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Tirta Susilo, Elinor McKone, Mark Edwards; Solving the upside-down puzzle: Inverted face aftereffects derive from shape-generic rather than face-specific mechanisms. Journal of Vision 2009;9(8):451. doi: 10.1167/9.8.451.
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Given the robust perceptual processing differences between upright and inverted faces (e.g. holistic effects), a somewhat peculiar finding in the face perception literature has been the consistent observation that inverted faces produce adaptation aftereffects very similar to those for upright faces. Although there is evidence that the specific neural populations supporting upright and inverted face aftereffects are partially separable (Rhodes et al., 2006), these different neural populations have produced, to date, no known qualitative differences between aftereffects for upright and inverted faces. Indeed, large inverted aftereffects occur not only for general shape distortions (Webster & Maclin, 1999) but also for distortions that might potentially be more face-specific, including gender (Watson & Clifford, 2006), identity relative to the average face (Leopold et al., 2001), and a ‘second-order relational’ distortion of eye height (Robbins et al., 2007); further, last year (Susilo, McKone, Edwards, VSS 2008) we reported that aftereffects for inverted faces, just like those for upright, comply with predictions of opponent neural coding. So, are there any important differences between aftereffects for upright and inverted faces? Here, we show there are. We tested whether the origin of the face aftereffects is face-specific or shape-generic. We manipulated eye heights (eyes up/down) and T-shape forms (vertical bar up/down), and compared the magnitude of the aftereffects following adaptation in same-stimuli conditions (adapt-face/test-face, adapt-T/test-T) with that in across-stimuli conditions (adapt-T/test-face, adapt-face/test-T). Our rationale was that the magnitude of adaptation transfer between faces and shapes reflects the degree of their shared neural representation. We found limited transfer of adaptation between eye heights and T-shapes when the stimuli were upright, but almost complete transfer when they were upside-down. Results argue that, while upright face aftereffects derive from high-level face-specific representations, inverted face aftereffects derive from shape-generic neural populations (possibly mid-level in origin).
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