Dichoptic viewing reveals distinct low- and high-level mechanisms for temporal phase discrimination

Conscious awareness of temporal visual structure is limited to frequencies below ∼10 Hz. Between ∼10 and 60 Hz, flicker is visible, but temporal phase cannot be accurately discriminated; above ∼60 Hz, stimuli are perceptually fused. Accurately estimating the cognitive limit of phase discrimination requires reducing the contribution of low-level motion mechanisms that can support phase-like judgments via short-range apparent motion. We examined how interstimulus spacing and hemifield placement affect temporal phase discrimination limits using a paired achromatic flicker task in two experiments. In Experiment 1 (binocular viewing), narrow interstimulus spacing produced higher frequency limits, implicating a local, low-level motion mechanism; wide spacing reduced this contribution and yielded lower frequency limits. In Experiment 2 (dichoptic viewing), the spacing effect persisted but was attenuated, indicating that the local mechanism operates on monocular signals, before binocular combination, and can be partially disrupted by presenting each stimulus to a separate eye. A distinct low-level mechanism contributed to performance when the stimuli were close together. Once its contribution was minimized, the upper limit of temporal phase discrimination for achromatic luminance modulations converged on approximately 7-10 Hz, a ceiling consistent with broader limits on temporal integration reported across perceptual and cognitive tasks.