Interhemispheric interactions between the human primary somatosensory cortices

Abstract

In the somatosensory domain it is still unclear at which processing stage information reaches the opposite hemispheres. Due to dense transcallosal connections, the secondary somatosensory cortex (S2) has been proposed to be the key candidate for interhemispheric information transfer. However, recent animal studies showed that the primary somatosensory cortex (S1) might as well account for interhemispheric information transfer. Using paired median nerve somatosensory evoked potential recordings in humans we tested the hypothesis that interhemispheric inhibitory interactions in the somatosensory system occur already in an early cortical processing stage such as S1. Conditioning right S1 by electrical median nerve (MN) stimulation of the left MN (CS) resulted in a significant reduction of the N20 response in the target (left) S1 relative to a test stimulus (TS) to the right MN alone when the interstimulus interval between CS and TS was between 20 and 25 ms. No such changes were observed for later cortical components such as the N20/P25, N30, P40 and N60 amplitude. Additionally, the subcortically generated P14 response in left S1 was also not affected. These results document the existence of interhemispheric inhibitory interactions between S1 in human subjects in the critical time interval of 20-25 ms after median nerve stimulation.

Figure 1. Experimental design of the paired median nerve somatosensory evoked potential recordings (PMNSEPs).

(A) Interhemispheric interactions between homologous primary somatosensory cortices (S1) were studied using paired median nerve somatosensory evoked potential recordings (PMNSEPs). In the paired median nerve paradigm, suprathreshold peripheral stimulation of the left median nerve (MN) served as conditioning stimulus (CS) and always preceded right MN stimulation (test stimulus (TS)) by 5–30 ms while recording SEPs over the left (target) S1. Additionally, SEP responses to a CS (left MN) and TS (right MN) alone were recorded over left S1 (for details see text). Analysis of PMNSEPs was performed at electrode CP3. (B) Example traces of an individual subject illustrating the subtraction method used. In brief, the response of a left MN CS alone stimulation (middle trace) over left S1 was subtracted from the raw CS+TS response over left S1 (upper trace). Final analysis of the PMNSEP data was performed on the CS+TS SEPs (CS+TS raw data – CS alone, lower trace). For details see text. Amplitudes of interest (P14, N20, N20/P25, N30, P40, N60) are marked on the lower trace.

Figure 2. Example traces of a single subject PMNSEP response at electrode CP3 for all CS+TS conditions (5–30ms, red trace) relative to TS alone (blue trace).

Please note that the PMNSEPs for all CS+TS conditions are superimposed (shifted) on the N20 onset of the TS alone condition for display purpose only. For average group data please see Table 1.

Figure 3. Effect of conditioning the right S1 on N20 response of the contralateral (left) S1.

Conditioning the right S1 by electrical median nerve stimulation of the left MN (CS) resulted in a statistically significant reduction of the N20 response in the target (left) S1 relative to a test stimulus to the right MN (TS) alone when the interstimulus interval between CS and TS was between 20 and 25 ms. (A) Non-normalized N20 amplitudes for all conditions tested (Ts alone, 5, 10, 15, 20, 25 and 30 ms) (B) Normalized N20 amplitudes of the left S1 are displayed as a ratio between the CS+TS conditions (5–30 ms ISI) relative to TS alone (CS+TS/TS ratio). Asterisks represent significant differences relative to TS alone (significance level p<0.008, corrected for multiple comparisons).