We assessed the usefulness of stereopsis across the visual field by quantifying how retinal eccentricity and distance from the horopter affect humans' relative dependence on monocular and binocular cues about 3D orientation. The reliabilities of monocular and binocular cues both decline with eccentricity, but the reliability of binocular information decreases more rapidly. Binocular cue reliability also declines with increasing distance from the horopter, whereas the reliability of monocular cues is virtually unaffected. We measured how subjects integrated these cues to orient their hands when grasping oriented discs at different eccentricities and distances from the horopter. Subjects relied increasingly less on binocular disparity as targets' retinal eccentricity and distance from the horopter increased. The measured cue influences were consistent with what would be predicted from the relative cue reliabilities at the various target locations. Our results showed that relative reliability affects how cues influence motor control and that stereopsis is of limited use in the periphery and away from the horopter because monocular cues are more reliable in these regions. Keywords: binocular vision, spatial vision, 3D surface and shape perception, grasping, cue integration Citation: Introduction Most conclusions about visual perception have been based on foveal vision since this is where visual acuity and thus performance on most tasks is best, and it is well established that stereopsis contributes to perception and motor control when stimuli are in the central portion of the visual field. However, peripheral regions of the visual field also significantly impact how we navigate through and interact with the world. Information from the periphery is particularly important for planning and executing reaching movements. It helps us plan both the saccades that will move the eyes so that the desired objects project onto the foveae and the reaching movements themselves Surprisingly few studies have focused on stereoacuity, the ability to use binocular disparity as a depth cue, away from the fovea, although it is agreed that thresholds for stereopsis increase with retinal eccentricity. This decrease in sensitivity appears to reflect decreases in the amount of cortical representation in the periphery rather than the visual angle per se Journal of Vision We tested these predictions in three experiments that required human subjects to use monocular and binocular information to estimate the 3D orientations of stimuli at different retinal eccentricities and distances from the horopter. Our first experiment separately measured monocular and binocular thresholds for 3D orientation discrimination at different retinal eccentricities along the theoretical horopter. Then, we used a grasping task to quantify how subjects integrated monocular and binocular information about 3D orientation at these same positions and compared the cue integration strategies we observed with those predicted by sensitivity to the individual cues at each retinal location. In Experiment 3, we investigated Journal of Vision (2009) 9(2):11, 1-16 Greenwald & Knill 2 how increasing the targets' distance from the theoretical horopter affected the contribution of stereopsis to subjects' 3D orientation estimates. Experiment 1: Eccentric monocular and binocular slant thresholds We separately measured 3D orientation thresholds from aspect ratio, a monocular cue, and disparity, a binocular cue, at the fixation point and at two points in the periphery. This enabled us to predict how the relative influences of the cues should change as a function of eccentricity. Method Subjects The ten subjects in this experiment were laboratory staff, graduate students, or postdoctoral fellows in the Department of Brain & Cognitive Sciences and/or the Center for Visual Science at the University of Rochester. All subjects had normal or corrected-to-normal vision and binocular acuity of at least 40 arc seconds, provided written informed consent, and were paid /10 per hour. We used experienced psychophysical observers to obtain the best possible threshold estimates; although they were aware that the purpose of the experiment was to estimate psychophysical thresholds, they were not informed of the details of the staircases we used or of our hypotheses. All experiments reported here followed protocols specified by the University of Rochester Research Subjects Review Board. Apparatus Participants viewed a 20 in. display (1152 Â 864 resolution, 118 Hz refresh rate) through a half-silvered mirror as shown in Journal of Vision (2009) 9(2):11, 1-16 Greenwald & Knill 3 and we occluded their left eye with a patch during monocular trials. Each subject viewed the monocular stimuli with their right eye because stimuli appeared to the right of fixation and Calibration procedures We first identified the locations of each subject's eyes relative to the monitor. At the beginning of each session, the backing of the half-silvered mirror was removed, which allowed subjects to see the monitor and their hand simultaneously. Subjects positioned an infrared marker at a series of visually cued locations so that the marker and a symbol presented monocularly on the monitor appeared to be aligned. Thirteen positions were matched for each eye at two different depth planes, and we calculated the 3D position of each eye relative to the center of the display by minimizing the squared error between the measured position of the marker and the position we predicted from the estimated eye locations. Subjects then moved the marker around the workspace and confirmed that a symbol presented binocularly in depth appeared at the same location as the infrared marker. To calibrate the eyetracker, we recorded the positions of both eyes for binocular conditions or the right eye for monocular conditions as subjects fixated points in a 3 Â 3 grid displayed on the screen. The eyetracker was calibrated at the start of each experimental block and after subjects removed their head from the chinrest, and drift corrections were performed after every five fixation losses or as needed. Fixation losses occurred when subjects looked away from the fixation target or when their measured eye positions drifted significantly from the calibrated positions. Stimuli The stimulus in monocular trials (see In both conditions, a red wireframe sphere (RGB = (0.8,0,0)) with a diameter of 1 cm (approximately 1-of visual angle) served as a fixation target. It appeared 4 cm to the left and 8 cm below the center of the display and 4 cm behind the accommodative plane of the display so that stimuli were near this plane at all retinal eccentricitie
