Perception of global facial geometry in the inversion effect and prosopagnosia

We investigated how efficiently combinations of positional shifts in facial features were perceived and whether the effects of combinations on the overall geometry of the face were reflected in discriminative performance. We moved the eyes closer together or further apart, and moved the mouth up or down. Trials with combinations of changes to both the mouth and the eyes were contrasted with trials with single changes to either the mouth or the eyes. As a contrast, we also examined combinations of changes in eye colour (brightness) and the same spatial manipulations. In addition, we specifically contrasted spatial combinations that more severely distorted the original triangular relation of the mouth and eyes (e.g. eyes closer and mouth down) to those that better preserved the original aspect ratio (e.g. eyes farther and mouth down). This we termed the "geometric context effect". We found that combinations of two spatial changes were detected more quickly and accurately by normal subjects viewing upright faces but not when faces were inverted. In contrast, combinations of spatial shifts and eye colour changes showed no advantage over faces with only one type of change. Combinations of spatial changes that distorted overall facial geometry more were detected more efficiently than less distorting combinations, showing that the spatial shifts were perceived in the context of the global facial structure. Again, this was found for upright but not inverted faces. We also tested a prosopagnosic patient, who showed the advantage for two spatial changes over one but lacked this geometric context effect, implying that she did not integrate local spatial information into overall facial structure.