We sought the conditions where the horizontal-vertical illusion (HVI) takes place outdoors in an open field. Longitudinal distance from a subject to a building wall was adjusted to appear equal to the vertical or horizontal distance on the wall. To examine validity of previous theories (physiology, frame, depth, and gravity theories), boundary of visual field (ellipse and circle), bodily orientation (upright and lying), and orientation of visual objects (normal, 90 degrees-tilted, and inverse) were manipulated in eight experiments. These three independent variables affected the HVI effects, but their effects were not explained by the previous theories. We therefore proposed a model on the basis of discord among the retinal, visual, and gravitational orientations. We also found that longitudinal distance was adjusted as being consistently larger than the standard distance. This result was explained by the reduction of cues to distance and the HVI effect.

In three experiments, perceived vertical and horizontal distances in outdoor settings were investigated. Horizontal distances were adjusted by 70 subjects to make them appear equal to vertical distances ranging from 2 to 47 m. The results showed that (1) the matched horizontal distance is represented as a linear function of vertical distance; (2) the slope of the linear function is generally larger than unity, suggesting that when vertical distance is physically equal to horizontal distance, vertical distance appears larger than horizontal distance; (3) physiological muscular variables such as eye, head, and body position are not crucial in judging vertical and horizontal distances; (4) vertical distance of a building appears larger when viewed from afar than when viewed from nearby. 151The purpose of this study was to determine whether, in outdoor settings, horizontal distance or vertical distance would appear to be longer. We refer to horizontal distance as the distance along the ground surface and vertical distance as the distance along the direction of gravity . An example of horizontal distance is the distance from an observer to a building, and an example of vertical distance is the height of the building. Figure I shows four different viewing positions from which comparisons of vertical and horizontal distances can be made. Figure la illustrates the look-up condition, with the subject standing near the base of the building; Figure lb illustrates the look-at condition, with the subject standing on the ground away from the building; Figure lc illustrates the look-down condition, with the subject on the roof or a floor of the building looking down at the ground; Figure Id illustrates the Iie-down condition, with the subject lying on his/her back near the base of the building.A number of studies have been performed under the look-up and lie-down conditions. Morinaga (1935) showed that for a vertical standard distance of 1 m in the look-up condition, the matched horizontal distance increased by 4 % to 14% and did not change substantially with the position of eye and head. Makishita (1947, Experiment 1) determined equidistant curves as a function of elevation angle in the look-up condition, and showed that the matched vertical distance was smaller than the matched horizontal distance. Osaka (1947) demonstrated that for vertical standard distances of 4 to 12 m, the matched horizontal distance increased by 12%to 21 %in the lookup condition, but that this trend disappeared when the sub-

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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.

A preliminary and two main experiments designed to examine the perceptual properties of electrocutaneous stimulation are reported. The stimuli used were single short pulses varying in intensity and duration. In Experiment 1, the exponents of power functions fitted to electrocutaneous magnitude estimation data were determined together with the sensory qualities induced by electrical stimulation. The results showed that there was no correlation between the exponent values and the sensory qualities. The mean exponent was 1.2. In Experiment 2, an intensity-duration trading function was constructed from the data obtained from identifying the induced sensory qualities. The results showed that the critical duration increases from 30 to 300 msec with increasing sensation level. These findings are compared with the properties of other sense modalities.

Apparent orientation of the body tilted laterally in the frontal plane was studied with the methods of absolute judgments in four experiments. In Experiment 1, 17 subjects, who maintained the normal adaptation of body to gravity,estimated their body tilts under the condition of seeing the gravitational vertical and under the condition of eliminating it. The results showed that (1) there was not a significant difference between the two conditions and (2) the small tilts of less than 45°were exactly estimated, whereas the large tilts of 45°-108°were overestimated. In Experiment 2, 10subjects estimated their body tilts under three velocities of a rotating chair on which each subject was placed. Although both body tilt and chair velocity were found to influence tilt estimation, the effect of body tilt was overwhelmingly greater than that of chair velocity. In Experiment 3, 11 subjects adapted their bodies to a 72°left tilt for 10 min and then estimated various body tilts around the adapting tilt. The estimations obtained under the 72°adaptation were lower than those obtained under the 0°adaptation, and this reduction was greater for the test tilt that was farther away from the adapting tilt. In Experiment 4, 11 subjects adjusted their own body tilts to designated angles. The results confirmed the outcomes of absolute estimation in Experiments 1-3. From these findings and past literature, the judgments of body tilt were considered to be subserved by a single sensory process that was based on the cutaneous and muscular proprioceptors, rather than the vestibular and joint proprioceptors. 331We investigated apparent orientation of the body by tilting it laterally in the frontal plane.' The first concern was to establish psychophysical scale for body tilt under normal adaptation of the body to gravity. The second concern was to explore the processes and proprioceptors that make the perception ofbody tilt possible. For the latter concern, we made use of postural aftereffects or postural persistence: the sense of position of a limb, the head, or the eyes that may be temporarily affected when an asymmetrical posture is maintained for some time (F. 1. Clark & Horch, 1986;Howard, 1982). In this study, after being placed at a certain fixed tilt for some time, the subjects judged their various body tilts around the fixed tilt. On the basis of extent and magnitude of postural aftereffects, we assessed how many processes (single or double) exert in the perception of body tilt. Moreover, we considered how the processes are related to the nonvisual proprioceptors, such as the cutaneous, vestibular, joint, and muscular proprioceptors.?One conventional method of measuring apparent body tilt is to have the subject indicate his or her apparent bodyThe authors are grateful to Y.Ohta, who helped collect the data of Experiments 2 and 3, and to K. Shimono, who critically read the earlier manuscript. Correspondence should be addressed to K. Koga, Research Institute of Environmental Medicine, Nagoya University, Furo-cho Nagoya 464-0 I, Japan...

Two hypotheses about the effects of familiar size on judgments of size and distance, the cueconflict hypothesis and the viewing-attitude hypothesis, were examined. In Experiment 1, observers estimated the size and distance of familiar targets with apparent or assumptive instructions under three different spatial cue conditions. In Experiment 2, observers performed tasks similar to those of Experiment 1 with no specific instructions. The main results were: (1) Assumptive instructions facilitate the effects of familiar size in both size and distance judgments, but reducing spatial cues does not, and (2) viewing attitude changes from the apparent to the assumptive when available spatial cues are reduced. Thus, it was concluded that the viewingattitude hypothesis gives a better account of the effects of familiar size,but that the cue-conflict hypothesis cannot be abandoned, because the number of conflicting cues contributes to the formation of viewingattitude.

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 freebinocular, 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 mirrorreflected 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 0 .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.