Common processing for two perceptual tasks in different spatial dimensions in response to identical visual stimuli

Abstract

The frontoparallel orientation of an eccentric luminescent line presented in darkness causes visual mislocalizations in both the perception of elevation (VPEL) and the perception of the vertical in the frontoparallel-plane (VPV). However, when two such lines are centered at equal distances relative to the observer' median plane, then if the lines are parallel they influence VPV but not VPEL, and if the lines are counter-rolled (bilaterally-symmetrical) they influence VPEL but not VPV (Matin & Li, 1994). Thus, despite the similar influences of line orientation on both discriminations, the summation of influences from two lines implies distinct integration processes. The current study compared the VPEL and VPV settings of 30 observers in response to the same set of stimuli. Measurements were made on observers with monocular viewing with each of 5 orientations in the frontal plane, 0° (erect), ±7.5°, and ±15°, with each of four stimuli: 1) one left-line, 2) one right-line, 3) two parallel-lines, and 4) two counter-rolled-lines. Each stimulus line was 70°-long and centered at 25° horizontal eccentricity. We correlated across observers the effect of orientation (defined as the slope of setting-vs-orientation) of the same set of stimuli on both discriminations, and found substantial correlations between the two discriminations (average of absolute r-values=0.56). We also correlated across observers the effects of individual off-vertical orientations of the same set of stimuli on both discriminations, and found substantial correlations between the two discriminations (average of absolute r-values=0.43). The two separate correlational analyses agree and indicate a similar response to line orientation in both discriminations. In light of the evidence for distinct integration processes, these findings support the view that the two discriminations may share common orientation processes that later distribute to separate integration processes.