The Audiovisual Tau Effect in Infancy

Background

Perceived spatial intervals between successive flashes can be distorted by varying the temporal intervals between them (the “tau effect”). A previous study showed that a tau effect for visual flashes could be induced when they were accompanied by auditory beeps with varied temporal intervals (an audiovisual tau effect).

Methodology/Principal Findings

We conducted two experiments to investigate whether the audiovisual tau effect occurs in infancy. Forty-eight infants aged 5–8 months took part in this study. In Experiment 1, infants were familiarized with audiovisual stimuli consisting of three pairs of two flashes and three beeps. The onsets of the first and third pairs of flashes were respectively matched to those of the first and third beeps. The onset of the second pair of flashes was separated from that of the second beep by 150 ms. Following the familiarization phase, infants were exposed to a test stimulus composed of two vertical arrays of three static flashes with different spatial intervals. We hypothesized that if the audiovisual tau effect occurred in infancy then infants would preferentially look at the flash array with spatial intervals that would be expected to be different from the perceived spatial intervals between flashes they were exposed to in the familiarization phase. The results of Experiment 1 supported this hypothesis. In Experiment 2, the first and third beeps were removed from the familiarization stimuli, resulting in the disappearance of the audiovisual tau effect. This indicates that the modulation of temporal intervals among flashes by beeps was essential for the audiovisual tau effect to occur (Experiment 2).

Conclusions/Significance

These results suggest that the cross-modal processing that underlies the audiovisual tau effect occurs even in early infancy. In particular, the results indicate that audiovisual modulation of temporal intervals emerges by 5–8 months of age.     

‘When Birds of a Feather Flock Together’: Synesthetic Correspondences Modulate Audiovisual Integration in Non-Synesthetes

Background

Synesthesia is a condition in which the stimulation of one sense elicits an additional experience, often in a different (i.e., unstimulated) sense. Although only a small proportion of the population is synesthetic, there is growing evidence to suggest that neurocognitively-normal individuals also experience some form of synesthetic association between the stimuli presented to different sensory modalities (i.e., between auditory pitch and visual size, where lower frequency tones are associated with large objects and higher frequency tones with small objects). While previous research has highlighted crossmodal interactions between synesthetically corresponding dimensions, the possible role of synesthetic associations in multisensory integration has not been considered previously.

Methodology

Here we investigate the effects of synesthetic associations by presenting pairs of asynchronous or spatially discrepant visual and auditory stimuli that were either synesthetically matched or mismatched. In a series of three psychophysical experiments, participants reported the relative temporal order of presentation or the relative spatial locations of the two stimuli.

Principal Findings

The reliability of non-synesthetic participants'' estimates of both audiovisual temporal asynchrony and spatial discrepancy were lower for pairs of synesthetically matched as compared to synesthetically mismatched audiovisual stimuli.

Conclusions

Recent studies of multisensory integration have shown that the reduced reliability of perceptual estimates regarding intersensory conflicts constitutes the marker of a stronger coupling between the unisensory signals. Our results therefore indicate a stronger coupling of synesthetically matched vs. mismatched stimuli and provide the first psychophysical evidence that synesthetic congruency can promote multisensory integration. Synesthetic crossmodal correspondences therefore appear to play a crucial (if unacknowledged) role in the multisensory integration of auditory and visual information.     

Aging,Spatial Disparity,and the Sound-Induced Flash Illusion

The present study examined age-related differences in multisensory integration and the effect of spatial disparity on the sound-induced flash illusion—-an illusion used in previous research to assess age-related differences in multisensory integration. Prior to participation in the study, both younger and older participants demonstrated their ability to detect 1–2 visual flashes and 1–2 auditory beep presented unimodally. After passing the pre-test, participants were then presented 1–2 flashes paired with 0–2 beeps that originated from one of five speakers positioned equidistantly 100cm from the participant. One speaker was positioned directly below the screen, two speakers were positioned 50cm to the left and right from the center of the screen, and two more speakers positioned to the left and right 100cm from the center of the screen. Participants were told to report the number of flashes presented and to ignore the beeps. Both age groups showed a significant effect of the beeps on the perceived number of flashes. However, neither younger nor older individuals showed any significant effect of spatial disparity on the sound-induced flash illusion. The presence of a congruent number of beeps increased accuracy for both older and younger individuals. Reaction time data was also analyzed. As expected, older individuals showed significantly longer reaction times when compared to younger individuals. In addition, both older and younger individuals showed a significant increase in reaction time for fusion trials, where two flashes and one beep are perceived as a single flash, as compared to congruent single flash trials. This increase in reaction time was not found for fission trials, where one flash and two beeps were perceived as two flashes. This suggests that processing may differ for the two forms for fission as compared to fusion illusions.     

