Perceptual organization at attended and unattended locations

This study examined the effects of attention on forming perceptual units by proximity grouping and by uniform connectedness (UC). In Experiment 1 a row of three global letters defined by either proximity or UC was presented at the center of the visual field. Participants were asked to identify the letter in the middle of stimulus arrays while ignoring the flankers. The stimulus onset asynchrony (SOA) between stimulus arrays and masks varied between 180 and 500 ms. We found that responses to targets defined by proximity grouping were slower than to those defined by UC at median SOAs but there were no differences at short or long SOAs. Incongruent flankers slowed responses to targets and this flanker compatibility effect was larger for UC than for proximity-defined flankers. Experiment 2 examined the effects of spatial precueing on discrimination responses to proximity- and UC-defined targets. The advantage for targets defined by UC over targets defined by proximity grouping was greater at uncued relative to cued locations. The results suggest that the advantage for UC over proximity grouping in forming perceptual units is contingent on the stimuli not being fully attended, and that paying attention to the stimuli differentially benefits proximity grouping.     

Perceptual organization at attended and unattended locations

In order to perceive complex visual scenes, the human perceptual system has to organize discrete enti-ties in the visual field into chunks or perceptual units for higher-level processing. Perceptual organization is governed by Gestalt principles such as proximity, similarity, and continuity[1]. Thus spatially close ob-jects tend to be grouped together, as do elements that are similar to one another. Grouping based on the Ge-stalt laws (particularly proximity) is critical for the perception of…     

Evidence for Enhanced Multisensory Facilitation with Stimulus Relevance: An Electrophysiological Investigation

Currently debate exists relating to the interplay between multisensory processes and bottom-up and top-down influences. However, few studies have looked at neural responses to newly paired audiovisual stimuli that differ in their prescribed relevance. For such newly associated audiovisual stimuli, optimal facilitation of motor actions was observed only when both components of the audiovisual stimuli were targets. Relevant auditory stimuli were found to significantly increase the amplitudes of the event-related potentials at the occipital pole during the first 100 ms post-stimulus onset, though this early integration was not predictive of multisensory facilitation. Activity related to multisensory behavioral facilitation was observed approximately 166 ms post-stimulus, at left central and occipital sites. Furthermore, optimal multisensory facilitation was found to be associated with a latency shift of induced oscillations in the beta range (14–30 Hz) at right hemisphere parietal scalp regions. These findings demonstrate the importance of stimulus relevance to multisensory processing by providing the first evidence that the neural processes underlying multisensory integration are modulated by the relevance of the stimuli being combined. We also provide evidence that such facilitation may be mediated by changes in neural synchronization in occipital and centro-parietal neural populations at early and late stages of neural processing that coincided with stimulus selection, and the preparation and initiation of motor action.     

Modality distribution of sensory neurons in the feline caudate nucleus and the substantia nigra

Despite extensive analysis of the motor functions of the basal ganglia and the fact that multisensory information processing appears critical for the execution of their behavioral action, little is known concerning the sensory functions of the caudate nucleus (CN) and the substantia nigra (SN). In the present study, we set out to describe the sensory modality distribution and to determine the proportions of multisensory units within the CN and the SN. The separate single sensory modality tests demonstrated that a majority of the neurons responded to only one modality, so that they seemed to be unimodal. In contrast with these findings, a large proportion of these neurons exhibited significant multisensory cross-modal interactions. Thus, these neurons should also be classified as multisensory. Our results suggest that a surprisingly high proportion of sensory neurons in the basal ganglia are multisensory, and demonstrate that an analysis without a consideration of multisensory cross-modal interactions may strongly underrepresent the number of multisensory units. We conclude that a majority of the sensory neurons in the CN and SN process multisensory information and only a minority of these units are clearly unimodal.     

Audio-Tactile Integration and the Influence of Musical Training

Perception of our environment is a multisensory experience; information from different sensory systems like the auditory, visual and tactile is constantly integrated. Complex tasks that require high temporal and spatial precision of multisensory integration put strong demands on the underlying networks but it is largely unknown how task experience shapes multisensory processing. Long-term musical training is an excellent model for brain plasticity because it shapes the human brain at functional and structural levels, affecting a network of brain areas. In the present study we used magnetoencephalography (MEG) to investigate how audio-tactile perception is integrated in the human brain and if musicians show enhancement of the corresponding activation compared to non-musicians. Using a paradigm that allowed the investigation of combined and separate auditory and tactile processing, we found a multisensory incongruency response, generated in frontal, cingulate and cerebellar regions, an auditory mismatch response generated mainly in the auditory cortex and a tactile mismatch response generated in frontal and cerebellar regions. The influence of musical training was seen in the audio-tactile as well as in the auditory condition, indicating enhanced higher-order processing in musicians, while the sources of the tactile MMN were not influenced by long-term musical training. Consistent with the predictive coding model, more basic, bottom-up sensory processing was relatively stable and less affected by expertise, whereas areas for top-down models of multisensory expectancies were modulated by training.     

