Depth perception of stereoscopic transparent stimuli with frame manipulation.

Depth perception is crucial in human vision, allowing us to move and interact with our 3-D surroundings. We used a stereoscopic transparent stimulus comprising parallel overlapping transparent stereoscopic surfaces (POTS) to understand depth perception better. The study focused on exploring the effect of a surrounding frame on the perceived depth of a POTS configuration. The research was based on a proposed idea that explains an "off-frame" effect: a frame at a different depth from a 2-D photograph depicting a 3-D scene increases its apparent depth qualitatively. The idea assumes that processing the disparity between a frame and a photo reduces the reliability of the photograph's flatness cues and increases depth magnitude in depth cue integration. We examined whether the idea can be applied to a 3-D POTS with the flatness cue as the constant accommodation. Through three experiments, the study showed that frames impact the perceived depth magnitude of a POTS configuration. More specifically, the depth magnitude increases as the frame's disparity concerning the monitor plane increases and decreases as the frame's size increases. We discussed the results in the context of depth cue combination.

Effect of 3-D depth structure, element size, and area containing elements on total-element overestimation phenomenon.

The number of elements distributed in a three-dimensional stimulus is overestimated compared to a two-dimensional stimulus when both stimuli have the same number of elements. We examined the effect of the properties of a three-dimensional stimulus (the number of overlapping stereo surfaces, size of the elements, and size of the area containing elements, on the overestimation phenomenon in four experiments. The two stimuli were presented side-by-side with the same diameters. Observers judged which of the three-dimensional standard and two-dimensional comparison had more elements. The results showed that (a) the overestimation phenomenon occurred for the three-dimensional standard stimuli, (b) the size of the areas affected the amount of overestimation, while the number of overlapping stereo surfaces and size of elements did not, and (c) the amount of overestimation increased when the stimuli included more than 100 elements. Implications of these findings were discussed in the framework of back-surface bias, occlusion, and disparity-processing interference models.

A frame at a different depth than a photograph enhances the apparent depth in the photograph.

Two experiments showed that a photo frame placed at a distance from the photo itself enhances the apparent depth of contents within the photo. In Experiment 1, 32 observers rated the apparent depth of 60 successively presented photos of scenes with pictorial depth cues. In a separate block of trials, the photos were presented either with a frame in front of each photo or without a frame. Observers also assessed apparent depth of the same 60 photos by choosing the photo that had greater apparent depth when a framed and a frameless photo of the same scene were presented side by side. We found that mean depth ratings were higher for framed than for frameless photos, and framed photos were chosen more often than frameless photos. In Experiment 2, 12 observers rated the apparent depth of 20 different photos that were successively presented with or without a frame. The frame was placed in front of, at the same distance as, or behind each photo. Mean ratings for front-framed and behind-framed photos were higher than those for equidistant-framed or frameless photos, and mean ratings increased with the distance between the photo and the frame. We hypothesize that having to process the relative depth between a photo and a frame reduces the effectiveness of flatness information provided by the photo.

Interaction of disparity size and depth structure on perceived numerosity in a three-dimensional space.

The number of elements in two stereo-surfaces parallelly overlapped in depth is overestimated compared to that in a single flat surface, even when both have the same number of elements. Using stereoscopic pairs of elements, we evaluated two hypotheses on the overestimation: one that a higher-order process, forming a background surface, increases the number of perceived elements, and the other that the number of elements potentially occluded by the elements on a front surface is taken accounted for. The data from four experiments showed that (a) when binocular disparity between (or among) stereoscopic elements was small, the overestimation occurred for the stimuli we used-a two-surface-overlapping stimulus, where the likelihood for the process to operate was manipulated by changing the averaged luminance of each surface, a volumetric stimulus, where the likelihood for the background surface to be formed would decrease, and a two-non-overlapping-surface stimulus, where the surfaces in depth were not overlapped-, and (b) when binocular disparity was large, the overestimation occurred for the two-surfaces-overlapping stimulus, when the averaged luminance of the two surfaces were the same, and for the volumetric stimulus, but diminished for the surface-overlapping stimulus, when the averaged luminance differed between the surfaces and for the surfaces-non-overlapping stimulus. These results cannot be explained either hypothesis only. We explain the results by postulating that the sensory system processing disparities of elements interferes with that estimating the number of elements, resulting in an overestimation of the elements in a stereo-stimulus, and the disparity range within which the interference occurs may depend on the stimulus depth structure.

