The role of object affordances and center of gravity in eye movements toward isolated daily-life objects

International audienceThe purpose of the current study was to investigate to what extent low-level versus high-level effects determine where the eyes land on isolated daily-life objects. We operationalized low-level effects as eye movements toward an object's center of gravity (CoG) or the absolute object center (OC) and high-level effects as visuomotor priming by object affordances. In two experiments, we asked participants to make saccades toward peripherally presented photographs of graspable objects (e.g., a hammer) and to either categorize them (Experiment 1) or to discriminate them from visually matched nonobjects (Experiment 2). Objects were rotated such that their graspable part (e.g., the hammer's handle) pointed toward either the left or the right whereas their action-performing part (e.g., the hammer's head) pointed toward the other side. We found that early-triggered saccades were neither biased toward the object's graspable part nor toward its action-performing part. Instead, participants' eyes landed near the CoG/OC. Only longer-latency initial saccades and refixations were subject to high-level influences, being significantly biased toward the object's action-performing part. Our comparison with eye movements toward visually matched nonobjects revealed that the latter was not merely the consequence of a low-level effect of shape, texture, asymmetry, or saliency. Instead, we interpret it as a higher-level, object-based affordance effect that requires time, and to some extent also foveation, in order to build up and to overcome default saccadic-programming mechanisms

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La fovéa est la seule zone de la rétine où l’acuité visuelle est maximale. Les mécanismes qui visent à déterminer à quels endroits vont avoir lieu les prochaines fixations restent débattus. Certains auteurs suggèrent que les yeux sont principalement attirés par les zones les plus saillantes de la scène. D’autres suggèrent qu’ils sont orientés vers les objets. Les propriétés du système oculomoteur contraignent également les mouvements oculaires. L’objectif de cette thèse était de départager ces différents points de vue en examinant la contribution respective et le décours temporel des facteurs visuo-moteurs de bas niveau et des facteurs cognitifs de plus haut niveau dans la détermination des mouvements oculaires. Nous nous sommes focalisés sur trois phénomènes comportementaux clés : la tendance à bouger les yeux à proximité du centre des objets (Prefered-Viewing Location), les répercussions qu’ont ces positions d’atterrissage sur les durées initiales des fixations (Inverted Optimal-Viewing-Position) et les probabilités de refixation (Optimal-Viewing-Position). Nous avons observé que ces trois effets émergeaient, et ce, quel que soit le type de stimulus testé: des objets, des mots ou des objets sans signification. Cela suggère que ces effets reflètent les propriétés de bas niveau des systèmes visuel et oculomoteur. Par ailleurs, nous avons observé que l’endroit où les yeux atterrissaient au sein de l’objet était également influencé au fil du temps par les traitements de plus haut-niveau des propriétés du stimulus. Les saccades initiées plus tardivement atterrissaient plus proches de la zone la plus informative de l’objet.High-quality vision is restricted to the fovea - a small region at the center of gaze. The mechanisms that determine which locations in a scene are selected for fixation remain debated. Some suggest that eye movements are mainly driven by the salient features in a scene. Others suggest that eye guidance is object based. The properties of the oculomotor system also strongly constrain eye behavior, but these have been neglected in most existing models. The purpose of this thesis was to disentangle between these different views, by investigating how low-level visuomotor factors versus higher-level cognitive factors contribute to eye movements towards and within isolated objects, and with which time course. We focused on three viewing-position effects: the tendency to move the eyes near the centers of objects (the PVL effect), and the repercussions these initial landing positions have on initial fixation durations (the I-OVP effect) and refixation probabilities (the OVP effect). We found that these three viewing-position effects emerged, and were comparable, in all stimulus types that we tested in this thesis: objects, words and even meaningless non-objects. This suggests that the effects reflect low-level properties of the visual and oculomotor systems. Furthermore, we found that where the eye moved within objects became influenced by ongoing processing of higher-level stimulus properties (e.g., object affordances) over time. Later- compared to early-triggered initial saccades, and even more so within-object refixations, were biased towards the most informative part of the objects, and away from their center of gravity

On the optimal viewing position for object processing

International audienceNumerous studies have shown that a visually presented word is processed most easily when participants initially fixate just to the left of the word's center. Fixating on this optimal viewing position (OVP) results in shorter response times and a lower probability of making additional within-word refixations (OVP effects), but also longer initial-fixation durations (an inverted-OVP or I-OVP effect), as compared to initially fixating at the beginning or the end of the word. Thus, typical curves are u-shaped (or inverted-u-shaped), with a leftward bias. Most researchers explain the u-shape in terms of visual constraints, and the leftward bias in terms of language constraints. Previous studies have demonstrated that (I)-OVP effects are not specific to words, but generalize to object viewing. We further investigated this by comparing the strength and (a)symmetry of (I-)OVP effects for words and objects. To this purpose, we gave participants an object- versus word-naming task in which we manipulated the position at which they initially fixated the stimulus (i.e., a line drawing or the written name of an object). Our results showed that object viewing, just as word viewing, resulted in u-shaped (I-)OVP curves. However, the effect was weaker than for words. Furthermore, for words, the curves were biased to the left, whereas they were symmetrical for objects. This might indicate that part of the (I-)OVP effect for words is language specific, and that (I-)OVP effects for objects are a purer measure of the effect of visual constraints

Viewing-position effects in meaningless object viewing

<b>van der Linden, L. Zelinsky, G. J. & Vitu, F. (2015). <i>Viewing-position effects in meaningless object viewing.</i><br><br>Poster presented at VSS 2016, St. Pete Beach, Florida.<br>And at the 2015 NVP winter conference, Egmond aan Zee, The Netherlands.</b><br

Workshop OpenSesame Part 1: Amorçage catégorique

<p><strong>van der Linden, L.</strong> (November 2014). <em>Une introduction au logiciel OpenSesame</em>. Workshop OpenSesame, Master 1 Psychology, part 1 (French).</p

The role of object affordances and center of gravity in eye movements towards isolated daily-life objects

<p><strong>van der Linden, L.</strong>, Mathôt, S. & Vitu, F. (2015). <em>Low-level versus high-level effects on saccadic landing positions on daily-life objects.</em> Talk presented at the 18th European Conference on Eye Movements (ECEM), Vienna, Austria.</p

The influence of low-level visuomotor factors versus high-level cognitive factors on object viewing

<b>Van der Linden, L.</b>, <i>The influence of low-level visuomotor factors versus high-level cognitive factors on object viewing </i>(2018).<div><br></div><div>Thesis defended at the Aix-Marseille Université on February 20th 2018.</div

Thesis defense Lotje van der Linden

Slides PhD defense Lotje van der Linden <div>Aix-Marseille Université</div><div>February 20th 2018</div