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.     

Inertial acceleration as a measure of linear vection: An alternative to magnitude estimation

The present study focused on the development of a procedure to assess perceived self-motion induced by visual surround motion—vection. Using an apparatus that permitted independent control of visual and inertial stimuli, prone observers were translated along their headx-axis (fore/aft). The observers’ task was to report the direction of self-motion during passive forward and backward translations of their bodies coupled with exposure to various visual surround conditions. The proportion of “forward” responses was used to calculate each observer’s point of subjective equality (PSE) for each surround condition. The results showed that the moving visual stimulus produced a significant shift in the PSE when data from the moving surround condition were compared with the stationary surround and no-vision condition. Further, the results indicated that vection increased monotonically with surround velocities between 4 and 40°/sec. It was concluded that linear vection can be measured in terms of changes in the amplitude of whole-body inertial acceleration required to elicit equivalent numbers of “forward” and “backward” self-motion reports.     

Circular vection as a function of the relative sizes, distances, and positions of two competing visual displays

In studies where it is reported that illusory self-rotation (circular vection) is induced more by peripheral displays than by central displays, eccentricity may have been confounded with perceived relative distance and area. Experiments are reported in which the direction and magnitude of vection induced by a central display in the presence of a surround display were measured. The displays varied in relative distance and area and were presented in isolation, with one moving and one stationary display, or with both moving in opposite directions. A more distant display had more influence over vection than a near display. A central display induced vection if seen in isolation or through a 'window' in a stationary surrounding display. Motion of a more distant central display weakened vection induced by a nearer surrounding display moving the other way. When the two displays had the same area their effects almost cancelled. A moving central display nearer than a textured stationary surround produced vection in the same direction as the moving stimulus. This phenomenon is termed 'contrast-motion vecton' because it is probably due to illusory motion of the surround induced by motion of the centre. Unequivocal statements about the dominance of an eccentric display over a central display cannot be made without considering the relative distances and sizes of the displays and the motion contrast between them.     

Velocity not acceleration of self-motion mediates vestibular-visual interaction

We investigated the influence of vestibular stimulation with different angular accelerations and velocities on the perception of visual motion direction. Constant accelerations resulting in different angular velocities and constant angular velocities obtained at different accelerations were combined in twenty healthy subjects. Random-dot kinematograms with coherently moving pixels and randomly moving pixels were used as visual stimuli during whole-body rotations. The smallest percentage of coherently moving pixels leading to a clear perception of motion direction was taken as the perception threshold. Perception thresholds significantly increased with increasing angular velocity. Increased acceleration, however, had no significant effect on the perception thresholds. We conclude that the achieved angular velocity, and not acceleration, is the predominant factor in the processing of vestibular-visual interaction.     

Vestibular sensitivity and vection chronometry along the spinal axis in erect man

A study is reported of the relations between vestibular sensitivity and vection chronometry in healthy human adults. Twenty-three subjects were examined. For both vestibular and vection investigations, the subjects were seated in an armchair with the spinal axis aligned with the earth vertical and the head normally erect. The subjects' vestibular thresholds for detection of vertical upward accelerations were assessed by a double-staircase psychophysical method. The subjects' vection onset latencies were measured for both upward and downward directions. Since the vection onset latencies are presumed to be shortened by the decrease of the conflict between visual and vestibular afferents, the less-vestibular-sensitive subjects were hypothesised to have shorter vection onset latencies than the more-vestibular-sensitive ones. As expected, the results indicate a negative correlation between vestibular thresholds and vection onset latencies: the higher the vestibular thresholds, the lower the vection onset latencies.     

Visual acceleration detection: effect of sign and motion orientation

Thresholds for the detection of constant acceleration and deceleration of a discrete object moving along horizontal and vertical axes were studied. A staircase methodology was used to determine thresholds for three average velocities (0.7, 1.2, and 1.7 deg/sec). Thresholds, expressed as the proportion of velocity change, did not differ significantly among the average velocities; thus, a consistent Weber-like fraction is suggested by the data. Furthermore, there was an interaction between the axis of motion (horizontal or vertical) and the sign of the velocity change (acceleration or deceleration): accelerations were easier to detect along the vertical axis, decelerations along the horizontal axis.     

The power law for the perception of rotation by airline pilots

The purpose of this study was to determine the power laws for the perception of rotation about the three major body axes. Eighteen airline pilots made magnitude estimates of 5-sec pulses of nine angular accelerations having a range of acceleration x time of 10–150 deg/sec. The results showed that (1) the power law with an exponent of 1.4 describes the subjective motion of these pilots for all three major body axes, (2) the power law also describes the perception of motion for individual pilots with a substantial range of exponents, (3) there were significant correlations among the exponents for the three body axes, and (4) the data suggest that the power law over the wide range used may be more complex than implied by a formula with a single exponent.     

