Oculomotor Evidence for Top-Down Control following the Initial Saccade

The goal of the current study was to investigate how salience-driven and goal-driven processes unfold during visual search over multiple eye movements. Eye movements were recorded while observers searched for a target, which was located on (Experiment 1) or defined as (Experiment 2) a specific orientation singleton. This singleton could either be the most, medium, or least salient element in the display. Results were analyzed as a function of response time separately for initial and second eye movements. Irrespective of the search task, initial saccades elicited shortly after the onset of the search display were primarily salience-driven whereas initial saccades elicited after approximately 250 ms were completely unaffected by salience. Initial saccades were increasingly guided in line with task requirements with increasing response times. Second saccades were completely unaffected by salience and were consistently goal-driven, irrespective of response time. These results suggest that stimulus-salience affects the visual system only briefly after a visual image enters the brain and has no effect thereafter

Stimulus overview of Experiment 1.

<p>The figure depicts a simplified overview of the different conditions in Experiment 1. In the single condition only one singleton is presented whereas in the dual condition two singletons are presented simultaneously. The singletons have an identical orientation contrast of 22.5° relative to the background lines across both conditions.</p

A typical trial sequence.

<p>Participants maintained fixation on a centrally presented cross until a stable fixation was detected. After pressing spacebar, a drift correction was performed and subsequently a trial was initiated. The search display was presented for 2000</p

Tree diagrams in Experiment 2.

<p>The tree diagrams correspond to a) the single, b) the dual same, and c) the dual different condition and depict the probabilities that the singleton is available for selection (italics) for each bin (in brackets), leading to a given response outcome (R) for each individual trial. The response outcome is determined through the path leading to it, which equals the product of probabilities on that path. Note that, for reasons of simplicity, the diagrams in a) and b) correspond to the salient singleton only. denotes the probability that the singleton is available in the single most condition, denotes the probability that the singleton is available in the dual most condition, and and denote the probability that the singleton is available in the dual different condition, for the more and less salient singleton, respectively.</p

Parameter values in Experiment 2.

<p>The parameter values represent the probability that a singleton is available for selection separately per condition for each Bin <i>i</i>, averaged over participants. The values in a) correspond to the parameter estimates obtained for the salient singleton for each of the three conditions, whereas b) depicts the parameter estimates for the non-salient singleton for each of the three conditions. Error bars represent 95% confidence intervals. Note that based on the assumption that the availability of both singletons within each dual same condition is identical, Bin 5 in these conditions adds up to 1 by definition.</p

Large Pupils Predict Goal-Driven Eye Movements

International audienceHere we report that large pupils predict fixations of the eye on low-salient, inconspicuous parts of a visual scene. We interpret this as showing that mental effort, reflected by a dilation of the pupil, is required to guide gaze toward objects that are relevant to current goals, but that may not be very salient. When mental effort is low, reflected by a constriction of the pupil, the eyes tend to be captured by high-salient parts of the image, irrespective of top-down goals. The relationship between pupil size and visual saliency was not driven by luminance or a range of other factors that we considered. Crucially, the relationship was strongest when mental effort was invested exclusively in eye-movement control (i.e., reduced in a dual-task setting), which suggests that it is not due to general effort or arousal. Our finding illustrates that goal-driven control during scene viewing requires mental effort, and that pupil size can be used as an online measure to track the goal-drivenness of behavior

Salience-driven selection in initial saccades in Experiment 2.

<p>Proportions of initial saccades directed towards each of the three singletons (22.5°, 45°and 67.5°), separately for each bin of the response time distribution and irrespective of target identity.</p

A typical trial sequence in Experiment 1.

<p>Prior to each trial, participants maintained fixated on a centrally presented disk until a stable fixation was detected. Upon depression of the spacebar, a drift correction was applied and a trial was initiated with the presentation of a centrally presented fixation cross. Following 500 ms, the fixation cross was replaced by the search display, which was presented for 3000 ms. Following each block, feedback regarding the speed of participants' initial saccade was provided and participants were given the opportunity to take a short break.</p