Flow Structure Versus Retinal Location in the Optical Control of Stance

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1 Journal of Experimental Psychology: Human Perception and Performance 1985 Vol. 1], No. 5, Copyright 1985 by the American Psychological Association, Inc /85/J00.75 Flow Structure Versus Retinal Location in the Optical Control of Stance Thomas A. Stoffregen Cornell University In four experiments I examined the importance of the retinal center and periphery in the pickup of optical information for controlling stance as a function of the dynamic geometrical structure of the optical flow. All experiments were performed in a moving room so that the magnitude of compensatory sway in response to room movements could be measured. In Experiments 1 and 2 I found stronger sway response to flow having a largely lamellar structure that was presented to the retinal periphery than to more radially structured flow in the center. In Experiment 3 observers turned their heads to face the right wall of the room, placing radial flow in the periphery and lamellar flow in the center of the visual field. Radial flow presented to the retinal periphery induced no compensatory sway. Lamellar flow in the center of the retina produced some sway. Row structure apparently interacts with the exposed retinal area in controlling stance. A number of experiments have demonstrated that optical motions, like those produced when an observer moves through an environment, have an effect on postural stability or stance (Berthoz, Lacour, Soechting, & Vidal, 1979; Lee & Aronson, 1974; Lee & Lishman, 1975; Lestienne, Soechting, & Berthoz, 1977). In these studies observers stand inside a moving room, a large enclosure suspended just above the floor. When the room moves slightly along the observer's line of sight, he or she sways slightly in response. The direction and magnitude of the sway are related to that of the room, indicating that the visual system has taken the optical motion of the surround as if it resulted from movement of the self (egomotion). The sway acts to compensate for a perceived motion of the observer. In another study Lishman and Lee (1973) used a moving room to demonstrate effects of These experiments were done as part of the author's doctoral dissertation. The data were reported in a talk at a meeting of the International Society for Ecological Psychology. Hartford, Connecticut, May, This work was carried out in part under a grant from the Sigma Chi Society to the author. Special thanks are due to James Cutting for his generous help with the manuscript, and to Ed Reed, David Irwin, and Eleanor Gibson for helpful comments on earlier drafts. Requests for reprints should be sent to Thomas A. Stoffregen, who is now at AFAMRL/HEF, Wright Patterson Air Force Base, Ohio optical flow on perceived (or induced) egomotion. At the same time Brandt, Dichgans, and Koenig (1973) began to ask whether one part of the retina was more important than others for the pickup of such optical information. Brandt et al. placed observers inside a drum rotating around the vertical axis and found that with the periphery blocked, exposures of up to 60 of visual angle of flow to the central retina and fovea produced little or no perceived egorotation. In the periphery (with the center blocked out) even much smaller exposures yielded strong sensations of rotation. Held, Dichgans, and Bauer (1975), found similar center/periphery effects on perceived egomotion, using a plane of optical flow rotating around the observer's line of sight. Johansson (1977) also found center/periphery differences with flow that moved vertically. Dichgans and Brandt (1978) and others (e.g., Howard, 1982) interpreted these results as indicating that the retinal periphery is dominant for pickup of visual information for the perception of egomotion (and, by implication, for the control of posture in the moving room studies). There is a potential problem, however, with this retinally based analysis. In the rotary motion situation used by Brandt et al., the stimulus is the same over the entire field of view: It is geometrically uniform in the optic array. (The same is true of the vertical flow used by Johansson, 1977, although the two 554

2 FLOW STRUCTURE IN THE CONTROL OF STANCE 555 flows differ in other ways.) But when a perceiver moves forward in the world (or stands in a moving room), the optical flow that is generated does not have the same geometrical structure everywhere (Gibson, 1979). Linear motion generates the melon-shaped family of curves described by Gibson. He suggested that all optical flow radiates outward from that point in the optic array that is spatially coincident with the direction of motion. Its structure near this point is radial relative to the observer. At the edges of the field of view, however, the lines of flow are nearly parallel to the line of motion: They have a lamellar arrangement (Cutting, in press) like lines of longitude at the equator on a globe. Flow near the line of motion expands in front of the observer, but at the edges of the field of view flow sweeps laterally past the observer. Between the center of radial expansion and the edges of the field there is a continuous change from expanding to lamellar flow, resembling the change in structure of longitudal lines from a pole (radial) to the equator (lamellar). When the eyes look in the direction of the movement, predominantly radial flow is projected to the central regions of the retina, whereas in the periphery the lines of flow have a lamellar structure. The peripheral flow is in some ways geometrically similar to the flow used by Brandt et al. (1973) (although their case simulated rotary and not linear motion), and we might therefore expect that peripheral stimulation during linear motion would induce perceived linear motion in the same way that perceived rotary motion was induced in their experiments. However, the radially expanding flow that falls on the retinal center during linear motion is very different from the