VERNIER AND LETTER ACUITIES FOR LOW-PASS FILTERED MOVING STIMULI

Vernier and letter acuities are both susceptible to degradation by ima ge motion. In a previous study, we showed that the worsening of Vernie r acuity for stimuli moving up to 4 degrees/s is accounted for primari ly by a shift of visual sensitivity to mechanisms of lower spatial fre quency. The purposes of this study were to extend the previous results for Vernier acuity to higher stimulus contrast and velocities, and to determine if a shift in spatial scare can similarly explain the degra dation of letter acuity for moving stimuli. We measured Vernier discri mination for a pair of vertical abutting thin lines and letter resolut ion for a four-orientation letter 'T' as a function of stimulus veloci ty ranging from 0 to 12 degrees/s. Stimuli were presented at 20 times the detection threshold, determined for each velocity. To determine th e spatial-frequency mechanism that mediates each task at each velocity , we measured Vernier and letter acuities with low-pass filtered stimu li (cut-off spatial-frequency: 17.1-1.67 c/deg) and analyzed the data using an equivalent blur analysis. Our results show that the empirical ly determined, equivalent intrinsic blur associated with both tasks in creases as a function of stimulus velocity, suggesting corresponding i ncreases in the size of optimally responding mechanisms. This progress ive increase in mechanism size can account for the worsening of Vernie r and letter acuities with velocity. Vernier discrimination is found t o be more susceptible to degradation by various stimulus parameters th an letter resolution, suggesting that different mechanisms are involve d in the two tasks. We conclude that the elevations in Vernier and let ter acuities for moving stimuli are the consequence of a shift of visu al sensitivity toward mechanisms of lower spatial frequencies. (C) 199 8 Elsevier Science Ltd. All rights reserved.