Academic journal article Attention, Perception and Psychophysics

Global-Motion Aftereffect Does Not Depend on Awareness of the Adapting Motion Direction

Academic journal article Attention, Perception and Psychophysics

Global-Motion Aftereffect Does Not Depend on Awareness of the Adapting Motion Direction

Article excerpt

Published online: 16 January 2014

# Psychonomic Society, Inc. 2014

Abstract It has been shown that humans cannot perceive more than three directions from a multidirectional motion stimulus. However, it remains unknown whether adapting to such imperceptible motion directions could generate motion aftereffects (MAEs). A series of psychophysical experiments were conducted to address this issue. Using a display consisting of randomly oriented Gabors, we replicated previous findings that observers were unable to perceive the global directions embedded in a five-direction motion pattern. However, adapting to this multidirectional pattern induced both static and dynamic MAEs, despite the fact that observers were unaware of any global motion directions during adaptation. Furthermore, by comparing the strengths of the dynamic MAEs induced at different levels of motion processing, we found that spatial integration of local illusory signals per se was sufficient to produce a significant global MAE. These psychophysical results show that the generation of a directional global MAE does not require conscious perception of the global motion during adaptation.

Keywords Adaptation . Aftereffects . Motion integration . Visual awareness

A hallmark of human motion perception is the ability to inte- grate locally ambiguous motion signals over space and time to perceive globally coherent motion patterns. Humans show not only high sensitivity to global motion (Amano, Edwards, Badcock, & Nishida, 2009; Freeman & Harris, 1992;Lee& Lu, 2010; Morrone, Burr, & Vaina, 1995), but also remarkable adaptability to the dynamics of the recent history of motion stimulation (Bex, Metha, & Makous, 1999;Lee&Lu,2012; Scarfe & Johnston, 2011; Smith, Scott-Samuel, & Singh, 2000). The famous "waterfall" illusion is often used to demon- strate adaptation-induced aftereffects following prolonged exposure to global motion (for reviews, see Anstis, Verstraten, &Mather,1998; Mather, Pavan, Campana, & Casco, 2008). In many seminal studies on global-motion adaptation, researchers employed visible and salient motion-adapting stimuli, so that observers were consciously aware of the global motion direc- tion(s) during adaptation (Anstis & Reinhardtrutland, 1976; Bex et al., 1999; Blake & Hiris, 1993; Snowden & Milne, 1997; van der Smagt, Verstraten, & van de Grind, 1999). Even though the subsequent test stimulus did not display any coher- ent motion (i.e., a static image or random motion), it is possible that the clear awareness of the global motion during adaptation caused the perceived motion aftereffect (MAE) in the subse- quent test. If awareness of the global motion direction were necessary for yielding adaptation-induced MAEs, we would expect such aftereffects to disappear when the global motion becomes invisible during adaptation.

To examine whether a global-motion aftereffect depends on visual awareness of the adaptor, previous research has used binocular rivalry to manipulate phenomenal invisibility of the adapting motion stimuli. Although a rival stimulus suppressed an adapting stimulus from visual awareness, researchers found that such invisible motion adaptors still elicited an MAE (Lehmkuhle & Fox, 1975; Maruya, Watanabe, & Watanabe, 2008;O'Shea & Crassini, 1981). Interestingly, with low con- trast of the adapting stimuli, the strength of MAE weakened when the adapting stimulus was removed from visual aware- ness for a longer suppression duration (Blake, Tadin, Sobel, Raissian, & Chong, 2006; Wiesenfelder & Blake, 1990). These results provided important evidence that global-motion adaptation depends on visual awareness of the motion adap- tors, and also suggested that the neural activity in V1 may contribute to the dependency of the visual aftereffect on the adapting contrast (Albrecht & Hamilton, 1982; Sclar, Maunsell, & Lennie, 1990). This finding is consistent with one aspect of a computational account in which low-contrast displays introduce more ambiguity in local motion analysis, which may modulate the activity pattern in V1. …

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