Vernier acuity experiments were carried out to obtain estimates of the minimal number of spatial samples extrated, and of the equivalent noise figure characterizing inherent limitations of the visual system in relative position detection tasks. Results are interpreted by modelling the visual system as an ideal detector limited by external noise and internal position uncertainty resulting from structural limitations and neural noise. Accordingly, we estimate that less than ten samples per line are extracted by the observer and that the equivalent noise figure or position uncertainty is of the order of 20 arc s.
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