rotary flow to which the central retina was exposed by Brandt et al. (1973). This difference in flow structure in different parts of the optic array corresponds to the central/peripheral division of the retina proposed by Dichgans and Brandt (1978); it is therefore possible that differences in flow structure could be in part responsible for the effects observed in earlier studies. Indeed, Andersen and Braunstein (1985) have recently demonstrated that perceived linear egomotion can be elicited by exposures of radially expanding flow limited to central retina. A related question concerns whether the established sensitivity of peripheral retina to lamellar flow would extend to radially structured flow as well. Finally, would radial flow in the periphery be useful as information for postural control, as has proved to be true with lamellar peripheral flow in the moving-room studies discussed above? With the moving rooms that have been used, lamellar flow normally projects to the retinal periphery, while radial flow projects to the retinal center. But if the observer were to turn to face the side of the room, this relation would be reversed: Lamellar flow would fall on the retinal center, with radial flow in the periphery. Using this technique, it is possible to assess the relative importance of different flow structures and retinal projections on postural stability. Brandt et al. (1973) also assessed the effects on perceived egomotion of varying the amount of available flow; this manipulation was repeated in the present experiments to determine its effect on postural adjustments. General Method All experiments reported here were performed in a moving room similar to that devised by Lishman and Lee (1973). This room was a cube about 2.5 m on a side, built of wood studs and mounted on four wheels so that it could be rolled along the floor (clearance: 0.5 cm). On the interior of the room, the ceiling and three side walls were faced with rigid cardboard, which was itself covered with an optically textured (marble pattern) plastic adhesive paper. One wall remained open, and the room itself had no floor. The room was illuminated by three incandescent lamps, two shining through a translucent plastic screen mounted in the center of the ceiling, and one in a reflector directed through a hole in the ceiling just in front of the midpoint of the front wall (opposite the open wall). All lamps were rigidly attached to the room, so that they moved when it did. Room motion was along an axis perpendicular to the open wall. Standing within the room, but physically separate from it, were three open wooden frames, linked together. One stood immediately in front of each of the three vertical walls. A variety of sizes and shapes of plain rigid cardboard could be attached to the wood frames in several configurations. Both the cardboard and the frames were stationary with respect to room movements. Observer responses were measured by a sway meter, consisting of a potentiometer mounted on a rigid stand with its axle at right angles to the direction of room motion. A grooved wheel was mounted on the axle, and a string circled this wheel and was held around the observer's neck by a clasp. A small weight at the other end of the string maintained tension. With this set-up when the observer moved in the anterior/posterior direction, the string caused the potentiometer to turn correspondingly. Resolution of the potentiometer was about 1 mm. A second potentiom-

3 556 THOMAS A. STOFFREGEN eter was mounted near the first, attached by a string to a structural member of the experimental room, and was used to register its motion. Outputs from both potentiometers were fed into a PDF-11 /24 computer for storage and analysis. The movement selected for use in all experiments reported here was a sinusoidal (back and forth) motion with a total excursion of 2.5 cm and a period of 12 s per cycle. Room motion was generated by the experimenter, who knelt on the floor behind the open back wall of the room, grasped the bottom stud of the room (running the width of the wall) near its left end, and moved the room slowly forward and back. The experimenter was not visible to observers at any time during trials. Motion was generated with reference to a ruler placed on the floor beneath the cross beam, and marked off with the 2.5-cm distance of movements. Accuracy and consistency of the room movements was high; 12 randomly selected pairs of complete records of room motion from experimental trials in different conditions correlated at or above.97. Each observer was run individually and served as his or her own control, having one trial in each of the conditions. Each was brought into the experimental room and asked to stand in the center of the open wall, facing into the room. He or she was then given a general outline of the experimental procedure, with instructions to stand comfortably during trials, with weight on both feet and arms still. At this lime he or she was given general instructions on where to look during trials (detailed below). Observers wore a baseball-type cap, which they were asked to adjust so that the bill occluded the ceiling from view. At the beginning of each trial, after a ready signal, the sway meter string was clasped about the observer's neck and calibrated so that their current position corresponded to the middle of the potentiometer's scale. Then a recording stripchart was turned on; the experimenter started a timer and began moving the room. Over the course of the 60-s trial the room traveled through five cycles. No attempt was made prior to Trial 1 either to inform the observer that room motion would be involved in the experiment or to keep him or her from knowing this. The room apparatus was easily visible, but the wheels were difficult to see and not likely to be noticed on casual glance. Observers were debriefed following the experimental session. Experiment 1 Experiment 1 was designed to serve two purposes. The first concerns the location of the available flow relative to the