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This study demonstrated effects of stimulus order on single-cell responses in the macaque primary and secondary somatosensory and 7b cortical areas. As part of a study of tactile attention, two monkeys (Macaca mulatta) received similar constant amplitude, sinusoidal tactile vibration patterns (125 Hz) at two glabrous skin, hand locations. The stimuli started asynchronously with offsets of 150 or 300 ms. In cells with bilateral receptive fields and increased firing to a stimulus, we observed an average lowering of 30% in the firing rates to the contralateral stimulus when preceded by stimulation of the ipsilateral hand. Some cells with only contralateral receptive fields showed similar depressed responses to a contralateral stimulus when preceded by an ipsilateral stimulus. Stimulus order effects were more prominent during dual stimulation of the receptive field on one hand. In six cells whose background activity was inhibited by the first stimulus, higher rates appeared at the onset of the second stimulus. These results suggest a possible substrate for psychophysical findings of stimulus masking in which a preceding stimulus depresses detection thresholds. The spatial and temporal characteristics of in-field inhibitory mechanisms best account for the observed stimulus order effects.

This study analyzed neuronal responses in the second somatosensory (SII) and 7b cortical areas during a selective attention task. Cues directed attention to one of three simultaneous stimuli: vibrotactile stimuli applied to mirror sites on both hands or to a similarly timed auditory tone. Two stimulus patterns appeared with equal probability for the cued stimulus: a constant amplitude sinewave or the latter with a superimposed brief amplitude pulse midway in the trial. Uncued stimuli always contained amplitude pulses. Monkeys demonstrated whether an amplitude pulse at the cued location was present or absent by making appropriately rewarded up and down foot pedal movements. Cue location and stimulus pattern varied trial-wise and pseudo-randomly. Average firing rates to vibrotactile stimuli in 82 of 181 SII cells and 13 of 22 area 7b cells differed significantly during at least one epoch for trials cued to the contralateral hand when compared to trials cued to the ipsilateral hand or auditory stimulus. Predominant were relatively suppressed firing rates during times prior to the epoch containing the amplitude pulses or enhanced activity during and after these pulses. Generally, different cells showed suppression early vs enhancement later in a trial. Analyses of the ratio between firing rates before and during the amplitude pulses suggested improved evoked signals to the amplitude pulses. The discussion considers attention as a mechanism for reducing distractions, early in the trial through suppressing these signals, or for selectively increasing response magnitudes in the cued channel, especially around times when amplitude pulses were present or absent.

Responses of 66 neurons in primary somatosensory cortex (SI) of three anesthetized monkeys (Macaca mulatta) were characterized with grating patterns of 550- to 2900-mm groove width (Gw) and 250-mm ridge width, and/or pairs of 3-mm-wide ridges (bars) spaced 1-20 mm apart. Surfaces were stroked across single fingertips at parametrically varied levels of force (25-150 g) and velocity (25-100 mm/sec). The average firing rates (AFRs) of many cells varied with Gw, but force and velocity altered response functions (e.g., from linear to plateau or inverted). Slowly adapting (SA) cells were more sensitive to force, rapidly adapting (RA) cells to velocity. Force and velocity affected all cells sensitive to Gw, which suggests that response independence (e.g., AFR correlated with Gw but not force or velocity) may require active touch. Discharge intervals of many cells replicated stimulus temporal period. This temporal fidelity in SAs far exceeded examples reported for active touch. However, discharge burst duration and AFR increased with Gw, supporting a neural rate rather than temporal code for roughness. Force and velocity altered the Gw at which some cells fired once in phase to stimulus cycle ("tuning point"). Responses to bar edges suggest cortical replication of peripheral mechanoreceptor sensitivity to skin curvature, leading to this temporal fidelity in some cortical cells. Graded RA responses to Gw without obvious stimulus temporal replication may reflect early stages of integrative processing in supra- and infragranular layers that blur obvious temporal patterning and lead to a rate code correlated with spatial variation and proportional to perceived roughness.

Human and monkey performance on discriminating tactile gratings revealed comparable cross-species Weber functions. Neural data obtained while monkeys performed discriminations revealed some matching of neural and psychometric functions. Nearly constant firing rate differences occurred at discrimination threshold for unequal groove widths. Firing rate differences of some cells decreased on trials preceding discrimination errors, and thus predicted performance.

Responses of single neurons were recorded from the ventroposterolateral nucleus (VPL) of the thalamus while a monkey stroked its fingertips over gratings. Monkeys were trained to stroke the gratings with consistent downward applied force and velocity of hand motion. Neurons were selected with receptive fields on the glabrous digits. Average firing rate was computed for a range of grating groove widths; groove width corresponded to roughness. Force and velocity were measured. VPL responses were compared to previously reported responses in primary somatosensory cortex (SI) under identical stimulus conditions, and to reports of peripheral afferent fiber responses to passively applied gratings. VPL responses more closely resembled those of peripheral afferent fibers than those of SI in important respects: lack of independent responses to roughness, force, and velocity; high temporal and force fidelity; and response patterns that closely followed the shape of elevated metal strips used to separate pairs of gratings. The presence in cortex of response patterns not seen in the thalamus, such as response independence and negative correlations to groove width, suggests that they stem from cortical processing.

Areas in the second somatic sensory cortex (SII) of cats that responded vigorously to low-amplitude, high-frequency vibratory stimulation were mapped with respect to the surrounding somatotopic organization. Neurons with these properties were found in the posterior and medial parts of the distal forelimb zone and were judged as receiving input from Pacinian mechanoreceptors. The responses of these neurons to sinusoidal vibrotactile stimulation were studied during iontophoretic administration of glutamate or bicuculline methiodide (BMI) to determine if the temporal fidelity of these cortical neurons was controlled by inhibitory circuits that used gamma-aminobutyric acid (GABA) as a neurotransmitter. The data from 19 Pacinian-sensitive neurons were analyzed for changes in the mean firing rate, the percentage of entrainment, and the pattern of periodicity as revealed by autocorrelograms and interval histograms. Iontophoresis of BMI or glutamate caused significant increases in mean firing rates during low- and high-frequency vibratory stimulation. The pattern of increased activity produced by BMI was characterized by a small, yet significant, reduction in the percentage of entrainment, whereas glutamate caused smaller and fewer significant changes in this measure. Analysis of autocorrelation and interval histograms suggested that BMI increased the probability of firing on consecutive stimulus cycles in small segments of the stimulus duration.