Auditory motion information drives visual motion perception

Background

Vision provides the most salient information with regard to the stimulus motion. However, it has recently been demonstrated that static visual stimuli are perceived as moving laterally by alternating left-right sound sources. The underlying mechanism of this phenomenon remains unclear; it has not yet been determined whether auditory motion signals, rather than auditory positional signals, can directly contribute to visual motion perception.

Methodology/Principal Findings

Static visual flashes were presented at retinal locations outside the fovea together with a lateral auditory motion provided by a virtual stereo noise source smoothly shifting in the horizontal plane. The flash appeared to move by means of the auditory motion when the spatiotemporal position of the flashes was in the middle of the auditory motion trajectory. Furthermore, the lateral auditory motion altered visual motion perception in a global motion display where different localized motion signals of multiple visual stimuli were combined to produce a coherent visual motion perception.

Conclusions/Significance

These findings suggest there exist direct interactions between auditory and visual motion signals, and that there might be common neural substrates for auditory and visual motion processing.     

A first- and second-order motion energy analysis of peripheral motion illusions leads to further evidence of "feature blur" in peripheral vision

Background

Anatomical and physiological differences between the central and peripheral visual systems are well documented. Recent findings have suggested that vision in the periphery is not just a scaled version of foveal vision, but rather is relatively poor at representing spatial and temporal phase and other visual features. Shapiro, Lu, Huang, Knight, and Ennis (2010) have recently examined a motion stimulus (the “curveball illusion”) in which the shift from foveal to peripheral viewing results in a dramatic spatial/temporal discontinuity. Here, we apply a similar analysis to a range of other spatial/temporal configurations that create perceptual conflict between foveal and peripheral vision.

Methodology/Principal Findings

To elucidate how the differences between foveal and peripheral vision affect super-threshold vision, we created a series of complex visual displays that contain opposing sources of motion information. The displays (referred to as the peripheral escalator illusion, peripheral acceleration and deceleration illusions, rotating reversals illusion, and disappearing squares illusion) create dramatically different perceptions when viewed foveally versus peripherally. We compute the first-order and second-order directional motion energy available in the displays using a three-dimensional Fourier analysis in the (x, y, t) space. The peripheral escalator, acceleration and deceleration illusions and rotating reversals illusion all show a similar trend: in the fovea, the first-order motion energy and second-order motion energy can be perceptually separated from each other; in the periphery, the perception seems to correspond to a combination of the multiple sources of motion information. The disappearing squares illusion shows that the ability to assemble the features of Kanisza squares becomes slower in the periphery.

Conclusions/Significance

The results lead us to hypothesize “feature blur” in the periphery (i.e., the peripheral visual system combines features that the foveal visual system can separate). Feature blur is of general importance because humans are frequently bringing the information in the periphery to the fovea and vice versa.     

Neural Responses in Parietal and Occipital Areas in Response to Visual Events Are Modulated by Prior Multisensory Stimuli

The effect of multi-modal vs uni-modal prior stimuli on the subsequent processing of a simple flash stimulus was studied in the context of the audio-visual ‘flash-beep’ illusion, in which the number of flashes a person sees is influenced by accompanying beep stimuli. EEG recordings were made while combinations of simple visual and audio-visual stimuli were presented. The experiments found that the electric field strength related to a flash stimulus was stronger when it was preceded by a multi-modal flash/beep stimulus, compared to when it was preceded by another uni-modal flash stimulus. This difference was found to be significant in two distinct timeframes – an early timeframe, from 130–160 ms, and a late timeframe, from 300–320 ms. Source localisation analysis found that the increased activity in the early interval was localised to an area centred on the inferior and superior parietal lobes, whereas the later increase was associated with stronger activity in an area centred on primary and secondary visual cortex, in the occipital lobe. The results suggest that processing of a visual stimulus can be affected by the presence of an immediately prior multisensory event. Relatively long-lasting interactions generated by the initial auditory and visual stimuli altered the processing of a subsequent visual stimulus.     

