Sachio Nakamizo

Greater depth seen with phantom stereopsis is coded at the early stages of visual processing

A visual search task was used to investigate the spatially parallel coding of depth from binocular disparity and from binocularly unmatched features. Experiment 1, using disparity noise, showed that detectability is higher for illusory phantom targets defined by unmatched features than for disparity-defined targets, although the two targets were equated as to theoretically minimum depth. Experiment 2, using binocularly unmatched noise whose width was equal to the disparity of the noise used in Experiment 1, showed that noise severely interferes with the detection of both the disparity and the phantom targets. These results are consistent with the idea that the greater depth seen with phantom stereopsis is coded at the early stages of visual processing.

Stereoscopic depth aftereffects without retinal position correspondence between adaptation and test stimuli.

To clarify whether stereo-slant aftereffects are independent of stimulated retinal position, two experiments compared the magnitude of aftereffects between the following two conditions: when the adaptation and test stimulus fell on (1) the same retinal position, and (2) on different retinal positions separated by 0.5 degrees -20 degrees . In Experiment 1, disc- or ring-shaped surface consisting of random-dots was presented at the central or peripheral visual fields. In Experiment 2, rectangular surface was presented at the upper or lower visual fields. After two minutes inspection of a random-dot stereogram depicting a +/-30 degrees slanted surface, the observer adjusted the slant of the test stimulus to appear fronto-parallel. The results of the experiments showed that significant aftereffects were observed similarly in both conditions. Moreover, the separation nor the stimulus shape scarcely affected the magnitude of the aftereffects. Based on these results we concluded that the depth processing mechanism which operates independently from the stimulated retinal position is responsible for the depth aftereffects we found.

A long-distance stereoscopic detector for partially occluding surfaces

An external noise technique was used to investigate the stereoscopic process that generates an illusory phantom occluder from binocularly unmatched elements. Observers were required to identify the quadrant in which a binocularly defined target was presented. We had three targets: (a) two vertical binocular bars with the unmatched portions arranged to induce a stable phantom occluder (valid), (b) the same stimuli except the image for the left eye was switched with that for the right eye therefore not inducing a stable occluder (invalid), and (c) a single binocular bar with the same unmatched portion (single-bar). For each target, the luminance contrast of the signal required for 75% correct responses was measured at four levels of external interocular noise. Contrast thresholds were found to be lower for the valid target than for both the invalid and the single-bar targets. The results suggest that the visual system has a stereoscopic detector that responds to stimuli that meet a long-distance requirement for the perception of partially occluding surfaces.

Depth scaling in phantom and monocular gap stereograms using absolute distance information.

The present study aimed to investigate whether the visual system scales apparent depth from binocularly unmatched features by using absolute distance information. In Experiment 1 we examined the effect of convergence on perceived depth in phantom stereograms [Gillam, B., & Nakayama, K. (1999). Quantitative depth for a phantom surface can be based on cyclopean occlusion cues alone. Vision Research, 39, 109-112.], monocular gap stereograms [Pianta, M. J., & Gillam, B. J. (2003a). Monocular gap stereopsis: manipulation of the outer edge disparity and the shape of the gap. Vision Research, 43, 1937-1950.] and random dot stereograms. In Experiments 2 and 3 we examined the effective range of viewing distances for scaling the apparent depths in these stereograms. The results showed that: (a) the magnitudes of perceived depths increased in all stereograms as the estimate of the viewing distance increased while keeping proximal and/or distal sizes of the stimuli constant, and (b) the effective range of viewing distances was significantly shorter in monocular gap stereograms. The first result indicates that the visual system scales apparent depth from unmatched features as well as that from horizontal disparity, while the second suggests that, at far distances, the strength of the depth signal from an unmatched feature in monocular gap stereograms is relatively weaker than that from horizontal disparity.

An Empirical Test of Formal Equivalence between Emmert’s Law and the Size-Distance Invariance Hypothesis

Emmerts law and the size-distance invariance hypothesis have been said to be formally equivalent, provided that Emmerts law means that the perceived size of an afterimage is proportional to the perceived distance of the projected surface of the afterimage. However, there have been very few studies that have attempted to verify this formal equivalence empirically. We measured both the perceived size and distance of afterimages and real objects with the same proximal size. Nineteen participants projected afterimages of 1 deg in visual angle on the wall located at distances of 1 to 23 meters from the participants. They also observed real objects, disc-shaped and made from a sheet of Styrofoam board, with the same proximal size as that of the afterimages, which were located at the same physical distances as those of the wall on which the afterimages were projected. Each participant reproduced the apparent sizes of the afterimages and real objects using the reproduction method and estimated the apparent distances using the magnitude estimation method. When the mean apparent sizes of the afterimages and real objects, represented as a function of apparent distance, were fitted to a linear function, the slopes for the afterimages and real objects did not differ significantly. These results are interpreted as evidence for the formal equivalence of Emmerts law and the size-distance invariance hypothesis.

