Double Flash Illusions: Current Findings and Future Directions

Self as a prior: The malleability of Bayesian multisensory integration to social salience

Our everyday perceptual experiences are grounded in the integration of information within and across our senses. Due to this direct behavioural relevance, cross-modal integration retains a certain degree of contextual flexibility, even to social relevance. However, how social relevance modulates cross-modal integration remains unclear. To investigate possible mechanisms, Experiment 1 tested the principles of audio-visual integration for numerosity estimation by deriving a Bayesian optimal observer model with perceptual prior from empirical data to explain perceptual biases. Such perceptual priors may shift towards locations of high salience in the stimulus space. Our results showed that the tendency to over- or underestimate numerosity, expressed in the frequency and strength of fission and fusion illusions, depended on the actual event numerosity. Experiment 2 replicated the effects of social relevance on multisensory integration from Scheller & Sui, 2022 JEP:HPP, using a lower number of events, thereby favouring the opposite illusion through enhanced influences of the prior. In line with the idea that the self acts like a prior, the more frequently observed illusion (more malleable to prior influences) was modulated by self-relevance. Our findings suggest that the self can influence perception by acting like a prior in cue integration, biasing perceptual estimates towards areas of high self-relevance.

Exploring the correlation and causation between alpha oscillations and one-second time perception through EEG and tACS

Alpha oscillations have been implicated in time perception, yet a consensus on their precise role remains elusive. This study directly investigates this relationship by examining the impact of alpha oscillations on time perception. Resting-state EEG recordings were used to extract peak alpha frequency (PAF) and peak alpha power (PAP) characteristics. Participants then performed a time generalization task under transcranial alternating current stimulation (tACS) at frequencies of PAF-2, PAF, and PAF+2, as well as a sham condition. Results revealed a significant correlation between PAP and accuracy, and between PAF and precision of one-second time perception in the sham condition. This suggests that alpha oscillations may influence one-second time perception by modulating their frequency and power. Interestingly, these correlations weakened with real tACS stimulations, particularly at higher frequencies. A second analysis aimed to establish a causal relationship between alpha peak modulation by tACS and time perception using repeated measures ANOVAs, but no significant effect was observed. Results were interpreted according to the state-dependent networks and internal clock model.

Audio-Visual Fission Illusion and Individual Alpha Frequency: Perspective on Buergers and Noppeney (2022)

Integrating visual and auditory information is an important ability in various cognitive processes, although its neural mechanisms remain unclear. Several studies indicated a close relationship between one's temporal binding window (TBW) for audio-visual interaction and their alpha rhythm in the brain (individual alpha frequency or IAF). A recent study by Buergers and Noppeney [Buergers, S., & Noppeney, U. The role of alpha oscillations in temporal binding within and across the senses. Nature Human Behaviour, 6, 732-742, 2022], however, challenged this view using a new approach to analyze behavioral data. Conforming to the same procedures by Buergers and Noppeney, here, I analyzed the data of my previous study and examined a relationship between TBW and IAF. In contrast to Buergers and Noppeney, a significant correlation was found between occipital IAF and a new behavioral measure of TBW. Some possibilities that caused these opposing results, such as a variability of "alpha band" across studies and a large inter-individual difference in magnitude of the fission illusion, are discussed.

Perceptual Training as Means to Assess the Effect of Alpha Frequency on Temporal Binding Window

For decades, it has been shown that alpha frequency is related to temporal binding window, and currently, such is the mainstream viewpoint [Noguchi, Y. Individual differences in beta frequency correlate with the audio-visual fusion illusion. Psychophysiology, 59, e14041, 2022; Gray, M. J., & Emmanouil, T. A. Individual alpha frequency increases during a task but is unchanged by alpha-band flicker. Psychophysiology, 57, e13480, 2020; Hirst, R. J., McGovern, D. P., Setti, A., Shams, L., & Newell, F. N. What you see is what you hear: Twenty years of research using the sound-induced flash illusion. Neuroscience & Biobehavioral Reviews, 118, 759-774, 2020; Keil, J. Double flash illusions: Current findings and future directions. Frontiers in Neuroscience, 14, 298, 2020; Migliorati, D., Zappasodi, F., Perrucci, M. G., Donno, B., Northoff, G., Romei, V., & Costantini, M. Individual alpha frequency predicts perceived visuotactile simultaneity. Journal of Cognitive Neuroscience, 32, 1-11, 2020; Keil, J., & Senkowski, D. Individual alpha frequency relates to the sound-induced flash illusion. Multisensory Research, 30, 565-578, 2017; Minami, S., & Amano, K. Illusory jitter perceived at the frequency of alpha oscillations. Current Biology, 27, 2344-2351, 2017; Cecere, R., Rees, G., & Romei, V. Individual differences in alpha frequency drive crossmodal illusory perception. Current Biology, 25, 231-235, 2015]. However, recently, this stance has been challenged [Buergers, S., & Noppeney, U. The role of alpha oscillations in temporal binding within and across the senses. Nature Human Behaviour, 6, 732-742, 2022]. Moreover, both stances appear to have their limitations regarding the reliability of results. Therefore, it is of paramount importance to develop new methodology to gain more reliable results. Perceptual training seems to be such a method that also offers significant practical implications.

