Lauritzen J S, Little J-A, O'Gara E, Saunders K J, 2005, "Measuring Vernier acuity with the use of a contrast-masking protocol" Perception 34 ECVP Abstract Supplement
Measuring Vernier acuity with the use of a contrast-masking protocol
J S Lauritzen, J-A Little, E O'Gara, K J Saunders
Contrast masking of a sinusoid target by a grating is dependent on the phase of the stimulus relative to the mask, with the classical view being that a greater phase offset will produce more masking, although performance is dependent on subjects' detection strategy (eg Foley and Chen, 1999 Vision Research 39 3855 - 3872). We found that, with appropriately chosen stimulus parameters, a target presented between 45° and 90° out of phase with a grating of the same spatial frequency can produce lower thresholds than when the target is presented at 0°. This effect appears to be due to the presence of Vernier cues in the stimulus, displaying 'sub-pixel' Vernier offsets. Modified contrast-masking protocols were used to display sub-pixel offsets with a variety of stimulus configurations. The target was moved out of phase by different offsets relative to the mask. Contrast thresholds were obtained with a QUEST adaptive staircase procedure, by using the psychophysics toolbox for MATLAB (Brainard, 1997 Spatial Vision 10 433 - 436). Perceived Vernier offset was calculated as follows:
(Mathematical expressions may not appear as intended) Offset = |x – π/2| radians ,
where x = arctan[(0.5+kcosφ)/ksinφ], k is contrast threshold, and φ is phase offset. We obtained an optimal stimulus configuration when the mask was a Gabor patch and the target a small Gaussian-edged square grating, spatial frequency 1.7 cycles deg-1, presented 90° out of phase (see also Little et al, 2005, paper presented at ARVO 2005, abstract 5647/B850). We show that thresholds obtained in this task cannot be explained in terms of contrast-masking mechanisms, that results correlate well with traditional measures of Vernier acuity, and that the target is resistant to blur compared to contrast masking with 0° offset. This novel protocol can be used to present Vernier stimuli at short test distances, overcoming the resolution limit that CRT monitors impose on traditional Vernier tasks.
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