Visual Directional Anisotropy Does Not Mirror the Directional Anisotropy Apparent in Postural Sway

Abstract

Abstract.Presenting a large optic flow pattern to observers is likely to cause postural sway. However, directional anisotropies have been reported, in that contracting optic flow induces more postural sway than expanding optic flow. Recently, we showed that the biomechanics of the lower leg cannot account for this anisotropy (Holten, Donker, Verstraten, & van der Smagt, 2013, Experimental Brain Research, 228, 117-129). The question we address in the current study is whether differences in visual processing of optic flow directions, in particular the perceptual strength of these directions, mirrors the anisotropy apparent in postural sway. That is, can contracting optic flow be considered to be a perceptually stronger visual stimulus than expanding optic flow? In the current study we use a breaking continuous flash suppression paradigm where we assume that perceptually stronger visual stimuli will break the flash suppression earlier, making the suppressed optic flow stimulus visible sooner. Surprisingly, our results show the opposite, in that expanding optic flow is detected earlier than contracting optic flow.

Does direction of walking impact binocular rivalry between competing patterns of optic flow?

“…3a), and they are accompanied by intervening periods of mixtures. We would note, too, that other studies of motion-induced rivalry have successfully used RDCs of the sort we employed (Blake et al, 1985(Blake et al, , 1998Chen et al, 2001;Parker & Alais, 2007;Malek et al, 2012;Holten et al, 2015). We are thus satisfied that the stimuli we employed are sufficient to generate robust binocular rivalry.…”

“…Ours is not the first study to employ complex optic flow patterns as rival stimuli, but it is the first to do so in the context of self-motion. In several earlier studies using rival targets simulating complex motion patterns, it was found that visual motion simulating expansion enjoyed a bias during rivalry over dots simulating contraction, although there was some disagreement among these studies about the specific dependence of that bias on the form of the motion stimuli (Parker & Alais, 2007;Malek et al, 2012;Conrad et al, 2013;Holten et al, 2015). Our aim, however, was to learn whether the predominance of expansion and contraction was dependent on one's walking direction.…”

Forward optic flow is prioritised in visual awareness independently of walking direction

“…Such effects have been found in previous binocular rivalry studies which did not involve bodily actions, but which did employ similarly structured (looming/expanding), though small-sized, visual stimuli. In all but one study [83] , expanding random-dot patterns appeared to have greater access to visual awareness compared to analogous receding stimuli [84,85,86] . This perceptual preference for expansion was even more pronounced when more naturalistic stimuli (such as concentric gratings or textures) were used [83,85] .…”

“…The literature also suggests an alternative explanation of the effects of perceptual preference for expansion [84,86] , based on the fact that visual patterns related to self-locomotion are asymmetrically represented in the brain. There is strong neurophysiological evidence that the cells which respond to expansion outnumber the cells attuned to contraction in higher level visual motion areas, such as the ventral intraparietal (VIP) area [90,91] and the dorsal subdivision of the medial superior temporal (MSTd) area [92,93,94] .…”

Forward optic flow is prioritised in visual awareness independently of walking direction

“…Such effects have been found in previous binocular rivalry studies which did not involve bodily actions, but which did employ similarly structured (looming/expanding), though small-sized, visual stimuli. In all but one study [ 92 ], expanding random-dot patterns appeared to have greater access to visual awareness compared to analogous receding stimuli [ 93 – 95 ]. This perceptual preference for expansion was more pronounced when more naturalistic stimuli (such as concentric gratings or textures) were used [ 92 , 94 ].…”