Face recognition under ambiguous visual stimulation: fMRI correlates of "encoding styles".

Object categorization during ambiguous sensory stimulation generally depends on the activity of extrastriate sensory areas as well as top-down information. Both reflect internal representations of prototypical object knowledge against which incoming sensory information is compared. However, besides these general mechanisms, individuals might differ in their readiness to impose internal representations onto incoming ambiguous information. These individual differences might be based on what was referred to as "Schema Instantiation Threshold" (SIT; Lewicki et al. [1992]: Am Pshycol 47:796-801), defining a continuum from very rapid (low threshold) to a rather controlled application of internal representations (high threshold). We collected fMRI scans while subjects with low SIT ("internal encoders") and subjects with high SIT ("external encoders") made gender categorizations of ambiguous facial images. Internal encoders made faster gender decisions during high sensory ambiguity, showed higher fusiform activity, and had faster BOLD responses in the fusiform (FFA) and occipital face area (OFA) indicating a faster and stronger application of face-gender representations due to a low SIT threshold. External encoders made slower gender decisions and showed increased medial frontal activity, indicating a more controlled strategy during gender categorizations and increased decisional uncertainties. Internal encoders showed higher functional connectivity of the orbito-frontal cortex (OFC) to seed activity in the FFA which might indicate both more readily generated predictive classificatory guesses and the subjective impressions of accurate classifications. Taken together, an "internal encoding style" is characterized by the fast, unsupervised and unverified application of primary object representations, whereas the opposite seems evident for subjects with an "external encoding style".