Audio-visual multisensory integration and haptic perception are altered in adults with developmental coordination disorder

Developmental Coordination Disorder (DCD) is a movement disorder in which atypical sensory processing may underly movement atypicality. However, whether altered sensory processing is domain-specific or global in nature, are unanswered questions. Here, we measured for the first time, different aspects of sensory processing and spatiotemporal integration in the same cohort of adult participants with DCD (N = 16), possible DCD (pDCD, N = 12) and neurotypical adults (NT, N = 28). Haptic perception was reduced in both DCD and the extended DCD + pDCD groups when compared to NT adults. Audio-visual integration, measured using the sound-induced double flash illusion, was reduced only in DCD participants, and not the DCD + pDCD extended group. While low-level sensory processing was altered in DCD, the more cognitive, higher-level ability to infer temporal dimensions from spatial information, and vice-versa, as assessed with Tau-Kappa effects, was intact in DCD (and extended DCD + pDCD) participants. Both audio-visual integration and haptic perception difficulties correlated with the degree of self-reported DCD symptoms and were most apparent when comparing DCD and NT groups directly, instead of the expanded DCD + pDCD group. The association of sensory difficulties with DCD symptoms suggests that perceptual differences play a role in motor difficulties in DCD via an underlying internal modelling mechanism.

Crossmodal interference on counting performance: Evidence for shared attentional resources

During the act of counting, our perceptual system may rely on information coming from different sensory channels. However, when the information coming from different sources is discordant, such as in the case of a de-synchronization between visual stimuli to be counted and irrelevant auditory stimuli, the performance in a sequential counting task might deteriorate. Such deterioration may originate from two different mechanisms, both linked to exogenous attention attracted by auditory stimuli. Indeed, exogenous auditory triggers may infiltrate our internal "counter", interfering with the counting process, resulting in an overcount; alternatively, the exogenous auditory triggers may disrupt the internal "counter" by deviating participants' attention from the visual stimuli, resulting in an undercount. We tested these hypotheses by asking participants to count visual discs sequentially appearing on the screen while listening to task-irrelevant sounds, in systematically varied conditions: visual stimuli could be synchronized or de-synchronized with sounds; they could feature regular or irregular pacing; and their speed presentation could be fast (approx. 3/sec), moderate (approx. 2/sec), or slow (approx. 1.5/sec). Our results support the second hypothesis since participants tend to undercount visual stimuli in all harder conditions (de-synchronized, irregular, fast sequences). We discuss these results in detail, adding novel elements to the study of crossmodal interference.

Medial temporal lobe functional network architecture supports sleep-related emotional memory processing in older adults

Memory consolidation occurs via reactivation of a hippocampal index during non-rapid eye movement slow-wave sleep (NREM SWS) which binds attributes of an experience existing within cortical modules. For memories containing emotional content, hippocampal-amygdala dynamics facilitate consolidation over a sleep bout. This study tested if modularity and centrality-graph theoretical measures that index the level of segregation/integration in a system and the relative import of its nodes-map onto central tenets of memory consolidation theory and sleep-related processing. Findings indicate that greater network integration is tied to overnight emotional memory retention via NREM SWS expression. Greater hippocampal and amygdala influence over network organization supports emotional memory retention, and hippocampal or amygdala control over information flow are differentially associated with distinct stages of memory processing. These centrality measures are also tied to the local expression and coupling of key sleep oscillations tied to sleep-dependent memory consolidation. These findings suggest that measures of intrinsic network connectivity may predict the capacity of brain functional networks to acquire, consolidate, and retrieve emotional memories.

Visual adaptation changes the susceptibility to the fission illusion

Sound-induced flash illusion (SiFI) is the illusion that participants perceive incorrectly that the number of visual flashes is equal to the number of auditory beeps when presented within 100 ms. Although previous studies found that repetition suppression can reduce an individual's perceptual sensitivity to the SiFI, there is not yet a consensus as to how visual adaptation affects the SiFI. In the present study, we added prolonged adapting visual stimuli prior to the presentation of audiovisual stimuli to investigate whether the bottom-up factor of adaptation affects the SiFI. The adapting visual stimuli consisted of one or two of the same visual stimuli that lasted for 2 minutes in succession, followed by the audiovisual stimuli. Both adaptation conditions showed SiFI effects. The accuracy of adapting double-flashes was significantly lower than that of in adapting a single flash for the fission illusion. Our analyses indicated that such a pattern could be attributed to a lower d' in adapting double-flashes than in adapting a single flash. However, the accuracy, discriminability and criterion were not significantly different between the two adaptation conditions because of the instability of the fusion illusion. Thus, the present study indicated that the reduced perceptual sensitivity based on visual adaptation could enhance the fission illusion in multisensory integration.

Crossmodal plasticity following short-term monocular deprivation

A brief period of monocular deprivation (MD) induces short-term plasticity of the adult visual system. Whether MD elicits neural changes beyond visual processing is yet unclear. Here, we assessed the specific impact of MD on neural correlates of multisensory processes. Neural oscillations associated with visual and audio-visual processing were measured for both the deprived and the non-deprived eye. Results revealed that MD changed neural activities associated with visual and multisensory processes in an eye-specific manner. Selectively for the deprived eye, alpha synchronization was reduced within the first 150 ms of visual processing. Conversely, gamma activity was enhanced in response to audio-visual events only for the non-deprived eye within 100-300 ms after stimulus onset. The analysis of gamma responses to unisensory auditory events revealed that MD elicited a crossmodal upweight for the non-deprived eye. Distributed source modeling suggested that the right parietal cortex played a major role in neural effects induced by MD. Finally, visual and audio-visual processing alterations emerged for the induced component of the neural oscillations, indicating a prominent role of feedback connectivity. Results reveal the causal impact of MD on both unisensory (visual and auditory) and multisensory (audio-visual) processes and, their frequency-specific profiles. These findings support a model in which MD increases excitability to visual events for the deprived eye and audio-visual and auditory input for the non-deprived eye.

