Cerebrally lateralized mental representations of hand shape and movement

Previous psychophysical and neuroimaging studies suggest that perceiving the handedness of a visually presented hand depends on sensorimotor processes that are specific to the limb of the stimulus and that may be controlled by the cerebral hemisphere contralateral to the limb. Therefore, it was hypothesized that disconnection between cerebral hemispheres would disrupt mental simulation of a hand presented to the ipsilateral, but not the contralateral, hemisphere. This hypothesis was examined by the present study in which two callosotomy patients and eight healthy controls judged the handedness of drawings of left and right hands in various positions, without moving or inspecting their own hands. Stimuli were presented for 150 msec in the right or left visual hemifield. As predicted, for each hemisphere, patients' accuracy was high when the hand was contralateral to the perceiving hemisphere, but it was not above chance when it was ipsilateral to the perceiving hemisphere. Controls' accuracy was high in both conditions. Response time analyses indicate patients, like controls, mentally simulated reaching into stimulus postures. When the stimulus laterality was ipsilateral to the perceiving hemisphere, patients imagined the hand contralateral to the perceiving hemisphere reaching into the stimulus posture but did not detect the mismatch, guessing with a response bias or responding on the basis of shape similarity. We conclude that each hemisphere could represent the shape and movement of the contralateral hand but could not for the ipsilateral hand. Mentally simulating one's action and discriminating body part handedness both depend on lateralized sensorimotor and somatosensory representations.