Modulation of somatosensory evoked responses in the primary somatosensory cortex produced by intracortical microstimulation of the motor cortex in the monkey

Enhanced amplitude reduction of somatosensory evoked potentials by voluntary movement in the elderly

We studied the effects of aging on modification of the median nerve somatosensory evoked potentials (SEPs) by voluntary movement in 17 aged (66.5 +/- 8.9 years, mean +/- SD) and 12 young normal humans (27.5 +/- 5.0 years). The amplitudes of cortical SEP components were generally larger in the aged group than in the young group. Following isometric contraction of the thenar muscle, the aged group showed significant attenuation of the prerolandic P22-N28-P45 and the postrolandic P24-N30-P45, while the young group only demonstrated significant reduction of the prerolandic P22-N28 amplitude. In the prerolandic N28-P45 and the postrolandic P24-N30 and N30-P45, amplitudes reduced by voluntary movement (gated amplitude) significantly correlated with amplitudes at rest (resting amplitude) and with the age of subjects. The effects of stimulus intensity and frequency on gating supported the correlative changes between gated and resting amplitudes. These results suggest that the magnitude of gating depends on SEP amplitudes at rest, and that augmented gating in the aged group is a result of enlarged SEPs. Since the cervical and Erb's potentials were not changed by movement, and passive movement did not significantly affect the SEPs, a centrifugal mechanism is probably responsible for gating in this study.

Interactions between motor commands and somatic perception in sensorimotor cortex

For many years, it has been postulated that interactions between motor commands and somatic perception in the sensorimotor cortices exist, but they have been difficult to demonstrate. Recent studies have made demonstration of this interaction easier and suggest that cortical activity related to somatic sensation and perception is modified by movement-generating mechanisms. Corollary discharge and efference copy may also play a role in motor behavior.

Somatosensory evoked potentials during the ideation and execution of individual finger movements

Scalp somatosensory evoked potentials (SEPs) were recorded in 10 volunteers after median nerve stimulation, in four experimental conditions of hand movements performance/ideation, and compared with the baseline condition of full relaxation. The experimental conditions were (a) self-improvised hand-finger sequential movements; (b) the same movements according to a read sequence of numbers; (c) mental ideation of finger movements; and (d) passive displacement of fingers in complete relaxation. Latencies and amplitudes of the parietal (N20, P25, N33, and P45) and frontal peaks (P20-22, N30, and P40) were analyzed. Latencies did not vary in any of the paradigms. Among the parietal complexes, only the P25-N33 amplitude was significantly reduced in (a), (b), (c), and (d) and the N20-P25 was reduced in (a) and (d); among frontal waves, N30 and P40 were significantly reduced (20-75%) in (a) and (b). Coronal electrodes showed amplitude decrements maximal at the frontal-rolandic positions contralateral to the stimulated side.

Functional magnetic resonance imaging mapping of the sensorimotor cortex with tactile stimulation

The purpose of this study is to compare tactile stimulation of the palm with voluntary movement of the fingers as paradigms for mapping the sensorimotor cortex in functional magnetic resonance imaging. In 22 subjects, 24 sets of functional magnetic resonance images were obtained with echoplanar acquisitions and cross-correlation image processing techniques. Two tasks were employed: a motor task in which subjects moved the thumb and index finger of one hand and a sensory task in which the palm was scratched by another person. The activation from the two tasks coincided entirely in 20 sets and partially in 3 sets. In one case, no activation was seen with the motor task. The study suggests that tactile stimulation of the palm is useful and reliable for mapping the sensorimotor cortex.

