Vestibulo-perceptual influences upon the vestibulo-spinal reflex.

The vestibular system facilitates gaze and postural stability via the vestibulo-ocular (VOR) and vestibulo-spinal reflexes, respectively. Cortical and perceptual mechanisms can modulate long-duration VOR responses, but little is known about whether high-order neural phenomena can modulate short-latency vestibulo-spinal responses. Here, we investigate this by assessing click-evoked cervical vestibular myogenic-evoked potentials (VEMPS) during visual roll motion that elicited an illusionary sensation of self-motion (i.e. vection). We observed that during vection, the amplitude of the VEMPs was enhanced when compared to baseline measures. This modulation in VEMP amplitude was positively correlated with the subjective reports of vection strength. That is, those subjects reporting greater subjective vection scores exhibited a greater increase in VEMP amplitude. Control experiments showed that simple arousal (cold-induced discomfort) also increased VEMP amplitude but that, unlike vection, it did not modulate VEMP amplitude linearly. In agreement, small-field visual roll motion that did not induce vection failed to increase VEMP amplitude. Taken together, our results demonstrate that vection can modify the response of vestibulo-collic reflexes. Even short-latency brainstem vestibulo-spinal reflexes are influenced by high-order mechanisms, illustrating the functional importance of perceptual mechanisms in human postural control. As VEMPs are inhibitory responses, we argue that the findings may represent a mechanism whereby high-order CNS mechanisms reduce activity levels in vestibulo-collic reflexes, necessary for instance when voluntary head movements need to be performed.

Effects of luminance contrast, averaged luminance and spatial frequency on vection.

Changing the speed, size and material properties of optic flow can significantly alter the experience of vection (i.e. visually induced illusions of self-motion). Until now, there has not been a systematic investigation of the effects of luminance contrast, averaged luminance and stimulus spatial frequency on vection. This study examined the vection induced by horizontally oriented gratings that continuously drifted downwards at either 20° or 60°/s. Each of the visual motion stimuli tested had one of: (a) six different levels of luminance contrast; (b) four different levels of averaged luminance; and (c) four different spatial frequencies. Our experiments showed that vection could be significantly altered by manipulating each of these visual properties. Vection strength increased with the grating's luminance contrast (in Experiment 1), its averaged luminance (in Experiment 2), and its spatial frequency (in Experiment 3). Importantly, interactions between these three factors were also found for the vection induced in Experiment 4. While simulations showed that these vection results could have been caused by effects on stimulus motion energy, differences in perceived grating visibility, brightness or speed may have also contributed to our findings.

Expansion of space for visuotactile interaction during visually induced self-motion.

Peripersonal space (PPS), which refers to space immediately around an individual's body, plays an important role in interacting with external objects and avoiding unsafe situations. Studies suggest that, during self-motion perception, PPS expands in the direction in which a person perceives himself/herself to be traveling. In the present study, we built on this by investigating, using visually induced self-motion (vection), how visual self-motion information modulates PPS representation. In our experiment, large-field visual motion was presented through a head-mounted display that caused observers to perceive themselves as moving forward in a tunnel (LF condition). To clarify the effects of self-motion information, we compared the findings for this condition with those of another condition, in which small-field visual motion was presented; here, only the central visual field represented motion, which caused the observers to perceive relatively little self-motion (SF condition). Two speeds were tested for both conditions: 1.5 m/s and 6.0 m/s. For measurement, we used a visuotactile-interaction task in which participants, while observing a visual probe object approaching from various distances, were instructed to press a response key as soon as they detected tactile stimuli delivered to their chest. We measured the distance at which the visual approaching probe object facilitated tactile detection (visual-facilitation effect); this was determined through comparisons with trials when no probe was presented. The results showed that the visual facilitation effects were observed for larger distance in the LF than SF conditions, irrespective of tested speeds. These results suggest that visual self-motion information can modulate PPS representation. This finding fits well with the view that PPS representation contributes to protecting the body from potential threats in the environment.

Hot Wind to the Body Can Facilitate Vection Only When Participants Walk Through a Fire Corridor Virtually.