Slant of a Surface Shifts Binocular Visual Direction.

We demonstrate how the slant of a surface affects the relative visual direction between binocular stimuli. In two experiments, we measured the visual direction of a binocular stimulus at different distances in the mid-sagittal plane or in the transverse plane at eye level relative to the center of the stimulus field. Experiment 1 showed that when a binocular stimulus (a vertical bar) was presented in front of or behind a surface slanted along the vertical center of the surface, its visual direction shifted toward the surface. Experiment 2 showed that when a binocular stimulus (a horizontal bar) was presented in front of or behind a surface slanted along the horizontal center of the surface, its visual direction also shifted toward the surface. These results indicate that the slant of a surface should be listed among the variables that contribute to the binocular visual direction, as well as the retinal loci of the stimulus, binocular eye position, the location of the visual egocenter, and stimulus properties.

Visual Presentation Effects on Identification of Multiple Environmental Sounds.

This study examined how the contents and timing of a visual stimulus affect the identification of mixed sounds recorded in a daily life environment. For experiments, we presented four environment sounds as auditory stimuli for 5 s along with a picture or a written word as a visual stimulus that might or might not denote the source of one of the four sounds. Three conditions of temporal relations between the visual stimuli and sounds were used. The visual stimulus was presented either: (a) for 5 s simultaneously with the sound; (b) for 5 s, 1 s before the sound (SOA between the audio and visual stimuli was 6 s); or (c) for 33 ms, 1 s before the sound (SOA was 1033 ms). Participants reported all identifiable sounds for those audio-visual stimuli. To characterize the effects of visual stimuli on sound identification, the following were used: the identification rates of sounds for which the visual stimulus denoted its sound source, the rates of other sounds for which the visual stimulus did not denote the sound source, and the frequency of false hearing of a sound that was not presented for each sound set. Results of the four experiments demonstrated that a picture or a written word promoted identification of the sound when it was related to the sound, particularly when the visual stimulus was presented for 5 s simultaneously with the sounds. However, a visual stimulus preceding the sounds had a benefit only for the picture, not for the written word. Furthermore, presentation with a picture denoting a sound simultaneously with the sound reduced the frequency of false hearing. These results suggest three ways that presenting a visual stimulus affects identification of the auditory stimulus. First, activation of the visual representation extracted directly from the picture promotes identification of the denoted sound and suppresses the processing of sounds for which the visual stimulus did not denote the sound source. Second, effects based on processing of the conceptual information promote identification of the denoted sound and suppress the processing of sounds for which the visual stimulus did not denote the sound source. Third, processing of the concurrent visual representation suppresses false hearing.

Overestimation of the number of elements in a three-dimensional stimulus.

Observers' numerosity judgments in binocular stereopsis were examined in four experiments, using random-dot stereograms (RDSs) that depicted a two-dimensional (2-D) stimulus side-by-side with a three-dimensional (3-D) stimulus. When the RDSs were correctly fused, a single surface and two (or three) transparent surfaces were observed for the 2-D and 3-D stimuli, respectively. Observers completed a numerosity discrimination task, where they judged which of the two stimuli had a greater number of dot elements. Results showed that (a) the 3-D stimulus was judged to contain more elements than the 2-D stimulus, even when both had the same number of elements, (b) the amount of overestimation increased as a function of the number of elements and the binocular disparity between the front and back surfaces of the 3-D stimulus, (c) the ratio of the physical number of elements in the front surface to that in the back surface of the 3-D stimulus had no effect on the magnitude of overestimation, and (d) when the number of elements for the two surfaces were judged separately, the ratio had more effect on the judged number of elements in the back surface than in the front surface. These results indicate that the extent of overestimation in the numerosity judgment of a set of elements in a stimulus depends on the number of depth layers in which the elements are embedded.

Magnitude of perceived depth of multiple stereo transparent surfaces.

According to the geometric relational expression of binocular stereopsis, for a given viewing distance the magnitude of the perceived depth of objects would be the same, as long as the disparity magnitudes were the same. However, we found that this is not necessarily the case for random-dot stereograms that depict parallel, overlapping, transparent stereoscopic surfaces (POTS). The data from five experiments indicated that (1) the magnitude of perceived depth between the two outer surfaces of a three- or a four-POTS configuration can be smaller than that for an identical pair of stereo surfaces of a two-POTS configuration for the range of disparities that we used (5.2-19.4 arcmin); (2) this phenomenon can be observed irrespective of the total dot density of a POTS configuration, at least for the range that we used (1.1-3.3 dots/deg(2)); and (3) the magnitude of perceived depth between the two outer surfaces of a POTS configuration can be reduced as the total number of stereo surfaces is increased, up to four surfaces. We explained these results in terms of a higher-order process or processes, with an output representing perceived depth magnitude, which is weakened when the number of its surfaces is increased.

