Visual Search: Physiological Experiments
The saliency of visual objects is based on the center to background contrast. Particularly objects differing in one feature from the background may be perceived as more salient. It is not clear to what extent this so called ‘‘pop-out’’ effect observed in humans and primates governs saliency perception in non-primates as well. In this study we searched for neural correlates of pop-out perception in neurons located in the optic tectum of the barn owl. We measured the responses of tectal neurons to stimuli appearing within the visual receptive field, embedded in a large array of additional stimuli (the background). Responses were compared between contrasting and uniform conditions. In a contrasting condition the center was different from the background while in the uniform condition it was identical to the background. Most tectal neurons responded better to stimuli in the contrasting condition compared to the uniform condition when the contrast between center and background was the direction of motion but not when it was the orientation of a bar. Tectal neurons also preferred contrasting over uniform stimuli when the center was looming and the background receding but not when the center was receding and the background looming. Therefore, our results do not support the hypothesis that tectal neurons are sensitive to pop-out per-se.
However, if the position of the target bar in the array was randomly shuffled across trials so that it occasionally appeared in the RF, as in freely viewing conditions where the owls actively scan their surroundings, then orientation contrast sensitivity emerged. The effect started to become significant after 3-4 positional changes of the target bar and strengthened with additional trials. We further showed that this effect arises due to stimulus-specific-adaptation (SSA) of tectal neurons to the orientation of the bar inside their RF. These findings suggest that barn owls, by active scanning of the scene, can induce adaptation of the tectal circuitry to the common orientation and by thus achieve a “pop-out” of rare orientations.