Vection has been reported to be enhanced by wind, as long as the wind is a normal temperature and not hot. However, here we report that a hot wind can facilitate vection, as long as it is natural and consistent with the visual stimulus. We created a fire-corridor stimulus that was consistent with a hot wind and a control stimulus composed of cubes, which were irrelevant to a hot wind. We compared the vection strength induced by a fire-corridor (fire condition) visual stimulus with that induced by geometric cubes (no-fire condition) visual stimulus. There were three wind type conditions: a normal temperature wind, hot wind, and no wind. The results showed that a normal temperature wind facilitated vection and that a hot wind (but not a normal wind) highly enhanced vection when a fire-corridor stimulus was presented. These results suggest that vection is highly affected and modulated by high-level cognitive processes.

Modulation of alpha waves in sensorimotor cortical networks during self-motion perception evoked by different visual-vestibular conflicts.

Visually induced illusion of self-motion (vection) has been used as a tool to address neural correlates of visual-vestibular interaction. The extent to which vestibular cortical areas are deactivated during vection varies from one study to another. The main question in this study is whether such deactivation depends on the visual-vestibular conflict induced by visual motion. A visual motion about the line of sight (roll motion) induces a visual-canal conflict in upright and supine observers. An additional visual-otolith conflict arises in the upright position only, with the graviceptive inputs indicating that the head is stationary. A 96-channel electroencephalogram (EEG) was recorded in 21 participants exposed to roll motion in seated and supine positions. Meanwhile, perceptual state of self-motion was recorded. Results showed a transient decrease in the cortical sensorimotor networks' alpha activity at the onset of vection whatever the participant's position, and therefore the visual-vestibular conflict. During vection, an increase in alpha activity over parieto-occipital areas was observed in the upright condition, that is, in a condition of visual-otolith conflict. The modulation of alpha activity may be predictive of the illusion of self-motion but also may reflect the level of inhibition in the sensorimotor networks needed to reduce potential interference from vestibular conflicting inputs.NEW & NOTEWORTHY For the first time, we explored the neural correlates of different visuo-vestibular conflicts induced by visual motion using EEG. Our study highlighted a neuronal signature for illusory self-motion (vection) in the sensorimotor networks. Strong alpha activity may predict successful vection but also reflects the level of inhibition of sensorimotor networks needed to reduce potential interfering vestibular inputs. These findings would be of prime importance for simulator and virtual reality systems that induce vection.

Shift in lateralization during illusory self-motion: EEG responses to visual flicker at 10 Hz and frequency-specific modulation by tACS.

Self-motion perception is a key aspect of higher vestibular processing, suggested to rely upon hemispheric lateralization and alpha-band oscillations. The first aim of this study was to test for any lateralization in the EEG alpha band during the illusory sense of self-movement (vection) induced by large optic flow stimuli. Visual stimuli flickered at alpha frequency (approx. 10 Hz) in order to produce steady state visually evoked potentials (SSVEPs), a robust EEG measure which allows probing the frequency-specific response of the cortex. The first main result was that differential lateralization of the alpha SSVEP response was found during vection compared with a matched random motion control condition, supporting the idea of lateralization of visual-vestibular function. Additionally, this effect was frequency-specific, not evident with lower frequency SSVEPs. The second aim of this study was to test for a causal role of the right hemisphere in producing this lateralization effect and to explore the possibility of selectively modulating the SSVEP response. Transcranial alternating current stimulation (tACS) was applied over the right hemisphere simultaneously with SSVEP recording, using a novel artefact removal strategy for combined tACS-EEG. The second main result was that tACS enhanced SSVEP amplitudes, and the effect of tACS was not confined to the right hemisphere. Subsequent control experiments showed the effect of tACS requires the flicker frequency and tACS frequency to be closely matched and tACS to be of sufficient intensity. Combined tACS-SSVEPs are a promising method for future investigation into the role of neural oscillations and for optimizing tACS.

Examining potential effects of arousal, valence, and likability of music on visually induced motion sickness.

