ATTENTIONAL RESOLUTION AND THE LOCUS OF VISUAL AWARENESS

VISUAL spatial resolution is limited by factors ranging from optics to neuronal filters in the visual cortex(1,2), but it is not known to wh at extent it is also limited by the resolving pow er of attention. To investigate this, we studied adaptation to lines of specific orientati on, a process that occurs in primary visual cortex(3). When a single g rating is presented in the periphery of the visual field, human observ ers are aware of its orientation, but when it is flanked by other simi lar gratings ('crowding'), its orientation becomes impossible to disce rn(4,5). Nevertheless, we show that orientation-specific adaptation is not affected by crowding, implying that spatial resolution is limited by an attentional filter acting beyond the primary visual cortex. Con sistent with this, we find that attentional resolution is greater in t he lower than in the upper visual field, whereas there is no correspon ding asymmetry in the primary visual cortex. We suggest that the atten tional filter acts in one or more higher visual cortical areas to rest rict the availability of visual information to conscious awareness(6).