Slow changing postural cues cancel visual field dependence on self-tilt detection.

Interindividual differences influence the multisensory integration process involved in spatial perception. Here, we assessed the effect of visual field dependence on self-tilt detection relative to upright, as a function of static vs. slow changing visual or postural cues. To that aim, we manipulated slow rotations (i.e., 0.05° s(-1)) of the body and/or the visual scene in pitch. Participants had to indicate whether they felt being tilted forward at successive angles. Results show that thresholds for self-tilt detection substantially differed between visual field dependent/independent subjects, when only the visual scene was rotated. This difference was no longer present when the body was actually rotated, whatever the visual scene condition (i.e., absent, static or rotated relative to the observer). These results suggest that the cancellation of visual field dependence by dynamic postural cues may rely on a multisensory reweighting process, where slow changing vestibular/somatosensory inputs may prevail over visual inputs.

How do visual and postural cues combine for self-tilt perception during slow pitch rotations?

Self-orientation perception relies on the integration of multiple sensory inputs which convey spatially-related visual and postural cues. In the present study, an experimental set-up was used to tilt the body and/or the visual scene to investigate how these postural and visual cues are integrated for self-tilt perception (the subjective sensation of being tilted). Participants were required to repeatedly rate a confidence level for self-tilt perception during slow (0.05°·s(-1)) body and/or visual scene pitch tilts up to 19° relative to vertical. Concurrently, subjects also had to perform arm reaching movements toward a body-fixed target at certain specific angles of tilt. While performance of a concurrent motor task did not influence the main perceptual task, self-tilt detection did vary according to the visuo-postural stimuli. Slow forward or backward tilts of the visual scene alone did not induce a marked sensation of self-tilt contrary to actual body tilt. However, combined body and visual scene tilt influenced self-tilt perception more strongly, although this effect was dependent on the direction of visual scene tilt: only a forward visual scene tilt combined with a forward body tilt facilitated self-tilt detection. In such a case, visual scene tilt did not seem to induce vection but rather may have produced a deviation of the perceived orientation of the longitudinal body axis in the forward direction, which may have lowered the self-tilt detection threshold during actual forward body tilt.

Spatial localization investigated by continuous pointing during visual and gravitoinertial changes.

In order to accurately localize an object, human observers must integrate multiple sensory cues related to the environment and/or to the body. Such multisensory integration must be repeated over time, so that spatial localization is constantly updated according to environmental changes. In the present experimental study, we examined the multisensory integration processes underlying spatial updating by investigating how gradual modifications of gravitoinertial cues (i.e., somatosensory and vestibular cues) and visual cues affect target localization skills. These were assessed by using a continuous pointing task toward a body-fixed visual target. The "single" rotation of the gravitoinertial vector (produced by off-axis centrifugation) resulted in downward pointing errors, which likely were related to a combination of oculogravic and somatogravic illusions. The "single" downward pitch rotation of the visual background produced an elevation of the arm relative to the visual target, suggesting that the rotation of the visual background caused an illusory target elevation (induced-motion phenomenon). Strikingly, the errors observed during the "combined" rotation of the visual background and of the gravitoinertial vector appeared as a linear combination of the errors independently observed during "single" rotations. In other words, the centrifugation effect on target localization was reduced by the visual background rotation. The observed linear combination indicates that the weights of visual and gravitoinertial cues were similar and remained constant throughout the stimulation.

Virtual reality and claustrophobia: multiple components therapy involving game editor virtual environments exposure.

The effectiveness of a multiple components therapy regarding claustrophobia and involving virtual reality (VR) will be demonstrated through a trial which immersed six claustrophobic patients in multiple context-graded enclosed virtual environments (VE) using affordable VR apparatus and software. The results of the questionnaires and behavior tests exhibited a significant reduction in fear towards the enclosed space and quality of life improvement. Such gains were maintained at 6-month follow-up. Presence score indicated the patients felt immersed and present inside the game editor VE.

Spatial scale of motion segmentation from speed cues.

