Viewing-distance invariance of movement detection

Abstract

Since visual movement information is often presented in electronic displays or films it is amazing that there is a paucity of research on the influence of viewing distance on motion detection in cinematograms. We report a relatively high degree of detection constancy with changing viewing distance for coherent motion in random-pixel cinematograms. A constant performance irrespective of viewing-distance is called 'distance-invariance' and for motion detection it proves to hold reasonably well for a relatively wide range of viewing distances both for foveal and eccentric vision. The limits of this viewing-distance invariance are explored as a function of screen velocity. Detection performance is quantified by a threshold signal-to-noise-ratio (SNR-) value, S, which is determined as a function of velocity for a range of viewing distances from 53 to 13,476 mm for foveal vision and from 60 to 1925 mm at 24 degrees eccentricity on the nasal horizontal meridian of the right eye's retina. The data can be explained, at least qualitatively, by a model in which a spatial-resolution stack has a stack of velocity-tuned motion detectors at every resolution layer. Such a 'stack-of-stacks' model is in line with proposals for contrast-detection stack-models, but it suggests that the usual hypothesis that motion perception is based on the activity of two separate systems, the short-range and the long-range system, might be superfluous. This two-systems distinction was largely based on the different performance found for moving random dot patterns and moving form-defined stimuli. A moving random pixel array viewed at very close range (e.g. 6 cm) presents the subject with relatively large almost square 'blobs', which are less dissimilar from the phi-stimuli used in classic motion perception studies than random dot stimuli at the usual medium to large viewing distances. It leads to maximum displacement threshold (Dm-) values that are not untypical of the 'long-range' system, but by gradually increasing the viewing-distance and thus decreasing the pixel-size a continuous change is found from typical long-range to typical short-range values of Dm. The two-systems distinction for motion detection appears to refer to the stimulus rather than to the visual system: The motion-detection system might be forced into a local or a global 'mode of operation' by the choice of stimulus.