The role of eye movements in a contour detection task

Abstract

Vision combines local feature integration with active viewing processes, such as eye movements, to perceive complex visual scenes. However, it is still unclear how these processes interact and support each other. Here, we investigated how the dynamics of saccadic eye movements interact with contour integration, focusing on situations in which contours are difficult to find or even absent. We recorded observers' eye movements while they searched for a contour embedded in a background of randomly oriented elements. Task difficulty was manipulated by varying the contour's path angle. An association field model of contour integration was employed to predict potential saccade targets by identifying stimulus locations with high contour salience. We found that the number and duration of fixations increased with the increasing path angle of the contour. In addition, fixation duration increased over the course of a trial, and the time course of saccade amplitude depended on the percept of observers. Model fitting revealed that saccades fully compensate for the reduced saliency of peripheral contour targets. Importantly, our model predicted fixation locations to a considerable degree, indicating that observers fixated collinear elements. These results show that contour integration actively guides eye movements and determines their spatial and temporal parameters.

Presaccadic EEG activity predicts visual saliency in free‐viewing contour integration

“…These modes have been associated with, respectively, dorsal stream (“where” pathway) and ventral stream processing (“what” pathway; Pannasch, Helmert, Roth, Herbold, & Walter, ; Unema, Pannasch, Joos, & Velichkovsky, ). These modes were also observed in our previous contour detection study (see Figures , and the corresponding discussion in Van Humbeeck et al[ ]). Likewise, the current contour‐present condition involved mostly the ambient mode; the contour‐absent condition included both ambient and focal modes but the portion of the focal mode was 2.5 times larger than the ambient one.…”

“…A possible reason is the higher heterogeneity of association strength in the contour‐present than the contour‐absent condition, as described above. Small saccades associated with regions with high association strength correspond to our previous observation that participants make progressively smaller saccades when their gaze approaches the contour (Van Humbeeck et al, ). Probably such a specific pattern of eye movement in the contour‐present condition introduces a dependency between saccade size and association strength.…”

“…In the contour integration task, the relevant locations on a display are those associated with high saliency or, according to our model, with high association strength. Previously, we found that association strength guides saccade planning (Van Humbeeck et al, 2013). We may thus expect that association strength will affect saccade-related EEG activity.…”

“…Previously, we introduced a model to predict how a salience map guides saccades in free viewing during a contour integration task (Van Humbeeck, Schmitt, Hermens, Wagemans, & Ernst, 2013). To identify a global contour, participants had to detect aligned orientations across several Gabor elements hidden among randomly oriented ones.…”

“…In these displays, a smooth target contour is either present or absent. Using the model of Van Humbeeck et al (2013), we compute saliency in terms of the association strength values at the fixation locations (i.e., the saccadic end points) in contour-present and contour-absent trials. We measure EEG activity during the presaccadic interval and determine the relationship with the association strength value.…”