Art - Braun (T)

375 Although attention profoundly alters visual perception1, it is not equally important to all aspects of vision. For example, attention is of little or no help to many detection tasks (for example, detecting a luminance increment2), and the degree to which it benefits discrimination tasks varies widely with the discriminated attribute (for example, discriminating color, orientation, form3,4). Here we report how attention alters thresholds for discriminating contrast, orientation and spatial frequency of simple patterns. Based on earlier work, we expected markedly different effects on different thresholds5. In addition, we describe how attention changes thresholds for detecting one pattern in the presence of another, superimposed pattern of different orientation or spatial frequency. Together, these measurements characterize the visual mechanisms that underlie basic pattern vision. Perceptual thresholds for stimulus contrast, orientation and spatial frequency have been studied for several decades6–8. Quantitative accounts of these thresholds have become increasingly refined and usually involve a population of ‘noisy filters’ tuned to different orientations and spatial frequencies. Although earlier models postulated filters that are independent of each other, there are serious shortcomings to this approach9,10. More recent models postulate an interaction between filters with spatially overlapping receptive fields11–13, specifically, the normalization of individual filter responses relative to the total response of the local filter population (‘divisive inhibition’14). This normalization accounts naturally for several otherwise puzzling observations, among them the initial decrease and later increase of contrast discrimination thresholds with increasing stimulus contrast12,13 (‘dipper function’6,8) and the relative constancy of orientation and spatial frequency thresholds over a wide range of stimulus contrasts9,15. An intriguing parallel to these perceptual accounts can be found in certain models of visual cortical responses to stimulus contrast and orientation16,17. Despite marked differences in detail, the models in question consider a population of neurons with overlapping receptive fields, broadly tuned to a range of different orientations, and normalize individual responses relative to Attention activates winner-take-all competition among visual filters