Effects of cross-modal manipulations of attention on the ability of human subjects to discriminate changes in texture

Cross-modal manipulations of attention significantly affect the detectability of tactile stimuli, but the effects on a more complex perceptual task, the discrimination of surface texture, are unknown. This study sought to examine whether attention influences the ability to discriminate a change in texture during passive touch. Twelve subjects were trained to perform two discrimination tasks: discriminating an increase in the intensity of a visual stimulus, and discriminating a change in the texture of a surface that was displaced beneath the tip of one digit. The texture change consisted of an increase in the spatial period between rectangular arrays of raised dots on Nyloprint surfaces, and for each subject an increment close to his or her discrimination threshold was employed in the experiment. Each trial began with the presentation of two baseline stimuli: A standard voltage illuminated the visual stimulus (an array of yellow light-emitting diodes (LEDs), and a standard texture (3-mm spatial period) was displaced under the tip of digit 3. For visual trials, three different increments in luminous intensity were presented, at one of three different delays following the initial presentation of the baseline stimuli. For texture trials, a single increment in spatial period was presented at one of three delays after the onset of the baseline stimuli. In any one trial, only one modality changed in intensity. The subject's task was to signal, as quickly as possible, the occurrence of the change. Instructional cues (red and green LEDs) were used to direct the subjects' attention toward the modality that changed (valid cue), to divide the subjects' attention between the visual and tactile modalities (neutral cue) as either might change, or to direct the subjects' attention toward the modality that did not change (invalid cue). Two measures of performance were employed: accuracy (percentage correct) and reaction time (speed with which the subject responded). The results indicated that cue condition significantly influenced the ability of subjects to discriminate a change in texture: Both accuracy and speed were significantly improved when subjects' attention was selectively directed toward the textured surface, as compared to when it was misdirected toward the visual modality. Performance was intermediate when attention was divided between the two modalities. The results were compared with those obtained previously in a tactile detection task using a similar attentional manipulation.(ABSTRACT TRUNCATED AT 400 WORDS)

The modulation of somatosensory evoked potentials during the foreperiod of a forewarned reaction time task

During the foreperiod of a forewarned reaction time (RT) task reflexes in the executing limb increase to a lesser extent than those in the contralateral limb. This is possibly due to input modulation. The present study investigates the possibility of cutaneous sensory modulation during motor preparation by studying the amplitudes of somatosensory evoked potentials (SEPs). Eighteen subjects performed a forewarned RT task with the same fingers as the ones which were electrically stimulated. SEPs evoked during the 4 sec preparatory period were compared to those evoked during movement execution and during the resting period after the motor response respectively. During response execution most SEP components showed smaller amplitudes, i.e., they were gated, which agrees with other studies. In the first part of the foreperiod no SEP modulation was observed. Towards the end of the foreperiod, 500 msec before the response stimulus (RS), the amplitude of the contralateral parietal N70-P100 was significantly decreased, while the P45-N70 showed a similar tendency. However, at the same time the P100-N140 was increased in amplitude. The decrease of the intermediate latency components towards the end of the foreperiod is discussed in terms of gating, while the increase in the long latency component is discussed with respect to a decrease in RT on trials where the fingers were stimulated just before the RS, pointing to the role of attentional mechanisms.

Modulation of cutaneous cortical evoked potentials during isometric and isotonic contractions in the monkey

The effects of the direction of movement (flexion vs extension) and the nature of the motor task (isotonic vs isometric) on the modulation of sensory cortical evoked responses to cutaneous stimulation were investigated in one monkey. Sensory responses were assessed by measuring the magnitude of the short latency component of air puff-evoked potentials recorded intracortically in the arm representation of areas 3b and 1 in the primary somatosensory cortex. At most recording sites, it was found that the amplitude of the air puff-evoked potential was decreased in a non-specific manner by motor activity. Neither the timing nor the depth of the modulation were found to vary with either the direction or the type of contraction. The effects were widespread since inputs from practically the entire forelimb (hairy skin) were diminished during the motor tasks. These results thus show that the modulation was more closely linked to the central motor output than to the peripheral input generated by muscle force and/or limb displacement. It is suggested that signals originating from central motor structures, acting in a feedforward manner, play a major role in 'gating' cutaneous inputs during movement. It is further suggested that the centrally mediated effects are exerted via a final common pathway upon which the 'gating' signals converge.