Interhemispheric interactions can be measured during both movement and rest. Here we test two new procedures to (i) probe how cutaneous sensory information is transmitted between hemispheres using the phenomenon of ‘short latency afferent inhibition’ (SAI); and (ii) modulate interhemispheric interactions of the sensorimotor network via transcranial alternating current stimulation (tACS).
In Exp. 1 (n¼15) we evoked SAI via cutaneous stimulation of FDI followed by a single TMS pulse applied either to the contralateral or the ipsilateral primary motor cortex (M1) relative to the stimulated finger. We probed contra- and ispilateral SAI for a large range of inter stimulus intervals to test interhemispheric transmission. In Exp. 2 (n¼20) we placed two electrodes over left and right M1, respectively, and one return electrode over the inion. tACS signals were tailored to the individual’s alpha rhythm but fluctuated in amplitude according to a 1
Hz power envelope. We used resting state functional magnetic resonance imaging to measure functional connectivity across the brain.
We show in Exp.1 that SAI can be detected in ipislateral M1 and the timing suggests that approx. 18ms are required for the sensory information to travel between S1 of the one and M1 of the other hemisphere (p<0.05). Interestingly, this form of interhemipsheric sensorimotor communication was only observed for the upper but not for the lower limbs. Furthermore, we found in Exp. 2 that synchronizing slowly fluctuating power envelopes of fast electrophysiological signals significantly increases interhemispheric coupling strength within the sensorimotor network (p<0.05). This effect is anatomically specific, outlasts the duration of the stimulation, and is largest in individuals with weak natural interhemispheric coupling.
Our findings suggest a high degree of interhemispheric communication between upper limb areas when participants are at rest, which might be modulated by synchronizing slow-wave oscillations betweenhemispheres.