Visual stimulus locking of EEG is modulated by temporal congruency of auditory stimuli

Lower multisensory temporal acuity in individuals with high schizotypal traits: a web-based study

Natural events are often multisensory, requiring the brain to combine information from the same spatial location and timing, across different senses. The importance of temporal coincidence has led to the introduction of the temporal binding window (TBW) construct, defined as the time range within which multisensory inputs are highly likely to be perceptually bound into a single entity. Anomalies in TBWs have been linked to confused perceptual experiences and inaccurate filtering of sensory inputs coming from different environmental sources. Indeed, larger TBWs have been associated with disorders such as schizophrenia and autism and are also correlated to a higher level of subclinical traits of these conditions in the general population. Here, we tested the feasibility of using a web-based version of a classic audio-visual simultaneity judgment (SJ) task with simple flash-beep stimuli in order to measure multisensory temporal acuity and its relationship with schizotypal traits as measured in the general population. Results show that: (i) the response distribution obtained in the web-based SJ task was strongly similar to those reported by studies carried out in controlled laboratory settings, and (ii) lower multisensory temporal acuity was associated with higher schizotypal traits in the “cognitive-perceptual” domains. Our findings reveal the possibility of adequately using a web-based audio-visual SJ task outside a controlled laboratory setting, available to a more diverse and representative pool of participants. These results provide additional evidence for a close relationship between lower multisensory acuity and the expression of schizotypal traits in the general population.

Perceived Congruency in Audiovisual Stimuli Consisting of Gabor Patches and AM and FM Tones

Experiments that focus on how humans perceive temporal, spatial or synaesthetic congruency in audiovisual sensory information have often employed stimuli consisting of a Gabor patch and an amplitude (AM) or frequency (FM)-modulated sound. Introducing similarity between the static and dynamic features of the Gabor patch and the (carrier) frequency or modulation frequency of the sound is often assumed to be effective enough to induce congruency. However, comparative empirical data on perceived congruency of various stimulus parameters are not readily available, and in particular with respect to sound modulation, it is still not clear which type (AM or FM) induces perceived congruency best in tandem with various patch parameters. In two experiments, we examined Gabor patches of various spatial frequencies with flickering (2, 3 and 4 flickers/s) or drifting (0.5, 1.0 and 1.5 degrees/s) gratings in combinations with AM or FM tones of 2-, 3- and 4-Hz modulation and 500-, 1000- and 2000-Hz carrier frequencies. Perceived congruency ratings were obtained by asking participants to rate stimulus (in)congruency from 1 (incongruent) to 7 (congruent). The data showed that varying the spatial frequency of the Gabor patch and the carrier frequency of the modulated tone had comparatively little impact on perceived congruency. Similar to previous findings, similarity between the temporal frequency of the Gabor patch and the modulated tone effectively promoted perceived congruency. Furthermore, direct comparisons convincingly showed that AM tones in combination with flickering Gabor patches received significantly higher audiovisual congruency ratings compared to FM tones.

On the Timing of Signals in Multisensory Integration and Crossmodal Interactions: a Scoping Review

A scoping review was undertaken to explore research investigating early interactions and integration of auditory and visual stimuli in the human brain. The focus was on methods used to study low-level multisensory temporal processing using simple stimuli in humans, and how this research has informed our understanding of multisensory perception. The study of multisensory temporal processing probes how the relative timing between signals affects perception. Several tasks, illusions, computational models, and neuroimaging techniques were identified in the literature search. Research into early audiovisual temporal processing in special populations was also reviewed. Recent research has continued to provide support for early integration of crossmodal information. These early interactions can influence higher-level factors, and vice versa. Temporal relationships between auditory and visual stimuli influence multisensory perception, and likely play a substantial role in solving the 'correspondence problem' (how the brain determines which sensory signals belong together, and which should be segregated).

Multisensory integration, body representation and hyperactivity of the immune system

Multisensory stimuli are integrated over a delimited window of temporal asynchronies. This window is highly variable across individuals, but the origins of this variability are still not clear. We hypothesized that immune system functioning could partially account for this variability. In two experiments, we investigated the relationship between key aspects of multisensory integration in allergic participants and healthy controls. First, we tested the temporal constraint of multisensory integration, as measured by the temporal binding window. Second, we tested multisensory body representation, as indexed by the Rubber Hand Illusion (RHI). Results showed that allergic participants have a narrower temporal binding window and are less susceptible to the RHI than healthy controls. Overall, we provide evidence linking multisensory integration processes and the activity of the immune system. The present findings are discussed within the context of the effect of immune molecules on the brain mechanisms enabling multisensory integration and multisensory body representation.

