Distinct Contributions of Alpha and Beta Oscillations to Context-Dependent Visual Size Perception

Abstract

Previous studies have proposed two cognitive mechanisms responsible for the Ebbinghaus illusion effect, i.e., contour interaction and size contrast. However, the neural underpinnings of these two mechanisms are largely unexplored. The present study introduced binocular depth to the Ebbinghaus illusion configuration and made the central target appear either in front of or behind the surrounding inducers in order to disturb size contrast instead of contour interaction. The results showed that the illusion effect, though persisted, was significantly reduced under the binocular depth conditions. Notably, the target with a larger perceived size reduced early alpha-band power (8-13 Hz, 0-100 ms after stimulus onset) at centroparietal sites irrespective of the relative depth of the target and the inducers, with the parietal alpha power negatively correlated with the illusion effect. Moreover, the target with a larger perceived size increased the occipito-parietal beta-band power (14-25 Hz, 200-300 ms after stimulus onset) under the no-depth condition, and the beta power was positively correlated with the illusion effect when the depth conditions were subtracted from the no-depth condition. The findings provided neurophysiological evidence in favor of the two cognitive mechanisms of the Ebbinghaus illusion by revealing that early alpha power is associated with low-level contour interaction and late beta power is linked to high-level size contrast, supporting the claim that neural oscillations at distinct frequency bands dynamically support different aspects of visual processing.

Keywords: Alpha oscillation; Beta oscillation; Binocular depth; EEG; Ebbinghaus illusion.

Fig. 1

Illusory configurations in the conditions of crossed (near depth), zero (no depth), and uncrossed (far depth) disparities.

Fig. 2

Experimental procedures. In Experiment 1 (A), stimuli were dichoptically presented via a stereoscope mirror, and observers were required to sequentially perform depth discrimination and size-matching tasks. In Experiment 2 (B), stimuli were binocularly presented at the screen center. In Experiment 3 (C), stimuli were simultaneously presented to separate eyes, with the four surrounding inducers presented at different depths. In Experiment 4 (D), stimuli were dichoptically presented for 0.5 s, and observers had to perform the depth discrimination task only.

Fig. 3

Behavioral results. The illusion strength under each of the three conditions in Experiment 1 (A) and the corresponding three conditions without a stereoscope mirror in Experiment 2 (B), as well as the comparison of the illusion effect across Experiments 1, 2, and 3 (C). The perceived size of the target as a function of target depth in Experiment 3 (D). Error bars represent the SEM. *P < 0.05, **P < 0.01, and ***P < 0.001, two-tailed t-test (N = 22, 5, and 8 for Experiments 1 to 3, respectively).

Fig. 4

ERP results of binocular disparity. Topography of horizontal disparity effect on N1 peak (A) and the mean amplitude averaged across parietal and occipital electrodes (P7, P4, P6, P8, PO7, PO5, PO3, POZ, PO4, PO6, PO8, O1, OZ, O2; panel B). The pink dots indicate the electrodes with a significant horizontal disparity effect (crossed vs zero or uncrossed vs zero, P <0.05, two-tailed t-test, FDR corrected).

Fig. 5

ERP results of size of inducers. Topography of size of inducers (large vs small) on N1 peak averaged across three depth conditions (A), and the wave at electrode CP6 as a function of size of inducers (B). Topography of size of inducers on the P2 peak averaged across three depth conditions (C) and the mean wave of parietal and occipital electrodes (P3, P4, PO7, PO5, PO3, POZ, PO4, O1, OZ, O2) as a function of size of inducers (D). The pink dots indicate the electrodes with significant comparisons (P < 0.05, two-tailed t-test, FDR corrected).

Fig. 6

Results of time-frequency analysis. Topography of illusion effect on alpha power (8–13 Hz) in the early time window (0–100 ms) for the three depth conditions (A), and the correlation of alpha power at parietal electrodes (CP2 and CP4) and behavioral illusion effect (B). Topography of illusion effect on beta power (14–25 Hz) in the late time window (200–300 ms) for the zero disparity condition (C), and the correlation of beta power at parietal electrodes (P3, PO3, POZ) and the behavioral illusion effect with the horizontal-disparity condition subtracted from the zero-disparity condition (D). The pink dots indicate the electrodes with significant comparisons (P < 0.01, two-tailed t-test, uncorrected).