Mind the step: complementary effects of an implicit task on eye and head movements in real-life gaze allocation
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RESEARCH ARTICLE
Mind the step: complementary effects of an implicit task on eye and head movements in real-life gaze allocation Bernard Marius ’t Hart • Wolfgang Einha¨user
Received: 8 February 2012 / Accepted: 30 August 2012 / Published online: 22 September 2012 Ó Springer-Verlag 2012
Abstract Gaze in real-world scenarios is controlled by a huge variety of parameters, such as stimulus features, instructions or context, all of which have been studied systematically in laboratory studies. It is, however, unclear how these results transfer to real-world situations, when participants are largely unconstrained in their behavior. Here we measure eye and head orientation and gaze in two conditions, in which we ask participants to negotiate paths in a real-world outdoor environment. The implicit task set is varied by using paths of different irregularity: In one condition, the path consists of irregularly placed steps, and in the other condition, a cobbled road is used. With both paths located adjacently, the visual environment (i.e., context and features) for both conditions is virtually identical, as is the instruction. We show that terrain regularity causes differences in head orientation and gaze behavior, specifically in the vertical direction. Participants direct head and eyes lower when terrain irregularity increases. While head orientation is not affected otherwise, vertical spread of eye-in-head orientation also increases significantly for more irregular terrain. This is accompanied by altered patterns of eye movements, which compensate for the lower average gaze to still inspect the visual environment. Our results quantify the importance of implicit task demands for gaze allocation in the real world, and imply qualitatively distinct contributions of eyes and head in gaze allocation. This underlines the care that needs to be taken when inferring real-world behavior from constrained laboratory data.
B. M. ’t Hart (&) W. Einha¨user Neurophysics, Philipps-University Marburg, Karl-von-Frisch-Str. 8a (Altes MPI), 35032 Marburg, Germany e-mail: [email protected]
Keywords Gaze Eye movements Head orientation Natural environment Terrain negotiation Visuomotor routines
Introduction During natural behavior, humans shift their gaze to sample the visual environment in detail to gather information needed for skilled behavior. Research on natural gaze shifts has largely focused on eye movements (Buswell 1935; Yarbus 1967), although humans typically shift gaze by coordinated movements of head and eyes. Indeed, even laboratory experiments that match stimuli to real-world input have only limited predictive power about real-world gaze allocation (’t Hart et al. 2009; Foulsham et al. 2011). To what extent do real-world behavioral constraints implicitly influence eye-in-head and head-in-world movements when selecting relevant information? Bottom-up models of gaze allocation assign a probability to attract attention (‘‘saliency’’) to each stimulus region based on its visual features (Itti and Koch 2000), possibly incl
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