Grey matter volume in early human visual cortex predicts proneness to the sound-induced flash illusion

Visual perception can be modulated by sounds. A drastic example of this is the sound-induced flash illusion: when a single flash is accompanied by two bleeps, it is sometimes perceived in an illusory fashion as two consecutive flashes. However, there are strong individual differences in proneness to this illusion. Some participants experience the illusion on almost every trial, whereas others almost never do. We investigated whether such individual differences in proneness to the sound-induced flash illusion were reflected in structural differences in brain regions whose activity is modulated by the illusion. We found that individual differences in proneness to the illusion were strongly and significantly correlated with local grey matter volume in early retinotopic visual cortex. Participants with smaller early visual cortices were more prone to the illusion. We propose that strength of auditory influences on visual perception is determined by individual differences in recurrent connections, cross-modal attention and/or optimal weighting of sensory channels.     

Distortions of subjective time perception within and across senses

Background

The ability to estimate the passage of time is of fundamental importance for perceptual and cognitive processes. One experience of time is the perception of duration, which is not isomorphic to physical duration and can be distorted by a number of factors. Yet, the critical features generating these perceptual shifts in subjective duration are not understood.

Methodology/Findings

We used prospective duration judgments within and across sensory modalities to examine the effect of stimulus predictability and feature change on the perception of duration. First, we found robust distortions of perceived duration in auditory, visual and auditory-visual presentations despite the predictability of the feature changes in the stimuli. For example, a looming disc embedded in a series of steady discs led to time dilation, whereas a steady disc embedded in a series of looming discs led to time compression. Second, we addressed whether visual (auditory) inputs could alter the perception of duration of auditory (visual) inputs. When participants were presented with incongruent audio-visual stimuli, the perceived duration of auditory events could be shortened or lengthened by the presence of conflicting visual information; however, the perceived duration of visual events was seldom distorted by the presence of auditory information and was never perceived shorter than their actual durations.

Conclusions/Significance

These results support the existence of multisensory interactions in the perception of duration and, importantly, suggest that vision can modify auditory temporal perception in a pure timing task. Insofar as distortions in subjective duration can neither be accounted for by the unpredictability of an auditory, visual or auditory-visual event, we propose that it is the intrinsic features of the stimulus that critically affect subjective time distortions.     

Dynamic perceptual changes in audiovisual simultaneity

Background

The timing at which sensory input reaches the level of conscious perception is an intriguing question still awaiting an answer. It is often assumed that both visual and auditory percepts have a modality specific processing delay and their difference determines perceptual temporal offset.

Methodology/Principal Findings

Here, we show that the perception of audiovisual simultaneity can change flexibly and fluctuates over a short period of time while subjects observe a constant stimulus. We investigated the mechanisms underlying the spontaneous alternations in this audiovisual illusion and found that attention plays a crucial role. When attention was distracted from the stimulus, the perceptual transitions disappeared. When attention was directed to a visual event, the perceived timing of an auditory event was attracted towards that event.

Conclusions/Significance

This multistable display illustrates how flexible perceived timing can be, and at the same time offers a paradigm to dissociate perceptual from stimulus-driven factors in crossmodal feature binding. Our findings suggest that the perception of crossmodal synchrony depends on perceptual binding of audiovisual stimuli as a common event.     

Benefits of stimulus congruency for multisensory facilitation of visual learning

Background

Studies of perceptual learning have largely focused on unisensory stimuli. However, multisensory interactions are ubiquitous in perception, even at early processing stages, and thus can potentially play a role in learning. Here, we examine the effect of auditory-visual congruency on visual learning.

Methodology/Principle Findings

Subjects were trained over five days on a visual motion coherence detection task with either congruent audiovisual, or incongruent audiovisual stimuli. Comparing performance on visual-only trials, we find that training with congruent audiovisual stimuli produces significantly better learning than training with incongruent audiovisual stimuli or with only visual stimuli.

Conclusions/Significance

This advantage from stimulus congruency during training suggests that the benefits of multisensory training may result from audiovisual interactions at a perceptual rather than cognitive level.     

The Duration of a Co-Occurring Sound Modulates Visual Detection Performance in Humans

Background

The duration of sounds can affect the perceived duration of co-occurring visual stimuli. However, it is unclear whether this is limited to amodal processes of duration perception or affects other non-temporal qualities of visual perception.

