Somatotopic finger mapping using MEG: Toward an optimal stimulation paradigm

ObjectiveIn non-invasive somatotopic mapping based on neuromagnetic source analysis, the recording time can be shortened and accuracy improved by applying simultaneously vibrotactile stimuli at different frequencies to multiple body sites and recording multiple steady-state responses. This study compared the reliability of sensory evoked responses, source localization performance, and reproducibility of digit maps for three different stimulation paradigms.MethodsVibrotactile stimuli were applied to the fingertip and neuromagnetic steady-state responses were recorded. Index and middle fingers were stimulated either sequentially in separate blocks, simultaneously at different frequencies, or in alternating temporal order within a block.ResultsResponse amplitudes were largest and source localization was most accurate between 21 and 23 Hz. Separation of adjacent digits was significant for all paradigms in all participants. Suppressive interactions occurred between simultaneously applied stimuli. However, when frequently alternating between stimulus sites, the higher stimulus novelty resulted in increased amplitudes and superior localization performance.ConclusionsWhen receptive fields are strongly overlapping, the alternating stimulation is preferable over recording multiple steady state responses.SignificanceThe new paradigm improved the measurement of the distance of somatotopic finger representation in human primary somatosensory cortex, which is an important metric for neuroplastic reorganization after learning and rehabilitation training.

► The somatotopic finger representations in human SI were precisely localized with neuromagnetic steady-state responses to 20-Hz vibrotactile stimuli. ► Cortical sources of adjacent digits were separated significantly in each individual. ► Rapidly alternating the stimulation site overcomes the effects of suppressive interactions in simultaneous digit stimulation and improved source localization accuracy.