Searching for symmetry: Eye movements during a difficult symmetry detection task

Abstract

Bilateral symmetry does not pop-out of a noisy background as an indicator of object location and orientation should. We recorded eye movements to examine observers' behavior when searching for symmetry. If symmetry is detected automatically and in parallel across a display, then eye movements early in the search should be directed towards the target region. In Experiment 1, symmetric regions appeared at one of seven locations along the horizontal meridian. In Experiment 2, the symmetric regions were presented at locations throughout the display. In both experiments, 100 by 100 pixel symmetric dot patterns embedded in 800 by 600 pixel random backgrounds were presented while eye movements were monitored using an ASL Series 504 remote eye tracker. Symmetry was detected within 5 fixations on 20% of the trials, and after more than 15 fixations on over half of the trials. Response times and first target fixation latencies were longer when symmetry was farther from the starting fixation point in both Experiments. Targets were fixated an average of 5 s before a response in Experiment 1, which increased to 10 s in Experiment 2. No speed advantage was observed for detecting symmetry when present along the horizontal or vertical meridian compared to other locations. On some trials it was clear that fixations gradually approached the symmetric target, but this was inconsistent, and the results were not suggestive of preconscious symmetry detection. Accidental symmetries in the random background may have produced the slow search for the target symmetric regions.

Footnotes

 Supported in part by BAE Systems grant to JBP and AMH