Relative sensitivities to large-field optic-flow patterns varying in direction and speed

Motion in depth results in radial optic-flow patterns. Forward motion results in radially expanding patterns, whereas backward motion generates contracting patterns. Radial optic-flow patterns are typically represented with a positive speed gradient, ie zero speed at the point of fixation, and maximum speed at the periphery. However, the actual speed profile in such a stimulus will depend upon the relative depth of objects in the scene. Using large-field stimuli (82 deg diameter) we determined relative sensitivities to radial expansion and contraction patterns and also to various types of speed gradients: positive, negative, random, and flat. We found that, even when large-field stimuli are used, observers are more sensitive to radially contracting patterns than to expanding patterns. Sensitivity to the positive speed gradient was not consistently different from either the negative or random gradients. Sensitivity to the flat gradient depended upon the speed of the stimuli. The finding of greater sensitivity to radial contraction is discussed in terms of the functional requirements involved in the use of optic-flow signals in maintaining balance. On the basis of the present findings, the utility of comparing psychophysical results based on thresholds against physiological data based on suprathreshold stimuli is also discussed.     

Vestibular stimulation affects optic-flow sensitivity

Typically, multiple cues can be used to generate a particular percept. Our area of interest is the extent to which humans are able to synergistically combine cues that are generated when moving through an environment. For example, movement through the environment leads to both visual (optic-flow) and vestibular stimulation, and studies have shown that non-human primates are able to combine these cues to generate a more accurate perception of heading than can be obtained with either cue in isolation. Here we investigate whether humans show a similar ability to synergistically combine optic-flow and vestibular cues. This was achieved by determining the sensitivity to optic-flow stimuli while physically moving the observer, and hence producing a vestibular signal, that was either consistent with the optic-flow signal, eg a radially expanding pattern coupled with forward motion, or inconsistent with it, eg a radially expanding pattern with backward motion. Results indicate that humans are more sensitive to motion-in-depth optic-flow stimuli when they are combined with complementary vestibular signals than when they are combined with conflicting vestibular signals. These results indicate that in humans, like in nonhuman primates, there is perceptual integration of visual and vestibular signals.     

Inconsistent locomotion inhibits vection

We measured the strength of illusory self-motion perception (vection) with and without locomotion on a treadmill. The results revealed that vection was inhibited by inconsistent locomotion, but facilitated by consistent locomotion.     

Properties of curvilinear vection

Approximately linear relationships were observed between contrast, spatial frequency, temporal frequency, or velocity of stimulation and perceived velocity of curvilinear vection—that is, a visually induced self-motion in a curved path. Similarly, linear relationships were also found between the perceived degree of curvature of curvilinear vection and spatial frequency or velocity of stimulation. Since the perceived velocity of curvilinear vection varies with contrast, spatial frequency, temporal frequency, and angular velocity, and the perceived degree of curvature of curvilinear vection varies only with spatial frequency and angular velocity, peripheral vision is not sufficient for computing accurately the curvilinear component of induced self-motion in a curved path. Concurrently, it was shown that the perceived direction of curvilinear vection is not always unambiguously perceived (Sauvan & Bonnet, 1989). Consequently, it is suggested that two different types of visual processing, which involve the peripheral or the central vision, underlie the processing of curvilinear vection.     

Attentional load inhibits vection

In this study, we examined the effects of cognitive task performance on the induction of vection. We hypothesized that, if vection requires attentional resources, performing cognitive tasks requiring attention should inhibit or weaken it. Experiment 1 tested the effects on vection of simultaneously performing a rapid serial visual presentation (RSVP) task. The results revealed that the RSVP task affected the subjective strength of vection. Experiment 2 tested the effects of a multiple-object-tracking (MOT) task on vection. Simultaneous performance of the MOT task decreased the duration and subjective strength of vection. Taken together, these findings suggest that vection induction requires attentional resources.     