observer's retina. Would a given amount of flow be more effective in inducing sway when available to the retinal periphery as opposed to the center? Previous studies of such retinal location effects investigated only perceived egomotion, not the use of flow information for postural control. Second, the moving room, together with the occluding system described above, permits assessment of the effectiveness of radial flow, available to the retinal center with the periphery blocked, in inducing sway. Twelve observers took part in Experiment 1. Observers were recruited through sign-up sheets and by personal contact, and were paid for their participation. They were told to keep their gaze within a square about 6 of visual angle on a side outlined on the front wall of the room (centered left to right about 150 cm above the floor), and not to turn their heads or look away during trials. Conditions Experiment 1 had nine conditions. The first measured baseline postural stability and had the observer standing normally in the moving room, but with his or her eyes closed for the entire trial (eyes-closed condition). In the fullroom condition the entire room was visible (except the ceiling, which was blocked out by the brim of the hat, and the small areas blocked by the wooden frames of the occluding system); observers looked straight ahead. In the side condition, the entire room was visible, but observers turned only their head toward the right wall, their body remaining oriented toward the front wall. Because the head turn would ordinarily have brought into view the laboratory outside the experimental room, a small cardboard panel was attached to the hat's brim to block this out. Observers were told the purpose of this panel and were instructed to hold their heads so that they could not see out of the experimental room. A square similar to the one on the front wall was outlined on the right wall for observers to gaze at during side trials. This square was positioned slightly to the right of center on the wall. There were two groups of experimental conditions, three providing exposures of flow from the front wall of the room, which were available to the retinal center, and three with exposures of the left and right walls, providing flow to the periphery of both retinas. The front wall was exposed using three apertures concentric around the center of the wall. Apertures were cut in the cardboard sheets of the occluding system described above. The smallest aperture (Fl) was 62.5 cm wide and 74 cm high, subtending a visual angle at the observer's head of approximately 20 X 23". The medium-sized aperture in F2 was 125 cm high and 144 cm wide, subtending a visual angle of

4 FLOW STRUCTURE IN THE CONTROL OF STANCE 557 Full Room Subject r = Room ' 2 seconds Figure 1. Sample records of room and observer movement. (Scale is the same on both records.) 38 X 43. When the full front wall was visible (F3), the edges of the frame were 188 cm apart and 214 cm high, for a visual angle of 54 by 61. During front wall exposures, the side walls were completely occluded. In the three conditions in which flow reached the retinal periphery (PI, P2, and P3) exposures were not concentric. Instead, floorto-ceiling strips of both left and right walls were exposed, beginning at the most peripheral edge, with larger exposures adding visible flow in toward the front wall (which was completely occluded). In condition P3 the entire side walls were exposed. In P2 strips of cardboard occluded the first third of the left and right walls (closest to the front wall), subtracting 10 of visual angle of flow from each side. In condition PI, cardboard occluded two thirds of the side walls, for a total occluded area (including the front wall) of 115. The floor inside the room was always visible but did not move. Results For each trial the data consisted of two time series: one generated by the potentiometer attached to the room, the other by the one attached to the observer. The output of the potentiometers was sampled at 0.5-s intervals. The records for a typical trial are shown in Figure 1. The two time series were correlated, with lags ranging from 0 to ±2 s, at '/2-s intervals. Two analyses of the resulting correlations were performed. In the first, correlations for zero lag were compared separately. In the second, out of the nine correlations computed for each trial (from -2 to +2 s), the largest was selected, and these were compared. Analysis showed that taking the maximum correlation (thereby eliminating phase effects) produced no significant differences from the zero-lag data in the results for any of the experiments presented here (indicating that there were no time lag, or phase, effects in observers' responses to room motion). Therefore, zero-lag data will be presented throughout. Analysis of variance revealed a significant effect for conditions, F(8, 88) = 10.45, p < The mean correlations across observers for each condition are presented in Figure 2. In the full-room condition, with the room fully visible and the observer looking straight ahead, the correlation between room movement and observer sway was.516. This shows that the room by itself (with only the interior frame occluding any part of the walls), provides, when in motion, unambiguous information for egomotion that is used by the observer in maintaining posture. By contrast, with the eyes closed, the correlation between room movement and observer sway was.039, illustrating that the room effect is a visual one. In general, the magnitude of sway increased as size of exposures increased, a finding consistent with that of Brandt et al. (1973). Frontal/peripheral effects. Post hoc multiple comparisons performed using the Tukey test (Winer, 1971) revealed several significant differences between conditions. Two of the peripheral conditions (P2 and P3) produced sig-

