Bilateral cutaneous stimulation of the somatosensory system in hemidecorticate rats.

An enduring somatosensory consequence of extensive neocortex injury in people is "simultaneous extinction," which is an interhemispheric perceptual interaction that is operationally distinguishable from neglect. A cutaneous stimulus presented on the contralateral side of the body is readily detected when presented singly but is actively masked during bilateral stimulation. In hemidecorticate rats, small adhesive stimuli were attached to the radial surface of each forelimb simultaneously, and the latencies to contact and to remove each stimulus were recorded. Neglect of the contralateral stimulus lasted 2-3 days. Thereafter the ipsilateral stimulus was removed first, followed immediately by the contralateral stimulus. This ipsilateral sensorimotor bias lasted several months. Further analysis with unremovable tactile stimuli provided evidence for true extinction. For a limited period (during the first 2 postoperative weeks), the contralateral stimulus did not appear to be detected in the presence of the ipsilateral stimulus. Rather than switching back and forth between the two stimuli, the animals ignored the stimulus on the contralateral limb and persisted in their attempts to remove the ipsilateral stimulus. A key feature of the "extinction" was its complete reversibility. Simply by adjusting the sensory fields occupied by the contralateral (C) and ipsilateral (I) stimuli (specifically, by increasing the C/I ratio), the sensorimotor bias was totally shifted to contralateral. During recovery, the size of the C/I ratio necessary to reverse sensorimotor asymmetry gradually decreased. Asymmetrical sensorimotor behavior and amphetamine-induced circling were examined in three additional groups of hemidecorticate rats, which were given their initial behavioral tests at 2, 12, or 52 postoperative weeks. This experiment confirmed the above findings and controlled for practice. Over the course of a year, apparently complete recovery occurred; however, there were residual effects. At each period, previously recovered symptoms were reinstated simply by turning on the room lights and opening the home cage slightly. These data underscore the importance of detailed behavioral analysis and the value of the rat as a model in studies of recovery of sensorimotor function.