Solving the upside-down puzzle: Why do upright and inverted face aftereffects look alike?

Face aftereffects for upright faces have been widely assumed to derive from face space and to provide useful information about its properties. Yet remarkably similar aftereffects have consistently been reported for inverted faces, a problematic finding because other paradigms argue that inverted faces are processed by different mechanisms from upright faces. Here, we identify a qualitative difference between upright and inverted face aftereffects. Using eye-height aftereffects, we tested for opponent versus multichannel coding of face dimensions by manipulating distance of the adaptor from the average, and face-specific versus shape-generic contributions via transfer of aftereffects between faces and simple T-shapes. Our results argue that (i) inverted face aftereffects derive entirely from shape-generic mechanisms, (ii) upright face aftereffects derive partly from shape-generic mechanisms but also have a substantial face space component, and (iii) both face-specific and shape-generic multidimensional spaces use opponent coding.