5 558 THOMAS A. STOFFREGEN Condition Full Room Eyes Closed Experimental Setting F L R F L R 1 Side only Left Front Right Experiment I Experiment 2 (N = I2) (N=I5) Mean nificant sway (more than with the eyes closed), which was also not different from that produced by the full-room condition. Only one front-wall condition produced any sway at all, and that, though consistent, was not significantly greater than in the eyes-closed control condition. This pattern of results is almost identical to those found by Brandt et al. (1973), who used exposures with similar visual angles. Clearly, even in this rather gross assessment, flow in the far visual periphery is more effective in inducing compensatory sway than is flow in the near periphery and foveal regions. It should be stressed that the visual angle of available flow in the two larger frontal conditions (F2 and F3, about 40 and 60, respectively) was substantially larger than the visual angle normally associated with foveal and even parafoveal areas of the retina. Side condition. The side condition produced nearly as much sway as did the full room (.473 and.516, respectively, both significantly higher than the eyes-closed control). The significance of this will be discussed below. Experiment ".123 A second group of 15 observers was run using a subset of the conditions from Experiment 1. The conditions used were the full room, frontal exposure conditions Fl and F3, and peripheral exposure conditions PI and P3. These five conditions were ordered in a Latinsquare design to control for possible order effects. The design was run three times. Results.371 Figure 2. Conditions and results for Experiments 1 and 2. (The experimental setting shows a top view of the room and the observer's placement within it. The right panel shows the observer's placement and orientation during the side condition, including the panel used to block vision out of the open back wall of the room. Below this is shown the general schematic layout of the walls. The illustrations of each condition conform to this format. The small squares in the schematic layout and in the side condition indicate the area of gaze. The diagonal line through the left wall in the side condition indicates that that wall was out of sight behind the observer's head. The data shown are mean correlations between room movement and observer sway. [*p <.05. "p <.01.] L = left wall; F = front wall; R = right wall.) By analysis of variance we found significant effects for condition, F(4, 60) = 17.87, p < There was no effect of either condition order or design replication. Mean correlations between room and observer movement for Experiment 2 are given in Figure 2. The same basic pattern of results can be found here as in Experiment 1: The larger peripheral exposure induced the same magnitude of sway as did the full room, and neither central exposure induced any significant sway. Discussion The primary result of Experiments 1 and 2 is that this moving room induced, through its

6 FLOW STRUCTURE IN THE CONTROL OF STANCE 559 motion, postural readjustments in standing observers. In the full-room condition observers swayed in response to room motion in a strong and consistent way. This result accords well with the findings of Lee and Lishman (1975), although they used a different measure of sway and a room that was suspended from the ceiling, rather than rolling along the floor. A few observers reported that they were aware that the room was moving, but none were able to determine the magnitude or periodicity of the motion, or even whether it occurred on all trials, and none reported seeing movements of the shadows cast by the stationary screens and frames relative to the room. By contrast, many of observers reported that they felt themselves to be moving on some trials; some felt that both they and the room were moving. Approximately one quarter of observers reported some symptoms of motion sickness, either at the conclusion of the experimental session or up to an hour later. Lee and Lishman (1975) also reported that some observers experienced slight motion sickness as a result of room motion. Another issue concerns lighting levels within the room. With the front wall of the room occluded in the peripheral conditions, the forward light was occluded; the room was noticeably dimmer in these conditions. Studies by Berthoz, Pavard, and Young (1975), and by Leibowitz, Shupert-Rodemer, and Dichgans (1979) found that perceived egomotion remains robust over a very wide range of illumination, even down to the threshold for stationary visual pattern detection. It therefore seems unlikely that the moderate variation in illumination in these experiments could have influenced the results. Leibowitz et al. (1979) also examined the role of acuity by defocusing the image and found no effect; as with illumination levels, any visible pattern at all appeared to be enough to bring out the sensation of egomotion. Frontal/peripheral effects. The results show clearly that exposures of the side walls of the room produced more compensatory sway than did similar exposures of the front wall. This would seem to suggest that the retinal periphery, which detected side wall flow, is dominant for the pickup of optical information for postural stability, a finding consistent with the claim of Dichgans and Brandt (1978) that the periphery is dominant in detecting optical information for egomotion. Side condition. When in the side condition, the observer, standing as in the other conditions, turned his or her head to face the right wall of the room, the previous frontal/radial, peripheral/lamellar relation between retinal position and flow structure was reversed. In the side condition, central flow was lamellar and peripheral flow was radial (on the left side only, as the right side was blocked to keep the observer from seeing outside the room). The side condition does not directly assess the problem of the relative importance of radial and lamellar flow in inducing sway, but it does determine how important it is that frontal flow be radial while peripheral flow remains lamellar. The results were unambiguous: the side condition produced nearly as much controlled sway as did the full-room