Saliency changes appearance

Abstract

Numerous studies have suggested that the deployment of attention is linked to saliency. In contrast, very little is known about how salient objects are perceived. To probe the perception of salient elements, observers compared two horizontally aligned stimuli in an array of eight elements. One of them was salient because of its orientation or direction of motion. We observed that the perceived luminance contrast or color saturation of the salient element increased: the salient stimulus looked even more salient. We explored the possibility that changes in appearance were caused by attention. We chose an event-related potential indexing attentional selection, the N2pc, to answer this question. The absence of an N2pc to the salient object provides preliminary evidence against involuntary attentional capture by the salient element. We suggest that signals from a master saliency map flow back into individual feature maps. These signals boost the perceived feature contrast of salient objects, even on perceptual dimensions different from the one that initially defined saliency.

Figure 1. Experimental results and illustration of experimental trials (drawn to scale).

Panels A, B, C, and D show the psychometric functions for Experiment 1 (Weber contrast), 2 (Michelson contrast), 3 (color saturation), and 5 (orientation of bar), respectively. The vertical lines represent the points of subjective equality (PSE) where the probability of judging the test to be brighter/more saturated/more tilted than the standard was 50%. Error bars have been omitted for clarity. Note that the illustrations on the right of each panel do not represent the PSE, but are random samples from the two conditions. The arrows in panel B illustrate the direction of motion, but were not shown in the actual experiments.

Figure 2. Results from Experiment 4.

Grand average waveforms for electrodes PO7 and PO8 were averaged across positions ipsilateral and contralateral to the target. The panel on the left shows results from the control task in which observers judged the orientation of the tilted bar. The panel on the right shows results from the contrast judgment task. The N2pc component was clearly visible in the control task, but not in the contrast judgment task. The ERPs displayed in the figures were digitally low-pass filtered at 40 Hz.