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Thomas Busigny, Markus Graf, Eugéne Mayer, Bruno Rossion; Acquired prosopagnosia as a face-specific disorder: Ruling out the visual similarity hypothesis. Journal of Vision 2010;10(7):595. doi: 10.1167/10.7.595.
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The understanding of the nature of prosopagnosia - classically defined as a disorder of visual recognition specific to faces following brain damage - can inform about how visual face recognition is performed in the normal human brain. However, according to a long-standing alternative view of prosopagnosia, the prosopagnosic impairment would rather reflect a general difficulty for fine-grained discrimination in visually homogenous object categories (Faust, 1955; Damasio et al., 1982; Gauthier et al., 1999). We tested this hypothesis stringently with a well-known brain-damaged prosopagnosic patient (PS, Rossion et al., 2003), in three delayed matching experiments in which the visual similarity between the target and distractor was manipulated parametrically. We used 3 kinds of stimuli: novel 3D geometric shapes manipulated on single or multiple dimensions, morphed common objects (Hahn et al., 2009), and morphed photographs of a highly homogenous familiar category (cars). In every experiment, there was no evidence of a steeper increase of error rates and RTs with increasing levels of visual similarity for the patient, relative to normal observers. These data categorically rule out an account of acquired prosopagnosia in terms of a general problem of fine-grained discrimination in a visually homogenous category. Finally, a fourth experiment with faces showed that, compared to normal observers, the patient's impairment with morphed faces was best revealed at the easiest levels of discrimination, i.e. when individual faces differ clearly in global shape rather than in fine-grained details. Overall, these observations indicate that the alternative view of prosopagnosia as a more general impairment for fine-grained discrimination in visually homogeneous object categories does not hold.
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