The present study investigated how valence, arousal, and subjective liking of music affect visually induced motion sickness (VIMS). VIMS is a common side effect when interacting with virtual environments, resulting in discomfort, dizziness, and/or nausea. Music has previously been shown to reduce VIMS, but the precise nature of this effect remains unknown. Eighty participants watched a video of a bicycle ride filmed from a first-person perspective. First, participants (n = 40) were randomly assigned to one of four groups that listened to pre-selected, classical music excerpts varying in valence and arousal (happy, peaceful, agitated, sad) while watching the video. Second, the level of subjective liking of music was maximized by asking participants to select their favourite music (n = 20), which was then played during the video. A control group (n = 20) watched the video without music. VIMS was measured using the Fast Motion Sickness Scale (FMS) and the Simulator Sickness Questionnaire (SSQ). No effects of valence or arousal on VIMS symptoms were found. Instead, we found that VIMS was significantly reduced when music liking was maximized: Participants who listened to their favourite music reported less VIMS compared to those who did not listen to music at all or to pre-selected music that they liked less. Music that is highly liked can, under certain circumstances, successfully reduce VIMS. These effects appear to be independent of the valence and arousal characteristics of the music.

Flying dreams stimulated by an immersive virtual reality task.

Despite a high prevalence and broad interest in flying dreams, these exceptional experiences remain infrequent. Our study aimed to (1) induce flying dreams using a custom-built virtual reality (VR) flying task, (2) examine their phenomenological correlates and (3) investigate their relations to participant state and trait factors. 137 participants underwent VR-flying followed by a morning nap. They also completed home dream journals for 5 days before and 10 days after the VR exposure. VR-flying successfully increased the reporting of flying dreams during the laboratory nap and on the following morning compared to both baseline frequencies and a control cohort. Flying dreams were also changed qualitatively, exhibiting higher levels of Lucid-control and emotional intensity, after VR exposure. Factors such as prior dream-flying experiences and level of VR sensory immersion modulated flying dream induction. Findings are consistent with a new vection-based explanation of dream-flying and may facilitate development of dream flight-induction technologies.

Manipulating the temporal locus and content of mind-wandering.

The human brain has a tendency to drift into the realm of internally-generated thoughts that are unbound by space and time. The term mind-wandering (MW) is often used describe such thoughts when they are perceptually decoupled. Evidence suggests that exposure to forward and backward illusory motion skews the temporal orientation of MW thoughts to either the future or past respectively. However, little is known about the impact of this manipulation on other features of MW. Here, using a novel experimental paradigm, we first confirmed that our illusory motion method facilitated the generation of MW thoughts congruent with the direction of motion. We then conducted content analyses which revealed that goal orientation and temporal distance were also significantly affected by the direction of illusory motion. We conclude that illusory motion may be an effective means of assaying MW and could help to elucidate this ubiquitous, and likely critical, component of cognition.

Motion sickness-susceptible participants exposed to coherent rotating dot patterns show excessive N2 amplitudes and impaired theta-band phase synchronization.

Visually induced motion sickness (VIMS) can occur via prolonged exposure to visual stimulation that generates the illusion of self-motion (vection). Not everyone is susceptible to VIMS and the neural mechanism underlying susceptibility is unclear. This study explored the differences of electroencephalographic (EEG) signatures between VIMS-susceptible and VIMS-resistant groups. Thirty-two-channel EEG data were recorded from 12 VIMS-susceptible and 15 VIMS-resistant university students while they were watching two patterns of moving dots: (1) a coherent rotation pattern (vection-inducing and potentially VIMS-provoking pattern), and (2) a random movement pattern (non-VIMS-provoking control). The VIMS-susceptible group exhibited a significantly larger increase in the parietal N2 response when exposed to the coherent rotating pattern than when exposed to control patterns. In members of the VIMS-resistant group, before vection onset, global connectivity from all other EEG electrodes to the right-temporal-parietal and to the right-central areas increased, whereas after vection onset the global connectivity to the right-frontal area reduced. Such changes were not observed in the susceptible group. Further, the increases in N2 amplitude and the identified phase synchronization index were significantly correlated with individual motion sickness susceptibility. Results suggest that VIMS susceptibility is associated with systematic impairment of dynamic cortical coordination as captured by the phase synchronization of cortical activities. Analyses of dynamic EEG signatures could be a means to unlock the neural mechanism of VIMS.

