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Haluk Ogmen, Murat Aydin, Michael Herzog; Differential perceived speeds explain the apparent compression in slit viewing. Journal of Vision 2007;7(9):197. doi: 10.1167/7.9.197.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose. When a figure moves behind a stationary narrow slit, observers often report seeing the figure as an integrated whole but compressed along the axis of motion (anorthoscopic perception). Under-estimation of the speed of the moving object was offered as an explanation for this apparent compression. Here, we present results inconsistent with this hypothesis and offer an alternative explanation: The apparent compression is caused by the fact that the trailing parts of the figure are perceived to move faster than the leading parts.
Methods. The perceived speed and width of an ellipse (7.1 deg wide, 5 deg tall) moving (3.6, 7.1 or 10.7 deg/s) behind a slit (21 arcmin wide) were measured. In separate sessions, the perceived speeds of the leading and trailing halves of the ellipse were also measured. Moreover, we quantified the visibilities of the leading and trailing parts of a 5x5 square grid array moving behind the slit.
Results. 1) For all speed values tested, the ellipse was perceived as compressed and the magnitude of the compression increased with speed. 2) However, this compression cannot be explained by the under-estimation of the speed, since the ellipse was always perceived to move faster, not slower, than its physical speed. 3) The trailing part of the ellipse was always perceived to move faster than its leading part. 4) The results of the visibility experiment show that the slower perceived speed for the leading part might be due to its reduced visibility.
Conclusion. The apparent compression of a figure in slit viewing results from differential perceived speeds of its parts. More specifically, the trailing parts of the figure are perceived to move faster than its leading parts. The differential visibility of the different parts might play a role in determining the perceived speeds.
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