Interaction of perceptual grouping and crossmodal temporal capture in tactile apparent-motion

Previous studies have shown that in tasks requiring participants to report the direction of apparent motion, task-irrelevant mono-beeps can "capture" visual motion perception when the beeps occur temporally close to the visual stimuli. However, the contributions of the relative timing of multimodal events and the event structure, modulating uni- and/or crossmodal perceptual grouping, remain unclear. To examine this question and extend the investigation to the tactile modality, the current experiments presented tactile two-tap apparent-motion streams, with an SOA of 400 ms between successive, left-/right-hand middle-finger taps, accompanied by task-irrelevant, non-spatial auditory stimuli. The streams were shown for 90 seconds, and participants' task was to continuously report the perceived (left- or rightward) direction of tactile motion. In Experiment 1, each tactile stimulus was paired with an auditory beep, though odd-numbered taps were paired with an asynchronous beep, with audiotactile SOAs ranging from -75 ms to 75 ms. Perceived direction of tactile motion varied systematically with audiotactile SOA, indicative of a temporal-capture effect. In Experiment 2, two audiotactile SOAs--one short (75 ms), one long (325 ms)--were compared. The long-SOA condition preserved the crossmodal event structure (so the temporal-capture dynamics should have been similar to that in Experiment 1), but both beeps now occurred temporally close to the taps on one side (even-numbered taps). The two SOAs were found to produce opposite modulations of apparent motion, indicative of an influence of crossmodal grouping. In Experiment 3, only odd-numbered, but not even-numbered, taps were paired with auditory beeps. This abolished the temporal-capture effect and, instead, a dominant percept of apparent motion from the audiotactile side to the tactile-only side was observed independently of the SOA variation. These findings suggest that asymmetric crossmodal grouping leads to an attentional modulation of apparent motion, which inhibits crossmodal temporal-capture effects.     

An fMRI Study of Audiovisual Speech Perception Reveals Multisensory Interactions in Auditory Cortex

Research on the neural basis of speech-reading implicates a network of auditory language regions involving inferior frontal cortex, premotor cortex and sites along superior temporal cortex. In audiovisual speech studies, neural activity is consistently reported in posterior superior temporal Sulcus (pSTS) and this site has been implicated in multimodal integration. Traditionally, multisensory interactions are considered high-level processing that engages heteromodal association cortices (such as STS). Recent work, however, challenges this notion and suggests that multisensory interactions may occur in low-level unimodal sensory cortices. While previous audiovisual speech studies demonstrate that high-level multisensory interactions occur in pSTS, what remains unclear is how early in the processing hierarchy these multisensory interactions may occur. The goal of the present fMRI experiment is to investigate how visual speech can influence activity in auditory cortex above and beyond its response to auditory speech. In an audiovisual speech experiment, subjects were presented with auditory speech with and without congruent visual input. Holding the auditory stimulus constant across the experiment, we investigated how the addition of visual speech influences activity in auditory cortex. We demonstrate that congruent visual speech increases the activity in auditory cortex.     

Recalibration of the Multisensory Temporal Window of Integration Results from Changing Task Demands

The notion of the temporal window of integration, when applied in a multisensory context, refers to the breadth of the interval across which the brain perceives two stimuli from different sensory modalities as synchronous. It maintains a unitary perception of multisensory events despite physical and biophysical timing differences between the senses. The boundaries of the window can be influenced by attention and past sensory experience. Here we examined whether task demands could also influence the multisensory temporal window of integration. We varied the stimulus onset asynchrony between simple, short-lasting auditory and visual stimuli while participants performed two tasks in separate blocks: a temporal order judgment task that required the discrimination of subtle auditory-visual asynchronies, and a reaction time task to the first incoming stimulus irrespective of its sensory modality. We defined the temporal window of integration as the range of stimulus onset asynchronies where performance was below 75% in the temporal order judgment task, as well as the range of stimulus onset asynchronies where responses showed multisensory facilitation (race model violation) in the reaction time task. In 5 of 11 participants, we observed audio-visual stimulus onset asynchronies where reaction time was significantly accelerated (indicating successful integration in this task) while performance was accurate in the temporal order judgment task (indicating successful segregation in that task). This dissociation suggests that in some participants, the boundaries of the temporal window of integration can adaptively recalibrate in order to optimize performance according to specific task demands.     

