Adaptation effects in facial expression recognition Shen-Mou Hsu and Andrew W. Young Department of Psychology, University of York, UK The effect of adaptation on facial expression recognition was investigated by measuring how identification performance of test stimuli falling along a particular expression continuum was affected after adapting to various prototype emotional faces or a control pattern. The results showed that for recognition of fear, happi- ness, and sadness, inhibition effects were observed on recognition of test expressions following 5 s adaptation to the same emotion, suggesting different neural populations tuned for the encoding of fearful, happy, and sad expressions. Facilitation of recognition of test stimuli differing in emotion to the adapting stimulus was also sometimes observed. The nature of these adaptation effects was investigated by introducing a size transformation or a delay between adapting and test stimuli and was found to survive these changes. The results of a further experiment argued against a criterion effect being the major source by demon- strating the importance of adapting time in generating the effects. Overall, the present study demonstrates the utility of adaptation effects for revealing functional characteristics of facial expression processing. The ability to recognize facial expressions is thought to be part of our evolu- tionary heritage and to rely upon finely tuned neural mechanisms in the brain. Neurophysiological studies in macaques have found that facial expressions may elicit selective responses from single neurons in the superior temporal sulcus Hasselmo, Rolls, & Baylis, 1989). Neuropsychological Adolphs, Tranel, Damasio, & Damasio, 1994; Sprengelmeyer et al., 1996) and neuroimaging data Breiter et al., 1996; Morris et al., 1996; Phillips et al., 1997) with humans have also suggested that recognition of some distinct facial expressions engages specific neural circuits. Although various brain regions have therefore been correlated with facial expression recognition, the nature of their contributions remains unresolved. The act of seeing is so effortless that it is difficult to appreciate the sophis- ticated mechanisms underlying it. However, the structure of the visual system VISUAL COGNITION, 2004, 11 7), 871±899 Please address all correspondence to A. W. Young, Department of Psychology, University of York, Heslington, York, YO10 5DD, UK. Email: awy1@york.ac.uk The first author was sponsored by Hsu and Shen Foundation. The work reported here forms part of PhD thesis at the University of York Hsu, 2003). # 2004 Psychology Press Ltd http://www.tandf.co.uk/journals/pp/13506285.html DOI: 10.1080/13506280444000030