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Avesh Raghunandan, Scott B. Stevenson; Binocular fusion and interocular motion direction discrimination.. Journal of Vision 2004;4(8):557. doi: 10.1167/4.8.557.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Minimum motion thresholds are substantially elevated when targets are presented dichoptically. The present study investigated whether interocular motion direction-discrimination thresholds are limited by the binocular fusional mechanism. Methods: All stimuli were bright horizontal Gaussian lines with sigma 1 arcmin, presented on a black background. Fusion ranges were measured by adjusting each of two binocularly presented lines to match the apparent positions of two dichoptically presented lines. A monocular control established the reliability and precision of the subjects' settings. The adjusting lines and the test lines were horizontally separated by 10 arcmin and were flashed on for 50 ms. Test lines were presented with one of 11 spatial offsets and with one of 4 ISI's: −50 ms (simultaneous), 0 ms (temporally abutting), 72 ms and 288 ms. Forced choice motion direction discrimination thresholds were measured with a Gaussian line that moved upward or downward in two frame motion, within a reference frame of random dots. The test lines were presented monocularly or interocularly with one of 11 offsets and with one of 4 ISIs (0 msec, 72 msec, 144 msec and 288 msec). Motion direction discrimination threshold was computed as the offset corresponding to 84% performance. Results: Settings from three subjects showed that test lines appeared single for separations of 1 to 3 arcmin at ISIs of −50 and 0 ms. Fusion ranges decreased to zero at the longest ISI. Interocular motion discrimination thresholds were around 1 arcmin and were roughly constant over the range of ISIs tested. Monocular thresholds increased from about 0.2 arcmin at 0 ISI to 1 arcmin at an ISI of 144 ms. Conclusion: Two aspects of our results suggest that binocular fusion does not limit interocular motion thresholds: 1) interocular motion detection thresholds are substantially smaller than fusion limits measured with the same stimuli, and 2) fusion and detection show a different dependence on ISI.
NEI grants P30EY07551 and EY12986, and an Erhart Fellowship to AR
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