Eye movements during gaze perception.

The gaze of other people is of interest to human observers, particularly in cases of direct gaze, that is, when it targets the observer. Gaze direction research has successfully clarified some of the mechanisms underlying gaze perception, but little is known about the active perception of direct gaze. Three eye-tracking experiments were conducted in which fixations and scan paths were recorded during the task to judge direct gaze. Somewhat surprisingly, judgments were issued after a single eye fixation only in a minority of trials. In most cases, observers fixated both eyes of a looker model, sometimes even scanning them repeatedly. Fixation duration showed a consistent pattern, where first fixations were longer when the task response followed immediately, and second fixations were shorter just before the response. A direct-gaze bias was tested but was not found: visiting the second eye was even more likely when the first fixation was on a straight-gazing rather than an averted eye. There was no systematic pattern in the final fixation, contradicting the expectation that it would fall on the abducting (leading) eye. It is argued that overt looking behavior during direct gaze judgments reflects a cumulative decision process that spans over consecutive fixations. Several factors may contribute to the high incidence of multiple-eye scans, including vergence and angle kappa. Vergence, in particular, is considered an important candidate, because the depth of fixation is ambiguous when only one eye is visible, but can be limited by probing the gaze direction of both eyes.