Influence of contrast on foveal and peripheral detection of coherent motion in moving random-dot patterns

The detection of coherent motion was studied in stroboscopically displayed moving random-dot patterns disturbed by incoherent noise. We determined the threshold signal-to-noise ratio S as a function of velocity V at eccentricities of 0 degrees, 3 degrees, 6 degrees, 12 degrees, 24 degrees, 48 degrees in the temporal visual field of the right eye. At each eccentricity the measurements of S = f(V) were repeated for a range of rms contrast values from 60% (0 dB) in steps of 3 dB down to 1.9% (-30 dB). All stimuli were scaled with eccentricity to keep the ratio of pixel size to acuity constant (about 2). It is shown that the S values in our paradigm are never determined by contrast-threshold effects. They are true correlational thresholds. Bilocal movement detectors are assumed to underlie the detection of coherent motion. The bilocal correlation proves to be rather insensitive to rms contrast down to contrast levels of about 10%. Despite the eccentricity scaling, which is quite effective at high contrast levels, differences between the eccentricities become noticeable at lower contrast levels (below about 30-20%). The fovea is the least, and the far periphery the most, resistent to contrast degradation.