Spatiotemporal boundaries of linear vection

Thresholds for the perception of linear vection were measured. These thresholds allowed us to define the spatiotemporal contrast surface sensitivity and the spatiotemporal domain of the perception of rectilinear vection (a visually induced self-motion in a straight line). Moreover, a Weber’s law was found, such that a mean relative differential threshold in angular velocity of about 41% is necessary to perceive curvilinear vection. This visually induced self-motion corresponds to the sensation of moving in a curved path. It is proposed that curvilinear vection is induced when the apparent velocity difference is detectable. The spatiotemporal domain of perception of rectilinear vection and its spatiotemporal contrast surface sensitivity are centered on low spatial frequencies. Concurrently, the values which correspond to the relative differential thresholds of curvilinear vection are low spatial frequencies. Accordingly, the peripheral ambient visual system seems to be involved in perceiving linear vection. It is argued further that the central ambient system might also be involved in the processing of linear vection.     

Inhibition of vection by red

We investigated the effects of colors on vection induction. Expanding optical flows during one’s forward self-motion were simulated by moving dots. The dots and the background were painted in equiluminant red and green. Experiments 1 and 2 showed that vection was weaker when the background was red than when the background was green. In addition, Experiment 3 showed that vection was weaker when the moving dots were red than when the dots were green. Experiment 4 demonstrated that red dots on a red background induced very weak vection, as compared with green dots on a green background. In Experiments 5 and 6, we showed that the present results could not be explained by a luminance artifact. Furthermore, Experiment 7 showed that a moving red grating induced weaker vection than did a green one. We concluded that a red visual stimulus inhibits vection.     

Induction and impairment of saturated yaw and surge vection

A flight simulator was used to investigate the perception of self-motion and visual scene motion during the induction of saturated 10 deg/sec yaw and 50 m/sec surge vection, and during subsequent impairment of saturated vection by inertial motions. The subjects (n = 5) did not perceive any self-acceleration or visual scene deceleration during the induction of saturated vection but perceived a rather sudden change in self-velocity and visual scene velocity. The mean group times to saturated vection were 3.0 sec for yaw and 2.7 sec for surge. Above certain inertial motion amplitudes, the subjects reported additional self-motion from the applied inertial motions while experiencing saturated vection. To impair saturated yaw vection, these amplitudes were 0.6 m/sec2, 0.4 m/sec2, 8 deg/sec2, and 5 deg/sec2, for surge, sway, roll and yaw motions, respectively. To impair saturated surge vection, these amplitudes were 0.6 m/sec2, 0.3 m/sec2, 5 deg/sec2, and 4 deg/sec2, respectively. The results indicate that saturated vection is more robust for translations than for rotations because the rotational inertial amplitudes were closer to the amplitudes at which the applied inertial motion was perceived than the translational inertial amplitudes.     

Global-perspective jitter improves vection in central vision

Previous vection research has tended to minimise visual-vestibular conflict by using optic-flow patterns which simulate self-motions of constant velocity. Here, experiments are reported on the effect of adding 'global-perspective jitter' to these displays--simulating forward motion of the observer on a platform oscillating in horizontal and/or vertical dimensions. Unlike non-jittering displays, jittering displays produced a situation of sustained visual-vestibular conflict. Contrary to the prevailing notion that visual-vestibular conflict impairs vection, jittering optic flow was found to produce shorter vection onsets and longer vection durations than non-jittering optic flow for all of jitter magnitudes and temporal frequencies examined. On the basis of these findings, it would appear that purely radial patterns of optic flow are not the optimal inducing stimuli for vection. Rather, flow patterns which contain both regular and random-oscillating components appear to produce the most compelling subjective experiences of self-motion.     

Circular vection is independent of stimulus eccentricity

The sensation of self-rotation induced by viewing a surround rotating about the observer's vertical axis (circular vection or CV) was investigated with equal-area stimuli located in either the central, the mid-peripheral, or the far-peripheral visual field. Magnitude estimation responses indicated greater CV with larger stimulus area, but no significant differences in CV sensations as a function of stimulus eccentricity. This pattern of results does not support the belief that CV is dominated by peripheral stimulation when equal-area stimuli are compared.     

