Bimodal discrimination tasks involving different hierarchical processing levels reveal a late stage interaction between multisensory integration and attention

Multisensory integration and attention have been studied independently and a vast \namount of literature exists for both phenomena. Researchers have recently raised \nquestions, however, as to how these two processes interact. For example, does \nmultisensory integration occur automatically without the need for attention; or does \nintegrative processing require attention for it to occur? If the latter, do attention and \nintegration act in parallel throughout all information processing levels, or does one \noperation need to exist to advance the other? The present study sought to answer these \nquestions through a series of within-subject tasks spanning multiple layers of the \nprocessing hierarchy. Forty-five participants completed three tasks involving audiovisual, \nintegrated stimuli in which they discriminated the location of a visual target stimulus \nfrom nontarget distractors while being simultaneously presented with congruent auditory \ntones. The first task involved the discrimination of shapes and was shown to be preattentive \nin nature, with no facilitatory effect being observed in response to \nsimultaneously presented visual and auditory stimuli. The second task involved the \ndiscrimination of die-point stimuli, which required high attentional demand. A trend \ntowards intersensory facilitation was observed in this task, but was not significant. The \nthird and final task involved the discrimination of integrated shapes and die-points, which \nalso required a high attentional load. Findings from this conjunction search revealed a \nsurprising reversal of intersensory facilitation. These results suggest that attention has a \nlimited capacity in terms of multisensory processing, and that specific intersensory \nfacilitation requires a unique amount of attentional involvement. Results are discussed in \nterms of feature integration theory, the perceptual load hypothesis, and attentional inhibition. This study also highlights the need for multisensory research to pay close \nattention to the influence of methodology, task sensitivity, and cognitive hierarchy when \ninterpreting results.