Frequency specificity in the cortical representation of thumb with respect to little finger: dynamics of regional activation and intra-regional synchronization

The aim of this work was to study the degree of synchronization within the portion of the somatosensory cortex devoted to hand control during external sensory stimulation. To this aim, we collected magnetoencephalograhic data from healthy subjects during separate stimulation of their thumbs and little fingers. Subsequently, we analyzed these data by means of a "synchronization index", whose definition we extended to include time dependence. The properties of this dynamic synchronization index within the beta [16-32 Hz] and gamma [36-44 Hz] bands suggest that the hand representation in humans' primary cortex follows a frequency coding, in addition to the somatotopic one, for discriminating different districts. Our results show that gamma synchronization is higher following stimulation of the thumb than of the little finger. The strength of gamma band synchronization appears to work as a code for functional prevalence. Comparative analysis of synchronization index and signal amplitude showed that this frequency selectivity regarded the synchrony among activated neurons and their number.