Methodology/Principal Findings

Here, we tested the hypothesis that visual sensitivity - rather than only the perceived duration of visual stimuli - can be affected by the duration of co-occurring sounds. We found that visual detection sensitivity (d’) for unimodal stimuli was higher for stimuli of longer duration. Crucially, in a cross-modal condition, we replicated previous unimodal findings, observing that visual sensitivity was shaped by the duration of co-occurring sounds. When short visual stimuli (∼24 ms) were accompanied by sounds of matching duration, visual sensitivity was decreased relative to the unimodal visual condition. However, when the same visual stimuli were accompanied by longer auditory stimuli (∼60–96 ms), visual sensitivity was increased relative to the performance for ∼24 ms auditory stimuli. Across participants, this sensitivity enhancement was observed within a critical time window of ∼60–96 ms. Moreover, the amplitude of this effect correlated with visual sensitivity enhancement found for longer lasting visual stimuli across participants.

Conclusions/Significance

Our findings show that the duration of co-occurring sounds affects visual perception; it changes visual sensitivity in a similar way as altering the (actual) duration of the visual stimuli does.     

Alternation of Sound Location Induces Visual Motion Perception of a Static Object

Background

Audition provides important cues with regard to stimulus motion although vision may provide the most salient information. It has been reported that a sound of fixed intensity tends to be judged as decreasing in intensity after adaptation to looming visual stimuli or as increasing in intensity after adaptation to receding visual stimuli. This audiovisual interaction in motion aftereffects indicates that there are multimodal contributions to motion perception at early levels of sensory processing. However, there has been no report that sounds can induce the perception of visual motion.

Methodology/Principal Findings

A visual stimulus blinking at a fixed location was perceived to be moving laterally when the flash onset was synchronized to an alternating left-right sound source. This illusory visual motion was strengthened with an increasing retinal eccentricity (2.5 deg to 20 deg) and occurred more frequently when the onsets of the audio and visual stimuli were synchronized.

Conclusions/Significance

We clearly demonstrated that the alternation of sound location induces illusory visual motion when vision cannot provide accurate spatial information. The present findings strongly suggest that the neural representations of auditory and visual motion processing can bias each other, which yields the best estimates of external events in a complementary manner.     

Neuromagnetic evidence for early auditory restoration of fundamental pitch

Background

Understanding the time course of how listeners reconstruct a missing fundamental component in an auditory stimulus remains elusive. We report MEG evidence that the missing fundamental component of a complex auditory stimulus is recovered in auditory cortex within 100 ms post stimulus onset.

Methodology

Two outside tones of four-tone complex stimuli were held constant (1200 Hz and 2400 Hz), while two inside tones were systematically modulated (between 1300 Hz and 2300 Hz), such that the restored fundamental (also knows as “virtual pitch”) changed from 100 Hz to 600 Hz. Constructing the auditory stimuli in this manner controls for a number of spectral properties known to modulate the neuromagnetic signal. The tone complex stimuli only diverged on the value of the missing fundamental component.

Principal Findings

We compared the M100 latencies of these tone complexes to the M100 latencies elicited by their respective pure tone (spectral pitch) counterparts. The M100 latencies for the tone complexes matched their pure sinusoid counterparts, while also replicating the M100 temporal latency response curve found in previous studies.

Conclusions

Our findings suggest that listeners are reconstructing the inferred pitch by roughly 100 ms after stimulus onset and are consistent with previous electrophysiological research suggesting that the inferential pitch is perceived in early auditory cortex.     

Dynamic Sounds Capture the Boundaries of Peripersonal Space Representation in Humans

Background

We physically interact with external stimuli when they occur within a limited space immediately surrounding the body, i.e., Peripersonal Space (PPS). In the primate brain, specific fronto-parietal areas are responsible for the multisensory representation of PPS, by integrating tactile, visual and auditory information occurring on and near the body. Dynamic stimuli are particularly relevant for PPS representation, as they might refer to potential harms approaching the body. However, behavioural tasks for studying PPS representation with moving stimuli are lacking. Here we propose a new dynamic audio-tactile interaction task in order to assess the extension of PPS in a more functionally and ecologically valid condition.

Methodology/Principal Findings

Participants vocally responded to a tactile stimulus administered at the hand at different delays from the onset of task-irrelevant dynamic sounds which gave the impression of a sound source either approaching or receding from the subject’s hand. Results showed that a moving auditory stimulus speeded up the processing of a tactile stimulus at the hand as long as it was perceived at a limited distance from the hand, that is within the boundaries of PPS representation. The audio-tactile interaction effect was stronger when sounds were approaching compared to when sounds were receding.

