Visual hyperacuity: spatiotemporal interpolation in human vision

Stroboscopic presentation of a moving object can be interpolated by our visual system into the perception of continuous motion. The precision of this interpolation process has been explored by measuring the vernier discrimination threshold for targets displayed stroboscopically at a sequence of stations. The vernier targets, moving at constant velocity, were presented either with a spatial offset or with a temporal offset or with both. The main results are: (1) vernier acuity for spatial offset is rather invariant over a wide range of velocities and separations between the stations (see Westheimer & McKee 1975); (2) vernier acuity for temporal offset depends on spatial separation and velocity. At each separation there is an optimal velocity such that the strobe interval is roughly constant at about 30 ms; optimal acuity decreases with increasing separation; (3) blur of the vernier pattern decreases acuity for spatial offsets, but improves acuity for temporal offsets (at high velocities and large separations); (4) a temporal offset exactly compensates the equivalent (at the given velocity) spatial offset only for a small separation and optimal velocity; otherwise the spatial offset dominates. A theoretical analysis of the interpolation problem suggests a computational scheme based on the assumption of constant velocity motion. This assumption reflects a constraint satisfied in normal vision over the short times and small distances normally relevant for the interpolation process. A reasonable implementation of this scheme only requires a set of independent, direction selective spatiotemporal channels, that is receptive fields with the different sizes and temporal properties revealed by psychophysical experiments. It is concluded that sophisticated mechanisms are not required to account for the main properties of vernier acuity with moving targets. It is furthermore suggested that the spatiotemporal channels of human vision may be the interpolation filters themselves. Possible neurophysiological implications are briefly discussed.