Relationship between vection and motion perception in depth

This study examined the effects of cues to motion in depth – namely, stereoscopic (i.e., changing-disparity cues and interocular velocity differences) and changing-size cues on forward and backward vection. We conducted four experiments in which participants viewed expanding or contracting optical flows with the addition of either or both cues. In Experiment 1, participants reported vection by pressing a button whenever they felt it. After each trial, they also rated the magnitude of the vection (from 0 to 100). In Experiments 2 and 3, the participants rated the perceived velocity and motion-in-depth impression of the flows relative to standard stimuli, respectively. In Experiment 4, the participants rated the perceived depth and distance of the display. We observed enhancements in vection, motion-in-depth impression, and perceived depth and distance when either or both types of cues indicated motion-in-depth, as compared to those when the cues did not (Experiments 1, 3, and 4). The perceived velocity changed with cue conditions only for the high velocity condition (Experiment 2). Correlational analyses showed that the vection can be best explained by the motion-in-depth impression. This was partially supported by the multiple regression analyses. These results indicate that the enhancement of vection caused by cues is related to the impression of motion-in-depth rather than the perceived velocity and perceived three-dimensionality.     

Factors affecting the onset and magnitude of linear vection

The role of central and peripheral vision in the production of linear vection was assessed by using displays in which flow structure and sources of internal and external depth information were manipulated. Radial optical flow was more effective for inducing self-motion in both central and peripheral visual fields than was lamellar flow in displays of the same size. The presence of external occlusion information was necessary to induce linear vection when small displays were composed of lamellar flow, whereas the effectiveness of small radial displays did not depend on the availability of occlusion edges.     

Roll vection analysis of suggestion-induced visual field narrowing

Instructions to simulate visual field narrowing resulted in apparently narrow visual fields when these were evaluated by means of conventional perimetry. However, when a pattern of stripes moving around the visual axis was viewed, the magnitude of the induced change in the subjective horizontal or vertical (roll vection) was unaffected. These results demonstrate that conventional perimetric techniques may in some instances be inadequate to demonstrate functional peripheral vision. Evaluation of the peripheral visual field in perimetry, the role of peripheral vision in visually guided behavior, and the effect of stressors on peripheral visual functions are discussed.     

Circular vection as a function of foreground-background relationships

It has previously been reported that illusory self-rotation (circular vection) is most effectively induced by the more distant of two moving displays. Experiments are reported in which the relative effectiveness of two superimposed displays in generating circular vection as a function of (i) the separation in depth between them, (ii) their perceived relative distances, and (iii) which display was in the plane of focus was investigated. Circular vection was governed by the motion of the display that was perceived to be the more distant, even when it was actually nearer. However, actual or perceived distance was found to be not the crucial factor in circular vection because even when the distance between the two displays was virtually zero, vection was controlled by the display perceived to be in the background. When the displays were well separated in depth, vection was not affected by whether the near or the far display was in the plane of focus, nor by which display was fixed or pursued by the eyes.     

Stimulus eccentricity and spatial frequency interact to determine circular vection.

While early research suggested that peripheral vision dominates the perception of self-motion, subsequent studies found little or no effect of stimulus eccentricity. In contradiction to these broad notions of 'peripheral dominance' and 'eccentricity independence', the present experiments showed that the spatial frequency of optic flow interacts with its eccentricity to determine circular vection magnitude--central stimulation producing the most compelling vection for high-spatial-frequency stimuli and peripheral stimulation producing the most compelling vection for lower-spatial-frequency stimuli. This interaction appeared to be due, in part at least, to the effect that the higher-spatial-frequency moving pattern had on subjects' ability to organise optic flow into related motion about a single axis. For example, far-peripheral exposure to this high-spatial-frequency pattern caused many subjects to organise the optic flow into independent local regions of motion (a situation which clearly favoured the perception of object motion not self-motion). It is concluded that both high-spatial-frequency and low-spatial-frequency mechanisms are involved in the visual perception of self-motion--with their activities depending on the nature and eccentricity of the motion stimulation.     

