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.