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Sarah Lagacé-Nadon, Rémy Allard, Jocelyn Faubert; Exploring the spatiotemporal properties of fractal rotation. Journal of Vision 2008;8(6):596. doi: 10.1167/8.6.596.
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© ARVO (1962-2015); The Authors (2016-present)
Motion perception of first- and second-order stimuli has been proposed to be mediated by separate mechanisms. Whereas luminance-based stimuli are analyze by energy-based motion detectors, uncertainty remains as to the mechanisms involved in the processing of second-order stimuli.
The aim of this experiment was to determine the nature of mechanisms accounting for detection of fractal rotation (Benton, O'Brien & Curran, 2007) in comparison with those responsible for first-order rotation. To reproduce such a stimulus, a rotating oriented filtered noise pattern was used, in which orientation varied from frame to frame. Noise was resampled for each frame. This stimulus should be invisible to first-order motion sensitive mechanisms considering the absence of energy movement. Rather, rotation is the only local cue available to motion detectors. Hence, motion perception would be based on the analysis of spatial structure, more specifically the orientation change over time. In comparison, we have used another stimulus composed of a single rotating oriented filtered noise frame where motion is detected by first-order sensitive mechanisms. First, we measured the temporal response of fractal rotation. Contrast thresholds were measured using a direction discrimination task at various temporal frequencies. First-order rotation was found to be band-pass, whereas fractal rotation was low-pass, as previously reported for contrast-, polarity- and spatial length-modulated motion. Hence, fractal rotation has second-order mechanism properties. Second, the nature of mechanisms responsible for detection of fractal rotation has been explored using a known paradigm where different energy levels are used by changing velocities (Seiffert & Cavanagh, 1998). Sensitivity to first-order stimuli is expected to change proportionately with energy levels while sensitivity to second-order properties is not. Results suggest a velocity-based mechanism account for perception of first-order motion but not for fractal rotation. This implies second-order mechanisms are sensitive to change of spatial orientation structure over time.
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