VERNIER IN MOTION - WHAT ACCOUNTS FOR THE THRESHOLD ELEVATION

Vernier acuity is susceptible to degradation by image motion, The purp ose of this study was to determine to what extent vernier thresholds a re elevated in the presence of image motion because of reduced stimulu s visibility, due to contrast smearing, or to a shift in the spatial s cale of analysis. To test the visibility hypothesis, we measured verni er thresholds as a function of stimulus velocity (0-6 deg/sec), for va rious levels of stimulus visibility, each normalized to the detection threshold at the respective velocity, Contrary to the prediction of th e visibility hypothesis, vernier thresholds worsen as the velocity inc reases, even when the stimuli are equally visible. To test the shift i n spatial scale hypothesis, we determined spatial frequency tuning fun ctions for vernier discrimination and line detection tasks, using a ma sking paradigm. We measured vernier and line detection thresholds as a function of spatial frequency of a sine-wave mask (0.5-32 c/deg), and for stimulus and mask velocities ranging from 0 to 4 deg/sec. Peak ma sking for both vernier discrimination and line detection, which indica tes the most sensitive band of spatial frequencies for each task, shif ts systematically toward lower spatial frequencies as the velocity inc reases. The progressive increase in spatial scale largely accounts for the worsening of vernier thresholds for moving stimuli. Differences b etween peak masking for vernier discrimination and line detection were found at 0 and 1 deg/sec, suggesting that different mechanisms mediat e the two tasks, at least at low velocities. The masking results are c onsistent with previous findings that directionally selective motion d etectors mediate detection of moving stimuli, but suggest that these d etectors do not analyze vernier offsets, We conclude that the elevatio n of vernier threshold for a moving stimulus is accounted for primaril y by a shift of sensitivity to mechanisms of lower spatial frequency, and not by decreased stimulus visibility. Copyright (C) 1996 Elsevier Science Ltd.