Mirror-image symmetry, distractor heterogeneity, and search asymmetry all can affect visual search performance, but each may affect a different stage of processing via different mechanisms. We tested 15 participants in a series of visual discrimination and search tasks to decompose processing into component parts. In each task the target was either a vertical (T: ) or tilted (T:/) line. To examine mirror-image symmetry between target and distractor, we compared a condition where the target was tilted clockwise and the distractors tilted counterclockwise (T:/ & D: ) to one where the distractors were vertical lines (T:/ & D: ). Target discrimination thresholds were much larger for the mirror-image condition, but once target salience was balanced across search conditions there was no difference in either the ability to divide attention (as measured by an attention-sharing index) or the magnitude of the set-size effects for the two conditions. To examine distractor heterogeneity effects, the target was a vertical line (T: ) and the tilted line distractors were either homogeneous (D: /) or heterogeneous (D: \ /) so that the distractors were mirror-images of each other. For the heterogeneous condition, the ability to divide attention plummeted and set-size effects were much larger. Finally, to examine effects of search asymmetry, we compared two conditions in which the roles of target and distractor were switched (T: & D:/ versus T:/ & D: ). Search asymmetry affected a higher level of processing where the " deviant" stimuli (tilted lines) attract attention away from the prototypical stimuli (vertical lines). That is, when the target was correctly detected ("hits"), there was less confusion about the target's spatial location for the "deviant" target (T: /) than for the prototypical target (T: ). In the near future we will compare results of ongoing correlated fMRI studies to our psychophysical results.
All Science Journal Classification (ASJC) codes
- Sensory Systems