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Klaus Landwehr, Robin Baurès, Daniel Oberfeld, Heiko Hecht; Visual discrimination of arrival times: Troublesome effects of stimuli and experimental regime. Journal of Vision 2010;10(7):804. doi: 10.1167/10.7.804.
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Discrimination thresholds for visually perceiving which of two objects, approaching head-on at constant velocity, will arrive earlier at one's station point, have been reported to range between 0.016 and 0.250 (Oberfeld & Hecht, 2008; Regan & Hamstra, 1993; Simpson, 1988; Todd, 1981). Values for lateral motion are typically in the lower range (Bootsma & Oudejans, 1993), and values for recession, moment of passage, and complex scenarios, in the higher range (Kaiser & Mowafy, 1993; Kim & Grocki, 2006). We compared Todd's (1981) original stimuli and his experimental regime with modified ones. Todd had presented outlines of two virtual squares, optically specified by 24 dots each, in 2AFC with a constant standard. We, in addition, used dot clouds with the same number of dots as Todd's squares, and a standard-free procedure. We also used narrower ranges of object sizes, velocities, and arrival-time differences, fewer trials, and naïve instead of trained observers. We obtained a minor effect of stimulus type and a large one of experimental regime. As verified by detailed analyses by conditions and levels of variables, huge differences in object size and velocity distract from the task. The weak effect of stimulus type is consistent with Simpson's (1988) contention that unspecialized optic-flow analyzers suffice for extracting temporal information; on the other hand, it might also mirror the flexibility of dedicated “looming detectors” – not requiring contours or outlined shapes for proper functioning (cf. Beverley & Regan, 1980; Koenderink, 1985). We observed excessive individual differences with Weber fractions ranging between 0.017 and 0.123. For 32 % of sessions, no psychometric functions could be fitted. We are currently extending our present work to include different trajectories, objects, and contexts, and also control measures of plain motion sensitivity, in order to test the generality of our findings and to better understand differences in performance.
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