Abstract

Short-latency saccades are often biased toward salient objects or toward the center of images, for example, when inspecting photographs of natural scenes. Here, we measured the contribution of salient objects and central fixation bias to visual selection over time. Participants made saccades to images containing one salient object on a structured background and were instructed to either look at (i) the image center, (ii) the salient object, or (iii) at a cued position halfway in between the two. Results revealed, first, an early involuntary bias toward the image center irrespective of strategic behavior or the location of objects in the image. Second, the salient object bias was stronger than the center bias and prevailed over the latter when they directly competed for visual selection. In a second experiment, we tested whether the center bias depends on how well the image can be segregated from the monitor background. We asked participants to explore images that either did or did not contain a salient object while we manipulated the contrast between image background and monitor background to make the image borders more or less visible. The initial orienting toward the image was not affected by the image-monitor contrast, but only by the presence of objects—with a strong bias toward the center of images containing no object. Yet, a low image-monitor contrast reduced this center bias during the subsequent image exploration.

Highlights

  • Human vision is characterized by the foveated nature of our visual system

  • Center-of-gravity responses caused by a distractor are said to arise from averaging across multiple possible saccade vectors and are strongest for saccades that are initiated in a time window approximately 100–300 ms after distractor onset

  • In Experiment 1 a salient object and the center of the image competed for visual selection

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Summary

Introduction

Human vision is characterized by the foveated nature of our visual system. Whereas the central part of the visual field, the fovea, allows humans to process objects with high scrutiny, acuity in the periphery declines rapidly with increasing eccentricity (for reviews, see Strasburger et al, 2011; Stewart et al, 2020). Rothkegel et al (2017) showed that the initial orienting toward the image center depends on time and can be reduced by delaying the initial saccade toward the image (Rothkegel et al, 2017; Peacock et al, 2020) This temporal dependency suggests that an explanation for the central fixation bias purely in terms of a strategical advantage is unlikely (Rothkegel et al, 2017), especially since voluntary/strategic control of eye movements is pronounced for long-latency saccades. In all conditions this could be achieved by making purely horizontal saccades and this information was explicitly provided to participants This facilitated making saccades to the instructed location and allowed us to attribute any systematic vertical bias in the time course of endpoints to (i) the salient objects or (ii) the image center. A systematic deviation in vertical saccade endpoints toward the image center in the look at object condition would reveal an involuntary scene-dependent center bias that goes beyond the location of salient objects in the image

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