The perceived direction of up depends on visual, gravity, and body cues, each of which is given a weighting by the brain (Dyde et al. 2006). Little work has been done, however, to demonstrate whether sound might also be usable by the brain as a cue to up. Here we assess the possible contribution of sound to perceived orientation by adding a sound cue to gravity. The perceptual upright, the direction in which a character is most easily recognized, was assessed using the Oriented Character Recognition Test (OCHART). Subjects identified the character ‘p’ that was presented in various orientations (0-360 degrees rotation) as either a ‘p’ or ‘d’. The orientations were chosen by a QUEST adaptive staircase procedure and the mean of the points of subjective equality was taken as the perceptual upright. Subjects lay on their side and viewed a laptop screen through a shroud. Thus, body and gravity cues were orthogonal and swings of the perceptual upright towards or away from gravity could be measured. Loudspeakers were mounted above and below the lying subject (opposite the left and right ears) and sounds were presented synchronized to the appearance of the character on the screen. Changes in the direction of the PU were recorded depending on whether a sound was present or not. We conclude that sounds can contribute to the perception of upright.

body orientation; audition; perceptual upright