Abstract
There has been longstanding interest from both experimental psychologists and cognitive neuroscientists in the potential modulatory role of various top–down factors on multisensory integration/perception in humans. One such top–down influence, often referred to in the literature as the ‘unity assumption,’ is thought to occur in those situations in which an observer considers that various of the unisensory stimuli that they have been presented with belong to one and the same object or event (Welch and Warren, 1980). Here, we review the possible factors that may lead to the emergence of the unity assumption. We then critically evaluate the evidence concerning the consequences of the unity assumption from studies of the spatial and temporal ventriloquism effects, from the McGurk effect, and from the Colavita visual dominance paradigm. The research that has been published to date using these tasks provides support for the claim that the unity assumption influences multisensory perception under at least a subset of experimental conditions. We then consider whether the notion has been superseded in recent years by the introduction of priors in Bayesian causal inference models of human multisensory perception. We suggest that the prior of common cause (that is, the prior concerning whether multisensory signals originate from the same source or not) offers the most useful way to quantify the unity assumption as a continuous cognitive variable.
Highlights
The ‘unity assumption’ is an observer’s assumption, or belief, that two or more unisensory cues belong together
We will critically assess whether research on the topics of crossmodal correspondences and semantic congruency should be considered as relevant to the debate concerning the role of the unity assumption in multisensory integration
A particular prior term may be able to match to a particular source of unity assumption as discussed in the section “Factors Leading to the Unity Assumption”
Summary
The ‘unity assumption’ is an observer’s assumption, or belief, that two or more unisensory cues belong together (i.e., that they come from the same object or event1, Welch and Warren, 1980, 1986; Spence, 2007; Chen and Vroomen, 2013) Such an assumption, or belief, on the part of the observer serves as a cognitive modulator of multisensory integration, leading to the empirical observations described as the ‘unity effect’. MacDonald, 1976), and the Colavita visual dominance effect (e.g., Colavita, 1974; Spence et al, 2011) These empirical results help answer the question of the conditions under which the unity assumption is formed and modulates multisensory integration in humans. We highlight some of the key questions awaiting future research in this area
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