Can holistic processing be learned for inverted faces?

The origin of "special" processing for upright faces has been a matter of ongoing debate. If it is due to generic expertise, as opposed to having some innate component, holistic processing should be learnable for stimuli other than upright faces. Here we assess inverted faces. We trained subjects to discriminate identical twins using up to 1100 exposures to each twin in different poses and images. In the upright orientation, twin discrimination was supported by holistic processing. Removal of a single face feature had no effect on performance, and a composite effect (Young, A. W., Hellawell, D., & Hay, D.C. (1987). Configurational information in face perception. Perception 16 (6), 747-759) was obtained. In the inverted orientation, however, above chance identification ability relied on (a) image specific learning, or (b) tiny local feature differences not noticed in the upright faces. The failure to learn holistic processing for inverted faces indicates that, in contrast to the situation for objects (Tarr, M.J., & Pinker, S. (1989). Mental rotation and orientation-dependence in shape recognition. Cognitive Psychology 21 (2), 233-282), orientation specificity of face processing is highly stable against practice.