Apparent depth of pictures reflected by a mirror: the plastic effect.

We investigated the plastic effect in picture perception, in which the apparent depth of a picture is increased when it is reflected by a mirror. The plastic effect was well known in the mid-18th century, but very few studies have elucidated its nature. In Experiment 1, we examined how often the plastic effect occurs in different ocular conditions. A group of 22 observers compared directly observed pictures and their mirror-reflected images in each of free-binocular, free-monocular, and restrictive-monocular conditions. When the observers were forced to choose the picture that appeared greater in depth, 73 % of them chose the reflected pictures, regardless of oculomotor condition. In Experiment 2, we examined how often the plastic effect is detected as a function of observation time. When 22 observers compared a directly watched movie and its mirror-reflected movie for 5 min, the number of observers who judged the reflected movie to be greater in depth was about 55 % at the onset of the trial but was 86 % at the end. In Experiment 3, we examined transfer of the plastic effect. Ten observers judged the change in apparent depth of directly observed pictures after prolonged exposure to the same reflected or actual pictures. Transfer was confirmed and was greater for pictures that represented greater depth (r = .88). We suggested that the plastic effect is mainly induced by the double apparent locations of a reflected picture. From the long incubation time and the transfer to real pictures, we also suggested that it involves perceptual learning regarding visual skill.

Dual-egocentre hypothesis on angular errors in visually directed pointing.

We examined the hypothesis that angular errors in visually directed pointing, in which an unseen target is pointed to after its direction has been seen, are attributed to the difference between the locations of the visual and kinesthetic egocentres. Experiment 1 showed that in three of four cases, angular errors in visually directed pointing equaled those in kinesthetically directed pointing, in which a visual target was pointed to after its direction had been felt. Experiment 2 confirmed the results of experiment 1 for the targets at two different egocentric distances. Experiment 3 showed that when the kinesthetic egocentre was used as the reference of direction, angular errors in visually directed pointing equaled those in visually directed reaching, in which an unseen target is reached after its location has been seen. These results suggest that in the visually and the kinesthetically directed pointing, the egocentric directions represented in the visual space are transferred to the kinesthetic space and vice versa.

Psychophysical evidence for a purely binocular color system.

Two adaptation experiments were conducted to examine a hypothesis for a purely binocular color system that responds only to simultaneous inputs from the two eyes and that inhibits the activities of a pair of monocular color systems with each receiving input from their respective eye. In the first experiment, after a red or green stimulus was presented to both eyes to adapt the hypothesized binocular system, its compensatory color was presented alternately to each eye to nullify the adaptation effect of the hypothesized monocular systems. Results showed that after adaptation, the color appearance of a test stimulus shifted more to that of the compensatory color in binocular viewing than in monocular viewing. In the second experiment, a red or green stimulus was presented either to both eyes or to the left eye, and then its compensatory color was presented only to the left eye. Comparison was made to the adaptation effect between the binocular presentation of the color stimulus and its monocular presentation. Results showed that the color appearance viewed with the left eye shifted toward the compensatory color for the binocular adaptation and was constant for the monocular adaptation. These results are consistent with the idea of a "purely" binocular color system inhibiting the activity of a pair of monocular systems.

Localization of monocular stimuli in different depth planes.

We examined the phenomenon in which two physically aligned monocular stimuli appear to be non-collinear when each of them is located in binocular regions that are at different depth planes. Using monocular bars embedded in binocular random-dot areas that are at different depths, we manipulated properties of the binocular areas and examined their effect on the perceived direction and depth of the monocular stimuli. Results showed that (1) the relative visual direction and perceived depth of the monocular bars depended on the binocular disparity and the dot density of the binocular areas, and (2) the visual direction, but not the depth, depended on the width of the binocular regions. These results are consistent with the hypothesis that monocular stimuli are treated by the visual system as binocular stimuli that have acquired the properties of their binocular surrounds. Moreover, partial correlation analysis suggests that the visual system utilizes both the disparity information of the binocular areas and the perceived depth of the monocular bars in determining the relative visual direction of the bars.

