The effects of blur and size on monocular and stereoscopic localization

Monocular localization of non-abutting stimuli and stereoscopic localizatio n of the same second-order targets are performed with the same precision (W ilcox, L.M. & Hess, R.F. (1996) Is the site of non-linear filtering in ster eopsis before or after binocular combination? Vision Research, 36, 391-399) . Further, both tasks show a similar dependence on the scale of the stimulu s. Since prior studies used Gaussian-enveloped stimuli, modifications of st imulus scale produced concurrent changes in edge blur. The experiments repo rted here assess the relative contributions of size and blur to the observe d dependence on envelope scale for both monocular localization and stereoac uity. Stereoacuity for first-order targets was found to be an order of magn itude better than stereoacuity for second-order targets and monocular acuit y for both first- and second-order targets. Further, while first-order ster eopsis was found to depend solely on blur, second-order stereoacuity and mo nocular acuity were affected by both size and blur. These results suggest t hat while stereoacuity for first-order stimuli may be determined by a corre lative process limited by early additive noise, stereoacuity for second-ord er stimuli and monocular acuity for non-abutting targets are more likely li mited by stimulus-dependent spatial subsampling. (C) 2000 Elsevier Science Ltd. All rights reserved.