A Systematic Review on the Elicitation and Enhancement of Vection Through Tactile Stimulation

Vection is classically defined as the illusory perception of self-motion induced via visual stimuli. The utility of vection research lies in its potential to enhance simulation fidelity as measured through presence and reduce the probability that motion sickness symptoms occur through the identification of differences and similarities in physiological responses to vection and motion sickness. Recent studies have shown a multimodal interaction of various sensory systems in facilitating vection. Moreover, the utility of co-stimulating some of these sensory systems along with the presentation of visual stimuli have been reviewed. However, a review on the tactile mediation of vection appears to be missing from literature. The purpose of this review is to evaluate the current methodologies, and utility of, tactile stimulation in vection research. We searched for articles through EBSCOHost, Scopus and Web of Science. Only studies were included wherein an experiment on the effect of tactile stimulation on human self-motion perception was detailed. Twenty-three studies were obtained and distilled in tabular form. The studies reviewed presented a strong variability in the a-priori definition of vection, methodological inhomogeneity, varying approaches to the assessment of subjective vection measures and a lack of objective measures. We discuss the need for more qualitative research to reduce methodological inhomogenities, recommend future research in tactile-mediated vection to investigate stimulation to the torso, and suggest further investigation into the utility of concurrently presenting different forms of tactile stimulation.

An Exploration into Vection During Virtual Helicopter Flight Using Varying Levels of Active Control and Passive Control

The illusory perception of self-motion, which is called vection, is a phenomenon that is traditionally measured in passive participants. Eliciting a compelling vection experience enhances the immersiveness of, and presence in, virtual environments. The investigation of vection during active control has mostly been done through subjective reports using single intensity stimuli for virtual locomotion or driving tasks, however, a vection study on virtual flight with varying difficulty levels appears to be missing from literature. Herein, we aimed to investigate how varying difficulty levels of active control as well as passive control of a virtual aircraft affects subjective vection intensity and objective physiological signals. Participants were visually and audibly immersed in a virtual environment in which either they or an AI controlled a virtual helicopter. Active control was facilitated by means of a stationary helicopter simulator. Galvanic skin response (GSR) and accelerations of the body core were recorded during flight and upon completion of each condition participants rated the intensity of vection and object-motion for 3 sensory modalities separately. Marginally higher vection intensity ratings compared to object-motion ratings were found and vection ratings were lightly affected by control difficulty due to weather conditions. Furthermore, participants reporting higher vection intensity ratings showed elevated changes in GSR compared to participants reporting lower vection intensity ratings. Our results show that vection can be elicited and modulated through active control with varying difficulty levels and including objective measures could elevate our understanding vection during active control tasks.

Proprioceptive Stimulation Added to a Walking Self-Avatar Enhances the Illusory Perception of Walking in Static Participants

When immersed in virtual reality, users who view their body as a co-located virtual avatar that reflects their movements, generally develop a sense of embodiment whereby they perceive the virtual body to be their own. One aspect of the sense of embodiment is the feeling of agency over the avatar, i.e., the feeling that one is producing the movements of the avatar. In contexts such as physical rehabilitation, telepresence and gaming, it may be useful to induce a strong sense of agency in users who cannot produce movements or for whom it is not practical to do so. Being able to feel agency over a walking avatar without having to produce walking movements could be especially valuable. Muscle vibrations have been shown to produce the proprioceptive perception of movements, without any movement on the part of the user. The objectives of the current study were to: 1-determine if the addition of lower-limb muscle-vibrations with gait-like patterns to a walking avatar can increase the illusory perception of walking in healthy individuals who are standing still; 2-compare the effects of the complexity of the vibration patterns and of their synchronicity on the sense of agency and on the illusory perception of walking. Thirty participants viewed a walking avatar from a first-person perspective, either without muscle vibrations or with one of four different patterns of vibrations. These five conditions were presented pairwise in a two-alternative forced choice paradigm and individually presented, after which participants answered an embodiment questionnaire. The displacement of center of pressure of the participants was measured throughout the experiment. The results show that all patterns of proprioceptive stimulation increased the sense of agency to a similar degree. However, the condition in which the proprioceptive feedback was realistic and temporally aligned with the avatar’s leg movements led to significantly larger anteroposterior sway of the center of pressure. The frequency of this sway matched the cadence of the avatar’s gait. Thus, congruent and realistic proprioceptive stimulation increases the feeling of agency, the illusory perception of walking and the motor responses of the participants when viewing a walking avatar from a first-person perspective.

