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

The purpose of the present study was to compare and contrast behavioral per formance on three different tasks of spatial cognition during unilateral an d bilateral reversible deactivation of posterior parietal cortex. Specifica lly, 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 visu al 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 blac k-and-white checkered, landmark box was closer to the right or left side of the testing apparatus. Following training on all tasks, cryoloops were imp lanted bilaterally within the pMS sulcus. Unilateral deactivation of pMS su lcal cortex resulted in virtually no responses to either moved or static st imuli and virtually no responses to landmarks presented in the contralatera l hemifield, and a profound contralateral hemifield neglect was induced. Re sponses to stimuli and landmarks presented in the ipsilateral hemifield wer e unimpaired. Additive, bilateral cooling of the homotopic region in the co ntralateral hemisphere, but not an adjacent region, resulted in reversal of the initial hemineglect for the moved stimulus, yet induced a complete fai lure to orient to peripheral static LED stimuli. Bilateral cooling also rev ersed the contralateral neglect of the landmark, but then cats could not ac curately determine position of the landmark anywhere in the visual field be cause performance was reduced to chance levels for all landmark loci in bot h hemifields. In this instance, as the contralateral neglect disappeared du ring bilateral cooling of pMS cortex, a new spatial discrimination deficit was revealed across the entire visual field. We conclude that pMS cortex co ntributes in multiple ways to the analyses of space, and that these contrib utions cannot be safely predicted from analyses of unilateral deactivations or from one task to another. Moreover, it is clear that other structures a re capable of guiding orienting to high contrast, moved targets when pMS co rtex 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 reversibl e deactivation of the superior colliculus on these same tasks is discussed.