Task-specific reversal of visual hemineglect following bilateral reversible deactivation of posterior parietal cortex: A comparison with deactivation of the superior colliculus

The purpose of the present study was to compare and contrast behavioral performance on three different tasks of spatial cognition during unilateral and bilateral reversible deactivation of posterior parietal cortex. Specifically, we examined posterior middle suprasylvian (pMS) sulcal cortex in adult cats during temporary and reversible cooling deactivation. In Task 1, the cats oriented to a high-contrast, black visual stimulus moved into the visual field periphery. In Task 2, the cats oriented to a static light-emitting diode (LED). Task 3 examined the cats' ability to determine whether a black-and-white checkered, landmark box was closer to the right or left side of the testing apparatus. Following training on all tasks, cryoloops were implanted bilaterally within the pMS sulcus. Unilateral deactivation of pMS sulcal cortex resulted in virtually no responses to either moved or static stimuli and virtually no responses to landmarks presented in the contralateral hemifield, and a profound contralateral hemifield neglect was induced. Responses to stimuli and landmarks presented in the ipsilateral hemifield were unimpaired. Additive, bilateral cooling of the homotopic region in the contralateral hemisphere, but not an adjacent region, resulted in reversal of the initial hemineglect for the moved stimulus, yet induced a complete failure to orient to peripheral static LED stimuli. Bilateral cooling also reversed the contralateral neglect of the landmark, but then cats could not accurately determine position of the landmark anywhere in the visual field because performance was reduced to chance levels for all landmark loci in both hemifields. In this instance, as the contralateral neglect disappeared during bilateral cooling of pMS cortex, a new spatial discrimination deficit was revealed across the entire visual field. We conclude that pMS cortex contributes in multiple ways to the analyses of space, and that these contributions cannot be safely predicted from analyses of unilateral deactivations or from one task to another. Moreover, it is clear that other structures are capable of guiding orienting to high contrast, moved targets when pMS cortex is eliminated from brain circuitry. However, these same structures are incapable of supporting either orienting to static stimuli or analyses of spatial relations as tested with the landmark task. The impact of reversible deactivation of the superior colliculus on these same tasks is discussed.