Abstract
During natural vision the human visual system has to process upcoming eye movements in parallel to currently fixated stimuli. Saccades targeting isolated faces are known to have lower latency and higher velocity, but it is unclear how this generalizes to the natural cycle of saccades and fixations during free-viewing of complex scenes. To which degree can the visual system process high-level features of extrafoveal stimuli when they are embedded in visual clutter and compete with concurrent foveal input? Here, we extracted about 50.000 relevant saccadic events from a large free-viewing dataset, containing close to 0.8 million saccades directed to 700 complex scenes by 101 observers. This rich dataset allowed us to employ stringent selection criteria when comparing the gaze dynamics of object- and face-directed saccades and to build a well-powered statistical model controlling for a range of low-level factors. We found strong evidence that face- vs. object-directed saccades are preceded by shorter fixations and have higher peak velocity. The effect on fixation duration was limited to small saccades following the trajectory of the preceding one, early in a trial. This matches a recently proposed model of saccade-related retinotopic shifts of attention. The effect on saccadic velocity, however, extended to very large saccades and increased with trial duration. These findings strongly suggest that multiple, independent mechanisms interact to process high-level features of extrafoveal targets and modulate the dynamics of natural vision.