The Challenge of Illusory Perception of Animals: The Impact of Methodological Variability in Cross-species Investigation

Although we live on the same planet, there are countless different ways of seeing the surroundings that reflect the different individual experiences and selective pressures. In recent decades, visual illusions have been used in behavioural research to compare the perception between different vertebrate species. The studies conducted so far have provided contradictory results, suggesting that the underlying perceptual mechanisms may differ across species. Besides the differentiation of the perceptual mechanisms, another explanation could be taken into account. Indeed, the different studies often used different methodologies that could have potentially introduced confounding factors. In fact, the possibility exists that the illusory perception is influenced by the different methodologies and the test design. Almost every study of this research field has been conducted in laboratories adopting two different methodological approaches: a spontaneous choice test or a training procedure. In the spontaneous choice test, a subject is presented with biologically relevant stimuli in an illusory context, whereas, in the training procedure, a subject has to undergo an extensive training during which neutral stimuli are associated with a biologically relevant reward. Here, we review the literature on this topic, highlighting both the relevance and the potential weaknesses of the different methodological approaches.

A binocular test of sensory eye dominance based on the Pulfrich effect

Many studies have shown inconsistent results among traditional measures of sensory ocular dominance (SED), seriously questioning the very concept of SED as a unitary aspect of the visual system (e.g., Laby & Kirschen, 2011; Mapp et al., 2003; Walls, 1951).The test outcomes may also change even if the same measure is repeated under different conditions, for example by varying the distance and eccentricity of the target (e.g., Rice et al., 2008; Kahn & Crawford, 2001).On the other hand, some authors suggest that such inconsistencies may result from uncontrolled variables. A candidate to produce confusing variables is the frequent format of these tests, often dichotomous and introspective.In this paper, we propose a possibility of measuring SED on a continuous scale and in a comparative way using a stimulus that induces a Pulfrich effect. Here a dichoptic motion stimulus, borrowed from a previous study (Reynaud & Hess, 2017), was used, which produced different degrees of 3D illusory perception strength through the variation of retinal disparity. We observed that the responses of the subjects varied according to their classical SED test outcomes and we estimated the differences in terms of the time delay of the information coming from the two eyes. In our sample, it appears that information from the dominant eye was processed 8.2±5.8 ms faster than that of the fellow eye. People with a left dominant eye showed more marked differences in processing time (6.8±2.0 ms) than people with a right dominant eye (1.8±0.9 ms). Eyes without a clear dominance did not show significant differences in processing time (1.2±1.7 ms). These results are consistent with the previous literature and could lead to the development of a new continuous-scaled SED test.

Evidence-Based Approach and Discussion of ‘Bioceramic Resonance’ to Induce Altered States of Consciousness with Illusory Perception: Possible Application as a Complementary and Alternative Therapy

By combing these results and our previous objective data of electroencephalographic brain wave activity and the locations of brain activation during 3T functional MRI scanning, we hypothesis that the phenomenon observed in this study mimics the psychotherapeutic effects of transcranial brain stimulation, which may probably explain by induction of cerebral electrical discharge and change of synchronous neuronal activity. We discussed the possibility of complementary and alternative therapy on different psychiatric and neurological disorders.

Rapid neural categorization of facelike objects predicts the perceptual awareness of a face (face pareidolia)

The human brain rapidly and automatically categorizes faces vs. other visual objects. However, whether face-selective neural activity predicts the subjective experience of a face – perceptual awareness – is debated. To clarify this issue, here we use face pareidolia, i.e., the illusory perception of a face, as a proxy to relate the neural categorization of a variety of facelike objects to conscious face perception. In Experiment 1, scalp electroencephalogram (EEG) is recorded while pictures of human faces or facelike objects – in different stimulation sequences – are interleaved every second (i.e., at 1 Hz) in a rapid 6-Hz train of natural images of nonface objects. Participants do not perform any explicit face categorization task during stimulation, and report whether they perceived illusory faces post-stimulation. A robust categorization response to facelike objects is identified at 1 Hz and harmonics in the EEG frequency spectrum with a facelike occipito-temporal topography. Across all individuals, the facelike categorization response is of about 20% of the response to human faces, but more strongly right-lateralized. Critically, its amplitude is much larger in participants who report having perceived illusory faces. In Experiment 2, facelike or matched nonface objects from the same categories appear at 1 Hz in sequences of nonface objects presented at variable stimulation rates (60 Hz to 12 Hz) and participants explicitly report after each sequence whether they perceived illusory faces. The facelike categorization response already emerges at the shortest stimulus duration (i.e., 17 ms at 60 Hz) and predicts the behavioral report of conscious perception. Strikingly, neural facelike-selectivity emerges exclusively when participants report illusory faces. Collectively, these experiments characterize a neural signature of face pareidolia in the context of rapid categorization, supporting the view that face-selective brain activity reliably predicts the subjective experience of a face from a single glance at a variety of stimuli.Highlights- EEG frequency-tagging measures the rapid categorization of facelike objects- Facelike objects elicit a facelike neural categorization response- Neural face categorization predicts conscious face perception across variable inputs

