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Multisensory processes are fundamental in scaffolding perception, cognition, learning, and behavior. How and when stimuli from different sensory modalities are integrated rather than treated as separate entities is poorly understood. We review how the relative reliance on stimulus characteristics versus learned associations dynamically shapes multisensory processes. We illustrate the dynamism in multisensory function across two timescales: one long term that operates across the lifespan and one short term that operates during the learning of new multisensory relations. In addition, we highlight the importance of task contingencies. We conclude that these highly dynamic multisensory processes, based on the relative weighting of stimulus characteristics and learned associations, provide both stability and flexibility to brain functions over a wide range of temporal scales. [ABSTRACT FROM AUTHOR]

It has been nearly 10 years since Ghazanfar and Schroeder (2006) proposed that the neocortex is essentially multisensory in nature. However, it is only recently that sufficient and hard evidence that supports this proposal has accrued. We review evidence that activity within the human primary visual cortex plays an active role in multisensory processes and directly impacts behavioural outcome. This evidence emerges from a full pallet of human brain imaging and brain mapping methods with which multisensory processes are quantitatively assessed by taking advantage of particular strengths of each technique as well as advances in signal analyses. Several general conclusions about multisensory processes in primary visual cortex of humans are supported relatively solidly. First, haemodynamic methods (fMRI/PET) show that there is both convergence and integration occurring within primary visual cortex. Second, primary visual cortex is involved in multisensory processes during early post-stimulus stages (as revealed by EEG/ERP/ERFs as well as TMS). Third, multisensory effects in primary visual cortex directly impact behaviour and perception, as revealed by correlational (EEG/ERPs/ERFs) as well as more causal measures (TMS/tACS). While the provocative claim of Ghazanfar and Schroeder (2006) that the whole of neocortex is multisensory in function has yet to be demonstrated, this can now be considered established in the case of the human primary visual cortex. [ABSTRACT FROM AUTHOR]

Abstract: Recent multisensory research has emphasized the occurrence of early, low-level interactions in humans. As such, it is proving increasingly necessary to also consider the kinds of information likely extracted from the unisensory signals that are available at the time and location of these interaction effects. This review addresses current evidence regarding how the spatio-temporal brain dynamics of auditory information processing likely curtails the information content of multisensory interactions observable in humans at a given latency and within a given brain region. First, we consider the time course of signal propagation as a limitation on when auditory information (of any kind) can impact the responsiveness of a given brain region. Next, we overview the dual pathway model for the treatment of auditory spatial and object information ranging from rudimentary to complex environmental stimuli. These dual pathways are considered an intrinsic feature of auditory information processing, which are not only partially distinct in their associated brain networks, but also (and perhaps more importantly) manifest only after several tens of milliseconds of cortical signal processing. This architecture of auditory functioning would thus pose a constraint on when and in which brain regions specific spatial and object information are available for multisensory interactions. We then separately consider evidence regarding mechanisms and dynamics of spatial and object processing with a particular emphasis on when discriminations along either dimension are likely performed by specific brain regions. We conclude by discussing open issues and directions for future research. [Copyright &y& Elsevier]

Multisensory memory traces established via single-trial exposures can impact subsequent visual object recognition. This impact appears to depend on the meaningfulness of the initial multisensory pairing, implying that multisensory exposures establish distinct object representations that are accessible during later unisensory processing. Multisensory contexts may be particularly effective in influencing auditory discrimination, given the purportedly inferior recognition memory in this sensory modality. The possibility of this generalization and the equivalence of effects when memory discrimination was being performed in the visual vs. auditory modality were at the focus of this study. First, we demonstrate that visual object discrimination is affected by the context of prior multisensory encounters, replicating and extending previous findings by controlling for the probability of multisensory contexts during initial as well as repeated object presentations. Second, we provide the first evidence that single-trial multisensory memories impact subsequent auditory object discrimination. Auditory object discrimination was enhanced when initial presentations entailed semantically congruent multisensory pairs and was impaired after semantically incongruent multisensory encounters, compared to sounds that had been encountered only in a unisensory manner. Third, the impact of single-trial multisensory memories upon unisensory object discrimination was greater when the task was performed in the auditory vs. visual modality. Fourth, there was no evidence for correlation between effects of past multisensory experiences on visual and auditory processing, suggestive of largely independent object processing mechanisms between modalities. We discuss these findings in terms of the conceptual short term memory (CSTM) model and predictive coding. Our results suggest differential recruitment and modulation of conceptual memory networks according to the sensory task at hand. [ABSTRACT FROM AUTHOR]

Abstract: Simple reaction times (RTs) to auditory–somatosensory (AS) multisensory stimuli are facilitated over their unisensory counterparts both when stimuli are delivered to the same location and when separated. In two experiments we addressed the possibility that top-down and/or task-related influences can dynamically impact the spatial representations mediating these effects and the extent to which multisensory facilitation will be observed. Participants performed a simple detection task in response to auditory, somatosensory, or simultaneous AS stimuli that in turn were either spatially aligned or misaligned by lateralizing the stimuli. Additionally, we also informed the participants that they would be retrogradely queried (one-third of trials) regarding the side where a given stimulus in a given sensory modality was presented. In this way, we sought to have participants attending to all possible spatial locations and sensory modalities, while nonetheless having them perform a simple detection task. Experiment 1 provided no cues prior to stimulus delivery. Experiment 2 included spatially uninformative cues (50% of trials). In both experiments, multisensory conditions significantly facilitated detection RTs with no evidence for differences according to spatial alignment (though general benefits of cuing were observed in Experiment 2). Facilitated detection occurs even when attending to spatial information. Performance with probes, quantified using sensitivity (d′), was impaired following multisensory trials in general and significantly more so following misaligned multisensory trials. This indicates that spatial information is not available, despite being task-relevant. The collective results support a model wherein early AS interactions may result in a loss of spatial acuity for unisensory information. [ABSTRACT FROM AUTHOR]

Abstract: The information conveyed by our senses can be combined to facilitate perception and behaviour. One focus of recent research has been on the factors governing such facilitatory multisensory interactions. The spatial register of neuronal receptive fields (RFs) appears to be a prerequisite for multisensory enhancement. In terms of auditory–somatosensory (AS) interactions, facilitatory effects on simple reaction times and on brain responses have been demonstrated in caudo-medial auditory cortices, both when auditory and somatosensory stimuli are presented to the same spatial location and also when they are separated by 100° in frontal space. One implication is that these brain regions contain large spatial RFs. The present study further investigated this possibility and, in particular, the question of whether AS interactions are restricted to frontal space, since recent research has revealed some fundamental differences between the sensory processing of stimuli in front and rear space. Twelve participants performed a simple reaction time task to auditory, somatosensory, or simultaneous auditory–somatosensory stimuli. The participants placed one of their arms in front of them and the other behind their backs. Loudspeakers were placed close to each hand. Thus, there were a total of eight stimulus conditions – four unisensory and four multisensory – including all possible combinations of posture and loudspeaker location. A significant facilitation of reaction times (RTs), exceeding that predicted by probability summation, was obtained following multisensory stimulation, irrespective of whether the stimuli were in spatial register or not. These results are interpreted in terms of the likely RF organization of previously identified auditory–somatosensory brain regions. [Copyright &y& Elsevier]