Schedule of Events | Symposia

Effects of forward and backward masking on feedforward and feedback processing: evidence from time- and frequency-resolved EEG decoding

Poster Session F - Tuesday, April 1, 2025, 8:00 – 10:00 am EDT, Back Bay Ballroom/Republic Ballroom

Yusuke Nakashima¹ (yusuke_nakashima@brown.edu), Yuka Sasaki, Takeo Watanabe; ¹Brown University

Distinguishing between feedforward and feedback visual processing in the brain is challenging. Some studies suggest that a mask following a stimulus (backward masking) disrupts feedback processing, while a mask preceding a stimulus (forward masking) affects feedforward processing. However, evidence supporting this hypothesis remains limited. To address this issue, we examined which frequency bands of neural oscillations are affected by forward and backward masking using EEG to decode orientation discrimination performance. This approach is inspired by recent neurophysiological studies suggesting that theta and gamma bands predominantly carry feedforward signals, while alpha and beta bands predominantly carry feedback signals. This finding led us to raise the possibility that forward and backward masking affect decoding accuracy in feedforward-related frequencies (theta or gamma) and feedback-related frequencies (alpha or beta), respectively. Participants performed an orientation discrimination task under three conditions: no masking, backward masking, and forward masking. Decoding accuracy for orientation classification was calculated at each time point and frequency band (theta, alpha, beta, and gamma) based on time-frequency coefficients. In the no masking condition, accuracy increased in the theta band during an earlier time window (from 100 ms) and in the alpha band during a later time window (from 200 ms), possibly reflecting feedforward and feedback signals, respectively. Backward masking did not affect theta accuracy but reduced alpha accuracy. Conversely, forward masking reduced theta accuracy but left alpha accuracy intact. These results suggest backward masking disrupts feedback processing reflected in the alpha band, while forward masking disrupts feedforward processing reflected in the theta band.

Topic Area: PERCEPTION & ACTION: Vision