Simulated angular head oscillation enhances vection in depth

Research has shown that adding simulated linear head oscillation to radial optic flow displays enhances the illusion of self-motion in depth (ie linear vection). We examined whether this oscillation advantage for vection was due to either the added motion parallax or retinal slip generated by insufficient compensatory eye movement during display oscillation. We constructed radial flow displays which simulated 1 Hz horizontal linear head oscillation (generates motion parallax) or angular head oscillation in yaw (generates no motion parallax). We found that adding simulated angular or linear head oscillation to radial flow increased the strength of linear vection in depth. Neither type of simulated head oscillation significantly reduced vection onset latencies relative to pure radial flow. Simultaneous eye-movement recordings showed that slow-phase ocular following responses (OFRs) were induced in both linear and angular viewpoint oscillation conditions. Vection strength was significantly reduced by active central fixation when viewing displays which simulated angular, but not linear, head oscillation. When these displays with angular oscillation were viewed without stable fixation, vection strength was found to increase with the velocity and regularity of the OFR. We conclude that vection improvements observed during central viewing of displays with angular viewpoint oscillation depend on the generation of eye movements.     

Consistent air flow to the face facilitates vection

We examined whether a somatosensory cue suggesting forward self-motion facilitated vection. We provided a consistent air flow to subjects' faces by using an electric fan.Vection strength was increased when the air flow was provided.     

Evidence for a boundary effect in roll vection

Large circular displays rotating around the line of sight produce an illusion in which the seen orientation of the true vertical is shifted in a direction opposite to the display’s motion. Two experiments were performed to determine whether the magnitude of this illusory tilt is a function of the area of display elements, of their boundary length, or of their spatial frequency. In Experiment 1, 12 subjects viewed each of nine displays across which the number and area of the circular elements were independently varied. Three of the displays were equated for the area of their elements. The results suggested that tilt magnitude and onset latency could be explained by a boundary length effect. A second experiment tested eight subjects on two displays, equated for element boundary length but differing in the spatial frequency of the elements. The displays produced closely similar illusory trite corroborating the view that, within broad limits, element boundary length—and not spatial frequency or area—determines the size and onset latency of illusory tilt. A third experiment confirmed previous research in finding greater tilt and more rapid onset with more peripherally projected displays.     

Motion direction distribution as a determinant of circular vection

Visually induced self-translation is called linear vection, while visually induced self-rotation is called circular vection. Impressions of circular vection and linear vection were measured using flow patterns presented on a flat screen. Subjects reported strong circular vection when the flow simulated a projected pattern of a rotating cylinder, which had gradients in speed and direction of moving elements on the screen. When speed gradients in a horizontal dimension were removed while not changing the direction distribution on the screen, strong circular vection was still reported. On the other hand, when the motion direction of all elements was the same (horizontal), having speed gradients, the circular vection was weak. The impression of linear vection showed the opposite trend. This result indicates not a speed distribution pattern but one of a two-dimensional direction on the retina determines the type of vection.     

Jitter and size effects on vection are immune to experimental instructions and demands

Both coherent perspective jitter and explicit changing-size cues have been shown to improve the vection induced by radially expanding optic flow. We examined whether these stimulus-based vection advantages could be modified by altering cognitions and/or expectations about both the likelihood of self-motion perception and the purpose of the experiment. In the main experiment, participants were randomly assigned into two groups-one where the cognitive conditions biased participants towards self-motion perception and another where the cognitive conditions biased them towards object-motion perception. Contrary to earlier findings by Lepecq et al (1995 Perception 24 435-449), we found that identical visual displays were less likely to induce vection in 'object-motion-bias' conditions than in 'self-motion bias' conditions. However, significant jitter and size advantages for vection were still found in both cognitive conditions (cognitive bias effects were greatest for non-jittering same-size control displays). The current results suggest that if a sufficiently large vection advantage can be produced when participants are expecting to experience self-motion, it is likely to persist in object-motion-bias conditions.