Prior Expectation Modulates Repetition Suppression without Perceptual Awareness

Abstract

Stimulus repetition induces attenuated brain responses. This phenomenon, termed repetition suppression (RS), is classically held to stem from bottom-up neuronal adaptation. However, recent studies suggest that RS is driven by top-down predictive mechanisms. It remains controversial whether these top-down mechanisms of RS rely on conscious strategies, or if they represent a more fundamental aspect of perception, coding for physical properties of the repeated feature. The presence of top-down effects in the absence of perceptual awareness would indicate that conscious strategies are not sufficient to explain top-down mechanisms of RS. We combined an unconscious priming paradigm with EEG recordings and tested whether RS can be modulated by the probability of encountering a repetition, even in the absence of awareness. Our results show that both behavioural priming and RS near occipital areas are modulated by repetition probability, regardless of prime awareness. This contradicts previous findings that have argued that RS modulation is a by-product of conscious strategies. In contrast, we found that the increase in theta-band power following unrepeated trials - an index of conflict detection - is modulated only by expectations during conscious primes, implicating the use of conscious strategies. Together, our results suggest that the influence of predictions on RS can be either automatic in sensory brain regions or dependent on conscious strategies.

Figure 1

Paradigm and Priming. (A) In each trial, participants were presented with a sequence of two gratings inside an annulus, separated by a brief pattern mask composed of concentric circles. Each grating was either tilted to the left or to the right (−45 and +45 degrees, respectively). Participants were instructed to identify as fast as possible the direction of the second grating (the target). Following their response, they had to select as accurately as possible the direction of the first grating (the prime). Trials were either repeated (same orientation for prime and target) or unrepeated (opposite prime and target orientations), and were grouped in two types of blocs: 80% of repeated trials (congruent context) and 80% of unrepeated trials (incongruent context). Also, in each trial, the prime was either made invisible by presenting for 33 ms (unconscious condition) or visible by presenting for 167 ms (conscious condition). Figures (B and C) show reaction times and accuracy for the target discrimination task, as a function of prime visibility and probabilistic context (paired two-tailed t-tests, ns: p > 0.05, *p < 0.05, **p < 0.01, ***p < 0.001; error bars represent s.e.m.).

Figure 2

Evoked Potentials. (A) Scalp topographies showing the difference between ERPs (time-locked to the target onset) evoked by unrepeated and repeated trials (i.e. repetition suppression or RS) collapsed across context and visibility. The first component is observed around 240 ms over electrodes in the vicinity of occipital areas. (B) Time-plot showing the ERPs for repeated and unrepeated trials for these electrodes (collapsed across context and visibility). (C) RS effect for each prime visibility and context (i.e. difference between unrepeated and repeated ERPs in the selected electrodes): significant clusters found only during congruent context, peaking around 280 ms, for both levels of awareness. (D) Difference between RS for each context (or Repetition Probability Effect, RPE), for each level of prime visibility. Significant clusters were found regardless of prime awareness. (E) Difference between conscious and unconscious RPEs: no significant cluster was found. Time plots are smoothed with a 50 ms moving average window for visualization (only μV information have been smoothed, not t-values). Horizontal solid lines in time plots close to time axis indicate significant cluster (monte-carlo p-value < 0.05).

Figure 3

Theta Activity. (A) Scalp topographies showing the effect of repetition (unrepeated minus repeated trials) for Theta band activity (3–8 Hz) elicited at target onset, collapsed across context and visibility. (B) Time-Frequency plots showing the activity in medial-frontal electrodes. (C) Difference between unrepeated and repeated trials for each prime visibility and context (i.e. theta band increase) showing a significant cluster found only for conscious primes in the congruent context. (D) Differences between theta band increase for each context (or theta-band repetition probability effect, RPE) for each prime visibility. Significant cluster peaking around 4.5 Hz and 360 ms was revealed only during conscious condition. (E) Difference between conscious and unconscious RPEs, with a significant cluster peaking around 4.5 Hz and 360 ms. In all Time-Frequency figures, contours indicate significant clusters (monte-carlo p-value < 0.05).

Figure 4

Unconscious priming without intermixed conscious repetitions. (A) shows reaction times and (B) shows accuracy for the target discrimination task, as a function of probabilistic context (paired two-tailed t-tests, ns: p > 0.05, *p < 0.05, **p < 0.01, ***p < 0.001; error bars represent s.e.m.).