Shifts in Audiovisual Processing in Healthy Aging

Purpose of Review

The integration of information across sensory modalities into unified percepts is a fundamental sensory process upon which a multitude of cognitive processes are based. We review the body of literature exploring aging-related changes in audiovisual integration published over the last five years. Specifically, we review the impact of changes in temporal processing, the influence of the effectiveness of sensory inputs, the role of working memory, and the newer studies of intra-individual variability during these processes.

Recent Findings

Work in the last five years on bottom-up influences of sensory perception has garnered significant attention. Temporal processing, a driving factors of multisensory integration, has now been shown to decouple with multisensory integration in aging, despite their co-decline with aging. The impact of stimulus effectiveness also changes with age, where older adults show maximal benefit from multisensory gain at high signal-to-noise ratios. Following sensory decline, high working memory capacities have now been shown to be somewhat of a protective factor against age-related declines in audiovisual speech perception, particularly in noise. Finally, newer research is emerging focusing on the general intra-individual variability observed with aging.

Summary

Overall, the studies of the past five years have replicated and expanded on previous work that highlights the role of bottom-up sensory changes with aging and their influence on audiovisual integration, as well as the top-down influence of working memory.

Audio-visual synchrony and spatial attention enhance processing of dynamic visual stimulation independently and in parallel: A frequency-tagging study

The neural processing of a visual stimulus can be facilitated by attending to its position or by a co-occurring auditory tone. Using frequency-tagging, we investigated whether facilitation by spatial attention and audio-visual synchrony rely on similar neural processes. Participants attended to one of two flickering Gabor patches (14.17 and 17 Hz) located in opposite lower visual fields. Gabor patches further "pulsed" (i.e. showed smooth spatial frequency variations) at distinct rates (3.14 and 3.63 Hz). Frequency-modulating an auditory stimulus at the pulse-rate of one of the visual stimuli established audio-visual synchrony. Flicker and pulsed stimulation elicited stimulus-locked rhythmic electrophysiological brain responses that allowed tracking the neural processing of simultaneously presented Gabor patches. These steady-state responses (SSRs) were quantified in the spectral domain to examine visual stimulus processing under conditions of synchronous vs. asynchronous tone presentation and when respective stimulus positions were attended vs. unattended. Strikingly, unique patterns of effects on pulse- and flicker driven SSRs indicated that spatial attention and audiovisual synchrony facilitated early visual processing in parallel and via different cortical processes. We found attention effects to resemble the classical top-down gain effect facilitating both, flicker and pulse-driven SSRs. Audio-visual synchrony, in turn, only amplified synchrony-producing stimulus aspects (i.e. pulse-driven SSRs) possibly highlighting the role of temporally co-occurring sights and sounds in bottom-up multisensory integration.

Humans treat unreliable filled-in percepts as more real than veridical ones

Humans often evaluate sensory signals according to their reliability for optimal decision-making. However, how do we evaluate percepts generated in the absence of direct input that are, therefore, completely unreliable? Here, we utilize the phenomenon of filling-in occurring at the physiological blind-spots to compare partially inferred and veridical percepts. Subjects chose between stimuli that elicit filling-in, and perceptually equivalent ones presented outside the blind-spots, looking for a Gabor stimulus without a small orthogonal inset. In ambiguous conditions, when the stimuli were physically identical and the inset was absent in both, subjects behaved opposite to optimal, preferring the blind-spot stimulus as the better example of a collinear stimulus, even though no relevant veridical information was available. Thus, a percept that is partially inferred is paradoxically considered more reliable than a percept based on external input. In other words: Humans treat filled-in inferred percepts as more real than veridical ones.

The cascading influence of multisensory processing on speech perception in autism

It has been recently theorized that atypical sensory processing in autism relates to difficulties in social communication. Through a series of tasks concurrently assessing multisensory temporal processes, multisensory integration and speech perception in 76 children with and without autism, we provide the first behavioral evidence of such a link. Temporal processing abilities in children with autism contributed to impairments in speech perception. This relationship was significantly mediated by their abilities to integrate social information across auditory and visual modalities. These data describe the cascading impact of sensory abilities in autism, whereby temporal processing impacts multisensory information of social information, which, in turn, contributes to deficits in speech perception. These relationships were found to be specific to autism, specific to multisensory but not unisensory integration, and specific to the processing of social information.

