Vision Res. Vol. 30, No. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 11, pp.IS%-1560, 1990 00424989/90 53.00 + 0.00 Printed zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA in Grest Britain. All rights reserved Copyright 0 1990 Pergamon Press pk THE SENSITIVITY OF SEPARATION DISCRIMINATION TO SPATIOTEMPORAL JITTER zyxwvutsrqponmlkjihgfedcbaZ DAVID R. BADCOCK’ and TERRENCE L. WONG* ‘Department of Psychology, University of Melbourne, Parkvillc, Victoria, Australia and 2Graduate Group in Biophysics, University of California, Berkeley, CA 94720, U.S.A. (Received 10 August 1989; in revtiedform 16 January 1990) Abstract-Differences of less than 20 set of visual at&e in the separation of a pair of closely spaced parallel lines can be reliably detected. This ability is known as a hyperacuity because the thresholds are smaller than the diameter of one fovea1 cone. It is shown that this ability does not requite a stationary pattern. Indeed, correlated horizontal jitter of the line pair has little detrimental effect on performance for jitter that ranges up to 8 min arc for two lines with a separation of only 6 min am Uncorrelated jitter of the two tines, which allows the actual separation to vary from moment to moment, causes performance to deteriorate at a rate similar to the rise of signal uncertainty. The results reflect the operation of a system which is not only extremely robust to oculomotor instability but is also robust to positional variation that could not be produced by eye movements. Hyperacuity Position Noise Separation discrimination INTRODUCI’ION The eyes of human observers are continually in motion (Riggs, Armington & Ratliff, 1954; Ditchbum, 1973; Hallett, 1986; Matin, 1986). In spite of this motion, observers are capable of extremely fine discriminations of the relative position of objects (Berry, 1948; Westheimer, 1975, 1979; Klein & Levi, 1985; Badcock & Westheimer, 1985a, b). Westheimer and McKee (1977) have shown that the ability to align two briefly presented vertical lines is not affected by sweeping the line pair across the retina at speeds up to approximately three degrees per second. Morgan, Watt and McKee (1983) replicated this observation for short stimulus presentations and, further, demonstrated that the improved performance gained by repeated stimulus pres- entation was affected by drifting the stimuli across the screen. In both cases, Morgan et al. (1983) still have thresholds less than 30 set arc, indicating that despite a degraded performance level with moving stimuli this is still a hyper- acuity task. Several investigators (Fahle & Poggio, 1981; Morgan & Watt, 1982; Morgan et al., 1983) have examined the mechanisms capable of in- terpolation along a linear path which could produce the performance reported by West- heimer and McKee (1977). The direction and extent of oculomotor jitter, however, is not uniform (Ditchbum, 1973). In spite of this, none of these studies have examined the more realistic situation in which the pattern moves randomly to and fro, rather than in a single direction with a fixed speed during a trial. If the ability to ignore motion of the retinal image is a consequence of a development in- tended to eliminate noise due to the motions of the eyes, then observers should also be able to ignore jitter that changes direction rapidly and at random. In order to explore this ability, a two line separation task was used. It will be shown that observers are remarkably adept at ignoring jitter, even if that jitter causes the separation between the two lines to randomly vary over time. METHOD In each condition a pair of parallel lines 10 min arc high and substantially narrower than the point spread function of the eye, were presented with a mean separation of 6 min arc for 6OOmsec. After a 1OOmsec dark inter- stimulus interval another pair of lines were shown for 600 msec. The second pair was either the same separation, 11.25,22.5 or 33.75 set arc wider or narrower than the first. The observers were asked to indicate by setting a switch whether the second line pair appeared narrower or wider than the first. 1555