Conclusion/Significance

This study provides a new method to dynamically assess PPS representation: The function describing the relationship between tactile processing and the position of sounds in space can be used to estimate the location of PPS boundaries, along a spatial continuum between far and near space, in a valuable and ecologically significant way.     

Mirror adaptation in sensory-motor simultaneity

Background

When one watches a sports game, one may feel her/his own muscles moving in synchrony with the player''s. Such parallels between observed actions of others and one''s own has been well supported in the latest progress in neuroscience, and coined “mirror system.” It is likely that due to such phenomena, we are able to learn motor skills just by observing an expert''s performance. Yet it is unknown whether such indirect learning occurs only at higher cognitive levels, or also at basic sensorimotor levels where sensorimotor delay is compensated and the timing of sensory feedback is constantly calibrated.

Methodology/Principal Findings

Here, we show that the subject''s passive observation of an actor manipulating a computer mouse with delayed auditory feedback led to shifts in subjective simultaneity of self mouse manipulation and auditory stimulus in the observing subjects. Likewise, self adaptation to the delayed feedback modulated the simultaneity judgment of the other subjects manipulating a mouse and an auditory stimulus. Meanwhile, subjective simultaneity of a simple visual disc and the auditory stimulus (flash test) was not affected by observation of an actor nor self-adaptation.

Conclusions/Significance

The lack of shift in the flash test for both conditions indicates that the recalibration transfer is specific to the action domain, and is not due to a general sensory adaptation. This points to the involvement of a system for the temporal monitoring of actions, one that processes both one''s own actions and those of others.     

Mechanisms and asymmetries in visual perception of simultaneity and temporal order

Temporally overlapping, spatially separated visual stimuli were used for studying perception of simultaneity and temporal order. Pairs of flashes each of 100 ms duration were presented with stimulus onset asynchronies of 0, 30, 50, and 70 ms. Three spatial arrangements of flash presentation were tested: 1) both flashes were presented foveally; 2) one flash was presented foveally and the other at 9 deg in the left visual hemifield; 3) one flash was presented foveally and the other at 8 deg in the right visual hemifield. Onset asynchronies of 30 and 50 ms were not sufficient for correct identification of temporal order although the flashes were not perceived as simultaneous. Analysis of the response distributions suggests the existence of two-independent mechanisms for evaluating temporal interrelations: one for detecting simultaneity and the other for identifying temporal order. A better detection of simultaneity was found when synchroneous flashes were presented together with pairs of flashes separated by larger onset asynchronies. Reading habits may explain only part of the left-right asymmetries of the response distributions. The possible lateralization of the two suggested mechanisms within the cerebral hemispheres is discussed.     

Direction of visual apparent motion driven solely by timing of a static sound

In temporal ventriloquism, auditory events can illusorily attract perceived timing of a visual onset [1-3]. We investigated whether timing of a static sound can also influence spatio-temporal processing of visual apparent motion, induced here by visual bars alternating between opposite hemifields. Perceived direction typically depends on the relative interval in timing between visual left-right and right-left flashes (e.g., rightwards motion dominating when left-to-right interflash intervals are shortest [4]). In our new multisensory condition, interflash intervals were equal, but auditory beeps could slightly lag the right flash, yet slightly lead the left flash, or vice versa. This auditory timing strongly influenced perceived visual motion direction, despite providing no spatial auditory motion signal whatsoever. Moreover, prolonged adaptation to such auditorily driven apparent motion produced a robust visual motion aftereffect in the opposite direction, when measured in subsequent silence. Control experiments argued against accounts in terms of possible auditory grouping, or possible attention capture. We suggest that the motion arises because the sounds change perceived visual timing, as we separately confirmed. Our results provide a new demonstration of multisensory influences on sensory-specific perception [5], with timing of a static sound influencing spatio-temporal processing of visual motion direction.     

The multisensory attentional consequences of tool use: a functional magnetic resonance imaging study

Background

Tool use in humans requires that multisensory information is integrated across different locations, from objects seen to be distant from the hand, but felt indirectly at the hand via the tool. We tested the hypothesis that using a simple tool to perceive vibrotactile stimuli results in the enhanced processing of visual stimuli presented at the distal, functional part of the tool. Such a finding would be consistent with a shift of spatial attention to the location where the tool is used.

