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Rick Gurnsey, Geneviève Pagé; The Pinna -Brelstaff Illusion is not optimal under self-motion conditions. Journal of Vision 2005;5(8):55. doi: 10.1167/5.8.55.
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Purpose: The Pinna-Brelstaff illusion (Pinna & Brelstaff, 2000, Vision Research) consists of two concentric rings of micropatterns that appear to counter-rotate when the observer moves towards and away from the image. There have been several anecdotal reports that the illusion is stronger when the retinal motion (expansion and contraction of the stimulus) is produced by self-motion rather than by expansion and contraction of the stimulus on the computer screen.
Method: Our displays consisted of two concentric rings of Gabor patches. Each micropattern was oriented ±? degrees from the line that connects it to the centre of the display (Gurnsey et al., 2002, Perception). Nine displays were created with θ ranging from 0 to 90 degrees. Subjects first made relative salience judgements (for all possible pairs of θ) under both self-motion and screen-motion conditions. Subjects then judged which of two identical retinal motions—one produced under conditions of screen motion and another under conditions of self motion—produced the stronger illusion.
Results: The relative salience judgements made under conditions of screen- and self-motion produced identical dependence on θ, with peak salience occurring at approximately θ = 31. For all values of θ screen motion produced the more salient illusion and in several cases the difference was statistically different from chance.
Conclusions: Contrary to anecdotal reports the Pinna-Brelstaff illusion is not strongest under conditions of self-motion; if anything, the opposite is true. Therefore, there is no need to consider a role for extraretinal inputs in determining the strength of the illusion. It should be noted however that the percepts differ under conditions of self motion and screen motion. Under conditions of self motion there is size constancy but not under conditions of screen motion. Nevertheless, the extra retinal contributions leading to size constancy do not affect the strength of the illusion.
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