Poster Session: Contrast polarity in photopic, mesopic, and scotopic vision.

The perception of dark and light shows considerable asymmetries. In scotopic vision, "white" patches appear gray - brightness perception is clipped at the upper end of the range. In photopic vision, darks are perceived as relatively more intense than lights. Here, we compared perception of contrast polarities in photopic, mesopic, and scotopic viewing. We tested the perception of positive and negative contrasts in the three viewing conditions, using a circular stimulus (2° radius) presented for 200 ms at 8° eccentricity. In a detection task, we obtained absolute thresholds for positive and negative contrasts. In a matching task, we obtained PSEs when participants compared stimuli with positive and negative contrasts at three contrast levels (0.2, 0.35, 0.5). Stimuli were defined in Weber's contrast. In the detection task, thresholds were highest in scotopic and lowest in photopic vision. Thresholds were higher for positive than negative contrasts in mesopic and scotopic but not in photopic viewing. In the matching task, the asymmetry was increased in scotopic compared to photopic viewing, such that even more positive contrast was required to match negative contrasts. Our results show that the asymmetry in contrast perception depends on the lighting condition, being largest in scotopic vision in both tasks. As signals from cone receptors are crucial for perceiving "white", the clipped range in scotopic vision also seems to impede the perception of positive contrasts.