For the accurate perception of multiple, potentially overlapping, surfaces or objects, the visual system must distinguish different local motion vectors and selectively integrate similar motion vectors over space to segment the retinal image properly. We recently showed that large differences in speed are required to yield a percept of motion transparency. In the present study, to investigate the spatial scale of motion segmentation from speed cues alone, we measured the speed-segmentation threshold (the minimum speed difference required for 75% performance accuracy) for 'corrugated' random-dot patterns, i.e. patterns in which dots with two different speeds were alternately placed in adjacent bars of variable width. In a first experiment, we found that, at large bar widths, a smaller speed difference was required to segment and perceive the corrugated pattern of moving dots, while at small bar-widths, a larger speed difference was required to segment the two speeds and perceive two transparent surfaces of moving dots. Both the perceptual and segmentation performance transitions occurred at a bar width of around 0.4 degrees. In a second experiment, speed-segmentation thresholds were found to increase sharply when dots with different speeds were paired within a local pooling area. The critical pairing distance was about 0.2 degrees in the fovea and increased linearly with stimulus eccentricity. However, across the range of eccentricities tested (up to 15 degrees ), the critical pairing distance did not change much and remained close to the receptive field size of neurons within the primate primary visual cortex. In a third experiment, increasing dot density changed the relationship between speed-segmentation thresholds and bar width. Thresholds decreased for large bar widths, but increased for small bar widths. All of these results are well fit by a simple stochastic model, which estimates the probabilities of having identical or different motion vectors within a local pooling area whose size is the same as that of primate V1 neurons. Altogether, these results demonstrate that speed-based segmentation can function well, even at small spatial scales (i.e. high-spatial frequencies of spatial corrugation) and thereby emphasizes the critical role of a local pooling process early in the cortical motion-processing pathway.

Lorazepam-induced modifications of saccadic and smooth-pursuit eye movements in humans: attentional and motor factors.

In a placebo-controlled, double-blind study, we measured the effects of low dose lorazepam on attentional and motor factors involved in saccadic and smooth pursuit eye movements. We manipulated the temporal interval between the extinction of the central fixation target and the appearance of a second eccentric target (gap/overlap step paradigm). The second target was either stationary (saccade trial) or moving in a direction opposite to the step (pursuit trial). Gap/overlap effects on the latency of saccadic and smooth pursuit eye movements were measured before and after oral intake of either lorazepam or placebo. Pharmacological effects on the dynamics and the accuracy of both types of eye movements were also investigated. In 14 healthy volunteers, we found that the temporal interval between fixation target offset and eccentric target onset modulates the latency of saccadic and smooth pursuit eye movements in a similar way. As compared to placebo, lorazepam significantly increased the latency of both types of eye movements, but did not modify the gap/overlap effect. Moreover, lorazepam significantly decreased the peak velocity of the first saccade towards the eccentric stationary target, as well as the gain of tracking towards the eccentric moving target. However, the overall accuracy of both behaviors was not significantly affected, indicating that systematic errors in foveating or tracking were detected and corrected by appropriate corrective or catch-up saccades, respectively. Results are discussed in terms of shared/different mechanisms for saccadic and pursuit systems in primates.

Temporal dynamics of motion integration for the initiation of tracking eye movements at ultra-short latencies.

The perceived direction of a grating moving behind an elongated aperture is biased towards the aperture's long axis. This "barber pole" illusion is a consequence of integrating one-dimensional (1D) or grating and two-dimensional (2D) or terminator motion signals. In humans, we recorded the ocular following responses to this stimulus. Tracking was always initiated at ultra-short latencies (approximately 85 ms) in the direction of grating motion. With elongated apertures, a later component was initiated 15-20 ms later in the direction of the terminator motion signals along the aperture's long axis. Amplitude of the later component was dependent upon the aperture's aspect ratio. Mean tracking direction at the end of the trial (135-175 ms after stimulus onset) was between the directions of the vector sum computed by integrating either terminator motion signals only or both grating and terminator motion signals. Introducing an elongated mask at the center of the "barber pole" did not affect the latency difference between early and later components, indicating that this latency shift was not due to foveal versus peripheral locations of 1D and 2D motion signals. Increasing the size of the foveal mask up to 90% of the stimulus area selectively reduced the strength of the grating motion signals and, consequently, the amplitude of the early component. Conversely, reducing the contrast of, or indenting the aperture's edges, selectively reduced the strength of terminator motion signals and, consequently, the amplitude of the later component. Latencies were never affected by these manipulations. These results tease apart an early component of tracking responses, driven by the grating motion signals and a later component, driven by the line-endings moving at the intersection between grating and aperture's borders. These results support the hypothesis of a parallel processing of 1D and 2D motion signals with different temporal dynamics.

