The Thatcher Illusion: Rotating the Viewer Instead of the Picture

Faces are difficult to recognise when presented upside down. This effect of face inversion was effectively demonstrated with the ‘Thatcher illusion’ by Thompson (1980 Perception9 483–484). It has been tacitly assumed that this effect is due to inversion relative to retinal coordinates. Here we tested whether it is due to egocentric (ie retinal) inversion or whether the orientation of the body with respect to gravity also influences the face-inversion effect. A 3-D human turntable was used to test subjects in 5 different body-tilt (roll) orientations: 0°, 45°, 90°, 135°, and 180°. The stimuli consisted of 4 ‘normal’ and 4 ‘thatcherised’ faces and were presented in 8 different orientations in the picture plane. The subjects had to decide in a yes – no task whether the faces were ‘normal’ or ‘thatcherised’. Analysis of the d' values revealed a significant effect of stimulus orientation and body tilt. The significant effect of body tilt was due to a drop in d' values in the 135° orientation. This result is compared to findings of studies on the subjective visual vertical, where larger errors occurred in body-tilt orientations between 90° and 180°. The present findings suggest that the face-inversion effect relies mainly on retinal coordinates, but that in head-down body-tilt orientations around 135° the gravitational reference frame has a major influence on the perception of faces