Comparisons of 1st- and 2nd-order stereopsis have typically employed isolat
ed, or local, narrow-band targets. While these experiments have revealed a
great deal about the distinction between these two types of processing, suc
h stimuli are rare in the natural environment. Instead, local disparity sig
nals are more likely to be part of extended surfaces that vary smoothly in
depth. The aim of the experiments presented here is to determine the relati
ve contribution of 1st- and 2nd-order stereopsis to the perception of depth
-modulated surfaces. Stereothresholds were measured under a range of condit
ions designed to isolate either 1st- or 2nd-order processing. The results d
emonstrate that while 2nd-order stereopsis provides local depth estimates f
or individual texture elements, 1st-order processing is essential to the gl
obal interpolation of those estimates across surfaces. (C) 1999 Elsevier Sc
ience Ltd. All rights reserved.