Postdiction: when temporal regularity drives space perception through pre-stimulus alpha oscillations

During postdiction, the last stimulus of a sequence changes the perception of the preceding ones: in the rabbit illusion, a three-flash series presented regularly in time, but not in space, is – illusory - perceived as spatially regular. Such a reorganization of (spatial) perception could be driven by internal priors, e.g. favoring slow motion for the rabbit illusion. Although postdiction is a ubiquitous phenomenon, its neural underpinnings remain poorly understood. Here, we focused on the role of priors during postdiction and hypothesized that these could be reflected by alpha oscillations (8-12Hz), previously observed to correlate with idiosyncratic biases. We presented human participants with ambiguous visual stimuli that elicited the rabbit illusion on about half the trials, allowing us to contrast MEG-EEG brain responses to the same physical events causing distinct percepts. Given that a strong prior will increase the overall probability of perceiving the illusion, we used the percentage of perceived illusion as a proxy for an individual’s prior. We found that high fronto-parietal alpha power was associated with perceiving the sequence according to individual biases: participants with high susceptibility to the illusion would report the illusion, while participants with low susceptibility would report the veridical sequence. Additionally, we found that pre-stimulus alpha phase in occipital and frontal areas dissociated illusory from non-illusory trials. These results point to a dissociated relation of the power and timing of alpha band activity to illusory perception, with power reflecting prior expectations and phase influencing behavioral performance, potentially due to the modulation of sensory uncertainty.Significance StatementLate events may sometimes influence how earlier events are perceived, as if the arrow of time was reversed in the brain. This surprising phenomenon, called postdiction, is observed in the rabbit illusion, and highlights a predominant mechanism for perceptual processes. Perception builds up from the combination of prior expectations with incoming sensory evidence, which takes time. We showed that pre-stimulus neural activity, and more specifically alpha oscillations (8-12Hz), play a double role in postdiction. Fronto-parietal alpha power reflects individual prior expectation, while occipital and frontal alpha phase predicts illusory perception. Postdiction might actually be a means of compensating for the neural delays inherent in perceptual processes, so that the arrow of perceptual time matches the arrow of physical time.

Homeostatic brain network mechanisms facilitate perceptual binding of audio-visual speech

In daily lives, speech perception requires binding of spatiotemporally disjoint auditory and visual cues. On the other hand, functional segregation and integration are the two complementary mechanisms that capture brain information processing. Here, we demonstrate using fMRI recordings that subjective perceptual experience of multisensory speech stimuli is dependent on a homeostatic balance of segregation and integration mechanisms. Previous reports conceptualized posterior superior temporal sulcus as the key brain region for binding signals from multiple sensory streams. However, we report an enhancement of segregated information processing in distributed brain regions, defined as the perceptual binding network. The seed-based whole brain functional connectivity of each node in this network was anti-correlated with higher propensity for illusory perception. Interestingly, the perceptual binding network was anti-correlated with other intrinsic brain networks, such as dorsal attention and default mode networks during cross-modal perception. The pattern disappeared for people who rarely reported the illusory perception, further strengthening the hypothesis of homeostatic balance. The cognitive theories of Bayesian causal inference and predictive coding hypothesis could explain the balance of segregative and integrative mechanisms during cross-modal perception.

The effect of the Müller–Lyer configuration on saccadic eye movements is not fully due to illusory perception

The Müller–Lyer illusion affects both perception and oculomotor control, but it is unknown whether these effects arise from the same or different underlying mechanisms. We developed a modified version of the Müller–Lyer configuration, which largely reduced the perceptual illusion effect compared with the typical configuration but reduced the saccadic effect to a much less extent. Such difference indicates that influence of the Müller–Lyer configuration on saccadic eye movements is not fully mediated by illusory perception.