Evidence for the correcting-mechanism explanation of the Kanizsa amodal shrinkage

An object phenomenally shrinks in its horizontal dimension when shown on a 2-D plane as if the central portion of the object were partially occluded by another vertical one in 3-D space (the Kanizsa amodal shrinkage). We examined the predictions of the correcting-mechanism hypothesis proposed by Ohtsuka and Ono (2002, Proceedings of SPIE 4864 167 – 174), which states that an inappropriate operation of the mechanism that corrects a phenomenal increase in monocularly visible areas accompanied by a stereoscopic occluder gives rise to the illusion. In this study we measured the perceived width (or height in experiment 3) of a square seen behind a rectangle, while controlling other factors which potentially influence the illusion, such as the division of space or depth stratification. The results of five experiments showed that (a) the perceived width was not influenced when the occluder had a relatively large binocular disparity, but was underestimated when the occluder did not have disparity, and (b) the shrinkage diminished when the foreground rectangle was transparent, was horizontally oriented, or contained no pictorial occlusion cues. These results support the hypothesis that the correcting mechanism, triggered by pictorial occlusion cues, contributes to the Kanizsa shrinkage.

The Pulfrich effect and depth constancy

Perceived depth of the Pulfrich effect was measured as a function of viewing distance and target velocity. The stimulus was a vertical rod, moving sinusoidally back and forth along a horizontal path 22.5 cm long, within the observers frontoparallel plane. The two independent variables were the viewing distance (1, 2, 3, and 4 m) and the target velocity (.2, .4, and .6 Hz). Observers viewed the stimulus binocularly through a neutral-density filter (1.0 OD) in front of the right eye and were then required to perform two tasks. The first was to adjust the position of a depth probe so that it appeared to be at the same location as the target in the center of its apparent path. The other was to reproduce the extent of the perceived depth of the target from the stimulus plane in the center of its path. The results with eight observers showed that the magnitude of the perceived depth for all target velocities in either task increased almost in proportion to the viewing distance. This result suggests that the visual system calibrates an equivalent of binocular disparity, which is assumed to be transformed from the visual latency difference in the Pulfrich effect, through the use of absolute distance information.

Visual Latency Difference Determined by Two ‘‘Rotating’’ Pulfrich Techniques

We tested the validity of Nickalls’ formula for determining visual latency difference by using two rotating Pulfrich techniques: (A) varying viewing distance while keeping target angular velocity constant (33 rev/min) and (B) varying the target angular velocity while keeping the viewing distance constant (180 cm). The formula predicts that the latency differences estimated by the two techniques are equal with a given neutral density filter. Observers were asked to judge whether or not the rotating target (clockwise) appeared to move back-and-forth from side-to-side with a neutral density filter (OD = 0.7, 1.0, 1.3) in front of the right eye. The results with ten observers showed that the mean visual latency differences associated with each technique for a given filter were not significantly different. These results further validate the Nickalls’ formula and, therefore, support the visual-latency hypothesis to account for the Pulfrich phenomenon.

Oculomotor responses of stereo‐anomalous observers to pulse and ramp disparities

We examined oculomotor responses to binocular disparities of one stereo-normal and three stereo-anomalous observers, who were identified through a stereoscopic depth-discrimination task, in two experimental conditions. In the pulse disparity condition, crossed and uncrossed disparities (1–6°) were briefly presented (for .25–2.0 s). In the ramp disparity condition, disparities were varied continuously with constant velocities (.7–2.0°/s) and with an amplitude of 10°. The stereo-normal observer showed vergence responses to both pulse and ramp disparities. The three stereo-anomalous observers showed a marked reduction or absence of vergence responses to pulse disparities but showed vergence responses to ramp disparities. The results suggest the existence of separate sub-systems mediating disparity vergence eye movements.

Fusional vergence hysteresis and sensorimotor connection of the two eyes

Three experiments were conducted to replicate and extend Helmholtzs observation indicating vergence hysteresis. The two stimuli were presented dichoptically to the eyes in the mirror stereoscope and moved temporalward or nasalward. Helmholtzs finding was confirmed in Experiment 1, in which the critical separations were measured for the stimuli divergently moving temporalward when fusion of their images was abruptly broken (breakaway point) and when fusion was re-established (refusion point) for the stimuli moving nasalward. In Experiment 2, in which only the stimulus for one eye moved temporalward while the stimulus for the other eye was stationary, the breakaway point of the moving stimulus was found to be linearly dependent on the position of the stationary stimulus. Experiment 3 showed that, in the asymmetric divergent tracking as in Experiment 2, the change of the perceived visual direction of the fused image was smaller than the angular magnitude of the movement of the tracking eye. These results were discussed as evidence indicating the sensory and motor connection of the two eyes.