Visual cortex responses reflect temporal structure of continuous quasi-rhythmic sensory stimulation

Neural processing of dynamic continuous visual input, and cognitive influences thereon, are frequently studied in paradigms employing strictly rhythmic stimulation. However, the temporal structure of natural stimuli is hardly ever fully rhythmic but possesses certain spectral bandwidths (e.g. lip movements in speech, gestures). Examining periodic brain responses elicited by strictly rhythmic stimulation might thus represent ideal, yet isolated cases. Here, we tested how the visual system reflects quasi-rhythmic stimulation with frequencies continuously varying within ranges of classical theta (4–7 Hz), alpha (8–13 Hz) and beta bands (14–20 Hz) using EEG. Our findings substantiate a systematic and sustained neural phase-locking to stimulation in all three frequency ranges. Further, we found that allocation of spatial attention enhances EEG-stimulus locking to theta- and alpha-band stimulation. Our results bridge recent findings regarding phase locking (“entrainment”) to quasi-rhythmic visual input and “frequency-tagging” experiments employing strictly rhythmic stimulation. We propose that sustained EEG-stimulus locking can be considered as a continuous neural signature of processing dynamic sensory input in early visual cortices. Accordingly, EEG-stimulus locking serves to trace the temporal evolution of rhythmic as well as quasi-rhythmic visual input and is subject to attentional bias.

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Dynamic visual stimuli constitute large parts of our perceptual experience.

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Strictly rhythmic dynamics condense in EEG-recorded mass-neural activity.

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We tested how stimuli with fluctuating rhythms reflect in the EEG.

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We found that the EEG allows tracing two quasi-rhythmic stimuli in parallel.

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Dynamics of attended stimuli may be tracked with greater temporal precision.

Interactions between space and effectiveness in human multisensory performance

Several stimulus factors are important in multisensory integration, including the spatial and temporal relationships of the paired stimuli as well as their effectiveness. Changes in these factors have been shown to dramatically change the nature and magnitude of multisensory interactions. Typically, these factors are considered in isolation, although there is a growing appreciation for the fact that they are likely to be strongly interrelated. Here, we examined interactions between two of these factors - spatial location and effectiveness - in dictating performance in the localization of an audiovisual target. A psychophysical experiment was conducted in which participants reported the perceived location of visual flashes and auditory noise bursts presented alone and in combination. Stimuli were presented at four spatial locations relative to fixation (0°, 30°, 60°, 90°) and at two intensity levels (high, low). Multisensory combinations were always spatially coincident and of the matching intensity (high-high or low-low). In responding to visual stimuli alone, localization accuracy decreased and response times (RTs) increased as stimuli were presented at more eccentric locations. In responding to auditory stimuli, performance was poorest at the 30° and 60° locations. For both visual and auditory stimuli, accuracy was greater and RTs were faster for more intense stimuli. For responses to visual-auditory stimulus combinations, performance enhancements were found at locations in which the unisensory performance was lowest, results concordant with the concept of inverse effectiveness. RTs for these multisensory presentations frequently violated race-model predictions, implying integration of these inputs, and a significant location-by-intensity interaction was observed. Performance gains under multisensory conditions were larger as stimuli were positioned at more peripheral locations, and this increase was most pronounced for the low-intensity conditions. These results provide strong support that the effects of stimulus location and effectiveness on multisensory integration are interdependent, with both contributing to the overall effectiveness of the stimuli in driving the resultant multisensory response.

Keeping time in the brain: Autism spectrum disorder and audiovisual temporal processing

A growing area of interest and relevance in the study of autism spectrum disorder (ASD) focuses on the relationship between multisensory temporal function and the behavioral, perceptual, and cognitive impairments observed in ASD. Atypical sensory processing is becoming increasingly recognized as a core component of autism, with evidence of atypical processing across a number of sensory modalities. These deviations from typical processing underscore the value of interpreting ASD within a multisensory framework. Furthermore, converging evidence illustrates that these differences in audiovisual processing may be specifically related to temporal processing. This review seeks to bridge the connection between temporal processing and audiovisual perception, and to elaborate on emerging data showing differences in audiovisual temporal function in autism. We also discuss the consequence of such changes, the specific impact on the processing of different classes of audiovisual stimuli (e.g. speech vs. nonspeech, etc.), and the presumptive brain processes and networks underlying audiovisual temporal integration. Finally, possible downstream behavioral implications, and possible remediation strategies are outlined. Autism Res 2016, 9: 720-738. © 2015 International Society for Autism Research, Wiley Periodicals, Inc.

