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Elliot D. Freeman; Attentional control of multi-stable aperture motion. Journal of Vision 2005;5(8):36. doi: 10.1167/5.8.36.
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© ARVO (1962-2015); The Authors (2016-present)
Ambiguous stimuli often look different in different contexts. This study measured the combined influence of visual and behavioural context on the subjective appearance of aperture motion. Subjects fixated the centre of a circularly-windowed drifting grating (2.9 deg diameter, orientation 45° clockwise from horizontal, with foveal mask), and continuously indicated the direction of perceived drift. Though many different distal motions could create the same proximal stimulation, only the vector orthogonal to the grating orientation was predominantly reported. However, adding two discrete horizontally-flanking 315° gratings (centre-to-centre separation 3.6 deg) produced spontaneous bistable switching between diagonal component motion and pattern motion, with the whole configuration occasionally appearing to drift upwards. Adding a second pair of vertically-flanking 135° gratings even produced tri-stable perception, with leftwards drift as the third mode. Thus, adding context increased rather than reduced the subjective ambiguity of aperture motion. Observers were now cued every 6 seconds to try to switch between leftwards and upwards pattern motion, by selectively attending to the vertical or horizontal configuration of three gratings respectively. Observers could rapidly switch between modes on demand, with dominance of the cued motion increasing markedly at the expense of the other two modes. However, behaviour was still constrained by stimulus factors modulating the appearance of pattern motion. As the extreme case, little or no control was possible with just a single isolated grating. Goal-directed attention could therefore resolve the subjective ambiguity, by integrating a subset of the local motion components into a behaviourally-relevant pattern. Such selective context integration results in dramatic but voluntarily-controlled changes in the observer's subjective state.
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