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Ian van der Linde, Abtine Tavassoli, Alan C. Bovik, Lawrence K. Cormack; Classification images reveal observer templates underlying the direct tilt illusion. Journal of Vision 2006;6(6):451. doi: 10.1167/6.6.451.
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© ARVO (1962-2015); The Authors (2016-present)
The tilt illusion demonstrates how the sensation of orientation is misperceived in the presence of an inducing distracter. A fast classification image (CI) technique (Tavassoli et al., in press) was used to visualise the impact of the direct tilt illusion in a visual search experiment. A search target (a sine-wave grating oriented at 0 deg) was embedded in a grid of 1/f noise tiles of 1 deg visual angle each. In an eye-tracker, observers searched the grid to find the target in a series of 5s time-limited trials. Two different background distracter orientations (−15 deg and +15 deg vertically offset sine-wave gratings) and a distracter-absent control condition were run, using two human observers at 500 trials per condition. For both observers a repulsive tilt in the CIs and spectral CIs was observed relative to the orientation of the background distracter, where no significant tilt was seen in the distracter-absent control condition. Earlier studies measured the tilt illusion at various eccentricities with 2AFC/yes-no psychophysics; our study reveals that this phenomenon is similarly pervasive in a naturalistic visual search scenario. Our results indicate that peripherally viewed tiles (saccade targets) are misperceived prior to foveal scrutiny, thus our results support the hypothesis that the direct tilt illusion is low level and originates early in the visual pathway. Work is underway to quantify magnitude of the tilt illusion in this experimental setup, relative to the eccentricity of the saccade preceding each tile's fixation.
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