Abstract
Not all detectable differences between face images correspond to a change in identity. Here we measure both sensitivity to change and the criterion difference that is perceived as a change in identity. Both measures are used to test between possible similarity metrics. Using a same/different task and the method of constant stimuli criterion is specified as the 50% “different” point (P50) and sensitivity as the difference limen (DL). Stimuli and differences are defined within a “face-space” based on principal components analysis of measured differences in three-dimensional shape. In Experiment 1 we varied views available. Criterion (P50) was lowest for identical full-face view comparisons that can be based on image differences. When comparing across views P50, was the same for a static 45° change as for multiple animated views, although sensitivity (DL) was higher for the animated case, where it was as high as for identical views. Experiments 2 and 3 tested possible similarity metrics. Experiment 2 contrasted Euclidean and Mahalanobis distance by setting PC1 or PC2 to zero. DL did not differ between conditions consistent with Mahalanobis. P50 was lower when PC2 changed emphasizing that perceived changes in identity are not determined by the magnitude of Euclidean physical differences. Experiment 3 contrasted a distance with an angle based similarity measure. We varied the distinctiveness of the faces being compared by varying distance from the origin, a manipulation that affects distances but not angles between faces. Angular P50 and DL were both constant for faces from 1 to 2 SD from the mean, consistent with an angular measure. We conclude that both criterion and sensitivity need to be considered and that an angular similarity metric based on standardized PC values provides the best metric for specifying what physical differences will be perceived to change in identity.
Highlights
Being able to detect differences between different face images is critical for recognition
For example in Experiment 1 we vary viewpoint, a manipulation well-known to affect sensitivity but one which does not affect underlying differences in shape. Both criterion and sensitivity are tested and defined within a “face-space” based on a principal components analysis (PCA) of measured variations in 3D face shape and, in Experiments 2 and 3, differences between conditions allow us to test between alternative distance metrics for relating physical to perceived differences
In Experiment 1 we use the average face as a reference and test whether P50 and difference limen (DL) are affected by the face views available
Summary
Being able to detect differences between different face images is critical for recognition. Direction was randomized with the constraint that all comparison faces were at the corners of concentric multi-dimensional hypercubes (Wilson et al, 2002) That is they were equal distances away on all PC dimensions when distance is specified in terms of the SD associated with each dimension. This experiment was designed as a test between Euclidean and Mahalanobis distance metrics again using the average face as a reference. This means that Euclidean distances will be larger when PC2 is zero and PC1 varies than when the SD0,1 In this experiment we test whether changes in angle capture changes in perceived identity better than changes in distance. For each of these reference faces comparison faces were “lateral caricatures” constructed by moving in a direction orthogonal to the reference p("Diff")
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