Summary One function of perceptual systems is to construct and maintain a reliable representation of the environment. A useful strategy intrinsic to modern “Bayesian” theories of perception 1 , 2 , 3 , 4 , 5 , 6 is to take advantage of the relative stability of the input and use perceptual history (priors) to predict current perception. This strategy is efficient 1 , 2 , 3 , 4 , 5 , 6 , 7 but can lead to stimuli being biased toward perceptual history, clearly revealed in a phenomenon known as serial dependence. 8 , 9 , 10 , 11 , 12 , 13 , 14 However, it is still unclear whether serial dependence biases sensory encoding or only perceptual decisions. 15 , 16 We leveraged on the “surround tilt illusion”—where tilted flanking stimuli strongly bias perceived orientation—to measure its influence on the pattern of serial dependence, which is typically maximal for similar orientations of past and present stimuli. 7 , 10 Maximal serial dependence for a neutral stimulus preceded by an illusory one occurred when the perceived, not the physical, orientations of the two stimuli matched, suggesting that the priors biasing current perception incorporate the effect of the illusion. However, maximal serial dependence of illusory stimuli induced by neutral stimuli occurred when their physical (not perceived) orientations were matched, suggesting that priors interact with incoming sensory signals before they are biased by flanking stimuli. The evidence suggests that priors are high-level constructs incorporating contextual information, which interact directly with early sensory signals, not with highly processed perceptual representations.

Recent evidence suggests that ongoing brain oscillations may be instrumental in binding and integrating multisensory signals. In this experiment, we investigated the temporal dynamics of visual-motor integration processes. We show that action modulates sensitivity to visual contrast discrimination in a rhythmic fashion at frequencies of about 5 Hz (in the theta range), for up to 1 s after execution of action. To understand the origin of the oscillations, we measured oscillations in contrast sensitivity at different levels of luminance, which is known to affect the endogenous brain rhythms, boosting the power of alpha-frequencies. We found that the frequency of oscillation in sensitivity increased at low luminance, probably reflecting the shift in mean endogenous brain rhythm towards higher frequencies. Importantly, both at high and at low luminance, contrast discrimination showed a rhythmic motor-induced suppression effect, with the suppression occurring earlier at low luminance. We suggest that oscillations play a key role in sensory-motor integration, and that the motor-induced suppression may reflect the first manifestation of a rhythmic oscillation.

Action and perception are intimately coupled systems. One clear case is saccadic suppression, the reduced visibility around the time of saccades, which is important in mediating visual stability; another is the oscillatory modulation of visibility synchronized with hand action. To suppress effectively the spurious retinal motion generated by the eye movements, it is crucial that saccadic suppression and saccadic onset be temporally synchronous. However, the mechanisms that determine this temporal synchrony are unknown. We investigated the effect of saccades on contrast discrimination sensitivity over a long period stretching over >1 s before and after saccade execution. Human subjects made horizontal saccades at will to two stationary saccadic targets separated by 20°. At a random interval, a brief Gabor patch was displayed between the two fixations in either the upper or lower visual field and the subject had to detect its location. Strong saccadic suppression was measured between -50 and 50 ms from saccadic onset. However, the suppression was systematically embedded in a trough of oscillations of contrast sensitivity that fluctuated rhythmically in the delta range (at ∼3 Hz), commencing ∼1 s before saccade execution and lasting for up to 1 s after the saccade. The results show that saccadic preparation and visual sensitivity oscillations are coupled and the coupling might be instrumental in temporally aligning the initiation of the saccade with the visual suppression. Saccades are known to produce a suppression of contrast sensitivity at saccadic onset and an enhancement after saccadic offset. Here, we show that these dynamics are systematically embedded in visual oscillations of contrast sensitivity that fluctuate rhythmically in the delta range (at ∼3 Hz), commencing ∼1 s before saccade execution and lasting for up to 1 s after the saccade. The results show that saccadic preparation and visual sensitivity oscillations are coupled and the coupling might be instrumental in aligning temporally the initiation of the saccade with the visual suppression.