Purpose. Linear image features were determined for a parafoveal two-line vernier task using the response classification technique described in Beard & Ahumada (SPIE, 1998).
Methods. Observers fixated 5 degrees to the right of two short, horizontally oriented vernier line features. Line contrast was set to maintain an error rate near 25%. Stimulus duration was 0.5 sec with abrupt onset and offset. The vernier stimulus was added to, and centrally located within, a 40 by 40-min (128 by 128 pixels) noise display area. Alignment thresholds were obtained using a two interval forced choice (2IFC) method of constant stimuli with one vernier offset value. On a given trial, the right vernier line was randomly either aligned with the left line or upwardly offset by 0.31 arc min. The observers were instructed to report if the two lines were aligned or vertically offset. After each trial, a tone gave the observer feedback. Trials were run in blocks of 100. There were four observers. On each trial the observers response, the stimulus type and the particular noise sample were tabulated. In two observers, the proportion of internal noise was determined. The noises were then averaged separately for each of the either four or six stimulus-response trials. These averaged noises were combined with appropriate signs to form a classification image illustrating the contribution of each noise image pixel to the decision.
Results. In fovea, although there are individual differences, all observers show discrete bipolar distributions as seen in our earlier research. Peripheral classification images showed one or more of the following characteristics: (1) a broader spatial spread in the vertical direction (2) an extension of relevant noise pixels toward the point of fixation (3) relevant noise pixels for the left reference line were clustered near the offset region.
Conclusions. The classification images for peripheral vernier acuity have greater spatial spread than do foveal classification images.
CR: none. Support: NASA RTOP 548-50-12.