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Richard T. Dyde, Shabnam Sadr, Michael R. Jenkin, Heather L. Jenkin, Laurence R. Harris; The perceived direction of “up” measured using a p/d letter probe. Journal of Vision 2004;4(8):385. doi: 10.1167/4.8.385.
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Determining the direction of “up” requires the observer to derive a reference from several visual and non-visual sources including the direction of gravity, the orientation of the body and visual cues. Here we describe a simple probe of the perceived direction of up that requires minimal assumptions about physical laws and which may have important practical applications in the study of reading and in the design of unusual environments. The only visible difference between the letters ‘p’ and ‘d’ is the orientation of the character relative to the observer. Subjects were shown the character in one of 18 orientations (ranging from 15 deg to 150 deg and from 210 deg to 330 deg in steps of 15 deg where 0 corresponds to a screen-upright ‘p’) and indicated whether they recognized it as a ‘p’ or a ‘d’. The perceived direction of up was calculated as the orientation half way between the two transition points (measured psychometrically) between these interpretations. The visual background was a highly polarized circular photograph (dia 40 deg) presented in 15 orientations in steps of 22.5 deg. The relationship between body and gravity was dissociated by repeating the experiment with observers in various orientations Both the orientation of the visual background and the orientation of the subject relative to gravity had a significant effect on the orientation of the transition zones between the p and d interpretations of the character. The corresponding “up” directions could be predicted from the weighted vector sum of the direction of gravity, orientation of the body and the orientation of the visual environment. A simple letter recognition task was influenced by both the orientation of the visual background and the orientation of the observer. The reference direction of up, to which many perceptions, including letter recognition, are referred can be predicted from a simple weighted sum of gravity, body orientation and visual orientation.
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