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Avesh Raghunandan, Jeremie Frazier, Siddharth Poonja, Austin Roorda, Scott B. Stevenson; The effect of retinal jitter on referenced and un-referenced motion discrimination thresholds. Journal of Vision 2005;5(8):442. doi: 10.1167/5.8.442.
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Referenced motion thresholds are significantly lower than un-referenced motion thresholds. To account for this, previous studies have postulated the existence of compensatory mechanisms, driven by the presence of a surround, that cancel the effects of eye movements. In the present study we used an Adaptive Optics Scanning Laser Ophthalmoscope (AOSLO) to investigate the effects of retinal jitter due to fixation eye movements on referenced and un-referenced motion thresholds in addition to the effect of external jitter on un-referenced motion thresholds. Methods: The stimuli were produced by modulation of the AOSLO imaging beam. In Experiment 1 subjects made up/down motion judgments of a dark horizontal bar presented against a stationary 1-degree bright background. In Experiment 2 un-referenced motion thresholds were measured with isolated bright horizontal bars in otherwise complete darkness. Experiment 3 was identical to Experiment 2, except that the eye was externally perturbed through the lower lid with a 2HZ square wave by a cotton tip applicator attached to a galvanometer. In all three experiments, AOSLO images for each trial were analyzed offline to extract retinal jitter. Results: For referenced motion, the results were consistent with complete compensation for eye movements by the visual system. In the un-referenced motion case eye movements adversely affected motion judgments however there was evidence of partial compensation for such eye movements. The results for the external jitter condition were also with consistent partial compensation for the external jitter. Conclusions: Compensatory processes completely cancel the effect of fixation jitter for referenced motion but such compensation is partial for un-referenced motion. The external jitter experiment suggests that proprioception may be a possible signal that could drive compensatory processes.
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