Perception & Psychophysics 1996,58 (8), JJ68-JJ76 Perceiving self-motion in depth: The role of stereoscopic motion and changing-size cues STEPHEN PALMISANO University ofNew South Wales, Kensington, New South Wales, Australia During self-motions, different patterns of optic flow are presented to the left and right eyes. Previ­ ous research has, however, focused mainly on the self-motion information contained in a single pattern of optic flow. The present experiments investigated the role that binocular disparity plays in the visual perception of self-motion, showing that the addition of stereoscopic cues to optic flow significantly im­ proves forward linear vection in central vision. Improvements were also achieved by adding changing­ size cues to sparse (but not dense) flow patterns. These findings showedthat assumptions in the head­ ing literature that stereoscopic cues facilitate self-motion only when the optic flow has ambiguous depth ordering do not apply to vection. Rather, it was concluded that both stereoscopic and changing­ size cues provide additional motion-in-depth information that is used in perceiving self-motion. Ofall the senses known to be involved in self-motion During self-motions, different patterns of optic flow perception-the vestibular, auditory, somatosensory, pro­ are presented to the left and right eyes (due to the sepa­ prioceptive, and visual systems-visionappears to play the ration ofthe eyes and their different angles ofregard; see dominant role (Benson, 1990; Howard, 1982). This is Figure 1). Theorists have, however, generally focused only demonstrated by the fact that compelling illusions of on the motion-perspective information contained in a sin­ self-motion can be induced by visual information alone. gle pattern, assuming that this is sufficient to accurately For example, when subjects are placed in a "swinging perceive self-motion (e.g., Gibson, 1950; Gibson et a1., room," where the walls and ceiling swing back and forth, 1955; Gordon, 1965; Heeger & Jepson, 1990; Koen­ they soon experience the illusion that they themselves are derink, 1990; Koenderink & van Doorn, 1981, 1987; Lee, swaying (Lee & Aronson, 1974; Lee & Lishman, 1975; 1980; Longuet-Higgins & Prazdny, 1980; Nakayama & Lishman & Lee, 1973). Similarly, when subjects are placed Loomis, 1974). Accordingly, the role that stereoscopic in­ inside a "rotating drum," a rotating cylinder with a pat­ formation plays in self-motion perception has received lit­ terned inner wall, they quickly experience an illusion of tle attention. self-rotation (Brandt, Dichgans, & Koenig, 1973; Mach, Recently, however, van den Berg and Brenner (1994b) 1875). These illusions occur because the swinging room have shown that, in some situations, heading perception and the rotating drum duplicate the visual stimulation (one aspect of self-motion perception) can be improved that normally occurs during real self-motions. by the addition ofstereoscopic cues. Their earlier research A major visual stimulus for self-motion perception is had found that heading estimates were error prone in the optic flow or the temporal change in the pattern of light presence of motion noise, when displays simulated ob­ intensities at the moving point of observation (Gibson, server motion through a cloud ofdots (van den Berg, 1992; 1966; Warren, Morris, & Kalish, 1988). Gradients ofop­ van den Berg & Brenner, 1994a). They subsequently dis­ tical velocity contain several potential sources ofinforma­ covered that when binocular disparities were added to tion about observer motion through three-dimensional these "cloud" displays, heading estimates became up to space (Gibson, Olum, & Rosenblatt, 1955). There is the four times more resistant to noise. Changing disparity was perspective change in location ofobjects in the optic array not essential for this improved heading performance: Sub­ (which shall be referred to as motion perspective) and jects performed just as well when each dot had a fixed their optical expansion/contraction (which shall be re­ retinal disparity for the duration of the display (in this ferred to as changing size). 