Vection induced by low-level motion extracted from complex animation films.

This study examined the contributions of low-, mid- and high-level visual motion information to vection. We compared the vection experiences induced by hand-drawn and computer-generated animation clips to those induced by versions of these movies that contained only their pure optic flow. While the original movies were found to induce longer and stronger vection experiences than the pure optic flow, vection onsets were not significantly altered by removing the mid- and high-level information. We conclude that low-level visual motion information appears to be important for vection induction, whereas mid- and higher-level display information appears to be important for sustaining and strengthening this vection after its initial induction.

Material surface properties modulate vection strength.

Realistic appearance and complexity in the visual field are known to affect the strength of vection (visually induced self-motion perception). Although surface properties of materials are, therefore, expected to be visual features that influence vection, to date, the results have been mixed. Here, we used computer graphics to simulate self-motion through rendered 3D tunnels constructed from nine different materials (bark, ceramic, fabric, fur, glass, leather, metal, stone, and wood). There are three ways in which the new stimuli are changed from those found in previous studies: (1) as they move, their appearances interactively change with the 3D structures of the simulated world, as do all the lighting effects and 3D geometric appearances, (2) they are colored, (3) and their components covered a large portion of the visual field. The entire inner surface of each tunnel was composed from one of the nine materials, and optic flow was evoked when an observer virtually moved through the tunnel. Bark, fabric, leather, stone, and wood effectively induced strong vection, whereas, ceramic, glass, fur, and metal did not. Regression analyses suggested that low-level image features such as the lighting and amplitude of spatial frequency were the main factors that modulated vection strength. Additionally, subjective impressions of the nine surface materials showed that the perceived depth, smoothness, and rigidity were related to the perceived vection strength. Overall, our results indicate that surface properties of materials do indeed modulate vection strength.

Perceived Rigidity Significantly Affects Visually Induced Self-Motion Perception (Vection).

When an observer sees a uniformly moving visual stimulus, he or she typically perceives an illusory motion of his or her body in the opposite direction (vection). In this study, the effects of the visual inducer's perceived rigidity were examined using a horizontal sine wave-like line stimulus moving horizontally. Lowering the sine wave amplitude resulted in the perception of a less rigid visual stimulus motion, although the stimulus was always set to move completely rigidly. The psychophysical experiment revealed that visual self-motion perception was weaker in the lower amplitude condition where the visual stimulus was perceived as less rigid. The follow-up experiments showed that the effects of sine wave amplitude manipulation were unrelated to the modulation of the perceived speed. Furthermore, small gaps inserted into the sine waves effectively increased the perceived rigidity and resulted in a strong self-motion perception even in the lower amplitude condition. The current investigation, together with previous studies, clearly demonstrated that perceived features, in addition to the physical ones, play a key role in visual self-motion perception. Visual stimuli, perceived as more rigid, provide a more reliable frame of reference in the observers' spatial orientation, determining their self-motion perception.

Sensitivity to Visual Speed Modulation in Head-Mounted Displays Depends on Fixation.

A primary cause of simulator sickness in head-mounted displays (HMDs) is conflict between the visual scene displayed to the user and the visual scene expected by the brain when the user's head is in motion. It is useful to measure perceptual sensitivity to visual speed modulation in HMDs because conditions that minimize this sensitivity may prove less likely to elicit simulator sickness. In prior research, we measured sensitivity to visual gain modulation during slow, passive, full-body yaw rotations and observed that sensitivity was reduced when subjects fixated a head-fixed target compared with when they fixated a scene-fixed target. In the current study, we investigated whether this pattern of results persists when (1) movements are faster, active head turns, and (2) visual stimuli are presented on an HMD rather than on a monitor. Subjects wore an Oculus Rift CV1 HMD and viewed a 3D scene of white points on a black background. On each trial, subjects moved their head from a central position to face a 15° eccentric target. During the head movement they fixated a point that was either head-fixed or scene-fixed, depending on condition. They then reported if the visual scene motion was too fast or too slow. Visual speed on subsequent trials was modulated according to a staircase procedure to find the speed increment that was just noticeable. Sensitivity to speed modulation during active head movement was reduced during head-fixed fixation, similar to what we observed during passive whole-body rotation. We conclude that fixation of a head-fixed target is an effective way to reduce sensitivity to visual speed modulation in HMDs, and may also be an effective strategy to reduce susceptibility to simulator sickness.