Audiovisual illusion training improves multisensory temporal integration

When we perceive external physical stimuli from the environment, the brain must remain somewhat flexible to unaligned stimuli within a specific range, as multisensory signals are subject to different transmission and processing delays. Recent studies have shown that the width of the 'temporal binding window (TBW)' can be reduced by perceptual learning. However, to date, the vast majority of studies examining the mechanisms of perceptual learning have focused on experience-dependent effects, failing to reach a consensus on its relationship with the underlying perception influenced by audiovisual illusion. The sound-induced flash illusion (SiFI) training is a reliable function for improving perceptual sensitivity. The present study utilized the classic auditory-dominated SiFI paradigm with feedback training to investigate the effect of a 5-day SiFI training on multisensory temporal integration, as evaluated by a simultaneity judgment (SJ) task and temporal order judgment (TOJ) task. We demonstrate that audiovisual illusion training enhances multisensory temporal integration precision in the form of (i) the point of subjective simultaneity (PSS) shifts to reality (0 ms) and (ii) a narrowing TBW. The results are consistent with a Bayesian model of causal inference, suggesting that perception learning reduce the susceptibility to SiFI, whilst improving the precision of audiovisual temporal estimation.

Sound-induced flash illusions at different spatial locations were affected by personality traits

Sound-induced flash illusion (SiFI) is an auditory-dominated effect in which observers will misperceive the number of flashes due to simultaneously presented beeps, which includes fission and fusion illusions. Although several individual differences have been found in SiFI, little is known about the effect of personality traits. In the present study, we presented flashes in near space and beeps in far space (Vnear_Afar) and flashes in far space and beeps in near space (Vfar_Anear) to better approximate the real world. We collected 103 participants' Big Five questionnaire results and their SiFI task performance to investigate the difference in trait level on the SiFI in the performance of accuracy, d' and c. The results show that all five personality traits had certain effects on the SiFI to different degrees, and different personality traits played different roles in the fission illusion and fusion illusion. The high agreeableness group was more prone to the fission illusion, and the report criteria were less strict. The report criteria of the low neuroticism group were stricter for the fusion illusion. The extraversion, conscientiousness and low openness groups were more prone to the fusion illusion in the Vnear_Afar condition than in the Vfar_Anear condition. The study indicated that personality traits were important but easily overlooked factors in multisensory illusion, which might make a difference between the fission illusion and the fusion illusion.

Differences in eccentricity for sound-induced flash illusion in four visual fields

A sound-induced flash illusion (SiFI) is a multisensory illusion dominated by auditory stimuli, in which the individual perceives that the number of visual flashes is equal to the number of auditory stimuli when visual flashes are presented along with an unequal number of auditory stimuli. Although the mechanisms underlying fission and fusion illusions have been documented, there is not yet a consensus on how they vary according to the different eccentricities. In the present study, by incorporating the classic SiFI paradigm into four different eccentricities, we aimed to investigate whether the SiFI varies under the different eccentricities. The results showed that the fission illusion varied significantly across the four eccentricities, with the perifovea (7°) and peripheral (11°) illusions being greater than the fovea and parafovea (3°) illusions. In contrast, the fusion illusion did not vary significantly across the four eccentricities. Our findings revealed that SiFI was affected by different visual fields and that there were differences between the fission and the fusion illusions. Furthermore, by examining the SiFI of eccentricity across visual fields, this study also suggests that bottom-up factors affect the SiFI.

Perceptual training modifies temporal sensitivity and a sense of agency

Perceptual training has been argued to be a potential means to modify temporal sensitivity (the ability to detect a time-based discrepancy between two stimuli) with previous studies providing preliminary evidence that perceptual training can lead to increased temporal sensitivity. However, previous studies have not employed a control group and therefore cannot rule out the possibility that the observed effects are due to repeated completion of the task, rather than the training itself. Moreover, despite temporal sensitivity being suggested to be an important aspect of the sense of agency, the effects of perceptual training on the sense of agency have not been explored. Therefore, this study aimed to explore the effects of perceptual training on the sense of agency and replicate previously observed effects on temporal sensitivity while utilizing a more rigorous methodology. Given the existing literature, it was predicted that the sense of agency and temporal sensitivity will be enhanced following perceptual training. Temporal sensitivity was only weakly modified by perceptual training when compared to the control condition. Sense of agency was significantly modulated by perceptual training, over and above the control condition. This study's findings present novel evidence indicating that perceptual training can influence high-level processes such as the sense of agency and temporal sensitivity.

Compensation Mechanisms May Not Always Account for Enhanced Multisensory Illusion in Older Adults: Evidence from Sound-Induced Flash Illusion

Sound-induced flash illusion (SiFI) is typical auditory dominance phenomenon in multisensory illusion. Although a number of studies have explored the SiFI in terms of age-related effects, the reasons for the enhanced SiFI in older adults are still controversial. In the present study, older and younger adults with equal visual discrimination were selected to explore age differences in SiFI effects, and to explore the neural indicators by resting-state functional magnetic resonance imaging (rs-fMRI) signals. A correlation analysis was calculated to examine the relationship between regional homogeneity (ReHo) and the SiFI. The results showed that both younger and older adults experienced significant fission and fusion illusions, and fission illusions of older adults were greater than that of younger adults. In addition, our results showed ReHo values of the left middle frontal gyrus (MFG), the right inferior frontal gyrus (IFG) and right superior frontal gyrus (SFG) were significantly positively correlated with the SiFI in older adults. More importantly, the comparison between older and younger adults showed that ReHo values of the right superior temporal gyrus (STG) decreased in older adults, and this was independent of the SiFI. The results indicated that when there was no difference in unisensory ability, the enhancement of multisensory illusion in older adults may not always be explained by compensation mechanisms.

