Visual Search for Flicker is Dependent on Stimulus Discriminability

Robert F. Dougherty, Mark R. Verardo, and Melanie J. Mayer
University of California, Santa Cruz, CA

Purpose. We examined visual search for flicker as a function of stimulus discriminability.

Methods. Visual search was performed using a two-interval, forced choice paradigm. The stimuli consisted of a total of 2, 4, 8 or 16 gaussian "blobs" arranged concentricly at 4° eccentricity and flickering at 2, 4, 8, 17 or 34 Hz. The target, which was always present in one of the two intervals, flickered at a different rate than the distractor(s). All 20 pairs of target/distractor flicker rate combinations were tested. The contrast of each flicker rate was set at twice threshold. The temporal phase of the distractors was randomized and each stimulus presentation ended with a brief, flickering mask. The stimulus duration was adjusted with a staircase procedure and the data for each condition were fit with a Weibull function to determine the critical duration necessary for 80% correct.

Results. Overall, search times were consistent with independently measured supra-threshold flicker discriminability. Generally, longer critical durations were obtained for less discriminable target/distractor flicker pairs. For easily discriminated flicker pairs, search times were relatively independent of distractor density. For the less discriminable pairs, search times increased with distractor density.

Conclusions. Our results are consistent with a model of flicker discrimination based on at least 2 temporal channels. Such a model, coupled with the idea that more difficult discriminations require more processing time, can account for our observed relation between target/distractor flicker difference and search time. The increase in search times with the number of items for some target/distractor flicker pairs suggests that focal attention is employed to help detect the target when stimulus discriminability is low. More generally, we demonstrate the crucial role of stimulus discriminability in the visual search task.

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