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Gordon Binsted, Kyle Brownell, Matthew Heath; It's all a matter of mass: Both the eye and hand know it. Journal of Vision 2008;8(6):58. doi: 10.1167/8.6.58.
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Recent findings suggest that oculomotor action can - partially - modulate perceptual bias in manual responses (e.g., Binsted et al 1999a: Exp Brain Res); others however posit the manual and saccadic systems operate independently (e.g., Thompson & Westwood 2007: Neurosci Letters). Common to these and other eye-hand derived accounts of perception-action dichotomies in vision is the reliance on Müller-Lyer (ML) figures: a stimulus that coincidentally generates an asymmetric center of mass (CoM). However, CoM has been shown to attract attention within a visual scene (Zhou et al 2006: Neuroreport), such attentional bias can both generate and reverse illusory effects (Coren & Porac 1983: Perception). The purpose of this study was to examine the degree to which CoM, can bias ocular and manual movements, independently and in the absence of illusory percept. Participants (n=27) were asked to point, saccade, or make a ‘normal eye-hand response’ to a visual target. Targets consisted of a central lobe with a directional tail roughly corresponding to the letters p, q, d, b; center of mass was moved to each quadrant of the target. Participants were asked to point as ‘quickly and accurately as possible’ to the center of the central ‘lobe’ of the target. Vision of the target and hand was available throughout all pointing trials. In all conditions (i.e. saccade; point; eye-hand coordinated), responses were consistently biased throughout the response trajectory consistent with the CoM shift. Thus, despite the mixed observations of ML effects on manual and ocular movements (Bruno et al, 2007; Neur Biobeh Rev; Binsted & Elliott 1999b; Hum Mov Sci), ML findings may simply reflect presence/absence of biases due to CoM and not bare any direct contribution to extant debates regarding the discrete nature of perceptual and motor visual processes.
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