Attention activates winner-take-all competition among visual filters

Shifting attention away from a visual stimulus reduces, but does not abolis h, visual discrimination performance. This residual vision with 'poor' atte ntion can be compared to normal vision with 'full' attention to reveal how attention alters visual perception. We report large differences between res idual and normal visual thresholds for discriminating the orientation or sp atial frequency of simple patterns, and smaller differences for discriminat ing contrast. A computational model, in which attention activates a winner- take-all competition among overlapping visual filters, quantitatively accou nts for all observations. Our model predicts that the effects of attention on visual cortical neurons include increased contrast gain as well as sharp er tuning to orientation and spatial frequency.