Probing the temporal dynamics of the exploration-exploitation dilemma of eye movements.

When scanning a visual scene, we are in a constant decision process regarding whether to further exploit the information content at the current fixation or to go on and explore the scene. The balance of these two processes determines the distribution of fixation durations. Using a gaze-contingent paradigm, we experimentally interrupt this process to probe its state. Here, we developed a guided-viewing task where only a single 3° aperture of an image ("bubble") is displayed. Subjects had to fixate the bubble for an experimentally controlled time (forced fixation time). Then, the previously fixated bubble disappeared, and one to five bubbles emerged at different locations. The subjects freely selected one of these by performing a saccade toward it. By repeating this procedure, the subjects explored the image. We modeled the resulting saccadic reaction times (choice times) from bubble offset to saccade onset using a Bayesian linear mixed model. We observed an exponential decay between the forced fixation time and the choice time: Short fixation durations elicited longer choice times. In trials with multiple bubbles, the choice time increased monotonically with the number of possible future targets. Additionally, we found only weak influences of the saccade amplitude, low-level stimulus properties, and saccade angle on the choice times. The exponential decay of the choice times suggests that the sampling and processing of the current stimulus were exhausted for long fixation durations, biasing toward faster exploration. This observation also shows that the decision process took into account processing demands at the current fixation location.