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Ikuya Murakami; Minimum motion threshold correlates with the fixation instability of the more wobbling eye. Journal of Vision 2010;10(7):830. doi: 10.1167/10.7.830.
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© ARVO (1962-2015); The Authors (2016-present)
Even if we look at a stationary object with maintained fixation, the eyes are actually making tiny random oscillations. Previously a correlation was found between the minimum detection threshold for unreferenced motion and fixation instability, such that observers with poorer fixation performances had higher thresholds (Murakami, 2004). Therefore fixation instability is a part of internal noise that limits motion perception. Which of the two eyes is more influential? To answer this question, the minimum motion threshold was measured in binocular viewing and was compared with the fixation instabilities of the two eyes. Within a blurred window at 0 and 8.5 deg eccentricities, a random-dot pattern moved in one of eight possible directions differing by 45 deg. The threshold was determined as the speed corresponding to the correct response rate of 53.3% in direction identification. Fixational eye movements of each observer were recorded and the SD of microsaccade-free instantaneous velocities was taken as the index of fixation instability. Inter-observer correlations were based on these data for 56 normal adults. The thresholds at both eccentricities positively correlated with the fixation instability of both eyes, duplicating the previous finding. Interestingly, the positive inter-observer correlation became more evident (r = 0.5, p <.0001) when the threshold was compared with the fixation instability of the more wobbling one of the two eyes. After partialing out this correlation component, there was no more residual correlation between the threshold and the fixation instability of the less wobbling eye. Even when the motion task was conducted in monocular viewing, the performance still correlated with the more wobbling eye's statistics, no matter which eye was actually open during the task. These results suggest that we are calibrated so as not to see motions slower than the velocity noise originating from the eye making larger fixational eye movements.
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