condition, indicating that with full flow available the relative placement of lamellar and radial portions of the flow is unimportant. Because there were no order effects in Experiment 2, it seemed appropriate to combine data across the two experiments for those conditions that were identical. The right-hand column of Figure 2 shows these combined data. Bonferroni tests on the combined means show that all of the relevant pairs of conditions were significantly different from one another. The full room produced more sway than did either of the front wall conditions, but not more than the larger side wall condition. The largest side wall exposure produced more sway than did either of the front wall exposures. The combined data also show that the largest front wall exposure produced significantly more sway than did the smallest exposure of that wall. Apparently, large exposures of radially structured optical flow to the central and near-peripheral areas of the retina (within 60 of the fovea) can induce some compensatory sway. This finding is consistent with the results reported by Andersen and Braunstein (1985) on perceived egomotion, and supports their suggestion that earlier studies were premature in concluding that central retina was not adept at picking up information for egomotion. None of these experiments, however, provide a complete test of the ability of nonglobal flow to control stance. The stationary screens of the occluding system do more than occlude the

7 560 THOMAS A. STOFFREGEN visual motion caused by the room; they substitute a stationary optical texture for a moving one. The effective information produced by the stationary screens is for observer stability, relative to the movements of the room, with respect to the areas of the optic array that they fill. Conditions employing the screens thus present ambiguous optical information: The exposed areas of the room walls specify observer motion, but the occluding surfaces (and the floor) specify observer nonmotion.' For this reason the walls of the room were covered with a bright and salient texture, whereas the occluding screens were left blank, dark, and relatively untextured. This was a rather crude control for the problem and almost certainly ineffective, given previous results showing that almost any discriminable texture can evoke a strong egomotion response (Berthoz et al., 1975). In summary, it appears that one part of the optic array specifies motion and another stability. This can be expected to confuse the results, producing a decrease in sway in the experimental conditions. However, this ambiguity hypothesis supports the notion of a possible role for central areas of the retina in postural control: Complete stability in the periphery (as in the largest front wall exposure, condition F3), which specified that postural adjustments in response to motion of the room were unnecessary, was not enough to entirely negate the postural adjustments that were induced by front wall motion. This issue will be taken up again in the General Discussion. Experiment 3 The first experiment raised the question of whether it is the area of the retina or the type of flow that is important for postural control (or some interaction of the two). The major manipulation in Experiment 3 was to present lamellar flow exclusively to the retinal center and radial flow exclusively to the retinal periphery in an attempt to determine the effectiveness of these two arrangements in inducing compensatory sway. The apparatus was the same as in Experiment 1. Eighteen observers took part. Conditions The eyes closed, full room, and side conditions used in Experiment 1 were repeated here. Six new experimental conditions were also run. Verbal descriptions of these conditions are inevitably confusing, so the reader is encouraged to consult Figure 3 for schematic depictions. For all six of the new conditions, observers turned their heads to face the right wall of the room, as in the side condition. Three of the conditions provided varying exposures of the front wall of the room, with the right wall blocked (the left wall was completely out of sight behind the observer's head). The radially structured flow generated by motion of this wall was projected to the periphery of the left visual field (the right field was blocked as in the side condition). The remaining three conditions provided varying exposures of the right wall of the room. Motion of the right wall generated lamellar optical flow that was available to the central areas of both the left and right retinas. As with the side condition, observers were instructed to turn their heads as far to the right as was comfortable (but not so far that they could see outside the open back wall of the room), and to maintain their gaze within a small square that was outlined on the right wall of the room and also at the same position on the relevant occluding screens. As can be seen in Figure 3, both front and side wall exposures consisted of apertures concentric about the center of the wall. To control for order effects, the conditions were arranged in a Latin-square design (the design had nine orders and was run twice). One procedural difference involved the placement of observers within the moving room. Because in the six experimental conditions observers could see only two walls of the room, they were placed equidistant from those two walls so that the visual angles of exposures would be the same. Results There was a significant effect for conditions only, F(8, 136) = 28.44, p <.0001, but not 1 In fact, natural postural instabilities will generate some observer motion relative to the stationary screens regardless of whether the room moves or not. Such motions are presumed to be randomly distributed across conditions and are discounted here for clarity. Areas occluded by panels attached to the observer's head do not provide information for any motion of the observer relative to the environment (except at the edges, where environmental optical texture may be dynamically occluded).

