Evidence for two distinct mechanisms directing gaze in natural scenes

Various models have been proposed to explain the interplay between bottom-up and top-down mechanisms in driving saccades rapidly to one or a few isolated targets. We investigate this relationship using eye-tracking data from subjects viewing natural scenes to test attentional allocation to high-level objects within a mathematical decision-making framework. We show the existence of two distinct types of bottom-up saliency to objects within a visual scene, which disappear within a few fixations, and modification of this saliency by top-down influences. Our analysis reveals a subpopulation of early saccades, which are capable of accurately fixating salient targets after prior fixation within the same image. These data can be described quantitatively in terms of bottom-up saliency, including an explicit face channel, weighted by top-down influences, determining the mean rate of rise of a decision-making model to a threshold that triggers a saccade. These results are compatible with a rapid subcortical pathway generating accurate saccades to salient targets after analysis by cortical mechanisms.