Abstract
A substantial body of research has demonstrated that mirror-reversed views of objects are especially difficult to distinguish (Corballis & Beale, 1976; Davidoff & Warrington, 2001; Rollenhagen & Olson, 2000). However, not all mirror images are equal: participants show a greater tendency to confuse reflections about the object's principal axis (OPA) than left-right reflections of the object about an external vertical axis (EVA) (Gregory & McCloskey, 2010; Gregory et al., 2011). Using an event-related fMRI adaptation paradigm, we examined mirror-image specificity and invariance in human object-selective cortex [lateral occipital (LO) and posterior fusiform (pFs)] and orientation-selective occipito-parietal junction (OPJ) (Valyear et al., 2006). Participants (N=17) viewed an object stimulus, followed by the same view repeated (Identical condition), an object-axis reflection (OPA), an external-axis reflection (EVA), or a different object (Diff). In LO, neither mirror image condition produced any adaptation (no difference from Different condition). In contrast, for pFs, both mirror image conditions were adapted relative to the Different condition (Diff vs. OPA: F(1,39) = 25.24, p <.05.; Diff vs. EVA: F(1,39) = 12.97, p <.05). Additionally, pFs adapted fully (no difference from Identical condition) to OPA reflections but not to EVA reflections (Identical vs. EVA: F(1,39) = 14.58, p <.05), and orientation-selective OPJ showed a similar trend towards releasing from adaptation only for EVA reflections, though OPA and EVA did not differ directly (both F's <3, n.s.). These results shed light on the neural representation of object orientation, and suggest the possibility of greater invariance to object-axis reflections than external-axis reflections in both ventral (pFs) and dorsal (OPJ) streams, consistent with the greater behavioral confusability for object-axis reflections (Gregory & McCloskey, 2010).
Meeting abstract presented at VSS 2014