Prestimulus somatosensory activation evoked by visually-based prediction

Brain activity was measured by magnetoencephalography (MEG) to investigate the effect of a visually-based prediction on somatosensory information processing. Based on the presence of early poststimulus responses modulated by predictions, it was hypothesized that a visually-based prediction of a somatosensory input would evoke prestimulus activity specific to the somatosensory modality if the input were easy to predict based on a visual cue when the length of the prediction time was sufficiently limited. A recorded static image of both hands was back-projected onto a screen located in front of subjects, who had their actual hands hidden. Each time a tip of either the left or right index finger in the image brightly lit, on the odd trials an electrical stimulus with a 400-ms delay corresponding to the cue was delivered to the actual finger; on the even trials, no electrical stimulus was delivered. Regardless of the presence of somatosensory input, in one set of trials the subjects were instructed to predict the stimulus based on the visual cue; they were instructed not to do so in the other set. Before the actual or the virtual onset of somatosensory input, only the prediction condition evoked continuous activity, with left hemisphere dominance irrespective of the stimulated side. The activity was observed in the bilateral primary and/or secondary somatosensory areas; the centre of gravity of the activated hemisphere contralateral to the stimulated side tended to lie towards the primary somatosensory area relative to that of the ipsilateral hemisphere. Since the contralateral primary and bilateral secondary somatosensory areas are involved in poststimulus processing, we conclude that somatosensory processing in conjunction with visual information begins even before stimulus reception. Moreover, these areas are activated by a visually-based prediction even if there is no somatosensory input when the visual and somatosensory information correspond with each other.