Abstract
Visual attention is not continuous over time. Both during spatial exploration and when sustained at a specific location, evidence suggests that attention rhythmically modulates information processing at low frequencies: theta (4-7Hz) during exploration and alpha (8-12Hz) for sustained attention. Yet, the specific mechanisms underlying such periodicity are unknown. Using a psychophysical reverse correlation approach, we tested the hypothesis that rhythms in performance are related to underlying fluctuations in the quality of sensory representation; specifically, changes in (1) tuning to relevant features, (2) enhancement (gain) of the target stimulus feature, and/or (3) suppression of irrelevant features. Participants performed a behavioral task in which endogenous (voluntary) attentional deployment was probed across time and spatial locations. On each trial, participants first, discriminated the position of gap in a cued (valid; 2/3) or uncued (invalid; 1/3) Landolt-C placed in the bottom left or right quadrant. Second, they detected the presence of a vertical Gabor inside each of two noise patches presented at the same two locations, at fourteen possible delays after the Landolt-C offset. The discrimination task manipulated attention, and the detection task probed attention state at each delay and location. We replicated previous results showing that sensitivity (d’) at each of the probed locations fluctuated rhythmically at the alpha frequency (~10Hz) in valid trials (attention was sustained at one location) and at theta frequency (~4Hz) in invalid trials (attention was reoriented to the opposite location; exploration). Critically, orientation tuning functions obtained from the probe task suggested that sustained attention rhythmically alters the representation of irrelevant features (extreme orientations), while exploratory attention rhythmically alters tuning to the relevant feature (vertical orientation). Our findings suggest that sustained and exploratory attention differ not only in the frequency at which they modulate performance, but also in what aspects of the underlying sensory representations they influence.