INTRINSIC UNCERTAINTY AND INTEGRATION EFFICIENCY IN BISECTION ACUITY

A spatial perturbation paradigm was used to determine equivalent intri nsic uncertainty and spatial integration efficiency in bisection. Spec ifically, three-line bisection thresholds were measured in the fovea o f four normal observers with stimulus lines comprised of discrete dark dots distributed randomly around the mean line position according to a Gaussian function. The standard deviation of the Gaussian distributi on (sigma(e)), the number (N), and the strength (C) of the dots as wel l as line separation were varied. Bisection thresholds were modeled by an ideal integrator, from which the magnitude of equivalent internal uncertainty (sigma(i)), the equivalent effective number of dots (k), a nd equivalent integration efficiency (k/N) were quantified. At the 2 m in are separation, sigma(i) decreases (down to a few sec arc) as N and /or C increases. The effects of both N and C can be accounted for by t he stimulus visibility (V, in multiples of detection threshold). At th e 16 min arc separation, sigma(i) is independent of N, C, or V, and is about 1 min arc. The two different forms of sigma(i) indicate that bi section judgments are limited by at least two separate sources of limi ting noise, consistent with the hypothesis of two separate mechanisms (i.e. spatial filters and local signs). A visibility dependent sigma(i ) at the 2 min are separation can be explained on the basis of contras t sensitive spatial filter mechanisms. A fixed sigma(i) at the 16 min arc separation indicates a genuine positional uncertainty, consistent with local-sign mechanisms. Interestingly, equivalent integration effi ciency (k/N) is very similar at the two line separations. k/N is criti cally dependent on, and proportional to C, indicating a common limitat ion in a detection mechanism.