On temporal hyperacuity in the human visual system.

The spatial grain of the human visual system has always been a central topic for visual sciences, and the optical and physiological basis of perceptual limitations are well described. In particular, we have thorough accounts of spatial hyperacuity, which refers to a precision in the spatial localisation of stimulus contours that is better than the photoreceptor grain that determines spatial resolution. However, although the temporal resolution of the human visual system is comparably well described, we have almost no direct knowledge about the precision of localising visual stimuli in time in the absence of correlated spatial cues. The present study addresses this question by comparing directly the temporal resolution of human observers with their temporal acuity as measured in a temporal bisection task. Despite some improvement with practice, temporal acuity in this task does not fall below 20-30 ms in the best case, which is similar to the temporal resolution limit, and performance does not improve for comparison tasks with multiple stimulus presentations. The absence of visual hyperacuity for purely temporal modulations as tested here contrasts with processing limitations for other types of visual information in comparable tasks, and with other sensory modalities, in particular to those of the auditory system. Such differences can be interpreted in the context of the ecological requirements for organising behaviour, and the functional design of nervous systems.