Multisensory perception is not influenced by previous concussion history in retired rugby union players

BACKGROUND

To assess whether concussion history adversely affects multisensory integration, we compared susceptibility to the Sound-Induced Flash Illusion (SIFI) in retired professional rugby players compared to controls.

METHODS

Retired professional rugby players ((N = 58) and retired international rowers (N = 26) completed a self-report concussion history questionnaire and the SIFI task. Susceptibility to the SIFI (i.e., perceiving two flashes in response to one flash paired with two beeps) was assessed at three stimulus onset asynchronies (70 ms, 150 ms or 230 ms).Logistic mixed-effects regression modeling was implemented to evaluate how athlete grouping, previous concussion history and total number of years playing sport, impacted the susceptibility to the SIFI task. The statistical significance of a fixed effect of interest was determined by a likelihood ratio test.

RESULTS

Former rugby players had significantly more self-reported concussions than the rower group (p < 0.001). There was no impact of athlete grouping (i.e., retired professional rugby players and retired international rowers), years participation in elite sport or concussion history on performance in the SIFI.

CONCLUSION

A career in professional rugby, concussion history or number of years participating in professional rugby was not found to be predictive of performance on the SIFI task.

The magnitude of the sound-induced flash illusion does not increase monotonically as a function of visual stimulus eccentricity

The sound-induced flash illusion (SIFI) occurs when a rapidly presented visual stimulus is accompanied by two auditory stimuli, creating the illusory percept of two visual stimuli. While much research has focused on how the temporal proximity of the audiovisual stimuli impacts susceptibility to the illusion, comparatively less research has focused on the impact of spatial manipulations. Here, we aimed to assess whether manipulating the eccentricity of visual flash stimuli altered the properties of the temporal binding window associated with the SIFI. Twenty participants were required to report whether they perceived one or two flashes that were concurrently presented with one or two beeps. Visual stimuli were presented at one of four different retinal eccentricities (2.5, 5, 7.5, or 10 degrees below fixation) and audiovisual stimuli were separated by one of eight stimulus-onset asynchronies. In keeping with previous findings, increasing stimulus-onset asynchrony between the auditory and visual stimuli led to a marked decrease in susceptibility to the illusion allowing us to estimate the width and amplitude of the temporal binding window. However, varying the eccentricity of the visual stimulus had no effect on either the width or the peak amplitude of the temporal binding window, with a similar pattern of results observed for both the “fission” and “fusion” variants of the illusion. Thus, spatial manipulations of the audiovisual stimuli used to elicit the SIFI appear to have a weaker effect on the integration of sensory signals than temporal manipulations, a finding which has implications for neuroanatomical models of multisensory integration.

Reward reduces the fission illusion in the sound-induced flash illusion

Pairing a single visual stimulus with multiple auditory stimuli will lead to the illusory perception of multiple visual stimuli, which is known as sound-induced flash illusion (SIFI). The present study adopted the classic SIFI paradigm to investigate whether value-associated tasks could affect the SIFI. By adjusting the sequence of reward and nonreward conditions, we also examined the effect of reward history on SIFI. The results showed that the fission illusion was reduced when associated with momentary reward, demonstrating significantly higher accuracy and discriminability than the nonreward condition. However, the fusion illusion was not affected by the momentary reward, and the explanation was that the fusion illusion was not as stable as the fission illusion and disappeared across different trials and conditions. Moreover, the robustness of reward history in the present study was not as strong as previous studies have suggested, indicating that the effect of sound on the perceptual representation of visual stimuli is strong and robust to reward history. These findings demonstrated that the reward could reduce the SIFI and broaden the existing dichotomy of SIFI. New evidence for the operation of value-driven attention mechanisms is also provided, suggesting that the underlying value-driven attention operates across multiple sensory systems.

Individual differences in beta frequency correlate with the audio-visual fusion illusion

Presenting one flash with two beeps induces a perception of two flashes (audio-visual [AV] fission illusion), while presenting two flashes with one beep induces a perception of one flash (fusion illusion). Although previous studies showed a relationship between the frequency of the alpha rhythm (alpha cycle) and one's susceptibility to the fission illusion, the relationship between neural oscillations and the fusion illusion is unknown. Using electroencephalography, here I investigated the frequency of oscillatory signals in the pre-stimulus period and found a significant correlation between the beta rhythm and the fusion illusion; specifically, participants with a lower beta frequency showed a larger fusion illusion. These data indicate two separate time windows of AV integration in the human brain, one defined by the alpha cycle (fission) and another defined by the beta cycle (fusion).

Urban and rural environments differentially shape multisensory perception in ageing

Recent studies suggest that the lived environment can affect cognition across the lifespan. We examined, in a large cohort of older adults (n = 3447), whether susceptibility to a multisensory illusion, the Sound-Induced Flash Illusion (SIFI), was influenced by the reported urbanity of current and childhood (at age 14 years) residence. If urban environments help to shape healthy perceptual function, we predicted reduced SIFI susceptibility in urban dwellers. Participants reporting urban, compared with rural, childhood residence were less susceptible to SIFI at longer Stimulus-Onset Asynchronies (SOAs). Those currently residing in urban environments were more susceptible to SIFI at longer SOAs, particularly if they scored low on general cognitive function. These findings held even when controlling for a several covariates, such as age, sex, education, social participation and cognitive ability. Exposure to urban environments in childhood may influence individual differences in perception and offer a multisensory perceptual benefit in older age.

