Hill Harold, Claes Peter, Corcoran Michelle, Walters Mark, Johnston Alan, Clement John Gerald
School of Psychology, University of Wollongong Wollongong, NSW, Australia.
Front Psychol. 2011 Mar 23;2:41. doi: 10.3389/fpsyg.2011.00041. eCollection 2011.
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.
面部图像之间所有可检测到的差异并不都对应于身份的变化。在这里，我们测量了对变化的敏感度以及被视为身份变化的标准差异。这两种测量方法都用于测试可能的相似性度量。使用相同/不同任务，并将恒定刺激法的标准指定为50%“不同”点（P50），将敏感度指定为差异阈限（DL）。基于对三维形状测量差异的主成分分析，在“面部空间”内定义刺激和差异。在实验1中，我们改变了可用视图。对于基于图像差异的相同全脸视图比较，标准（P50）最低。当跨视图比较时，对于静态45°变化和多个动画视图，P50相同，尽管对于动画情况敏感度（DL）更高，其与相同视图时一样高。实验2和3测试了可能的相似性度量。实验2通过将PC1或PC2设置为零来对比欧几里得距离和马氏距离。在与马氏距离一致的条件下，DL没有差异。当PC2变化时，P50较低，强调身份的感知变化不是由欧几里得物理差异的大小决定的。实验3将基于距离的度量与基于角度的相似性度量进行对比。我们通过改变与原点的距离来改变被比较面部的独特性，这种操作会影响面部之间的距离但不影响角度。对于距离均值1到2个标准差的面部，角度P50和DL都保持恒定，这与角度度量一致。我们得出结论，标准和敏感度都需要考虑，并且基于标准化PC值的角度相似性度量为指定哪些物理差异会被感知为身份变化提供了最佳度量。
