Spatial localization of cutaneous temporal order judgments involves the posterior parietal cortex

Abstract

Very little is known about how specific brain regions are involved with cutaneous temporal order judgments (TOJs). The processes underlying judgments in this modality are of special interest because the body surfaces to which such stimuli are delivered can be moved to different locations in space with respect to the midline of the body and may be encoded in multiple reference frames. Where and how the integration of these signals occurs is currently unknown. In the present experiment, we addressed this issue by using single pulse transcranial magnetic stimulation (TMS) at various intervals to disrupt processing in the right or left posterior parietal cortex (PPC) during a TOJ task with cutaneous vibrotactile stimuli applied to the index finger of each hand. The results demonstrated that when vibrotactile stimuli were delivered to the index finger contralateral to the site of TMS first, there was a significant increase in TOJ errors that were not present when the stimuli were delivered to the ipsilateral finger first. This was true for both the right and left PPC and more apparent when TMS was delivered prior to rather than during finger stimulation. Control experiments using sham TMS indicated that this increase in errors was not due to non-specific effects of the TMS drawing attention away from the contralateral side of space. To further investigate the spatial encoding of the cutaneously-encoded events, we had subjects cross his or her arms. We then observed an increase in errors ipsilateral to the side of stimulation. Taken together, these results demonstrate that both the right and left PPC contribute to the processing underlying cutaneous TOJ tasks and furthermore that this information is not encoded in a haptic reference frame but rather with respect to the body.