Multisensory integration precision is associated with better cognitive performance over time in older adults: A large-scale exploratory study

Age-related sensory decline impacts cognitive performance and exposes individuals to a greater risk of cognitive decline. Integration across the senses also changes with age, yet the link between multisensory perception and cognitive ageing is poorly understood. We explored the relationship between multisensory integration and cognitive function in 2875 adults aged 50 + from The Irish Longitudinal Study on Ageing. Multisensory integration was assessed at several audio-visual temporal asynchronies using the Sound Induced Flash Illusion (SIFI). More precise integration (i.e. less illusion susceptibility with larger temporal asynchronies) was cross-sectionally associated with faster Choice Response Times and Colour Trail Task performance, and fewer errors on the Sustained Attention to Response Task. We then used k-means clustering to identify groups with different 10-year cognitive trajectories on measures available longitudinally; delayed recall, immediate recall and verbal fluency. Across measures, groups with consistently higher performance trajectories had more precise multisensory integration. These findings support broad links between multisensory integration and several cognitive measures, including processing speed, attention and memory, rather than association with any specific subdomain.

Crossmodal Postdiction: Conscious Perception as Revisionist History

Postdiction occurs when later stimuli influence the perception of earlier stimuli. As the multisensory science field has grown in recent decades, the investigation of crossmodal postdictive phenomena has also expanded. Crossmodal postdiction can be considered (in its simplest form) the phenomenon in which later stimuli in one modality influence earlier stimuli in another modality (e.g., Intermodal Apparent Motion). Crossmodal postdiction can also appear in more nuanced forms, such as unimodal postdictive illusions (e.g., Apparent Motion) that are influenced by concurrent crossmodal stimuli (e.g., Crossmodal Influence on Apparent Motion), or crossmodal illusions (e.g., the Double Flash Illusion) that are influenced postdictively by a stimulus in one or the other modality (e.g., a visual stimulus in the Illusory Audiovisual Rabbit Illusion). In this review, these and other varied forms of crossmodal postdiction will be discussed. Three neuropsychological models proposed for unimodal postdiction will be adapted to the unique aspects of processing and integrating multisensory stimuli. Crossmodal postdiction opens a new window into sensory integration, and could potentially be used to identify new mechanisms of crossmodal crosstalk in the brain.

Neurofeedback Modulation of the Sound-induced Flash Illusion Using Parietal Cortex Alpha Oscillations Reveals Dependency on Prior Multisensory Congruency

Spontaneous neural oscillations are key predictors of perceptual decisions to bind multisensory signals into a unified percept. Research links decreased alpha power in the posterior cortices to attention and audiovisual binding in the sound-induced flash illusion (SIFI) paradigm. This suggests that controlling alpha oscillations would be a way of controlling audiovisual binding. In the present feasibility study we used MEG-neurofeedback to train one group of subjects to increase left/right and another to increase right/left alpha power ratios in the parietal cortex. We tested for changes in audiovisual binding in a SIFI paradigm where flashes appeared in both hemifields. Results showed that the neurofeedback induced a significant asymmetry in alpha power for the left/right group, not seen for the right/left group. Corresponding asymmetry changes in audiovisual binding in illusion trials (with 2, 3, and 4 beeps paired with 1 flash) were not apparent. Exploratory analyses showed that neurofeedback training effects were present for illusion trials with the lowest numeric disparity (i.e., 2 beeps and 1 flash trials) only if the previous trial had high congruency (2 beeps and 2 flashes). Our data suggest that the relation between parietal alpha power (an index of attention) and its effect on audiovisual binding is dependent on the learned causal structure in the previous stimulus. The present results suggests that low alpha power biases observers towards audiovisual binding when they have learned that audiovisual signals originate from a common origin, consistent with a Bayesian causal inference account of multisensory perception.

Impoverished Inhibitory Control Exacerbates Multisensory Impairments in Older Fallers

Impaired temporal perception of multisensory cues is a common phenomenon observed in older adults that can lead to unreliable percepts of the external world. For instance, the sound induced flash illusion (SIFI) can induce an illusory percept of a second flash by presenting a beep close in time to an initial flash-beep pair. Older adults that have enhanced susceptibility to a fall demonstrate significantly stronger illusion percepts during the SIFI task compared to those older adults without any history of falling. We hypothesize that a global inhibitory deficit may be driving the impairments across both postural stability and multisensory function in older adults with a fall history (FH). We investigated oscillatory activity and perceptual performance during the SIFI task, to understand how active sensory processing, measured by gamma (30–80 Hz) power, was regulated by alpha activity (8–13 Hz), oscillations that reflect inhibitory control. Compared to young adults (YA), the FH and non-faller (NF) groups demonstrated enhanced susceptibility to the SIFI. Further, the FH group had significantly greater illusion strength compared to the NF group. The FH group also showed significantly impaired performance relative to YA during congruent trials (2 flash-beep pairs resulting in veridical perception of 2 flashes). In illusion compared to non-illusion trials, the NF group demonstrated reduced alpha power (or diminished inhibitory control). Relative to YA and NF, the FH group showed reduced phase-amplitude coupling between alpha and gamma activity in non-illusion trials. This loss of inhibitory capacity over sensory processing in FH compared to NF suggests a more severe change than that consequent of natural aging.

