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K. K. De Valois, T. Takeuchi; Judging the velocity of an invisible moving object. Journal of Vision 2001;1(3):378. doi: 10.1167/1.3.378.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To determine the absolute apparent speed of a moving target without comparing unlike stimuli. Methods: Half the display contained opaque black/white random elements (RE); the other half was a uniform gray field. A thin vertical bar at the far edge of the RE field served as a goal line. On each trial, an incremental Gaussian blob began to move horizontally in the uniform field at a constant speed from a variable position. The blob disappeared at the edge of the RE field as though it moved behind a wall. At some variable time, the goal line flashed. Subjects were told to imagine the continued motion of the moving blob and judge whether it moved faster when it was seen or unseen. The duration between the disappearance of the blob and the flash of the goal line was fixed in each session. This duration was calculated by assuming the blob moved with constant velocity under the RE sheet at equivalent speeds of 2–10 deg/sec. An adaptive staircase was used to determine the point of subject equality between this equivalent speed and the speed of the visible moving blob by changing the physical speed of the visible blob. Blob luminance contrast and luminance adaptation level varied systematically. Results: The perceived velocity of a moving blob depends on both contrast and average luminance. Speed is significantly underestimated at high speeds. The error magnitude increases with absolute speed. When effective contrasts are equated, apparent speed increases linearly with real speed, but the slopes of the functions are not equal. At low contrasts, the blob appears to move more slowly than at high contrasts. At equal physical contrasts, perceived speed is higher at high light levels than at low light levels, and the difference is greater at higher velocities. Conclusion: The absolute apparent speed of a moving object rarely matches the real speed. It depends partly upon stimulus attributes. This method avoids certain problems of other speed matching paradigms.
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