1 case, only motion perspective simulated self-motion in depth). Van den Berg and Brenner concluded that stereo­ scopic vision improves heading perception indirectly by Portions of this research were presented at the 1995 Association for providing the depth order ofthe objects in the flow (rather Research in Vision and Ophthalmology Conference held in Fort Lau­ derdale, FL. The author thanks Barbara Gillam and Shane Blackburn than by providing additional motion in depth informa­ for their feedback and suggestions regarding this article and Keith tion). Furthermore, they argued that other depth cues, Llewellyn for the loan of his equipment. The author is also grateful to such as occlusion or texture gradients, might improve Myron Braunstein, George Andersen, and two anonymous reviewers for heading judgments in a similar fashion. their insightful comments on an earlier version of this article. Corre­ spondence should be addressed to S. Palmisano, School ofPsychology, There is reason to believe that stereoscopic informa­ University ofNew South Wales, POB I, Kensington, NSW 2033, Aus­ tion might also enhance the subjective experience ofself­ tralia (e-mail: [email protected]). motion, known as vection. In their study, Andersen and Copyright 1996 Psychonomic Society, Inc. 1168 PERCEIVING SELF-MOTION IN DEPTH 1169 • • •• • • ••• • •• • iii·.·· • • ...... ',. • .. .......• .. .." .. ' " . '. .. .. ..... ........ •... ·;····~::r·~·.·· ' ' .. " -,' .. ' . ... " .. •• " .. .... : ~ .. ....-. '. .... .. ..' • •• • •••• '. •.. .. '. .. : . ... .' . , _ '.' .•. " ... .•.... .. .. • • • ..- • Figure 1. A stereogram representing the different optic arrays presented to the left and right eyes at anyone point in time (converge in order to fuse the two images). Braunstein (1985) simulated forward self-motion through z-axis toward the observer (the projection plane and the observer's a three-dimensional cloud of dots. The perceived three­ viewpoint remained fixed). A constant density was maintained by dimensionality of these displays was manipulated by al­ replacing each object as it disappeared from view at the opposite tering the motion-based cues to self-motion in depth. They end of space (a simulated distance of 20 m). All displays had a frame rate of60 Hz. found that the more three-dimensional the inducing dis­ Stereoscopic displays presented horizontally disparate patterns plays appeared, the stronger the vection in central vision. of optic flow to the two eyes. This was achieved by presenting the Although Andersen and Braunstein did not investigate disparate views in different colors on a single display, which was the role ofstereoscopic cues on vection, an argument can then viewed through red--eyan anaglyph glasses. To fuse these dis­ be mounted on the basis oftheir data. Ifit is assumed that plays, the subjects needed to verge behind the screen. Thus, prior to adding stereoscopic information to inducing displays their presentation, the subjects were shown a pair ofvertically dis­ placed nonius lines (one red, one cyan) separated by a disparity rep­ makes them appear more three-dimensional, it follows resenting the farthest distance simulated by the display? (Hebbard, that such displays might produce stronger vection (in 1962; Mitchell & Ellerbrock, 1955). They then had to alter their central vision) than those with motion perspective alone. convergence until the nonius targets were aligned, before triggering The present experiments investigated whether the ad­ the stereoscopic display. dition ofstereoscopic information to optic flow would in­ Nonstereoscopic displays were oftwo types. Monocularly viewed crease forward linear vection in central vision. They were displays presented a single pattern ofoptic flow to one eye. Binoc­ ularly viewed nonstereoscopic displays presented the same pattern designed to determine whether any such increases were of moving objects to both eyes (producing slightly different flow due to improved depth ordering or to additional motion patterns due to the different positions ofthe two eyes). Prior to the in depth information. In addition, a further two experi­ monocularly viewed displays, the subjects were told to lower an eye ments examined whether another source of motion in patch over the right eye. Before binocularly viewed nonstereoscopic depth information (changing size) could also produce an displays, the subjects were presented with a pair ofnonius lines set advantage for vection. at zero disparity (since the subjects had to verge on the screen to view these displays). After lowering the eye patch or verging on the screen, the subjects then triggered the nonstereoscopic display. GENERAL METHOD EXPERIMENT 1 Subjects The subjects were students in an introductory psychology course who received course credit for their participation. All had normal Experiment I compared the vection induced by stereo­ or corrected-to-normal vision and had no previous laboratory ex­ scopic and monocularly viewed displays simulating self­ perience with illusions ofself-motion. Different subjects were used motion in depth. In the case of stereoscopic displays, in each ofthe four experiments. vergence was consistent with the presence of three­ dimensional, virtual space behind the screen. Nonstereo­ Visual Displays All displays simulated forward self-motion through a cloud of scopic displays were viewed monocularly to remove any randomly positioned