Metacontrast Masking Across Different Contrast Polarities: the Role of Late ERP Components

Metacontrast masking has found increasing applications as a powerful methodological tool in studies of information processing and visual perception. However, the cortical mechanisms underlying this important investigative tool are still subject to debate. Although substantial metacontrast can be obtained even when the target and mask have opposite contrast polarities, previous research also indicated that the masking strength is contrast-polarity specific. In the current study, using contrast polarity as a critical experimental factor, we aimed to identify the modulations of ERPs (event-related potentials) that parallel changes in the metacontrast masking functions. Accordingly, we employed a contour discrimination task in a metacontrast paradigm combined with EEG (Electroencephalography). The behavioral performance values indicated a typical U-shaped metacontrast function for the same polarity condition. However, when the target and mask had opposite contrast polarities, the masking function became a monotonic increasing function and the masking effect was strong at stimulus onset asynchronies less than 50 ms. This shift in metacontrast function has been mainly interpreted as an increase in intra-channel inhibition of the sustained activities (i.e., parvo-dominant pathway) associated with object visibility and identity. The cluster-based permutation test on the ERP waveforms revealed an early (160-300 ms, occipital and parieto-occipital scalp sites) and a late (300-550 ms, parietal and centro-parietal scalp sites) spatiotemporal cluster. Moreover, there was a robust correlation between the modulations of potentials in the late cluster time-range and the changes in performance values. Overall, these ERP findings indicate the involvement of late inhibitory mechanisms in metacontrast masking. Although the behavioral and ERP findings do not preclude other important proposed mechanisms underlying metacontrast (e.g., inter-channel inhibition), they together suggest that the late recurrent intra-channel inhibition within the sustained pathway also plays an important role in metacontrast masking.