Figure 4. Shift in hemispheric control of inhibition of bilateral percepts under conditions of left-hand stimulation.

A schematic of somatosensory processes under conditions of left-hand stimulation when the right hand is in contralesional (4a) compared to ipsilesional (4b) space relative to the trunk midline. Note that we include only the mechanisms and connectivity required to highlight the shift in hemispheric control that takes place under these circumstances. In each hemisphere, the lower large box represents early cortical somatosensory processes while the upper large box represents higher level integrative processes involved in body image and tactile awareness. The small boxes represent left and right hemisphere mechanisms involved in frame of reference representation and processing; these contribute to the inhibition of bilateral percepts from cross-hemispheric transmission of somatosensory information (as indicated by flat- topped arrows). Note that for clarity, only one reference frame mechanism is shown (trunk- or head-centered), although we propose that at least three separate mechanisms (trunk-centered, head-centered, and limb-relative) operate in a similar manner. Cross-hatched boxes and dashed lines indicate damaged mechanisms; we assume that DLE has damage to early left hemisphere somatosensory processes and inhibitory processes, but that at least some higher level processes are spared. Asterisk represents a stimulus. Thick red arrows and outlining identify the critical aspects of processing related to the unstimulated right hand; thick black arrows and outlining are used for critical aspects of processing related to the stimulated left hand.

In Figure 4a, DLE's unstimulated right hand is in right space relative to trunk and head midlines. This information is represented in the contralateral (and damaged) left hemisphere. Under those conditions, the inhibitory mechanisms responsible for inhibiting the bilateral percept are likely to fail and a synchiric phantom sensation on the right hand will be experienced. In Figure 4b, the right hand is in left space relative to head and body midlines. This information is represented in the contralateral (and intact) right hemisphere, resulting in the successful inhibition of the spurious bilateral percept.