Contraction of perceived size and perceived depth in mirrors

We investigated how size and depth are perceived in a plane or convexmirror. In Experiment 1, using a plane or convex mirror, 20 observersviewed a separation between two objects that were presented at a constantdistance and reproduced it by a separation between other two objects in anatural viewing situation. The mean matches generally approximated thereal size of the standard and did not equal either virtual size or visual angle ofthe standard. In addition, the mean matches obtained with convex mirrorswere reduced by about 7% in comparison with those obtained with the planemirror. In Experiment 2, we examined whether the perceived depth in aconvex mirror is comparable to that in a plane mirror. We presentedisosceles triangles on a table and required 12 observers to observe them witha plane or convex mirror. With the method of limits, we determined thetriangle that was perceived as an equilateral triangle. When the apexes ofisosceles triangles were directed to the observer or to depth, the ratio ofheight to base was larger in convex mirrors than in the plane mirror,whereas when the apexes were directed to left or to right, the ratio of heightto base was smaller in the convex mirrors than in the plane mirror. Thecontraction of perceived depth amounted to about 6% in convex mirrors.The results of both experiments suggest that although separation and depthin convex mirrors appear to reduce, there is a strong tendency that visualsystem recovers the optical distortions by convex mirrors

Mirror vision: perceived size and perceived distance of virtual images.

We investigated spatial perception of virtual images that were produced by convex and plane mirrors. In Experiment 1, 36 subjects reproduced both the perceived size and the perceived distance of virtual images for five targets that had been placed at a real distance of 10 or 20 m. In Experiment 2, 30 subjects verbally judged both the perceived size and the perceived distance of virtual images for five targets that were placed at each of five real distances of 2.5-45 m. In both experiments, the subjects received objective-size and objective-distance instructions. The results were that (1) size constancy was attained for a distance of up to 45 m, (2) distance was readily discriminated within this distance range, although virtual images produced by the mirror of strong curvature were judged to be farther away than those produced by the mirrors of less curvature, and (3) the ratio of perceived size to perceived distance was described as a power function of visual angle, and the ratio for the convex mirror was larger than that for the plane mirror. We compared the taking-into-account model and the direct perception model on the basis of a correlation analysis for proximal, virtual, and real levels of the stimuli. The taking-into-account model, which assumes that visual angle is transformed into perceived size by taking perceived distance into account, was supported by an analysis for the proximal level of stimuli. The direct perception model, which assumes that there is no inferential process between perceived size and perceived distance, was partially supported by an analysis for the distal level of the stimuli.

Monocular alignment in different depth planes.

We examined (a) whether vertical lines at different physical horizontal positions in the same eye can appear to be aligned, and (b), if so, whether the difference between the horizontal positions of the aligned vertical lines can vary with the perceived depth between them. In two experiments, each of two vertical monocular lines was presented (in its respective rectangular area) in one field of a random-dot stereopair with binocular disparity. In Experiment 1, 15 observers were asked to align a line in an upper area with a line in a lower area. The results indicated that when the lines appeared aligned, their horizontal physical positions could differ and the direction of the difference coincided with the type of disparity of the rectangular areas; this is not consistent with the law of the visual direction of monocular stimuli. In Experiment 2, 11 observers were asked to report relative depth between the two lines and to align them. The results indicated that the difference of the horizontal position did not covary with their perceived relative depth, suggesting that the visual direction and perceived depth of the monocular line are mediated via different mechanisms.

Stereoillusory motion concomitant with lateral head movements.

Stationary objects in a stereogram can appear to move when viewed with lateral head movements. This illusory motion can be explained by the motion-distance invariance hypothesis, which states that illusory motion covaries with perceived depth in accordance with the geometric relationship between the position of the stereo stimuli and the head. We examined two predictions based on the hypothesis. In Experiment 1, illusory motion was studied while varying the magnitude of binocular disparity and the magnitude of lateral head movement, holding viewing distance constant. In Experiment 2, illusory motion was studied while varying binocular disparity and viewing distance, holding magnitude of head movement constant. Ancillary measures of perceived depth, perceived viewing distance, and perceived magnitude of lateral head movement were also obtained. The results from the two experiments show that the extent of illusory motion varies as a function of perceived depth, supporting the motion-distance invariance hypothesis. The results also show that the extent of illusory motion is close to that predicted from the geometry in crossed disparity conditions, whereas it is greater than the predicted motion in uncrossed disparity conditions. Furthermore, predictions based on perceptual variables were no more accurate than predictions based on geometry.