The impact of joint attention on the sound-induced flash illusions

Humans coordinate their focus of attention with others, either by gaze following or prior agreement. Though the effects of joint attention on perceptual and cognitive processing tend to be examined in purely visual environments, they should also show in multisensory settings. According to a prevalent hypothesis, joint attention enhances visual information encoding and processing, over and above individual attention. If two individuals jointly attend to the visual components of an audiovisual event, this should affect the weighing of visual information during multisensory integration. We tested this prediction in this preregistered study, using the well-documented sound-induced flash illusions, where the integration of an incongruent number of visual flashes and auditory beeps results in a single flash being seen as two (fission illusion) and two flashes as one (fusion illusion). Participants were asked to count flashes either alone or together, and expected to be less prone to both fission and fusion illusions when they jointly attended to the visual targets. However, illusions were as frequent when people attended to the flashes alone or with someone else, even though they responded faster during joint attention. Our results reveal the limitations of the theory that joint attention enhances visual processing as it does not affect temporal audiovisual integration.

Disentangling early versus late audiovisual integration in adult ADHD: a combined behavioural and resting-state connectivity study

BACKGROUND

Studies investigating sensory processing in attention-deficit/hyperactivity disorder (ADHD) have shown altered visual and auditory processing. However, evidence is lacking for audiovisual interplay - namely, multisensory integration. As well, neuronal dysregulation at rest (e.g., aberrant within- or between-network functional connectivity) may account for difficulties with integration across the senses in ADHD. We investigated whether sensory processing was altered at the multimodal level in adult ADHD and included resting-state functional connectivity to illustrate a possible overlap between deficient network connectivity and the ability to integrate stimuli.

METHODS

We tested 25 patients with ADHD and 24 healthy controls using 2 illusionary paradigms: the sound-induced flash illusion and the McGurk illusion. We applied the Mann-Whitney U test to assess statistical differences between groups. We acquired resting-state functional MRIs on a 3.0 T Siemens magnetic resonance scanner, using a highly accelerated 3-dimensional echo planar imaging sequence.

RESULTS

For the sound-induced flash illusion, susceptibility and reaction time were not different between the 2 groups. For the McGurk illusion, susceptibility was significantly lower for patients with ADHD, and reaction times were significantly longer. At a neuronal level, resting-state functional connectivity in the ADHD group was more highly regulated in polymodal regions that play a role in binding unimodal sensory inputs from different modalities and enabling sensory-to-cognition integration.

LIMITATIONS

We did not explicitly screen for autism spectrum disorder, which has high rates of comorbidity with ADHD and also involves impairments in multisensory integration. Although the patients were carefully screened by our outpatient department, we could not rule out the possibility of autism spectrum disorder in some participants.

CONCLUSION

Unimodal hypersensitivity seems to have no influence on the integration of basal stimuli, but it might have negative consequences for the multisensory integration of complex stimuli. This finding was supported by observations of higher resting-state functional connectivity between unimodal sensory areas and polymodal multisensory integration convergence zones for complex stimuli.

Long-term training reduces the responses to the sound-induced flash illusion

The sound-induced flash illusion (SiFI) is a robust auditory-dominated multisensory integration phenomenon that is used as a reliable indicator to assess multisensory integration. Previous studies have indicated that the SiFI effect is correlated with perceptual sensitivity. However, to date, there is no consensus regarding how it corresponds to sensitivity with long-term training. The present study adopted the classic SiFI paradigm with feedback training to investigate the effect of a week of long-term training on the SiFI effect. Both the training group and control group completed a pretest and a posttest before and after the perceptual training; however, only the training group was required to complete 7-day behavioral training. The results showed that (1) long-term training could reduce the response of fission and fusion illusions by improving perceptual sensitivity and that (2) there was a "plateau effect" that emerged during the training stage, which tended to stabilize by the fifth day. These findings demonstrated that the SiFI effect could be modified with long-term training by ameliorating perceptual sensitivity, especially in terms of the fission illusion. Therefore, the present study supplements perceptual training in SiFI domains and provides evidence that the SiFI could be used as an assessment intervention to improve the efficiency of multisensory integration.

Temporal binding window and sense of agency are related processes modifiable via occipital tACS

The temporal binding window refers to the time frame within which temporal grouping of sensory information takes place. Sense of agency is the feeling of being in control of one’s actions, and their associated outcomes. While previous research has shown that temporal cues and multisensory integration play a role in sense of agency, no studies have directly assessed whether individual differences in the temporal binding window and sense of agency are associated. In all three experiments, to assess sense of agency, participants pressed a button triggering, after a varying delay, the appearance of the circle, and reported their sense of agency over the effect. To assess the temporal binding window a simultaneity judgment task (Experiment 1) and a double-flash illusion task (Experiment 2 and 3) was also performed. As expected, the temporal binding window correlated with the sense of agency window. In Experiment 3, these processes were modulated by applying occipital tACS at either 14Hz or 8Hz. We found 14Hz tACS stimulation was associated with narrower temporal biding window and sense of agency window. Our results suggest the temporal binding window and the time window of sense of agency are related. They also point towards a possible underlying neural mechanism (alpha peak frequency) for this association.

