Initial fixation to faces during gender identification is optimized for natural statistics of expressions

摘要

During face discrimination tasks, observers vary their initial fixation to faces in order to maximize task performance. For gender discrimination of faces with neutral expressions, the optimal point of fixation (OPF) is just below the eyes and is predicted by an optimal Bayesian model that takes into account the foveated nature of the visual system (foveated ideal observer, FIO; Peterson & Eckstein, 2012). Here, we investigate human OPFs for a gender task with an atypical relative frequency of facial expressions (50/50 - happyeutral) that alters the theoretical OPF predicted by the FIO. Methods: Five observers completed a gender discrimination task with 40 faces (20/20 male/female, and 10 of each with a happy expression) in luminance noise and 15 deg in height. In one condition, observers made free saccades in 3 blocks with presentation times of 350ms. In a second condition (forced-fixation), we assessed the human OPFs for the same task. Observers randomly fixated 1 of 5 horizontally centered positions on the face (forehead, eyes, nose, mouth, and an individual preferred fixation) for 200ms. Results: The forced-fixation condition showed that the human OPF is above the nose tip. However, human initial fixations in the free-saccade condition are suboptimal and directed to a higher point located below the eyes. This is consistent with the OPF for a gender discrimination task using the more frequent neutral-expression faces. Conclusions: Our findings suggest that observers optimize their initial fixation to faces, taking into account the statistics of occurrence of facial expressions. This fixation strategy seems to be inflexible to greatly altered facial expression statistics.