Audio-visual synchrony and feature-selective attention co-amplify early visual processing

Our brain relies on neural mechanisms of selective attention and converging sensory processing to efficiently cope with rich and unceasing multisensory inputs. One prominent assumption holds that audio-visual synchrony can act as a strong attractor for spatial attention. Here, we tested for a similar effect of audio-visual synchrony on feature-selective attention. We presented two superimposed Gabor patches that differed in colour and orientation. On each trial, participants were cued to selectively attend to one of the two patches. Over time, spatial frequencies of both patches varied sinusoidally at distinct rates (3.14 and 3.63 Hz), giving rise to pulse-like percepts. A simultaneously presented pure tone carried a frequency modulation at the pulse rate of one of the two visual stimuli to introduce audio-visual synchrony. Pulsed stimulation elicited distinct time-locked oscillatory electrophysiological brain responses. These steady-state responses were quantified in the spectral domain to examine individual stimulus processing under conditions of synchronous versus asynchronous tone presentation and when respective stimuli were attended versus unattended. We found that both, attending to the colour of a stimulus and its synchrony with the tone, enhanced its processing. Moreover, both gain effects combined linearly for attended in-sync stimuli. Our results suggest that audio-visual synchrony can attract attention to specific stimulus features when stimuli overlap in space.

Exploring how musical rhythm entrains brain activity with electroencephalogram frequency-tagging

The ability to perceive a regular beat in music and synchronize to this beat is a widespread human skill. Fundamental to musical behaviour, beat and meter refer to the perception of periodicities while listening to musical rhythms and often involve spontaneous entrainment to move on these periodicities. Here, we present a novel experimental approach inspired by the frequency-tagging approach to understand the perception and production of rhythmic inputs. This approach is illustrated here by recording the human electroencephalogram responses at beat and meter frequencies elicited in various contexts: mental imagery of meter, spontaneous induction of a beat from rhythmic patterns, multisensory integration and sensorimotor synchronization. Collectively, our observations support the view that entrainment and resonance phenomena subtend the processing of musical rhythms in the human brain. More generally, they highlight the potential of this approach to help us understand the link between the phenomenology of musical beat and meter and the bias towards periodicities arising under certain circumstances in the nervous system. Entrainment to music provides a highly valuable framework to explore general entrainment mechanisms as embodied in the human brain.

Seeing the song: left auditory structures may track auditory-visual dynamic alignment

Auditory and visual signals generated by a single source tend to be temporally correlated, such as the synchronous sounds of footsteps and the limb movements of a walker. Continuous tracking and comparison of the dynamics of auditory-visual streams is thus useful for the perceptual binding of information arising from a common source. Although language-related mechanisms have been implicated in the tracking of speech-related auditory-visual signals (e.g., speech sounds and lip movements), it is not well known what sensory mechanisms generally track ongoing auditory-visual synchrony for non-speech signals in a complex auditory-visual environment. To begin to address this question, we used music and visual displays that varied in the dynamics of multiple features (e.g., auditory loudness and pitch; visual luminance, color, size, motion, and organization) across multiple time scales. Auditory activity (monitored using auditory steady-state responses, ASSR) was selectively reduced in the left hemisphere when the music and dynamic visual displays were temporally misaligned. Importantly, ASSR was not affected when attentional engagement with the music was reduced, or when visual displays presented dynamics clearly dissimilar to the music. These results appear to suggest that left-lateralized auditory mechanisms are sensitive to auditory-visual temporal alignment, but perhaps only when the dynamics of auditory and visual streams are similar. These mechanisms may contribute to correct auditory-visual binding in a busy sensory environment.

Multisensory temporal integration: task and stimulus dependencies

The ability of human sensory systems to integrate information across the different modalities provides a wide range of behavioral and perceptual benefits. This integration process is dependent upon the temporal relationship of the different sensory signals, with stimuli occurring close together in time typically resulting in the largest behavior changes. The range of temporal intervals over which such benefits are seen is typically referred to as the temporal binding window (TBW). Given the importance of temporal factors in multisensory integration under both normal and atypical circumstances such as autism and dyslexia, the TBW has been measured with a variety of experimental protocols that differ according to criterion, task, and stimulus type, making comparisons across experiments difficult. In the current study, we attempt to elucidate the role that these various factors play in the measurement of this important construct. The results show a strong effect of stimulus type, with the TBW assessed with speech stimuli being both larger and more symmetrical than that seen using simple and complex non-speech stimuli. These effects are robust across task and statistical criteria and are highly consistent within individuals, suggesting substantial overlap in the neural and cognitive operations that govern multisensory temporal processes.

