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Talia Retter, Bruno Rossion; Mind the gap: behavioral measures and phenomenology of the composite face illusion. Journal of Vision 2014;14(10):575. doi: https://doi.org/10.1167/14.10.575.
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Holistic face perception is well-evidenced, although methods for its quantification remain debated. One measurement, provided by the composite face effect (CFE), reflects impaired performance at recognizing two upper face halves as identical when they are aligned (compared to misaligned) with lower face halves of differing identities (Young et al., 1987; Rossion, 2013 for review). The validity of this measure is challenged by two apparently contradictory goals: a composite face should be perceived as a whole entity, yet the precise area of an upper face half must be defined for task performance. Here, we investigated the impact of a small gap between upper and lower face halves in two complimentary experiments. First, 16 participants were tested with gap and no-gap stimuli in a standard delayed matching composite face task. Although both stimuli conditions produced a substantial CFE, this was significantly larger for no-gap stimuli. Second, we tested whether this larger effect reflected better integration of facial halves. Ten participants performed a forced-choice perceptual judgment, determining which of two simultaneously displayed faces was the veridical face (i.e., with both halves from the same original identity) and which the composite face. Perceptual judgments for no-gap stimuli approached ceiling (91%); in contrast, with a gap, participants were almost unable to identify the veridical face (61%). This effect was not only due to low-level segmentation cues at the border of no-gap face halves, because stimuli inversion decreased performance in both conditions (no-gap: 84%; gap: 52%). These observations indicate that, paradoxically, facial halves are integrated more naturally in stimuli with a gap. Therefore, the larger CFE for no-gap stimuli is likely to emerge primarily from a lack of clear definition of an upper face half. Taken together, these results indicate that composite face stimuli with a gap provide a more accurate measurement of holistic face perception.
Meeting abstract presented at VSS 2014
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