Visual stimulation affects the perception of voluntary leg movements during walking

When a limb is used for locomotion, patterns of afferent and efferent activity related to its own motion are present as well as visual, vestibular, and other proprioceptive information about motion of the whole body. A study is reported in which it was asked whether visual stimulation present during whole-body motion can influence the perception of the leg movements propelling the body. Subjects were tested in conditions in which the stepping movements they made were identical but the amount of body displacement relative to inertial space and to the visual surround varied. These test conditions were created by getting the subjects to walk on a rotatable platform centered inside a large, independently rotatable, optokinetic drum. In each test condition, subjects, without looking at their legs, compared, against a standard condition in which the floor and drum were both stationary, their speed of body motion, their stride length and stepping rate, the direction of their steps, and the perceived force they exerted during stepping. When visual surround motion was incompatible with the motion normally associated with the stepping movements being made, changes in apparent body motion and in the awareness of the frequency, extent, and direction of the voluntary stepping movements resulted.     

Power laws for the perception of rotation and the oculogyral illusion

Magnitude estimation was used to measure subjective motion for two indicators of vestibular function. Twelve as made estimates of 5-sec pulses of angular acceleration across the range of angular acceleration × time (at) =10-150 deg/sec. Results were: (1) the power law describes subjective motion for all individual as, (2) the power function exponent (1.41) for the perception of rotation is slightly greater than the exponent (1.25) for the oculogyral illusion, (3) a significant number of as gave higher exponents for the perception of rotation, and (4) the magnitude estimates of the oculogyral illusion and perception of rotation were highly correlated within and across as.     

The perception of an optical flow projected on the ground surface

In most experiments in which the importance of visual control on postural stability is studied, optical stimuli attached to vertical surfaces are used. Analyses of long-term standing readjustments generally involve back-and-forth movements of a visual scene or its projection on vertical circular screens. In a natural environment, however, visual information is largely available from the ground. The aim of the experiment reported was to assess the effect of a flow pattern simulating an open outdoor setting on motion perception and postural control. Subjects were presented with an optical texture projected onto the ground. Periods of motionless texture alternated with equivalent durations of unidirectional flows. The change of position of the subject's centre of gravity over time was recorded on a force platform. Results show that the direction of body sway corresponded with that of texture motion. Important aftereffects, as shown in linear vection experiments, were also observed. However, the latency of postural responses was much shorter than with prolonged unidirectional flows produced in other locations of the visual environment. The hypothesis of an ecological specificity of the flows perceived on the ground during terrestrial displacements is discussed.     

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.     

Perceived range, perceived velocity, and perceived duration of the body rotating in the frontal plane

We investigated perceived range, perceived velocity, and perceived duration of the body rotating in the frontal plane (in roll). Specifically, to examine how shear to the otoliths in the inner ears and tactile pressure to the trunk affect judgments of range and velocity, in two experiments, we manipulated rotating range (30°–160°), rotating velocity (1.8°/sec to 9.6°/sec), mean tilt of the body (?60°, 0°, and 60°), and exposure to the visual vertical. Thirty-three normal or blindfolded participants made verbal judgments of range, velocity, and duration for each combination of these factors. The exponents of the power functions fitted to these judgments were, as a first approximation, .94, .61, and .84 for range, velocity, and duration, respectively, and perceived velocity was proportional to the ratio of perceived range to perceived duration (r = .91). These results suggest that the vestibular and somatosensory inputs are effective on judgments of range, but less so on judgments of velocity, and that perceived velocity may be determined as a ratio of perceived range to perceived duration. In addition, we found that (1) when the range the body has traveled is constant, the perceived range increases as the objective velocity decreases (proprioceptive τ effect); (2) self-motion through the tilted roll sometimes enlarges perceived range and perceived duration but reduces perceived velocity; and (3) the exposure to the visual vertical reduces variability of judgments for range and velocity and also reduces perceived range and perceived velocity of self-motion within a small range through the vertical roll.     

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.     

Cyclopean stimulation can influence sensations of self-motion in normal and stereoblind subjects

Subjects experienced an illusion of self-motion when viewing the randomly patterned inner surface of a cylinder rotating about their main body axis. This sensation of rotation in a direction opposite to the direction of cylinder rotation is known as circular vection. An experiment was conducted to ascertain if the production of circular vection involved a binocular process in the visual system. Using dichoptic strobe illumination, stimuli were created that were identical monocularly but different binocularly. Groups of normal and stereoblind subjects were tested. The presence of purely binocular (cyclopean) stimulation increased the reported magnitude of vection for both groups. We conclude that a binocular process is involved in the production of circular vection and that this process retains its binocularity in stereoblind subjects.     

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.     

