Sequential stereopsis using high-pass spatial frequency filtered textures

Extra-retinal signals support the estimation of 3D motion

In natural settings, our eyes tend to track approaching objects. To estimate motion, the brain should thus take account of eye movements, perhaps using retinal cues (retinal slip of static objects) or extra-retinal signals (motor commands). Previous work suggests that extra-retinal ocular vergence signals do not support the perceptual judgments. Here, we re-evaluate this conclusion, studying motion judgments based on retinal slip and extra-retinal signals. We find that (1) each cue can be sufficient, and, (2) retinal and extra-retinal signals are combined, when estimating motion-in-depth. This challenges the accepted view that observers are essentially blind to eye vergence changes.

The wallpaper illusion explained

In the wallpaper illusion, a repetitive pattern appears to shift from one depth plane to the plane nearest fixation. We measured the timing of this shift for a 6 degrees wide, 3-cpd sinusoidal grating presented in a rectangular envelope; the edges (envelope) of the grating were presented at 20 arcmin of disparity (one period) behind the fixation plane. We asked observers to signal when the segment appeared to move from the edge plane forward to the fixation plane. Initially, the shift from the edge plane took 4-6 s, but after many trials, the shift became faster. Additional experiments demonstrated that the envelope was adapting, thereby permitting the alternative match. Our measurements for a range of spatial frequencies and disparities showed that these shifts to the fixation plane occurred only if the envelope disparity was more than one-half period of the carrier; that is, phase disparity >180 degrees. We also found that stereoacuity for the initial envelope-based match was poor, as might be expected for a target presented far off the fixation plane. However, once the perceived shift in depth occurred, stereoacuity improved fivefold without any change in the physical stimulus. We speculate that access to the most sensitive V1 neurons depends on the extrastriate processes that determine perceived depth--in this case, second-order envelope mechanisms.

Exploring sequential stereopsis for co-planarity tasks

We used the sequential stereopsis paradigm and apparatus described by Enright (Vision Research, 36, (1996) 307-312). The observer's task was to set targets to equidistance in Experiments 1-3, and to make them co-planar in Experiment 4. However, it is argued that in all experiments observers exploited a co-planarity setting strategy. Sequential stereopsis produced good performance throughout in terms of low disparity thresholds when head position was varied by rotations around three axes: vertical (azimuth condition); horizontal (elevation); and midline (tilt). It also produced good performance when the targets were shifted in position so that they both lay on one side of the median plane of the head. These results cannot be accounted for by Enright's isovergence hypothesis unless it is extended to incorporate other information about eye positions. Performance was better but not greatly so in control simultaneous stereopsis conditions, nor did it deteriorate much when the observer's view was restricted solely to the targets by removing visibility of the room in which the apparatus was located. Target settings were typically located on a concave arc centred on the median plane. This effect was quantitatively modelled using disparity correction for a relief task of co-planarity (Gârding, Porrill, Mayhew, & Frisby. Vision Research, 35 (1995) 703-722). This modelling indicated over-estimations of c.10-20 cm in fixation distance for target distances in the range 71.5-112.5 cm.