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Nestor Matthews, Jason Allen; The Role of Speed Lines in Subtle Direction Judgments. Journal of Vision 2004;4(8):556. doi: 10.1167/4.8.556.
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© ARVO (1962-2015); The Authors (2016-present)
Background & Purpose: Stimuli moving in slightly different directions trace trajectories that differ slightly in orientation. These different ‘speed lines’, in principle, could generate responses in orientation-tuned mechanisms, and such responses could determine how well we judge subtle direction differences. Alternatively, the ability to judge subtle direction differences could be determined by direction-tuned mechanisms. Distinguishing between these two possibilities was the purpose of this study: Method: Across trials, participants judged subtle orientation differences between random-dot motion trajectories having either the same or opposite motion signs. Three additional conditions were included. In the bi-directional condition, half the dots in the random-dot pattern moved coherently in one direction while the remaining dots moved coherently in the opposite direction. In the scrambled condition, coherent motion was destroyed by randomly shuffling the frame sequence. Finally, in the stationary condition, all frames of the motion sequence were played simultaneously. Results: The precision of orientation judgments (as indexed by d') was statistically indistinguishable in the “same” and “stationary” conditions. Relative to the “same” condition, however, orientation sensitivity was significantly reduced when the stimulus contained both directions simultaneously (t(19)=2.2, p=0.04), only the opposite direction (t(19)=2.35, p=0.03), or scrambled motion (t(19)=3.357, p=.003). Conclusions: Across our five conditions, the time-averaged motion (i.e., orientation) was identical; only the dynamic properties of the dots varied. The significant differences across our conditions, therefore, suggest that fine direction judgments are more likely limited by direction-tuned mechanisms than by orientation-tuned mechanisms responding to the orientation of speed lines.
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