Can the motor system resolve a premovement bias in grip aperture? Online analysis of grasping the M�ller-Lyer illusion

Abstract

The goal of the present investigation was to determine the time-course by which the motor system might resolve the context-dependent effects of a visual illusion [i.e., the Müller-Lyer (ML) figure]. Specifically, we asked participants to scale their grip aperture (GA) to the perceived size of an object embedded within a ML figure in advance of closed-loop (CL) and open-loop (OL) grasping movements. As a result, premovement GA was biased in a direction consistent with the perceptual effects of the illusion. We reasoned that such a manipulation might provide a novel opportunity to determine whether the motor system is able to resolve a biased GA immediately following the onset of a response [i.e., in accord with the perception/action model (PAM); Milner and Goodale 1995, The visual brain in action, Oxford University Press], or gradually as the action unfolds [i.e., in accord with the planning/control model (PCM); Glover and Dixon 2002, Percept Psychophys 64:266-278]. It was found that biasing GA in advance of movement resulted in a reliable effect of the ML figure throughout CL and OL trials (i.e., up to 80% of grasping time). Although the present findings appear contrary to the theoretical tenets of the PAM and the PCM, it is proposed that biasing GA in advance of movement leads to offline visual processing and a feedforward mode of grasping control, thus accounting for the illusion-induced effect throughout the grasping response.

Grasping visual illusions: Consistent data and no dissociation

“…This is consistent with a number of other studies that also came to similar negative evaluation of Glover and Dixon's (2002) model, using a number of different paradigms (Handlovsky, Hansen, Lee, & Elliott, 2004;Heath, Rival, & Binsted, 2004;Heath, Rival, Neely, & Krigolson, 2006;Heath, Rival, Westwood, & Neely, 2005;Mendoza, Elliott, Meegan, Lyons, & Welsh, 2006;Mendoza, Hansen, Glazebrook, Keetch, & Elliott, 2005). Note that Glover and Dixon's model is also at odds with the perception-versusaction model of Milner and Goodale (Goodale & Milner, 2004), such that, logically, at least one of the models must be wrong.…”

Target modality affects visually guided online control of reaching

“…And third, it limits the effects of MT variability on our results, for, unlike Khan et al (2003), we did not constrain MT in our task. Our choice of ten time points was somewhat arbitrary, but was guided by the belief that ten points would provide a relatively continuous picture of the movement and by the fact that normalization by tenths of movement time has previously been used in kinematic analysis (Heath, Rival, & Binsted, 2004). After the normalization of the movements we obtained the standard deviation of the stylus position in both the amplitude (x) and direction (y) dimensions of the reach at each time point.…”

Distinct Visual Cues Mediate Aperture Shaping for Grasping and Pantomime-Grasping Tasks

“…Importantly, however, PGA is a late occurring metric (i.e., ∼75% of grasping time; see Jeannerod, 1984) and it is therefore unclear whether aperture trajectories for pantomime-grasping are entirely reliant upon relative visual information (i.e., unitary visual processing) or engage dissociable visual information (i.e., relative and absolute) at different stages in the unfolding response. As well, between-condition differences in PGA size do not provide a direct measure of the visual information supporting aperture shaping; rather, differences may relate to task-based changes in attentional demands or sensory-based feedback processing (Heath, Rival, & Binsted, 2004;Hesse & Deubel, 2011;Jakobson & Goodale, 1991;Melmoth & Grant, 2012; see also Jeannerod, 1988).…”