Effects of acute nicotine on somatosensory change-related cortical responses.

Nicotine is known to have enhancing effects on some aspects of attention and cognition. As for the pre-attentive processes of detecting sensory changes, nicotine has significant effects on the auditory and visual systems implying that its pre-attentive effect is common among sensory modalities. The purpose of the present study was to elucidate whether acute nicotine administration has enhancing effects in the somatosensory system. Change-related cortical activity in response to an abrupt increase in stimulus intensity was recorded using magnetoencephalography. The test stimulus consisted of standard electrical pulses at 100 Hz for 500 ms applied to the dorsum of the left hand followed by 0.7-mA stronger pulses for 300 ms. Nicotine was administered in a gum (4 mg of nicotine). Eleven healthy nonsmokers were tested with a double-blind and placebo-controlled design. Effects of nicotine on the cortical response in the primary (S1) and secondary (S2) somatosensory cortices were investigated. Results showed that nicotine failed to affect the S1 response while it significantly increased the amplitude of S2 activity in the hemisphere ipsilateral to the stimulation, and shortened the peak latency of S2 activity in both hemispheres. Since cortical responses in the present study represent a pre-attentive automatic process to encode new somatosensory events, the results suggest that nicotine can exert beneficial cognitive effects without a direct impact on attention and that the effect of nicotine on the automatic change-detecting system is common across sensory modalities.

Characteristics of sensori-motor interaction in the primary and secondary somatosensory cortices in humans: a magnetoencephalography study.

We studied sensori-motor interaction in the primary (SI) and secondary somatosensory cortex (SII) using magnetoencephalography. Since SII in both hemispheres was activated following unilateral stimulation, we analyzed SIIc (contralateral to stimulation) as well as SIIi (ipsilateral to stimulation). Four tasks were performed in human subjects in which a voluntary thumb movement of the left or right hand was combined with electrical stimulation applied to the index finger of the left or right hand: L(M)-L(S) (movement of the left thumb triggered stimulation to the left finger), L(M)-R(S) (movement of the left thumb triggered electrical stimulation to the right finger), R(M)-R(S) (movement of the right thumb triggered electrical stimulation to the right finger), and R(M)-L(S) (movement of the right thumb triggered electrical stimulation to the left finger). Stimulation to the index finger only (S condition) was also recorded. In SI, the amplitude of N20m and P35m was significantly attenuated in the R(M)-R(S) and L(M)-L(S) tasks compared with the S condition, but that for other tasks showed no change, corresponding to a conventional gating phenomenon. In SII, the R(M)-L(S) task significantly enhanced the amplitude of SIIc but reduced that of SIIi compared with the S condition. The L(M)-L(S) and R(M)-R(S) tasks caused a significant enhancement only in SIIi. The L(M)-R(S) task enhanced the amplitude only in SIIc. The laterality index showed that SII modulation with voluntary movement was more dominant in the hemisphere ipsilateral to movement but was not affected by the side of stimulation. These results provided the characteristics of activities in somatosensory cortices, a simple inhibition in SI but complicated changes in SII depending on the side of movement and stimulation, which may indicate the higher cognitive processing in SII.

Attenuation of the effect of remote muscle contraction on the soleus H-reflex during plantar flexion.

OBJECTIVE: We investigated to what extent the facilitation of the soleus (Sol) Hoffmann (H-) reflex during a phasic voluntary wrist flexion (Jendrássik maneuver, JM) can be modulated by graded plantar flexion force and conditioning wrist flexion force. METHODS: The subjects were asked to perform phasic wrist flexion under a reaction time condition. Sol H-reflex was evoked by stimulating the right tibial nerve at various time intervals (50-400ms) after the 'Go' signal for initiating JM while the ankle was at rest and while plantarflexing. The level of tonic plantar flexion force (isometric contraction of 10, 20 and 30% of maximal EMG) and conditioning wrist flexion (isometric contraction of 30, 50 and 80% of maximum voluntary contraction) during JM was graded systematically. RESULTS: Although JM facilitation could be seen 80-120ms after the flexor carpi radialis (FCR) EMG onset even while plantarflexing, the magnitude of JM facilitation under plantar flexion was significantly decreased compared to that at rest. The degree of decrease in JM facilitation did not depend on the level of plantar flexion force. In contrast, the degree of JM facilitation was proportional to the level of wrist flexion force while the ankle was at rest and while plantarflexing, though the amount of JM facilitation significantly decreased while plantarflexing. CONCLUSIONS: JM facilitation of Sol H-reflex is decreased while performing tonic voluntary contraction of the homonymous muscle. The degree of decrease in JM facilitation is independent of the level of homonymous muscle contraction, but depends on the level of remote FCR contraction. In clinical application, when we intend to elicit a maximum stretch reflex by JM, full relaxation of homonymous muscle should be carefully confirmed. SIGNIFICANCE: Our results provide evidence for better understanding of the features of JM and insight into its clinical application.

Changes in the centrifugal gating effect on somatosensory evoked potentials depending on the level of contractile force.

In this study, we investigated the somatosensory evoked potentials (SEPs) during the preparatory period of self-initiated plantar flexion at different force levels of muscle contraction and elucidated the mechanism behind the centrifugal gating effect on somatosensory information processing. We recorded SEPs following stimulation of the tibial nerve at the popliteal fossa during the preparatory period of a 20% maximal voluntary contraction (MVC) and 50% MVC. The preparatory period was divided into two sub-periods based on the components of movement-related cortical potentials, the negative slope (NS sub-period) and the Bereitschaftspotential (BP sub-period). The subjects were instructed to concentrate on the movement and not to pay attention to the continuous electrical stimulation. Pre-movement SEPs were averaged separately during the two sub-periods under each MVC condition. The mean amplitudes of BP and NS were larger during the 50% MVC than the 20% MVC. As for the components of SEPs, during the NS sub-period the amplitude of P30 under the 50% MVC and N40 under both conditions were significantly smaller than that in the stationary sequence, and N40 amplitude was significantly smaller during the 50% MVC than the 20% MVC. During the BP sub-period, the amplitude of P30 and N40 during the 50% MVC was significantly smaller than during the stationary sequence, while it was not significantly different between the 20% and 50% MVCs. In conclusion, the extent of the centrifugal gating effect on SEPs was dependent on the activities of motor-related areas, which generated the NS and BP.