Effects of retinal image degradation on pre-attentive visual search (PAVS) efficiency for flicker, movement and orientation stimuli.

BACKGROUND Previous research has shown that several clinical conditions cause increased pre-attentive visual search (PAVS) times, implying reduced parallel search capabilities in glaucoma and DLB dementia. The purpose of the research reported here was to examine for the first time the resistance of PAVS to dioptric blur using targets differing from the background in terms of flicker, vertical displacement, and orientation. Resistance would enhance the applicability of PAVS as a screening method for glaucoma and other clinical conditions affecting performance of a substantial area of the retina. METHOD Computer generated flicker, orientation and vertical displacement targets were used to assess PAVS efficiency. The subject's task was to locate a small single target subtending 0.92 degrees as quickly as possible (embedded in a field of 119 distractors) on either the left- or right-hand side of a computer monitor. Average PAVS response times were calculated for 40 presentations of each target type presented randomly in any one of 120 positions within +/-15 degrees of fixation. Subjects performed the test using their distance spectacles (unless emmetropic), then three tests using positive lenses simulating myopia of up to -3 D, and finally using optimum correction again. RESULTS ANOVA revealed that blur of up to 3 D had no statistically significant effect on response times to the flicker target. Blur of over 2.0 D however resulted in increased response times for the displacement target, but only for eyes which were not cyclopleged. The orientation target became significantly more difficult to locate, response times becoming progressively slower with increasing levels of blur (p < 0.05%). CONCLUSIONS The present flicker and displacement targets are relatively resistant to the effects of reduced acuity, while the orientation target is only suitable for testing subjects with good visual acuity.