Abstract
It has been believed that human visual system is insensitive to accelerations in moving stimuli. The notion is notably supported by evidence that detection sensitivity for velocity modulation in moving stimuli exhibits low-pass characteristics against temporal frequency of velocity modulation (e.g., Werkhoven et al., 1992). On the other hand, recent studies demonstrate a biphasic temporal response of motion detectors, leading to a notion that the visual system is sensitive to accelerations at some processing levels. To revisit visual sensitivity for accelerations, we here re-examined the velocity modulation sensitivities for a wide range of conditions. Stimulus was a horizontal square-wave grating (0.5 cpd) that drifted at a temporal frequency of 10.7 Hz within a square window (3.2 x 3.2 deg) or within a vertically elongated window (H3.2 x V22.7 deg). For the both window conditions, detection thresholds were measured for sinusoidal velocity modulations of various temporal frequencies (0.25 - 8 Hz). The results showed that the modulation sensitivity function had a low-pass shape for stimuli with a long window, but was a band-pass shape for stimuli with a short window (peak at ~1 Hz). Similar band-pass characteristics were observed for Gabor stimuli. We also found that, even for stimuli with a long window, the low-pass sensitivity curve was altered into band-pass when observer's attention was removed by the concurrent letter identification task. An additional visual-search experiment also revealed that a target dot moving with a velocity modulation at relatively high temporal frequencies (~2 Hz) was the most easily detected among dots moving with various constant velocities. These results support a notion that high sensitivity to sluggish velocity modulations is a product of attentive tracking of continually moving stimuli, and that the visual system is primarily sensitive to accelerations and/or decelerations at the pre-attentive level.
Meeting abstract presented at VSS 2017