Imperfect size invariance in face discrimination

Multiple findings suggest that human visual perception displays extensive size invariance (i.e., insensitive to retinal image size changes) to multiple object categories. This is typically investigated when only retinal image size changes (by changing real world size and keeping a fixed viewing distance), a situation that does not reflect daily vision where objects retain their real-world size and their retinal image size changes as a function of viewing distance. Here size invariance was tested by examining if the ability of the visual system to discriminate subtle face changes (with morphed faces) survives retinal image size changes. Importantly, retinal image size changes were applied either naturalistically (when faces kept their world size but were more distant) or lab-like (where faces changed their world size while viewing distance was constant). We found that (i) the difference detection threshold (DT) was not influenced by the retinal size changes or viewing distances we manipulated, (ii) face discrimination was significantly worse when retinal image size was smaller (for both naturalistic and for lab-like conditions), (iii) worse face discrimination performance at smaller retinal image sizes were significantly associated between naturalistic and lab-like conditions. Our results indicate that face discrimination shows imperfect invariance to retinal size and importantly, this appears independent of viewing distance. Since face perception is based on fovea-related processes and may rely on face-dedicated mechanisms, future work should examine whether these results generalize to additional object categories.