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Chiara Listorti, Martina Poletti, Michele Rucci; Spatial localization during fixation does not depend on an extraretinal drift signal . Journal of Vision 2010;10(7):170. doi: 10.1167/10.7.170.
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© ARVO (1962-2015); The Authors (2016-present)
We are normally unaware of the eye movements which, during visual fixation, keep the retinal image continually in motion. How does the visual system disregard the retinal motion caused by these movements to achieve visual stability? According to extraretinal theories, stability is attained by means of motor/proprioceptive signals; according to retinal theories, eye motion is, instead, inferred directly from the spatiotemporal stimulus on the retina. In this study, we focus on the retinal motion caused by ocular drift. We have previously shown that motion detection does not rely on possible extraretinal drift signals. Here, we investigated whether this is true also for spatial localization. In a 2AFC experiment, subjects reported which one of two dots briefly displayed at distinct times during ocular drift was at the same spatial location of a reference presented at the beginning of each trial. One of the two dots was displayed at the same spatial (monitor) position of the reference, whereas the other was at the same retinal position. Stimuli were displayed in complete darkness. If an extraretinal signal is used, subjects should be able to correctly identify the dot at the same monitor position of the reference. Moreover, discrimination performance should increase with the extent of drift, as the extraretinal signal would also increase. Subjects systematically reported that the dot at the same retinal position of the reference was the one at the same spatial location. Furthermore, the probability of this error increased, rather than decreased, with the size of ocular drift. These results strongly support the predictions of retinal theories. Like motion detection, also spatial localization does not depend on an extraretinal drift signal, but relies instead on the spatiotemporal image on the retina to discard the retinal motion caused by ocular drift.
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