Thatcher Illusion: Dependence on Angle of Rotation

The expression of a face with its eyes and mouth inverted changes from ‘pleasant’ to ‘grotesque’ as the stimulus is rotated from 180° to 0° (Thatcher illusion). We determined the angular orientation at which this change occurred for three manipulated faces. Mean thresholds for eighteen observers were found to lie between 94° and 100° relative to the vertical with an average overlap of about 15° between an observer's ascending and descending thresholds. The sudden nature and relatively narrow zone of the changeover suggest a neuronal step-tuning of hypothetical face cells in the human brain, underlying the holistic (‘grotesque’) versus componential (‘pleasant’) processing of upright versus upside-down faces. Findings are discussed within the framework of cognitive, neuroimaging, and single-cell studies.