September 2011
Volume 11, Issue 11
Vision Sciences Society Annual Meeting Abstract  |   September 2011
Vergence, pin interval, and the double-nail illusion
Author Affiliations
  • Masahiro Ishii
    University of Toyama, Japan
  • Rie Igarashi
    University of Toyama, Japan
  • Masayuki Sato
    University of Kitakyushu, Japan
Journal of Vision September 2011, Vol.11, 327. doi:10.1167/11.11.327
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      Masahiro Ishii, Rie Igarashi, Masayuki Sato; Vergence, pin interval, and the double-nail illusion. Journal of Vision 2011;11(11):327. doi: 10.1167/11.11.327.

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      © ARVO (1962-2015); The Authors (2016-present)

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The double-nail illusion arises when the two eyes converge on halfway between the pins. Mallot and Bideau (1990) found that the illusion often arose even when the eyes converged on either pin and concluded that stereo matches in the fixation plane were preferred. This study investigated the appearance of double-nail display with varying pin interval. The stereograms that consisted of vertical bars were displayed on a mirror stereoscope in a darkroom. Each presentation was preceded by a stereoscopic fixation stimulus that was held 40 cm from the observer. Immediately after this fixation was turned off, the double-nail display was flashed for 200 msec. One pin replaced the preceding stimulus and the other appeared beyond or in front of the fixated pin. The task was formulated as a two alternative forced choice between “two lines visible” (frontoparallel percept) and “three lines visible” (medial percept). Three subjects performed 30 trials for each of the pin interval in a randomized sequence. The display with small pin interval was perceived as two lines. With larger interval, the frequency of three-lines percept increased. We then conducted another experiment to measure Panum's fusional area on the fiducial line. Immediately after a fixation was turned off, a pin was flashed for 200 msec. The pin appeared beyond or in front of the fixated target. The task was formulated as a two alternative forced choice between “two lines visible” and “one line visible”. Analyzing data, we found that the double-nail display was perceived as two lines when they were within the Panum's fusional area. This result indicates that a stimulus that the eyes converge does not always generate fused image.

Japan Science and Technology Agency. 

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