Methodology/Principal Findings

We tested this hypothesis by scanning healthy human participants'' brains using functional magnetic resonance imaging, while they used a simple tool to discriminate between target vibrations, accompanied by congruent or incongruent visual distractors, on the same or opposite side to the tool. The attentional hypothesis was supported: BOLD response in occipital cortex, particularly in the right hemisphere lingual gyrus, varied significantly as a function of tool position, increasing contralaterally, and decreasing ipsilaterally to the tool. Furthermore, these modulations occurred despite the fact that participants were repeatedly instructed to ignore the visual stimuli, to respond only to the vibrotactile stimuli, and to maintain visual fixation centrally. In addition, the magnitude of multisensory (visual-vibrotactile) interactions in participants'' behavioural responses significantly predicted the BOLD response in occipital cortical areas that were also modulated as a function of both visual stimulus position and tool position.

Conclusions/Significance

These results show that using a simple tool to locate and to perceive vibrotactile stimuli is accompanied by a shift of spatial attention to the location where the functional part of the tool is used, resulting in enhanced processing of visual stimuli at that location, and decreased processing at other locations. This was most clearly observed in the right hemisphere lingual gyrus. Such modulations of visual processing may reflect the functional importance of visuospatial information during human tool use.     

The effects of neck flexion on cerebral potentials evoked by visual,auditory and somatosensory stimuli and focal brain blood flow in related sensory cortices

Background

A flexed neck posture leads to non-specific activation of the brain. Sensory evoked cerebral potentials and focal brain blood flow have been used to evaluate the activation of the sensory cortex. We investigated the effects of a flexed neck posture on the cerebral potentials evoked by visual, auditory and somatosensory stimuli and focal brain blood flow in the related sensory cortices.

Methods

Twelve healthy young adults received right visual hemi-field, binaural auditory and left median nerve stimuli while sitting with the neck in a resting and flexed (20° flexion) position. Sensory evoked potentials were recorded from the right occipital region, Cz in accordance with the international 10–20 system, and 2 cm posterior from C4, during visual, auditory and somatosensory stimulations. The oxidative-hemoglobin concentration was measured in the respective sensory cortex using near-infrared spectroscopy.

Results

Latencies of the late component of all sensory evoked potentials significantly shortened, and the amplitude of auditory evoked potentials increased when the neck was in a flexed position. Oxidative-hemoglobin concentrations in the left and right visual cortices were higher during visual stimulation in the flexed neck position. The left visual cortex is responsible for receiving the visual information. In addition, oxidative-hemoglobin concentrations in the bilateral auditory cortex during auditory stimulation, and in the right somatosensory cortex during somatosensory stimulation, were higher in the flexed neck position.

Conclusions

Visual, auditory and somatosensory pathways were activated by neck flexion. The sensory cortices were selectively activated, reflecting the modalities in sensory projection to the cerebral cortex and inter-hemispheric connections.     

Ubiquitous crossmodal Stochastic Resonance in humans: auditory noise facilitates tactile, visual and proprioceptive sensations

Background

Stochastic resonance is a nonlinear phenomenon whereby the addition of noise can improve the detection of weak stimuli. An optimal amount of added noise results in the maximum enhancement, whereas further increases in noise intensity only degrade detection or information content. The phenomenon does not occur in linear systems, where the addition of noise to either the system or the stimulus only degrades the signal quality. Stochastic Resonance (SR) has been extensively studied in different physical systems. It has been extended to human sensory systems where it can be classified as unimodal, central, behavioral and recently crossmodal. However what has not been explored is the extension of this crossmodal SR in humans. For instance, if under the same auditory noise conditions the crossmodal SR persists among different sensory systems.

Methodology/Principal Findings

Using physiological and psychophysical techniques we demonstrate that the same auditory noise can enhance the sensitivity of tactile, visual and propioceptive system responses to weak signals. Specifically, we show that the effective auditory noise significantly increased tactile sensations of the finger, decreased luminance and contrast visual thresholds and significantly changed EMG recordings of the leg muscles during posture maintenance.

Conclusions/Significance

We conclude that crossmodal SR is a ubiquitous phenomenon in humans that can be interpreted within an energy and frequency model of multisensory neurons spontaneous activity. Initially the energy and frequency content of the multisensory neurons'' activity (supplied by the weak signals) is not enough to be detected but when the auditory noise enters the brain, it generates a general activation among multisensory neurons of different regions, modifying their original activity. The result is an integrated activation that promotes sensitivity transitions and the signals are then perceived. A physiologically plausible model for crossmodal stochastic resonance is presented.