Estimating the sensorimotor components of cybersickness.

The user base of the virtual reality (VR) medium is growing, and many of these users will experience cybersickness. Accounting for the vast interindividual variability in cybersickness forms a pivotal step in solving the issue. Most studies of cybersickness focus on a single factor (e.g., balance, sex, or vection), while other contributors are overlooked. Here, we characterize the complex relationship between cybersickness and several measures of sensorimotor processing. In a single session, we conducted a battery of tests of balance control, vection responses, and vestibular sensitivity to self-motion. Following this, we measured cybersickness after VR exposure. We constructed a principal components regression model using the measures of sensorimotor processing. The model significantly predicted 37% of the variability in cybersickness measures, with 16% of this variance being accounted for by a principal component that represented balance control measures. The strongest predictor was participants' sway path length during vection, which was inversely related to cybersickness [ r(28) = -0.53, P = 0.002] and uniquely accounted for 7.5% of the variance in cybersickness scores across participants. Vection strength reports and measures of vestibular sensitivity were not significant predictors of cybersickness. We discuss the possible role of sensory reweighting in cybersickness that is suggested by these results, and we identify other factors that may account for the remaining variance in cybersickness. The results reiterate that the relationship between balance control and cybersickness is anything but straightforward. NEW & NOTEWORTHY The advent of consumer virtual reality provides a pressing need for interventions that combat sickness in simulated environments (cybersickness). This research builds on multiple theories of cybersickness etiology to develop a predictive model that distinguishes between individuals who are/are not likely to experience cybersickness. In the future this approach can be adapted to provide virtual reality users with curated content recommendations based on more efficient measurements of sensorimotor processing.

More vection means more velocity storage activity: a factor in visually induced motion sickness?

Full-field visual rotation around the vertical axis induces a sense of self-motion (vection), optokinetic nystagmus (OKN), and, eventually, also motion sickness (MS). If the lights are then suddenly switched off, optokinetic afternystagmus (OKAN) occurs. This is due to the discharge of the velocity storage mechanism (VSM), a central integrative network that has been suggested to be involved in motion sickness. We previously showed that visually induced motion sickness (VIMS) following optokinetic stimulation is dependent on vection intensity. To shed light on this relationship, the current study investigated whether vection intensity is related to VSM activity, and thus, to the OKAN. In repetitive trials (eight per condition), 15 stationary participants were exposed to 120 s of visual yaw rotation (60°/s), followed by 90 s in darkness. The visual stimulus either induced strong vection (i.e., scene rotating normally) or weak vection (central and peripheral part moving in opposite directions). Eye movements and subjective vection intensity were continuously measured. Results showed that OKAN occurred less frequently and with lower initial magnitude in the weak-vection condition compared to the strong-vection condition. OKAN decay time constants were not significantly different. The results suggest that the stimuli that produced strong vection also enhanced the charging of the VSM. As VSM activity presumably is a factor in motion sickness, the enhanced VSM activity in our strong-vection condition hints at an involvement of the VSM in VIMS, and could explain why visual stimuli producing a strong sense of vection also elicit high levels of VIMS.

Inhibition of vection by grasping an object.

