Hemispheric asymmetries in categorical facial expression perception

Although many sensory phenomena vary continuously, humans tend to divide them into discrete categories with facial expressions being divided into categories such as happy, sad, anger, and fear. A critical effect of category use is categorical perception: same sized physical differences are better perceived if the difference occurs between two categories rather than within the same category. Here we investigate the lateralization of categorical perception of facial expressions. Categorical perception of facial expressions may be lateralized to the right hemisphere due to the intimacy of basic, possibly universal and innate, prototypical facial expression categories and face processing which is predominantly lateralized to the right hemisphere. Alternatively, facial expression categories may be a linguistic phenomenon and thus predominantly involve the left hemisphere. A visual half-field paradigm was used to assess categorical perception of morphed facial expressions running between happy-sad, and anger-fear. Across two experiments, the lateralization of categorical perception was found to be contingent upon previous exposure. Better between-than within-category discrimination of facial expressions was found in both visual half-fields when participants had previously been exposed to facial expression categories; but for naïve participants, categorical perception was restricted to the right visual hemifield, corresponding to the left cerebral hemisphere. Facial expressions presented to the right hemisphere are therefore not necessarily encoded in terms of their emotional expression category and such category-based encoding, when it does occur, may be due to the left hemisphere and so is likely to be subject to biases related to language. (PsycINFO Database Record (c) 2019 APA, all rights reserved).