Visual hemineglect and lesion-induced changes of top-down activity in the primary visual cortex

Visual selective attention is the fundamental cognitive ability to filter out irrele-vant sensory information in favor of relevant information. In order to make this possi-ble, an integration of exogenous, stimulus-dependent, and endogenous, context-dependent, attentional factors is necessary. The anatomical basis for this integration is formed by a highly complex network of cortical feedforward and feedback projections which connect areas at every sensory processing level. Lesions in distinct areas of this attention network lead to the clinical pathology of visual unilateral neglect, in which the failure of one network component results in the inability to orient or respond to stimuli appearing in the visual field contralateral to a lesion. Frequently, in humans as well as in animal models, a gradual improvement up to a complete restoration of the behavioral performance can be observed some time after the initial lesion. Based on findings from behavioral studies, it has been suggested that a significant imbalance of neuronal activity levels between the two hemispheres and a resulting unequal distri-bution of limited attention capacities could be the neural basis of visual neglect. Ac-cordingly, a functional recovery would be accompanied by a restoration of the activity balance between the two hemispheres. Current concepts of the underlying mechanisms of neglect are mainly based on results from behavioral experiments or data collected in anesthetized animals. To date, there are no neurophysiological studies in awake animals which consider the neglect syndrome and its recovery in the behavioral context. The aim of the present study was to close this gap on the one hand, and on the other, to provide a detailed characterization of feedback-mediated attention signals in the early visual cortex