The present study investigated whether vection could be modified by an object grasping movement. Twenty-five university students were asked to do one of the following four types of left-hand movements while they were viewing a radial optic flow: (1) grasping the hand-gripper strongly; (2) holding the hand-gripper; (3) clenching fist strongly; and (4) open hand without having anything in their left hands (normal hand condition). The participants' tasks were to keep pressing a button with their right hands while they were perceiving vection. After each trial, they estimated the subjective strength of vection on a 101-point scale. The result showed that the vection was inhibited by strongly grasping the hand-gripper task more than by the other hand movements. Vection could be weakened by the object grasping movement. It might be suggested that vection could be inhibited by the presence of an object being grasped and also by the grasping movement itself. We speculated that the mechanism underlying this inhibition might be related to cognitive pressure, attentional load, power and muscle tonus, and multisensory and proprioception interactions.

Predicting vection and visually induced motion sickness based on spontaneous postural activity.

Evidence is mounting that differences in postural instability can be used to predict who will experience strong illusory self-motions (vection) and become sick when exposed to global patterns of optical flow (e.g., Apthorp et al., PLoS One 9(12):e113897, 2014; Stoffregen and Smart, Brain Res Bull 47:437-448, 1998). This study compared the predictive ability of traditional and recurrence quantification analysis (RQA) based measures of postural activity. We initially measured spontaneous fluctuations in the centre of foot pressure (CoP) of our subjects as they stood quietly with their eyes open and closed. They were then repeatedly exposed to two different types of self-motion display. As expected, the oscillating self-motion displays were found to induce stronger vection and greater sickness than the smooth self-motion displays. RQA based measures of spontaneous postural activity proved to be superior predictors of both vection strength and visually induced motion sickness (VIMS). Participants who had displayed lower CoP recurrence rates when standing quietly were more likely to later report stronger vection and VIMS when exposed to both types of optical flow. Vection strength (but not VIMS) was also found to correlate significantly with three other RQA based measures of postural activity (determinism, entropy, and average diagonal line length). We propose that these RQA based measures of spontaneous postural activity could serve as useful diagnostic tools for evaluating who will benefit the most/least from exposure to virtual environments.

Smoothness of stimulus motion can affect vection strength.

We examined the effect of the smoothness of motion on vection strength. The smoothness of stimulus motion was modulated by varying the number of frames comprising the movement. In this study, a horizontal grating translated through 360° of phase in 1 s divided into steps of 3, 4, 6, 12, 20, 30, or 60 frames. We hypothesized that smoother motion should induce stronger vection because the smoother stimulus is more natural and contains more motion energy. We examined this effect of frame number on vection for both downward (Experiment 1) and expanding (Experiment 2) optical flow. The results clearly showed that vection strength increased with increasing frame rate, however, the rates of increase in the vection strength with frame rate are not constant, but rapidly increase in the low frame-rate range and appear to asymptote in the high range. The strength estimates saturated at lower frame rates for expanding flow than for downward flow. This might be related to the fact that to process expanding flow it is necessary to integrate motion signals across the visual field. We conclude that the smoothness of the motion stimulus highly affects vection induction.

Allocating less attention to central vision during vection is correlated with less motion sickness.

Visually induced motion sickness (VIMS) is a common discomfort response associated with vection-provoking stimuli. It has been suggested that susceptibility to VIMS depends on the ability to regulate visual performance during vection. To test this, 29 participants, with VIMS susceptibility assessed by Motion Sickness Susceptibility Questionnaire, were recruited to undergo three series of sustained attention to response tests (SARTs) while watching dot pattern stimuli known to provoke roll-vection. In general, SARTs performance was impaired in the central visual field (CVF), but improved in peripheral visual field (PVF), suggesting the reallocation of attention during vection. Moreover, VIMS susceptibility was negatively correlated with the effect sizes, suggesting that participants who were less susceptible to VIMS showed better performance in attention re-allocation. Finally, when trained to re-allocation attention from the CVF to the PVF, participants experienced more stable vection. Findings provide a better understanding of VIMS and shed light on possible preventive measures. Practitioner Summary: Allocating less visual attention to central visual field during visual motion stimulation is associated with stronger vection and higher resistance to motion sickness. Virtual reality application designers may utilise the location of visual tasks to strengthen and stabilise vection, while reducing the potential of visually induced motion sickness.