8 FLOW STRUCTURE IN THE CONTROL OF STANCE 561 Experimental Setting! Radial Peripheral Flow Lamellar Central Flow.486" Small j Medium Side Large Large Figure 3. Conditions and results for Experiment 3. (The experimental setting shows the placement and orientation of observers. Observers looked at the right wall of the room on all but full-room and eyes-closed trials, where they looked straight ahead. A diagonal line through a wall indicates that that wall was out of sight behind the observer's head. Data are mean correlations between room movement and observer sway. I'P <.05.]) for order or repetition of the design. The mean correlations between room movement and observer sway are presented in Figure 3. Planned comparisons using the Bonferroni test showed that four conditions produced more sway than did the eyes-closed control condition. These were the full room and side conditions, and the two larger exposures of the right wall. The full-room condition was also significantly greater than the largest exposures of the front and right walls. Finally, the side condition produced more sway than did the largest front wall exposure (the large peripheral radial condition), as did the largest right wall exposure. Discussion The most interesting result of Experiment 3 is the complete lack of compensatory sway elicited by even the largest exposure of radial flow to the retinal periphery. This finding indicates that retinal position is not the single controlling factor in the pickup of optical information for the control of stance. By contrast, levels of sway generated by lamellar central exposures of flow in Experiment 3 remained the same as those generated by radial central flow in Experiment 1 (.242 and.252, respectively); in Experiment 3 even the medium-sized exposure of central flow produced significantly more sway than did the eyesclosed control (.292). Either of the two larger right wall exposures produced as much sway as did the side condition itself, despite the fact that the side condition made available at least twice as much optical motion as the others. Two things are implied by these results. First, the central retina is modestly useful for picking up information for the control of stance from lamellar flow. Second and more surprising, the peripheral retina appears to be totally unable to pick up stance-related information from radial flow of this magnitude. One confound remains. In Experiments 1 and 2, where the observers faced forward, the optical flow generated by the front wall of the room was bilateral, that is, available on both left and right sides of the visual field. Exposures of the side walls were also bilateral because both the left and right walls could be seen. In Experiment 3, where the observer faced the right wall, central exposures (of the right wall itself) remained bilateral, but peripheral exposures of radial flow projected from the front wall were available only on the left side because the right periphery was blocked off by the hat panel (and there was no wall there to generate flow anyway). This unilateral availability of flow in the radial peripheral conditions could have been partly responsible for the lack of compensatory sway observed. That this unilateral factor is not likely to account for all of the effect is suggested by one of the results of Brandt et al. (1973). When they presented whole field, unilateral, and monocular rotary optical flow to their observ-

9 562 THOMAS A. STOFFREGEN ers, they found that perceived egomotion was almost as strong with unilateral stimulation as with bilateral stimulation. The use of monocular exposures did produce some decrement but not enough to eliminate the effect. This suggests that the restriction to unilateral flow was not responsible for the results of the present experiments. Nevertheless, it seemed important to devise some control for the unilateral exposure of the radial peripheral flow in the foregoing experiments. Experimental Setting Experiment 4 In Experiment 4 observers once again faced straight ahead in the room, looking at the front wall. The key conditions permitted a comparison between bilateral and unilateral exposures of the side walls. It was not possible with the present apparatus to provide bilateral radial flow, but it was hoped that the comparison of unilateral and bilateral peripheral lamellar flow would shed adequate light on the relative effectiveness of unilateral and bilateral peripheral radial flow. Two additional conditions were run in Experiment 4 as separate controls. These will be described below. The apparatus and general procedure in Experiment 4 were the same as for the previous experiments. Nine observers took part. Conditions There were nine conditions in the experiment, which are illustrated schematically in Figure 4. The eyes closed, full room, and side conditions were identical to those employed previously. The two larger, bilateral, side wall exposures from Experiment 1 were included (P2 and P3), as were unilateral versions of each, in which the right wall of the room was completely blocked off, resulting in flow being available only from the left wall. Two other conditions were also included. In the first of these the full room was available, but the observer's right eye was covered with an eye patch, creating a monocular condition. The last condition was intended to serve as a head position control for the original side condition: Observers faced and looked straight ahead with the full room, but the occluding panel on the hat blocked out their view of the right wall of the room (the side control condition). P3 Bilateral P3 Unilateral Figure 4. Conditions and results for Experiment 4. (The experimental setting shows placement and orientation of observers on all but side trials. The dotted line in the monocular condition indicates that most of the right wall was out of the observer's field of view on that trail. Data are mean correlations between room movement and observer sway. l"p <.05.J) Results and Discussion Again, there was a significant effect for conditions, F(8, 56) = 3.80, p <.002, but not for observers or order. Mean correlations between room movement and observer sway for each condition are presented in Figure 4. Post hoc multiple comparisons using the Tukey test revealed that four conditions produced significantly more sway than did the eyes-closed condition. These were the full room, the side, and side control, and the largest bilateral side wall exposure (P3). There were no other significant differences between conditions. Neither of the unilateral conditions produced significantly less sway than their bilateral counterparts; such differences as do exist did not approach significance. The major finding is that unilateral exposures of lamellar optical flow to the retinal periphery produced almost as much sway as did bilateral exposures. The implication is that the complete inability of unilateral exposures of radial flow in the periphery to induce sway (Experiment 3) was not the result of their being unilateral as opposed to bilateral. The reduced sway observed in the monocular condition is