Paving the way forward: integrating the senses through phase-resetting of cortical oscillations

Most, if not all, of the neocortex is multisensory, but the mechanisms by which different cortical areas - association versus sensory, for instance - integrate multisensory inputs are not known. The study by Lakatos et al. reveals that, in the primary auditory cortex, the phase of neural oscillations is reset by somatosensory inputs, and subsequent auditory inputs are enhanced or suppressed, depending on their timing relative to the oscillatory cycle.     

Phase reset affects auditory-visual simultaneity judgment

We continuously receive the external information from multiple sensors simultaneously. The brain must judge a source event of these sensory informations and integrate them. It is thought that judging the simultaneity of such multisensory stimuli is an important cue when we discriminate whether the stimuli are derived from one event or not. Although previous studies have investigated the correspondence between an auditory-visual (AV) simultaneity perceptions and the neural responses, there are still few studies of this. Electrophysiological studies have reported that ongoing oscillations in human cortex affect perception. Especially, the phase resetting of ongoing oscillations has been examined as it plays an important role in multisensory integration. The aim of this study was to investigate the relationship of phase resetting for the judgment of AV simultaneity judgement tasks. The subjects were successively presented with auditory and visual stimuli with intervals that were controlled as SOA_50% and they were asked to report whether they perceived them simultaneously or not. We investigated the effects of the phase of ongoing oscillations on simultaneity judgments with AV stimuli with SOAs in which the detection rate of asynchrony was 50 %. It was found that phase resetting at the beta frequency band in the brain area that related to the modality of the following stimulus occurred after preceding stimulus onset only when the subjects perceived AV stimuli as simultaneous. This result suggested that beta phase resetting occurred in areas that are related to the subsequent stimulus, supporting perception multisensory stimuli as simultaneous.     

Multisensory contributions to the 3-D representation of visuotactile peripersonal space in humans: evidence from the crossmodal congruency task.

In order to determine precisely the location of a tactile stimulus presented to the hand it is necessary to know not only which part of the body has been stimulated, but also where that part of the body lies in space. This involves the multisensory integration of visual, tactile, proprioceptive, and even auditory cues regarding limb position. In recent years, researchers have become increasingly interested in the question of how these various sensory cues are weighted and integrated in order to enable people to localize tactile stimuli, as well as to give rise to the 'felt' position of our limbs, and ultimately the multisensory representation of 3-D peripersonal space. We highlight recent research on this topic using the crossmodal congruency task, in which participants make speeded elevation discrimination responses to vibrotactile targets presented to the thumb or index finger, while simultaneously trying to ignore irrelevant visual distractors presented from either the same (i.e., congruent) or a different (i.e., incongruent) elevation. Crossmodal congruency effects (calculated as performance on incongruent-congruent trials) are greatest when visual and vibrotactile stimuli are presented from the same azimuthal location, thus providing an index of common position across different sensory modalities. The crossmodal congruency task has been used to investigate a number of questions related to the representation of space in both normal participants and brain-damaged patients. In this review, we detail the major findings from this research, and highlight areas of convergence with other cognitive neuroscience disciplines.     

Touch Influences Visual Perception with a Tight Orientation-Tuning

Stimuli from different sensory modalities are thought to be processed initially in distinct unisensory brain areas prior to convergence in multisensory areas. However, signals in one modality can influence the processing of signals from other modalities and recent studies suggest this cross-modal influence may occur early on, even in ‘unisensory’ areas. Some recent psychophysical studies have shown specific cross-modal effects between touch and vision during binocular rivalry, but these cannot completely rule out a response bias. To test for genuine cross-modal integration of haptic and visual signals, we investigated whether congruent haptic input could influence visual contrast sensitivity compared to incongruent haptic input in three psychophysical experiments using a two-interval, two-alternative forced-choice method to eliminate response bias. The initial experiment demonstrated that contrast thresholds for a visual grating were lower when exploring a haptic grating that shared the same orientation compared to an orthogonal orientation. Two subsequent experiments mapped the orientation and spatial frequency tunings for the congruent haptic facilitation of vision, finding a clear orientation tuning effect but not a spatial frequency tuning. In addition to an increased contrast sensitivity for iso-oriented visual-haptic gratings, we found a significant loss of sensitivity for orthogonally oriented visual-haptic gratings. We conclude that the tactile influence on vision is a result of a tactile input to orientation-tuned visual areas.     

