Abstract
Abstract: :
Purpose: It is accepted that vernier acuity reflects cortical processing (Levi et al 1985). This has been measured as low as 5.1 arcsec (Li et al 2000). Such low thresholds combined with the resolution limitations of commercially available CRTs have necessitated large test distances. We designed a protocol to measure vernier acuity using a CRT at short test distances. Methods: A modified contrast masking protocol was used to display a sub–pixel offset. The target stimulus was a small sine wave grating superimposed on a large masking sine wave grating. Both mask and target had either a Gaussian or square envelope. The target was moved out of phase by different offsets relative to the mask. Contrast thresholds were obtained using a QUEST adaptive staircase procedure. Perceived vernier offset was calculated as follows: Offset = |x – π/2| radians where x = tan–1 {(0.5 + k.cos Ø)/k.sin Ø}, k is contrast threshold and Ø is phase offset. To identify stimuli that provide a valid measure of vernier acuity the following criteria were applied: i) comparable thresholds to traditional vernier acuity, ii) plausible measures yielded, iii) resistance to blur. A variety of parameters influencing suitability of the stimulus were identified: spatial frequency, type of mask, size, phase and type of target (Gabor, Gaussian, and square–edged targets). Preliminary investigation isolated four promising stimuli. The following experiments revealed the optimal protocol for measuring vernier acuity at short test distances. 1) 4 subjects were tested using all four stimuli, each presented 100 times at 7 different offsets. The pattern of thresholds obtained enabled selection of a single testing protocol, subsequently referred to as Gabor–Vernier. 2) The impact of dioptric blur on the Gabor–Vernier stimulus and a contrast masking task was evaluated. 3) Gabor–Vernier thresholds were compared to traditional vernier acuity measures. Results: 1) Gabor–Vernier stimulus parameters: the mask is a Gabor patch and the target a small Gaussian–edged square grating, spatial frequency 1.7 cpd. A target 67.5° out of phase relative to the mask yielded the most plausible and repeatable vernier thresholds. 2) The Gabor–Vernier task produced thresholds more resistant to dioptric blur than the contrast masking task. 3) Thresholds obtained with the traditional vernier task compared well to Gabor–Vernier thresholds. A paired t–test revealed no statistically significant difference between the two thresholds (p=0.20). Conclusions: These results indicate that the Gabor–Vernier test will prove to be a novel and reliable method of testing vernier acuity at short test distances.
Keywords: visual cortex • pattern vision • perception