Cross-Modal Interaction Between Auditory and Visual Input Impacts Memory Retrieval

How we perceive and learn about our environment is influenced by our prior experiences and existing representations of the world. Top-down cognitive processes, such as attention and expectations, can alter how we process sensory stimuli, both within a modality (e.g., effects of auditory experience on auditory perception), as well as across modalities (e.g., effects of visual feedback on sound localization). Here, we demonstrate that experience with different types of auditory input (spoken words vs. environmental sounds) modulates how humans remember concurrently-presented visual objects. Participants viewed a series of line drawings (e.g., picture of a cat) displayed in one of four quadrants while listening to a word or sound that was congruent (e.g., "cat" or <meow>), incongruent (e.g., "motorcycle" or <vroom-vroom>), or neutral (e.g., a meaningless pseudoword or a tonal beep) relative to the picture. Following the encoding phase, participants were presented with the original drawings plus new drawings and asked to indicate whether each one was "old" or "new." If a drawing was designated as "old," participants then reported where it had been displayed. We find that words and sounds both elicit more accurate memory for what objects were previously seen, but only congruent environmental sounds enhance memory for where objects were positioned - this, despite the fact that the auditory stimuli were not meaningful spatial cues of the objects' locations on the screen. Given that during real-world listening conditions, environmental sounds, but not words, reliably originate from the location of their referents, listening to sounds may attune the visual dorsal pathway to facilitate attention and memory for objects' locations. We propose that audio-visual associations in the environment and in our previous experience jointly contribute to visual memory, strengthening visual memory through exposure to auditory input.

Individual differences in alpha frequency are associated with the time window of multisensory integration, but not time perception

Previous research provides some preliminary evidence to link the temporal binding window, the time frame within which multisensory information from different sensory modalities is integrated, and time perception. In addition, alpha peak frequency has been proposed to be the neural mechanism for both processes. However, these links are not well established. Hence, the aim of the current study was to explore to what degree, if any, time perception, the temporal binding window and the alpha peak frequency are related. It was predicted that as the width of the temporal binding window increases the size of the filled duration illusion increases and the alpha peak frequency decreases. We observed a significant relationship between the temporal binding window and peak alpha frequency. However, time perception was not linked with either of these. These findings are discussed with respect to the possible underlying mechanisms of multisensory integration and time perception.

Repetition Suppression in Visual and Auditory Modalities Affects the Sound-Induced Flash Illusion

Sound-induced flash illusion (SiFI) refers to the illusion that the number of visual flashes is equal to the number of auditory sounds when the visual flashes are accompanied by an unequal number of auditory sounds presented within 100 ms. The effect of repetition suppression (RS), an adaptive effect caused by stimulus repetition, upon the SiFI has not been investigated. Based on the classic SiFI paradigm, the present study investigated whether RS would affect the SiFI differently by adding preceding stimuli in visual and auditory modalities prior to the appearance of audiovisual stimuli. The results showed the auditory RS effect on the SiFI varied with the number of preceding auditory stimuli. The hit rate was higher with two preceding auditory stimuli than one preceding auditory stimulus in fission illusion, but it did not affect the size of the fusion illusion. However, the visual RS had no effect on the size of the fission and fusion illusions. The present study suggested that RS could affect the SiFI, indicating that the RS effect in different modalities would differentially affect the magnitude of the SiFI. In the process of multisensory integration, the visual and auditory modalities had asymmetrical RS effects.

Predictive Coding Over the Lifespan: Increased Reliance on Perceptual Priors in Older Adults-A Magnetoencephalography and Dynamic Causal Modeling Study

Aging is accompanied by unisensory decline. To compensate for this, two complementary strategies are potentially relied upon increasingly: first, older adults integrate more information from different sensory organs. Second, according to the predictive coding (PC) model, we form “templates” (internal models or “priors”) of the environment through our experiences. It is through increased life experience that older adults may rely more on these templates compared to younger adults. Multisensory integration and predictive coding would be effective strategies for the perception of near-threshold stimuli, which may however come at the cost of integrating irrelevant information. Both strategies can be studied in multisensory illusions because these require the integration of different sensory information, as well as an internal model of the world that can take precedence over sensory input. Here, we elicited a classic multisensory illusion, the sound-induced flash illusion, in younger (mean: 27 years, N = 25) and older (mean: 67 years, N = 28) adult participants while recording the magnetoencephalogram. Older adults perceived more illusions than younger adults. Older adults had increased pre-stimulus beta-band activity compared to younger adults as predicted by microcircuit theories of predictive coding, which suggest priors and predictions are linked to beta-band activity. Transfer entropy analysis and dynamic causal modeling of pre-stimulus magnetoencephalography data revealed a stronger illusion-related modulation of cross-modal connectivity from auditory to visual cortices in older compared to younger adults. We interpret this as the neural correlate of increased reliance on a cross-modal predictive template in older adults leading to the illusory percept.

The development of visuotactile congruency effects for sequences of events

Sensitivity to the temporal coherence of visual and tactile signals increases perceptual reliability and is evident during infancy. However, it is not clear how, or whether, bidirectional visuotactile interactions change across childhood. Furthermore, no study has explored whether viewing a body modulates how children perceive visuotactile sequences of events. Here, children aged 5-7 years (n = 19), 8 and 9 years (n = 21), and 10-12 years (n = 24) and adults (n = 20) discriminated the number of target events (one or two) in a task-relevant modality (touch or vision) and ignored distractors (one or two) in the opposing modality. While participants performed the task, an image of either a hand or an object was presented. Children aged 5-7 years and 8 and 9 years showed larger crossmodal interference from visual distractors when discriminating tactile targets than the converse. Across age groups, this was strongest when two visual distractors were presented with one tactile target, implying a "fission-like" crossmodal effect (perceiving one event as two events). There was no influence of visual context (viewing a hand or non-hand image) on visuotactile interactions for any age group. Our results suggest robust interference from discontinuous visual information on tactile discrimination of sequences of events during early and middle childhood. These findings are discussed with respect to age-related changes in sensory dominance, selective attention, and multisensory processing.