On the role of otoliths and semicircular canals in spatial orientation: Dynamics of the visually perceived eye level during gondola centrifugation

The visually perceived eye level (VPEL) was measured during gondola centrifugation. Subjects (N = 11) were seated upright, facing motion in a swing-out gondola The head was adjusted so that Reid's baseline was tilted 10 degrees anterior end up. The subjects were requested to adjust the position of a small luminous dot so that it was perceived as gravitationally at eye level. In the 1-g environment, the VPEL was a few degrees below the true gravitational eye level (M = -1.75 degrees, SD = 1.90 degrees). After rapid acceleration of the centrifuge to 2 G (vectorial sum of the earth gravity force and the centrifugal force), there was an exponentially increasing depression of the VPEL. The initial value was -6.4 degrees +/- 5.2 degrees. During 10 min at 2 G, the VPEL approached an asymptotic value of -24.8 degrees +/- 5.4 degrees. The time constant showed a large interindividual variability, ranging from 59 to 1,000 sec (M = 261 sec, median = 147 sec). The findings are discussed, taking into consideration otolith-semicircular-canal interaction, as well as memory functions of the vestibular system.     

The angle of visual roll motion determines displacement of subjective visual vertical

The effect of full-field sinusoidal visual roll motion stimuli of various frequencies and peak velocities upon the orientation of subjective visual vertical (SV) was studied. The angle of the optokinetically induced displacement of SV was found to be a linear function of the logarithm of the stimulus oscillation angle. Interindividual slopes of this function varied between 2 and 9. The logarithmic function is independent of stimulus frequency within the range of .02 Hz to .5 Hz and of peak stimulus velocity from 7.5°/sec to 170°/sec. It holds for oscillation angles up to 100°–140°. With larger rotational angles, saturation is reached. With small stimulus angles, a surprisingly high threshold (5°-8°) was observed in our experiments. This may reflect the unphysiological combination of visual roll stimuli without corroborating vestibular and proprioceptive inputs normally present when body sway produces visual stimulation. Under natural conditions, the visual feedback about spontaneous sway stabilizes body posture by integrating rotational velocity over stimulus duration which is equal to rotational angle.     

Self-motion sensation influenced by visual fixation

Subjects were exposed to angular decelerations of between 1 and 50 deg/sec’ (1) in total darkness, (2) in view of a dim subject-stationary fixation light, or (3) inside an illuminated subject-stationary striped cylinder (conflict stimulation). Vestibularly induced eye movements led to the oculogyral illusion of object motion. This phenomenon can be distinguished from that of the sensation of self-rotation. At the end of deceleration, the initial velocity of self-rotation sensation is similar during all three stimulus conditions, but is reduced in duration with the conflict stimulus. Differences of interpretation in the literature concerning these phenomena can be explained on the basis of the failure to distinguish between the oculogyral illusion and sensation of self-motion and the inability to fully suppress vestibular eye movements.     

Linear vection in the central visual field facilitated by kinetic depth cues.

Illusory self-motion (vection) is thought to be determined by motion in the peripheral visual field, whereas stimulation of more central retinal areas results in object-motion perception. Recent data suggest that vection can be produced by stimulation of the central visual field provided it is configured as a more distant surface. In this study vection strength (tracking speed, onset latency, and the percentage of trials where vection was experienced) and the direction of self-motion produced by displays moving in the central visual field were investigated. Apparent depth, introduced by using kinetic occlusion information, influenced vection strength. Central displays perceived to be in the background elicited stronger vection than identical displays appearing in the foreground. Further, increasing the eccentricity of these displays from the central retina diminished vection strength. If the central and peripheral displays were moved in opposite directions, vection strength was unaffected, and the direction of vection was determined by motion of the central display on almost half of the trials when the centre was far. Near centres produced fewer centre-consistent responses. A complete understanding of linear vection requires that factors such as display size, retinal locus, and apparent depth plane are considered.     

Chromaticity, spatial complexity, and self-motion perception

The effects of visual field color and spatial complexity on self-motion perception were investigated by placing observers inside a large rotating cylinder (optokinetic drum). Under optokinetic-drum conditions visually induced self-motion (vection) is typically perceived within 30 s, even though all forms of sensory input (eg vestibular, proprioceptive, auditory), except vision, indicate that the observer is stationary. It was hypothesized that vection would be hastened and vection magnitude increased by adding chromatic colors and spatial complexity to the lining of an optokinetic drum. Addition of these visual-field characteristics results in an array that shares more visual-field characteristics with our typical environment that usually serves as a stable frame of reference regarding self-motion perception. In the color experiment, participants viewed vertical stripes that were: (i) black and white, (ii) various gray shades, or (iii) chromatic. In the spatial complexity experiment, participants were presented with: (i) black-and-white vertical stripes, or (ii) a black-and-white checkerboard pattern. Drum rotation velocity was 5 rev. min(-1) (30 degrees s(-1)), and both vection onset and magnitude were measured for 60 s trials. Results indicate that chromaticity and spatial complexity hasten the onset of vection and increase its perceived magnitude. Chromaticity and spatial complexity are common characteristics of the environments in which our visual system evolved. The presence of these visual-field features in an optic flow pattern may be treated as an indicator that the scene being viewed is stationary and that the observer is moving.