Integrating information from different senses in the auditory cortex

Multisensory integration was once thought to be the domain of brain areas high in the cortical hierarchy, with early sensory cortical fields devoted to unisensory processing of inputs from their given set of sensory receptors. More recently, a wealth of evidence documenting visual and somatosensory responses in auditory cortex, even as early as the primary fields, has changed this view of cortical processing. These multisensory inputs may serve to enhance responses to sounds that are accompanied by other sensory cues, effectively making them easier to hear, but may also act more selectively to shape the receptive field properties of auditory cortical neurons to the location or identity of these events. We discuss the new, converging evidence that multiplexing of neural signals may play a key role in informatively encoding and integrating signals in auditory cortex across multiple sensory modalities. We highlight some of the many open research questions that exist about the neural mechanisms that give rise to multisensory integration in auditory cortex, which should be addressed in future experimental and theoretical studies.     

Cross-modal deactivations during modality-specific selective attention

Background  

Processing stimuli in one sensory modality is known to result in suppression of other sensory-specific cortices. Additionally, behavioral experiments suggest that the primary consequence of paying attention to a specific sensory modality is poorer task performance in the unattended sensory modality. This study was designed to determine how focusing attention on the auditory or visual modality impacts neural activity in cortical regions responsible for processing stimuli in the unattended modality.     

The sense of agency is action–effect causality perception based on cross-modal grouping

Sense of agency, the experience of controlling external events through one''s actions, stems from contiguity between action- and effect-related signals. Here we show that human observers link their action- and effect-related signals using a computational principle common to cross-modal sensory grouping. We first report that the detection of a delay between tactile and visual stimuli is enhanced when both stimuli are synchronized with separate auditory stimuli (experiment 1). This occurs because the synchronized auditory stimuli hinder the potential grouping between tactile and visual stimuli. We subsequently demonstrate an analogous effect on observers'' key press as an action and a sensory event. This change is associated with a modulation in sense of agency; namely, sense of agency, as evaluated by apparent compressions of action–effect intervals (intentional binding) or subjective causality ratings, is impaired when both participant''s action and its putative visual effect events are synchronized with auditory tones (experiments 2 and 3). Moreover, a similar role of action–effect grouping in determining sense of agency is demonstrated when the additional signal is presented in the modality identical to an effect event (experiment 4). These results are consistent with the view that sense of agency is the result of general processes of causal perception and that cross-modal grouping plays a central role in these processes.     

Evidence from functional magnetic resonance imaging of crossmodal binding in the human heteromodal cortex

BACKGROUND: Integrating information from the different senses markedly enhances the detection and identification of external stimuli. Compared with unimodal inputs, semantically and/or spatially congruent multisensory cues speed discrimination and improve reaction times. Discordant inputs have the opposite effect, reducing performance and slowing responses. These behavioural features of crossmodal processing appear to have parallels in the response properties of multisensory cells in the superior colliculi and cerebral cortex of non-human mammals. Although spatially concordant multisensory inputs can produce a dramatic, often multiplicative, increase in cellular activity, spatially disparate cues tend to induce a profound response depression. RESULTS: Using functional magnetic resonance imaging (fMRI), we investigated whether similar indices of crossmodal integration are detectable in human cerebral cortex, and for the synthesis of complex inputs relating to stimulus identity. Ten human subjects were exposed to varying epochs of semantically congruent and incongruent audio-visual speech and to each modality in isolation. Brain activations to matched and mismatched audio-visual inputs were contrasted with the combined response to both unimodal conditions. This strategy identified an area of heteromodal cortex in the left superior temporal sulcus that exhibited significant supra-additive response enhancement to matched audio-visual inputs and a corresponding sub-additive response to mismatched inputs. CONCLUSIONS: The data provide fMRI evidence of crossmodal binding by convergence in the human heteromodal cortex. They further suggest that response enhancement and depression may be a general property of multisensory integration operating at different levels of the neuroaxis and irrespective of the purpose for which sensory inputs are combined.     

