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Martina Poletti, Murat Aytekin, Michele Rucci; Binocular head/eye coordination during natural fixation. Journal of Vision 2014;14(10):199. doi: https://doi.org/10.1167/14.10.199.
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© ARVO (1962-2015); The Authors (2016-present)
Incessant eye movements (ocular drift and tremor) shift the image on the retina during the brief fixation intervals in between saccades, the very periods in which visual information is acquired and processed. Because of the difficulty in accurately measuring microscopic eye movements, ocular drift has been typically studied with the head restrained, an artificial condition that minimizes recording artifacts. In this condition, ocular drift resembles Brownian motion, with loosely correlated or uncorrelated motion in the two eyes. Here, we examine ocular drift during natural head-free fixation, when the eyes also continually translate in space because of microscopic head movements. Traces of head and eye movements were acquired by means of the Maryland Revolving Field Monitor, an experimental device which enables oculomotor recordings at resolution higher than 1 arcmin during normal head-free viewing. We show that, contrary to the widespread assumption, ocular drift is under oculomotor control and highly correlated in the two eyes. The two eyes drifted faster during intersaccadic head-free fixation than under traditional head-restrained conditions and moved together to finely compensate for head movements, even when eye and head speeds were well below 2 deg/s. As a consequence, the resulting motion on the retina was comparable to that measured with the head restrained, when ocular drift was the only contributor to retinal image motion. We show that in the absence of this precise head/eye compensation during fixation: (1) the spatiotemporal frequency content of retinal stimulation would be severely altered; and (2) the retinal projection of the stimulus would quickly leave the foveola. Thus, our results show that the smooth inter-saccadic movement of the eye is under oculomotor control. During natural fixation, ocular drift is part of a compensatory mechanism, which aims at maintaining retinal image motion within an optimal range for vision.
Meeting abstract presented at VSS 2014
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