Speed tuning of motion segmentation and discrimination.

Motion transparency requires that the visual system distinguish different motion vectors and selectively integrate similar motion vectors over space into the perception of multiple surfaces moving through or over each other. Using large-field (7 degrees x 7 degrees) displays containing two populations of random-dots moving in the same (horizontal) direction but at different speeds, we examined speed-based segmentation by measuring the speed difference above which observers can perceive two moving surfaces. We systematically investigated this 'speed-segmentation' threshold as a function of speed and stimulus duration, and found that it increases sharply for speeds above approximately 8 degrees/s. In addition, speed-segmentation thresholds decrease with stimulus duration out to approximately 200 ms. In contrast, under matched conditions, speed-discrimination thresholds stay low at least out to 16 degrees/s and decrease with increasing stimulus duration at a faster rate than for speed segmentation. Thus, motion segmentation and motion discrimination exhibit different speed selectivity and different temporal integration characteristics. Results are discussed in terms of the speed preferences of different neuronal populations within the primate visual cortex.

Ocular responses to motion parallax stimuli: the role of perceptual and attentional factors.

When human subjects are presented with visual displays consisting of random dots moving sideways at different velocities, they perceive transparent surfaces, moving in the same direction but located at different distances from themselves. They perceive depth from motion parallax, without any additional cues to depth, such as relative size, occlusion or binocular disparity. Simultaneously, large-field visual motion triggers compensatory eye movements which tend to offset such motion, in order to stabilize the visual image of the environment. In a series of experiments, we investigated how such reflexive eye movements are controlled by motion parallax displays, that is, in a situation where a complete stabilization of the visual image is never possible. Results show that optokinetic nystagmus, and not merely active visual pursuit of singular elements, is triggered by such displays. Prior to the detection of depth from motion parallax, eye tracking velocity is equal to the average velocity of the visual image. After detection, eye tracking velocity spontaneously matches the slowest velocity in the visual field, but can be controlled by attentional factors. Finally, for a visual stimulation containing more than three velocities, subjects are no longer able to perceptually dissociate between different surfaces in depth, and eye tracking velocity remains equal to the average velocity of the visual image. These data suggest that, in the presence of flow fields containing motion parallax, optokinetic eye movements are modulated by perceptual and attentional factors.

Is judging time-to-contact based on 'tau'?

An investigation was undertaken into whether judgments of time-to-contact between a laterally moving object and a bar are based on the direct perception of an optical variable (tau), or on the ratio between the perceived distance and perceived velocity of the object. A moving background was used to induce changes in the perceived velocities without changing the optical variables that specify time-to-contact. Background motion induced large systematic errors in the estimated time-to-contact. It is concluded that the judgment of time-to-contact is primarily based on the ratio between the perceived distance and the perceived velocity, and not on tau.

Effects of stationary and moving textured backgrounds on the visuo-oculo-manual tracking in humans.

We investigated the effects of stationary and moving textured backgrounds on ocular and manual pursuit of a discrete target that suddenly starts to move at constant speed (ramp motion). When a stationary textured background was superimposed to the target displacement, the gain of the steady-state eye smooth pursuit velocity was significantly reduced, while the latency of pursuit initiation did not vary significantly, as compared to a dark background condition. The initial velocity of the eye smooth pursuit was also lowered. Both the initial acceleration and the steady-state manual tracking angular velocity were slightly, but not significantly, lowered when compared to a dark background condition. Detrimental effects of the stationary textured background were of comparable amplitude (approximately 10%) for ocular and manual pursuit. In a second condition, we compared ocular and manual pursuit when the textured background was either stationary or drifting. Initial and steady-state eye velocities increased when the textured background moved in the same direction as the target. Conversely, when the background moved in the opposite direction, both velocities were decreased. Eye displacement gain remained however close to unity due to an increase in the occurrence of catch-up corrective saccades. The effects of the moving backgrounds on the initial and steady-state forearm velocities were inverse to that reported for smooth pursuit eye movements. Neither manual nor ocular smooth pursuit latencies were affected.

Effects of the spatio-temporal structure of optical flow on postural readjustments in man.