Memory Load Alters Perception-Related Neural Oscillations during Multisensory Integration

Integrating information across different senses is a central feature of human perception. Previous research suggests that multisensory integration is shaped by a context-dependent and largely adaptive interplay between stimulus-driven bottom-up and top-down endogenous influences. One critical question concerns the extent to which this interplay is sensitive to the amount of available cognitive resources. In the present study, we investigated the influence of limited cognitive resources on audiovisual integration by measuring high-density electroencephalography (EEG) in healthy participants performing the sound-induced flash illusion (SIFI) and a verbal n-back task (0-back, low load and 2-back, high load) in a dual-task design. In the SIFI, the integration of a flash with two rapid beeps can induce the illusory perception of two flashes. We found that high compared with low load increased illusion susceptibility and modulated neural oscillations underlying illusion-related crossmodal interactions. Illusion perception under high load was associated with reduced early β power (18-26 Hz, ∼70 ms) in auditory and motor areas, presumably reflecting an early mismatch signal and subsequent top-down influences including increased frontal θ power (7-9 Hz, ∼120 ms) in mid-anterior cingulate cortex (ACC) and a later β power suppression (13-22 Hz, ∼350 ms) in prefrontal and auditory cortex. Our study demonstrates that integrative crossmodal interactions underlying the SIFI are sensitive to the amount of available cognitive resources and that multisensory integration engages top-down θ and β oscillations when cognitive resources are scarce.SIGNIFICANCE STATEMENT The integration of information across multiple senses, a remarkable ability of our perceptual system, is influenced by multiple context-related factors, the role of which is highly debated. It is, for instance, poorly understood how available cognitive resources influence crossmodal interactions during multisensory integration. We addressed this question using the sound-induced flash illusion (SIFI), a phenomenon in which the integration of two rapid beeps together with a flash induces the illusion of a second flash. Replicating our previous work, we demonstrate that depletion of cognitive resources through a working memory (WM) task increases the perception of the illusion. With respect to the underlying neural processes, we show that when available resources are limited, multisensory integration engages top-down θ and β oscillations.

Gray matter volume in the right angular gyrus is associated with differential patterns of multisensory integration with aging

Multisensory perception might provide an important marker of brain function in aging. However, the cortical structures supporting multisensory perception in aging are poorly understood. In this study, we compared regional gray matter volume in a group of middle-aged (n = 101; 49-64 years) and older (n = 116; 71-87 years) adults from The Irish Longitudinal Study on Aging using voxel-based morphometry. Participants completed a measure of multisensory integration, the sound-induced flash illusion, and were grouped as per their illusion susceptibility. A significant interaction was observed in the right angular gyrus; in the middle-aged group, larger gray matter volume corresponded to stronger illusion perception while in older adults larger gray matter corresponded to less illusion susceptibility. This interaction remained significant even when controlling for a range of demographic, sensory, cognitive, and health variables. These findings show that multisensory integration is associated with specific structural differences in the aging brain and highlight the angular gyrus as a possible "cross-modal hub" associated with age-related change in multisensory perception.

Exposure to first-person shooter videogames is associated with multisensory temporal precision and migraine incidence

Adaptive interactions with the environment require optimal integration and segregation of sensory information. Yet, temporal misalignments in the presentation of visual and auditory stimuli may generate illusory phenomena such as the sound-induced flash illusion, in which a single flash paired with multiple auditory stimuli induces the perception of multiple illusory flashes. This phenomenon has been shown to be robust and resistant to feedback training. According to a Bayesian account, this is due to a statistically optimal combination of the signals operated by the nervous system. From this perspective, individual susceptibility to the illusion might be moulded through prolonged experience. For example, repeated exposure to the illusion and prolonged training sessions partially impact on the reported illusion. Therefore, extensive and immersive audio-visual experience, such as first-person shooter videogames, should sharpen individual capacity to correctly integrate multisensory information over time, leading to more veridical perception. We tested this hypothesis by comparing the temporal profile of the sound-induced illusion in a group of expert first-person shooter gamers and a non-players group. In line with the hypotheses, gamers experience significantly narrower windows of illusion (~87 ms) relative to non-players (~105 ms), leading to higher veridical reports in gamers (~68%) relative to non-players (~59%). Moreover, according to recent literature, we tested whether audio-visual intensive training in gamers could be related to the incidence of migraine, and found that its severity may be directly proportioned to the time spent on videogames. Overall, these results suggest that continued training within audio-visual environments such as first-person shooter videogames improves temporal discrimination and sensory integration. This finding may pave the way for future therapeutic strategies based on self-administered multisensory training. On the other hand, the impact of intensive training on visual-related stress disorders, such as migraine incidence, should be taken into account as a risk factor during therapeutic planning.

Causally linking neural dominance to perceptual dominance in a multisensory conflict

When different senses are in conflict, one sense may dominate the perception of other sense, but it is not known whether the sensory cortex associated with the dominant modality exerts directional influence, at the functional brain level, over the sensory cortex associated with the dominated modality; in short, the link between sensory dominance and neuronal dominance is not established. In a task involving audio-visual conflict, using magnetoencephalography recordings in humans, we first demonstrated that the neuronal dominance - auditory cortex functionally influencing visual cortex - was associated with the sensory dominance - sound qualitatively altering visual perception. Further, we found that prestimulus auditory-to-visual connectivity could predict the perceptual outcome on a trial-by-trial basis. Subsequently, we performed an effective connectivity-guided neurofeedback electroencephalography experiment and showed that participants who were briefly trained to increase the neuronal dominance from auditory to visual cortex showed higher sensory, that is auditory, dominance during the conflict task immediately after the training. These results shed new light into the interactive neuronal nature of multisensory integration and open up exciting opportunities by enhancing or suppressing targeted mental functions subserved by effective connectivity.