Saccadic Reaction Times to Audiovisual Stimuli Show Effects of Oscillatory Phase Reset

Initiating an eye movement towards a suddenly appearing visual target is faster when an accessory auditory stimulus occurs in close spatiotemporal vicinity. Such facilitation of saccadic reaction time (SRT) is well-documented, but the exact neural mechanisms underlying the crossmodal effect remain to be elucidated. From EEG/MEG studies it has been hypothesized that coupled oscillatory activity in primary sensory cortices regulates multisensory processing. Specifically, it is assumed that the phase of an ongoing neural oscillation is shifted due to the occurrence of a sensory stimulus so that, across trials, phase values become highly consistent (phase reset). If one can identify the phase an oscillation is reset to, it is possible to predict when temporal windows of high and low excitability will occur. However, in behavioral experiments the pre-stimulus phase will be different on successive repetitions of the experimental trial, and average performance over many trials will show no signs of the modulation. Here we circumvent this problem by repeatedly presenting an auditory accessory stimulus followed by a visual target stimulus with a temporal delay varied in steps of 2 ms. Performing a discrete time series analysis on SRT as a function of the delay, we provide statistical evidence for the existence of distinct peak spectral components in the power spectrum. These frequencies, although varying across participants, fall within the beta and gamma range (20 to 40 Hz) of neural oscillatory activity observed in neurophysiological studies of multisensory integration. Some evidence for high-theta/alpha activity was found as well. Our results are consistent with the phase reset hypothesis and demonstrate that it is amenable to testing by purely psychophysical methods. Thus, any theory of multisensory processes that connects specific brain states with patterns of saccadic responses should be able to account for traces of oscillatory activity in observable behavior.     

Multimodal sensory integration in weakly electric fish: a behavioral account.

The ability to integrate multisensory information is a fundamental characteristic of the brain serving to enhance the detection and identification of external stimuli. Weakly electric fish employ multiple senses in their interactions with one another and with their inanimate environment (electric, visual, acoustic, mechanical, chemical, thermal, and hydrostatic pressure) and also generate signals using some of the same stimulus energies (electric, acoustic, visual, mechanical). A brief overview provides background on the sensory and motor channels available to the fish followed by an examination of how weakly electric fish 'benefit' from integrating various stimulus modalities that assist in prey detection, schooling, foraging, courtship, and object location. Depending on environmental conditions, multiple sensory inputs can act synergistically and improve the task at hand, can be redundant or contradictory, and can substitute for one another. Over time, in repeated encounters with familiar surrounds, loss of one modality can be compensated for through learning. Studies of neuronal substrates and an understanding of the computational algorithms that underlie multisensory integration ought to expose the physiological corollaries to widely published concepts such as internal representation, sensory expectation, sensory generalization, and sensory transfer.     

Neonatal Restriction of Tactile Inputs Leads to Long-Lasting Impairments of Cross-Modal Processing

Optimal behavior relies on the combination of inputs from multiple senses through complex interactions within neocortical networks. The ontogeny of this multisensory interplay is still unknown. Here, we identify critical factors that control the development of visual-tactile processing by combining in vivo electrophysiology with anatomical/functional assessment of cortico-cortical communication and behavioral investigation of pigmented rats. We demonstrate that the transient reduction of unimodal (tactile) inputs during a short period of neonatal development prior to the first cross-modal experience affects feed-forward subcortico-cortical interactions by attenuating the cross-modal enhancement of evoked responses in the adult primary somatosensory cortex. Moreover, the neonatal manipulation alters cortico-cortical interactions by decreasing the cross-modal synchrony and directionality in line with the sparsification of direct projections between primary somatosensory and visual cortices. At the behavioral level, these functional and structural deficits resulted in lower cross-modal matching abilities. Thus, neonatal unimodal experience during defined developmental stages is necessary for setting up the neuronal networks of multisensory processing.     

Effects of cue-triggered expectation on cortical processing of taste

Animals are not passive spectators of the sensory world in which they live. In natural conditions they often sense objects on the bases of expectations initiated by predictive cues. Expectation profoundly modulates neural activity by altering the background state of cortical networks and modulating sensory processing. The link between these two effects is not known. Here, we studied how cue-triggered expectation of stimulus availability influences processing of sensory stimuli in the gustatory cortex (GC). We found that expected tastants were coded more rapidly than unexpected stimuli. The faster onset of sensory coding related to anticipatory priming of GC by associative auditory cues. Simultaneous recordings and pharmacological manipulations of GC and basolateral amygdala revealed the role of top-down inputs in mediating the effects of anticipatory cues. Altogether, these data provide a model for how cue-triggered expectation changes the state of sensory cortices to achieve rapid processing of natural stimuli.