The effects of visual training on multisensory temporal processing

The importance of multisensory integration for human behavior and perception is well documented, as is the impact that temporal synchrony has on driving such integration. Thus, the more temporally coincident two sensory inputs from different modalities are, the more likely they will be perceptually bound. This temporal integration process is captured by the construct of the temporal binding window-the range of temporal offsets within which an individual is able to perceptually bind inputs across sensory modalities. Recent work has shown that this window is malleable and can be narrowed via a multisensory perceptual feedback training process. In the current study, we seek to extend this by examining the malleability of the multisensory temporal binding window through changes in unisensory experience. Specifically, we measured the ability of visual perceptual feedback training to induce changes in the multisensory temporal binding window. Visual perceptual training with feedback successfully improved temporal visual processing, and more importantly, this visual training increased the temporal precision across modalities, which manifested as a narrowing of the multisensory temporal binding window. These results are the first to establish the ability of unisensory temporal training to modulate multisensory temporal processes, findings that can provide mechanistic insights into multisensory integration and which may have a host of practical applications.

Interactions between the spatial and temporal stimulus factors that influence multisensory integration in human performance

In natural environments, human sensory systems work in a coordinated and integrated manner to perceive and respond to external events. Previous research has shown that the spatial and temporal relationships of sensory signals are paramount in determining how information is integrated across sensory modalities, but in ecologically plausible settings, these factors are not independent. In the current study, we provide a novel exploration of the impact on behavioral performance for systematic manipulations of the spatial location and temporal synchrony of a visual-auditory stimulus pair. Simple auditory and visual stimuli were presented across a range of spatial locations and stimulus onset asynchronies (SOAs), and participants performed both a spatial localization and simultaneity judgment task. Response times in localizing paired visual-auditory stimuli were slower in the periphery and at larger SOAs, but most importantly, an interaction was found between the two factors, in which the effect of SOA was greater in peripheral as opposed to central locations. Simultaneity judgments also revealed a novel interaction between space and time: individuals were more likely to judge stimuli as synchronous when occurring in the periphery at large SOAs. The results of this study provide novel insights into (a) how the speed of spatial localization of an audiovisual stimulus is affected by location and temporal coincidence and the interaction between these two factors and (b) how the location of a multisensory stimulus impacts judgments concerning the temporal relationship of the paired stimuli. These findings provide strong evidence for a complex interdependency between spatial location and temporal structure in determining the ultimate behavioral and perceptual outcome associated with a paired multisensory (i.e., visual-auditory) stimulus.

Individual differences in the multisensory temporal binding window predict susceptibility to audiovisual illusions

Human multisensory systems are known to bind inputs from the different sensory modalities into a unified percept, a process that leads to measurable behavioral benefits. This integrative process can be observed through multisensory illusions, including the McGurk effect and the sound-induced flash illusion, both of which demonstrate the ability of one sensory modality to modulate perception in a second modality. Such multisensory integration is highly dependent upon the temporal relationship of the different sensory inputs, with perceptual binding occurring within a limited range of asynchronies known as the temporal binding window (TBW). Previous studies have shown that this window is highly variable across individuals, but it is unclear how these variations in the TBW relate to an individual's ability to integrate multisensory cues. Here we provide evidence linking individual differences in multisensory temporal processes to differences in the individual's audiovisual integration of illusory stimuli. Our data provide strong evidence that the temporal processing of multiple sensory signals and the merging of multiple signals into a single, unified perception, are highly related. Specifically, the width of right side of an individuals' TBW, where the auditory stimulus follows the visual, is significantly correlated with the strength of illusory percepts, as indexed via both an increase in the strength of binding synchronous sensory signals and in an improvement in correctly dissociating asynchronous signals. These findings are discussed in terms of their possible neurobiological basis, relevance to the development of sensory integration, and possible importance for clinical conditions in which there is growing evidence that multisensory integration is compromised.

Steady-state evoked potentials as an index of multisensory temporal binding

Temporal congruency promotes perceptual binding of multisensory inputs. Here, we used EEG frequency-tagging to track cortical activities elicited by auditory and visual inputs separately, in the form of steady-state evoked potentials (SS-EPs). We tested whether SS-EPs could reveal a dynamic coupling of cortical activities related to the binding of auditory and visual inputs conveying synchronous vs. non-synchronous temporal periodicities, or beats. The temporally congruent audiovisual condition elicited markedly enhanced auditory and visual SS-EPs, as compared to the incongruent condition. Furthermore, an increased inter-trial phase coherence of both SS-EPs was observed in that condition. Taken together, these observations indicate that temporal congruency enhances the processing of multisensory inputs at sensory-specific stages of cortical processing, possibly through a dynamic binding by synchrony of the elicited activities and/or improved dynamic attending. Moreover, we show that EEG frequency-tagging with SS-EPs constitutes an effective tool to explore the neural dynamics of multisensory integration in the human brain.