10 FLOW STRUCTURE IN THE CONTROL OF STANCE 563 consistent with the finding of Brandt et al. (1973) that monocular rotary flow reduces the strength of perceived egorotation. General Discussion Flow Structure Versus Retinal Location What is here called the peripheral dominance theory (Dichgans & Brandt, 1978) asserts that the periphery of the retina is dominant for the pickup of optical information for both the perception of egomotion and the control of stance. This theory is so appealing in its simplicity that it is subscribed to even by researchers who normally abjure retinally based explanations of perception: "The retinal periphery essentially seems to be an organ for recording and controlling the position and motion of the perceiver's head relative to the environment, thus locomotion or selfmotion" (Johansson, von Hofsten, & Jansson, 1980, p. 49). The major finding of the present studies, however, is that there are naturalistic circumstances under which the retinal periphery exhibits no facility whatever for picking up optical information for use in controlling stance. Experiment 3 provides evidence that the retinal periphery is completely unable to extract information for postural control when it is presented with radially expanding and contracting flow. Optical Velocities One possible problem with analyzing the present results in terms of flow geometry concerns the relative velocities of texture elements in different parts of the optic array. In the rotating drum used by Brandt et al. (1973), the velocity of all texture elements was essentially the same. But this is not true of the moving rooms used by Lishman and Lee (1975) and in the present studies, or, for that matter, in natural locomotion. In these cases there is a gradient of angular velocities over the optic array. Optical velocities (assuming an evenly cluttered environment) are lowest where elements radiate outward from the line of motion, and increase with increasing eccentricity from that line. Leibowitz, Johnson, and Isabelle (1972) determined the motion sensitivity threshold of the retina at eccentricities of up to 80 from the fovea. They found a maximum sensitivity of 1 min of arc per second near the fovea, trailing off to 10 min of arc per second at 80 eccentricity. This sensitivity curve neatly matches the gradient of optical velocities of points generated by locomotion (or by a moving room), with the result that when the eyes are oriented forward during locomotion, the effective motion sensitivity is equal across the retina. In the present studies in which observers faced forward in the moving room (Experiments 1, 2, and 4), the mean angular velocities of points on the side walls were at or slightly below the threshold sensitivity values reported by Leibowitz et al. (1972). The fact that the mean values were at threshold implies, for the sinusoidal room motion, that about half of the time the actual angular velocities were below the threshold values. Nonetheless, compensatory sway displayed the sinusoidal form of the room motion. Similarly, Lishman and Lee (1975) measured natural postural instabilities in standing adults and found a mean velocity for such movements of 1.42 mm/s. In their experimental situation such movements generated motion in the optic array having a maximum velocity of about 1.08 min of arc per second, far below Leibowitz et al.'s (1972) reported thresholds. There are two likely explanations for the discrepancy between the reported thresholds and observed effective velocities. First, the threshold determinations of Leibowitz et al. were made on the basis of observer's verbal reports of perceived object motion, whereas the response measure in the present studies (and in Lishman & Lee, 1975) assessed not awareness but compensatory sway (observers in the present studies were often not aware that either they or the room were moving). Second, the motion stimulus used by Leibowitz et al. was a small test square (2.0 of visual angle) that moved in an otherwise blank field. By contrast, the effective motion stimulus in the present studies never subtended fewer than 20 of visual angle. The motion in Leibowitz et al. would properly be perceived as object motion, whereas that in the present studies specified egomotion. It is possible that there might be different thresholds for the two types of motion because our responses to object motion and egomotion are often very different. For this reason it seems likely that the thresholds for the detection of object motion

11 564 THOMAS A. STOFFREGEN established by Leibowitz et al. do not apply here. In fact, it is possible that the gradient of motion detection thresholds across the retina reported by Leibowitz et al. may also not apply to the present case. It may be that large field motion sensitivity is in fact higher in the peripheral retina than in the center. Such a state would appear to be compatible with the peripheral dominance theory of Dichgans and Brandt (1978) because it would in fact make the peripheral retina more adept at detecting visual information for egomotion. The actual thresholds for large field motion sensitivity as a function of retinal eccentricity are not known, but there is reason to suspect that they may not follow the same pattern as those for small field or object motion. Brandt et al. (1973) held optical velocity constant across the optic array and still found that the retinal periphery dominated for the pickup of information specifying egomotion. It is therefore possible that the sensitivity function for egomotion may not be the same as that for object motion. The angular velocities of points on the front wall of the present room were much smaller than those of the side walls, having a maximum at the outermost edges of the front wall of about 4.0 min of arc per second (decreasing to zero at the center of the wall); that is below the threshold for foveal motion sensitivity reported by Leibowitz et al., which was on the order of 5 min of arc per second at that