How does the spatio-temporal structure of an oscillating radial optical flow affect postural stability? In order to investigate this problem, two different types of stimulus pattern were presented to human subjects. These stimuli were generated either with a constant spatial frequency or with a spatial frequency gradient providing monocular depth cues. When the stimulation was set in motion, the gain response of the antero-posterior postural changes depended upon the oscillation frequency of the visual scene. The amplitude of the postural response did not change with the amplitude of the visual scene motion. The spatial orientation of the postural sway (major axis of sway) depended strictly and solely on the structure of the visual scene. In static conditions, depth information resulting from the presence of a spatial frequency gradient enhanced postural stability. When set in motion, a visual scene with a spatial frequency gradient induced an organization of postural sway in the direction of the visual motion. Considering visual dynamic cues, postural instability depended linearly both on the logarithm of the velocity and on the logarithm of the temporal frequency. A nonlinear relationship existed between the amplitude of the fore-aft postural sway at the driving frequency and the temporal frequency, with a peak around 2-4 Hz. These results are discussed in terms of their implications for the separation of visual and biomechanical factors influencing visuo-postural control.

Low luminance contrast sensitivity: effects of training on psychophysical and optokinetic nystagmus thresholds in man.

We compared psychophysical contrast sensitivity function (psi-CSF) and optokinetic contrast sensitivity function (OKN-CSF) in man, for the combination of three spatial and three temporal frequencies. psi-CSF was defined as the inverse of the contrast threshold, that is the contrast value of a sinusoidal grating for which a subject was able to identify the width of a drifting grating. OKN-CSF was defined as the inverse of the contrast value of the grating which triggered an involuntary optokinetic nystagmus. In highly experienced subjects, OKN-CSF was overall higher than psi-CSF. More precisely, differences between both contrast sensitivity functions occurred mainly in the low spatio-temporal frequency range (below 4 c/deg and 9 Hz). In naive subjects, psi-CSF reached the level of OKN-CSF after two consecutive test sessions. OKN-CSF did not change with training. Similarly, high spatio-temporal frequency psychophysical thresholds did not change with training and, moreover, approximated OKN-CSF thresholds. Low spatio-temporal frequency psychophysical sensitivity was initially lower than corresponding OKN-CSF sensitivity; however, after only two training sessions, the two functions were indistinguishable due to a selective increase in psychophysical low spatio-temporal frequency sensitivity.

Catching balls: how to get the hand to the right place at the right time.

Information specifying the future passing distance of an approaching object is available (in units of object size) in the ratio of optical displacement velocity and optical expansion velocity. Despite empirical support for the assumption that object size can serve as a metric in the perception of passing distance, the present series of experiments reveals that in catching a ball subjects do not rely on such "point-predictive" information. The angle at which (real and simulated) balls approached the subject systematically affected verbal and manual estimates of future passing distance, as well as the kinematic characteristics of catching movements. To catch a ball, the actor uses momentary action-related information instead of spatiotemporal estimates. The hand velocity is geared to information specifying the currently required velocity. This secures ending up at the right place in the right time, regardless of where this may be.

Perception of optical flow in cortical blindness: a case report.

Motion perception was studied in a subject with bilateral lesion of the visual cortex, involving severe damage to cortical areas V1 and V4, but with no apparent damage to visual associative areas situated in occipito-parietal and lateral occipito-temporal (presumably V5) zones. He was able to perceive optical flow motions simulating motion in depth in "blind" parts of his visual field, provided that the stimulus-onset was temporally dissociated from its motion. Moreover, he was able to discriminate between different velocities and directions of motion. The results suggest that perimetrically "blind" parts of the visual field in this patient have true capacities to process visual motion. They are discussed in reference to the subject's ability to move freely in his environment and in reference to the role of extrastriate visual pathways in visual motion processing.

Perception of circular heading from optical flow.

Observers viewed random-dot optical flow displays that simulated self-motion on a circular path and judged whether they would pass to the right or left of a target at 16 m. Two dots in two frames are theoretically sufficient to specify circular heading if the orientation of the rotation axis is known. Heading accuracies were better than 1.5 degrees with a ground surface, wall surface, and 3D cloud of dots, and were constant over densities down to 2 dots, consistent with the theory. However, there was an inverse relation between the radius of the observer's path and constant heading error, such that at small radii observers reported heading 3 degrees to the outside of the actual path with the ground and to the inside with the wall and cloud. This may be an artifact of a small display screen.