Impaired intracortical inhibition in the primary somatosensory cortex in focal hand dystonia.

Somesthetic temporal discrimination (STD) is impaired in focal hand dystonia (FHD). We explored the electrophysiological correlate of the STD deficit to assess whether this is due to dysfunction of temporal inhibition in the somatosensory inhibitory pathway or due to dysfunction in structures responsible for nonmodality-specific timing integration. Eleven FHD patients and 11 healthy volunteers were studied. STD threshold was investigated as the time interval required for perceiving a pair of stimuli as two separate stimuli in time. We also examined the somatosensory-evoked potential (SEP) in a paired-pulse paradigm. We compared STD threshold and recovery function of SEP between the groups. STD thresholds were significantly greater in FHD than in healthy volunteers. The amount of P27 suppression in the 5 ms-ISI condition was significantly less in FHD. It was also found that the STD threshold and P27 suppression were significantly correlated: the greater the STD threshold, the less the P27 suppression. Significantly less suppression of P27 with a lack of significant change in N20 indicates that the impairment of somatosensory information processing in the time domain is due to dysfunction within the primary somatosensory cortex, suggesting that that the STD deficit in FHD is more attributable to dysfunction in the somatosensory pathway. (c) 2007 Movement Disorder Society.