eccentricity. The combined results of Experiments 1 and 2 show that some compensatory sway was induced despite such minimal velocities, suggesting again the inappropriateness of applying thresholds derived with object motion to situations involving large field motions. The low level of front wall angular velocities compared to those generated by side wall motion imply a confound of the results of Experiment 3 (in which observers turned to face the side wall of the moving room). The head turn placed the slowest available optical motion, that of the front wall, in the retinal periphery. If large field motion thresholds follow the pattern established by Leibowitz et al., then the optical velocities of points projected to the retinal periphery in Experiment 3 might have been below threshold, which could account for the failure of peripheral exposures to induce sway in that experiment. Because large field motion thresholds as a function of retinal location are not known, this possibility cannot currently be addressed. Influence of Central Retina Another surprising result of the present studies, from the point of view of peripheral dominance theory, is the small but consistent ability of the central retina to mediate postural adjustments. This result is not explicitly prohibited by the peripheral dominance view (indeed, Brandt, et al., 1973, found a similar effect with perceived egomotion), but it has not been stressed. The present results make it clear that the ability to visually modulate stance is not restricted to the retinal periphery, and they are in agreement with Andersen and Braunstein's (1985) finding that central retina can support perceived egomotion. It is possible that the central retina has an even greater stance-modulating capacity than was observed in the present studies. As discussed above, the stationary blocking screens used in these experiments presented information to specify that the observer was not moving. When these were present peripherally, they might well have suppressed centrally modulated sway responses to the moving front wall. The actual limit on the central retina's ability to pick up information for egomotion is therefore likely to be higher; how much higher is unclear, though it seems unlikely that central pickup could equal the performance of lamellar flow in the periphery. Conclusion The final assessment of retinally based theories in light of the present results is not entirely straightforward. There is a difference in sensitivity between different areas of the retina, but it is more complex than had been previously supposed. Centra] retina has a modest capacity to use radial and lamellar flow for the control of stance. Peripheral retina appears to be specialized for the pickup of information for postural control from flow having lamellar geometry, but it is unable to use radial flow for controlling posture at all. This makes sense because we typically orient our eyes at least generally in the direction in which we are moving, with the effect that the retinal periphery typically samples areas of the optic array

12 FLOW STRUCTURE IN THE CONTROL OF STANCE 565 containing lamellar flow. There is thus little reason for the retinal periphery to be adapted to detect stance-relevant motion information from the more radially structured flow found near the line of motion. It thus seems that the peripheral dominance approach to the optical control of stance is correct, but a more ecologically oriented approach, such as Gibson's (1979), is also correct. In explaining these results of the present experiments, one must take into account not only the function of the receptor organ but also the structure of the information in the ambient optic array. References Andersen, G., & Braunstein, M. (1985). Induced self-motion in central vision. Journal of Experimental Psychology: Human Perception and Performance, 11, Berthoz, A., Lacour, M., Soechting, J., & Vidal, P. (1979). The role of vision in the control of posture during linear motion. In R. Granit & O. Pompeiano (Eds.), Progress in brain research (pp ). New York: Elsevier. Berthoz, A., Pavard, B., & Young, L. R. (1975). Perception of linear horizontal self-motion induced by peripheral vision (linearvection). Experimental Brain Research, 23, Brandt, T., Dichgans, J., & Koenig, E. (1973). Differential effects of central versus peripheral vision on egocentric and exocentric motion perception. Experimental Brain Research. 16, Cutting, J. (in press). Perception with an eye toward motion. Cambridge, MA: MIT Press/Bradford Books. Dichgans, J., & Brandt, T. (1978). Visual-vestibular interaction: Effects on self-motion perception and postural control. In R. Held, H. Leibowitz, & H. Teuber (Eds.), Handbook of sensory physiology (Vol. 8, pp ). New York: Springer-Verlag. Gibson, J. J. (1979). The ecological approach to visual perception. Boston: Houghton-Mifflin. Held, R., Dichgans, J., & Bauer, J. (1975). Characteristics of moving visual scenes influencing spatial orientation. Vision Research, 15, Howard. I. (1982). Human visual orientation. New York: Wiley. Johansson, G. (1977). Studies on visual perception of locomotion. Perception, 6, Johansson, G., von Hofsten, C, & Jansson, G. (1980). Event perception. In Annual Review of Psychology, 31, Lee, D., & Aronson, E. (1974). Visual proprioceptive control of standing in human infants. Perception & Psychophysics, 15, Lee, D.. & Lishman, J. (1975). Visual proprioceptive control of stance. Journal of Human Movement Studies, Leibowitz, H., Johnson, C., & Isabelle, E. (1972). Peripheral motion detection and refractive error. Science, 177, Leibowitz, H., Shupert-Rodemer, C, & Dichgans. J. (1979). The independence of dynamic spatial orientation from luminance and refractive error. Perception & Psychophysics. 25, Lestienne, F., Soechting, J., & Berthoz, A. (1977). Postural readjustments induced by linear motion of visual scenes. Experimental Brain Research, 28, Lishman. J., & Lee, D. (1973). The autonomy of visual kinaesthesis. Perception, 2, Winer, B. (1971). Statistical principles in experimental design. New York: McGraw-Hill. Received December 21, 1984 Revision received March 27, 1985 Correction to Kolers and Brewster In the article "Rhythms and Responses" by Paul A. Kolers and Joan M. Brewster (Journal of Experimental Psychology: Human Perception and Performance, 1985, Vol. 11, No. 2, pp ), there is a typographical error on page 153 (line 32. left-hand column): "77 db re 2 /ibar" should read "77 db re /*bar."

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