April Pilipenko¹, Jason Samaha¹, Alexandra Mcgowan¹, Jacob Chaudhry¹, Vrishab Nukala¹, Emily Lincoln¹, Aishu Narayanan¹; ¹UC Santa Cruz

Trial-to-trial variability in alpha-band neural oscillations (8–13 Hz) have previously been linked to variability in conscious visual perception. However, the functional role of alpha phase in visual perception remains debated, with prior studies reporting effects of pre-stimulus phase on detection (i.e., hit rates) but also null results. Methodological differences such as stimulus properties (e.g., duration, size, location), and low trial counts may contribute to these inconsistencies. Additionally, previous studies have not distinguished whether alpha phase modulates perceptual sensitivity (d') or criterion, using signal detection theory (SDT). To address these gaps, we used a Yes/No detection paradigm with a 8ms vertical Gabor target (2 cycles per degree) embedded in filtered noise. On half of the trials only the noise patch was presented. Participants reported whether they perceived the target in the noise or only the noise. Each participant (n=5) completed 6,020 trials across 4-5 EEG sessions. We observed a significant effect of pre-stimulus alpha phase on d' for most individual subjects and at the group level with no accompanying effect on criterion. Examining detection across the alpha cycle indicates that these sensitivity modulations arise from an inverse phasic relationship between hit and false alarm rates, consistent with a reduction in internal noise during optimal alpha phases. This contrasts with a multiplicative gain model, which would predict a constant false alarm rate across phases and only a boost in hits. These findings suggest that alpha phase enhances perceptual sensitivity by decreasing internal noise, rather than amplifying the signal. Lastly, we created classification images of the optimal and suboptimal phases. These images showed small boosts in the sharpness of spatial frequency and orientation tuning during the optimal alpha phase, suggesting that alpha phase may modulate sensory tuning towards relevant stimulus properties.