Attention alters appearance in early vision: Contrast sensitivity, spatial resolution, and color saturation

Abstract

Background: Does attention alter appearance? This critical issue, debated for over a century, remains unsettled. From psychophysical evidence that covert attention affects early vision -it enhances contrast sensitivity and spatial resolution- and from neurophysiological evidence that attention increases the gain response, one could infer that attention changes stimulus appearance. Surprisingly, few studies have directly investigated this issue. Here we directly assess the phenomenological correlates of attention across several dimensions of early vision. Methods: We implemented a psychophysical paradigm that enabled us to directly assess the appearance of the relevant dimension -contrast, spatial frequency, or saturation. To preclude response bias, observers performed an orientation discrimination task contingent on the stimulus that appeared greater in magnitude -higher contrast, spatial frequency, or saturation. The task instructions and the experimental design emphasized the orientation judgment, when in fact we were interested in the appearance judgment. By assessing which stimulus observers perceived as being greater in magnitude, we obtained psychometric functions and the point of subjective equality (PSE). We measured these functions both when transient covert attention was directed to a particular location via a peripheral cue and when it was directed to the center of the display via a neutral cue. The peripheral cue was uninformative in terms of both stimulus orientation and intensity. Control experiments using a post-cue or ineffective cue timing ruled out a cue bias explanation of the results. Results & Discussion: We show that attention alters appearance; it enhances the apparent contrast, spatial frequency and color saturation. By implementing a paradigm that allowed us to assess the effects of spatial cueing on apparent magnitude, for the first time we can conclude that transient covert attention does intensify the sensory impression of a stimulus.

NSF #: BCS 9910734/HCP