate that SI neuron intertrial response variation is 1) stimulus tuned (intertrial response variation is lowest when the skin stimulus moves at 10 cm/s and traverses the neuron's RF center) and 2) NMDA receptor dependent (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).
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ABSTRACT: Neural representation of somatosensory events undergoes major transformation in the primary somatosensory cortex (SI) from its original, more or less isomorphic, form found at the level of peripheral receptors. A large body of SI optical imaging, neural recording and psychophysical studies suggests that SI representation of stimuli encountered in everyday life is a product of dynamic processes that involve competitive interactions at multiple levels of cortical organization. Such interactions take place among neighboring neurons, among local groups of minicolumns, among neighboring macrocolumns, between SI and SII, between Pacinian and non-Pacinian channels, and bilaterally between homotopic somatosensory regions of the opposite hemispheres. Together these interactions sharpen SI response to suprathreshold and time-extended tactile stimuli by funneling the initially widespread stimulus-triggered activity in SI into the local group of macrocolumns most directly driven by the stimulus. Those macrocolumns in turn fractionate into stimulus-specific patterns of differentially active minicolumns. Thus SI dynamically shapes its representation of a tactile stimulus by selecting among all of its neurons initially activated by the stimulus a subset of neurons with receptive-field and feature-tuning properties closely matching those of the stimulus. Through this stimulus-directed dynamical selection process, which operates on a scale of hundreds of milliseconds, SI achieves a more faithful representation of stimulus properties, which is reflected in improved performance on tactile perceptual tasks.Neuroscience & Biobehavioral Reviews 10/2009; 34(2):160-70. · 9.44 Impact Factor
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ABSTRACT: Psychophysical experiments and model simulations were performed to identify plausible neural codes representing stimulus magnitude in the Non-Pacinian I (NP I) tactile channel associated with rapidly adapting fibers. Sinusoidal mechanical displacements were applied on the fingertips of eight human subjects. The NP I channel was isolated by elevating the thresholds of the Pacinian (P) channel during forward masking. Psychophysical magnitude estimates were obtained at 40 Hz for the NP I channel and at 250 Hz for the P channel by using a small contactor (radius: 2 mm). The P channel was additionally tested with a larger contactor (radius: 4.3 mm) to compensate for the lower innervation density of the Pacinian fibers. The magnitude estimates were fitted by power functions. The exponent (1.02) obtained with the large contactor for the P channel was higher than the exponent (0.68) obtained with the small contactor, but it was not statistically different from the exponent (1.21) obtained with the small contactor for the NP I channel. This suggests that the exponent increases when more fibers are recruited in the P channel. Six hypothetical neural codes were tested by using a computational population model for the rapidly adapting afferents. The validity of each code was evaluated by comparing psychophysical and simulation exponents, by finding the correlations between the magnitude estimates and the neural code results, and by a novel distance metric for measuring the proximity between the data sets. The codes based on the number of active fibers, the total spike count, the mean and the standard deviation of the spike count distribution yielded the best results, while the codes based on the interspike intervals were not related with the magnitude estimates.Somatosensory & Motor Research 11/2012; · 1.11 Impact Factor
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ABSTRACT: Patients with chronic pain syndromes, like fibromyalgia (FM) complain of widespread pain and tenderness, as well as non-refreshing sleep, cognitive dysfunction, and negative mood. Several lines of evidence implicate abnormalities of central pain processing as contributors for chronic pain, including dysfunctional descending pain inhibition. One form of endogenous pain inhibition, diffuse noxious inhibitory controls (DNIC), has been found to be abnormal in some chronic pain patients and evidence exists for deficient spatial summation of pain, specifically in FM. Similar findings have been reported in patients with localized musculoskeletal pain (LMP) disorders, like neck and back pain. Whereas DNIC reduces pain through activation of nociceptive afferents, vibro-tactile pain inhibition involves innocuous A-beta fiber. To assess whether patients with localized or widespread chronic pain disorders have dysfunctional A-beta related pain inhibition we enrolled 28 normal pain-free controls (NC), 29 FM patients, and 19 subjects with neck or back pain. All received 10s sensitivity-adjusted noxious heat stimuli to the forearms as test stimuli. To assess endogenous analgesic mechanisms of study subjects, vibro-tactile conditioning stimuli were simultaneously applied with test stimuli either homotopically or heterotopically. Additionally, the effect of distraction on experimental pain was assessed. Homotopic vibro-tactile stimulation resulted in 40% heat pain reductions in all subject groups. Distraction did not seem to affect experimental pain ratings. Conclusions: Vibro-tactile stimulation effectively recruited analgesic mechanisms not only in NC but also in patients with chronic musculoskeletal pain, including FM. Distraction did not seem to contribute to this analgesic effect.European journal of pain (London, England) 02/2011; 15(8):836-42. · 3.37 Impact Factor