Bridging the gap: Synaesthesia and multisensory processes.

Although the study of multisensory processes and the study of synaesthesia both represent burgeoning fields of inquiry, there has been little attempt to bridge between these two research topics. This is somewhat surprising, as these two are undoubtedly heavily interrelated from both a psychological and neuroscience perspective. The goal of the present issue is explore these interrelationships, and focus on the state-of-the-field within these closely connected research domains. Synaesthesia is a fascinating phenomenon in which perception of different senses are mixed. For synaesthetes, specific sensory stimuli (automatically) trigger additional perceptual experiences that are not normally perceived by non-synaesthetes (e.g. letters elicit colours or words elicit tastes). Theories about the cause of this condition range from altered pruning and changes in anatomical connectivity to the disinhibition of typical feedback mechanisms common to all of us. The study of synaesthesia by itself has yielded many valuable insights over the past few decades (e.g. Ward, 2013), but a more current question in the field is how synaesthesia relates to other processes such as multisensory integration and crossmodal processing. The field of multisensory processing is in a period of rapid growth. Although the behavioral and perceptual benefits attributable to having information available from multiple senses has been long known (Welch and Warren, 1980; Sumby and Pollack, 1954), only recently has there been a concerted effort to better understand the psychological phenomena associated with multisensory functions and to link them to their neural correlates (Stein, 2012; Murray and Wallace, 2012). It is now well established that information from each of the senses converges at many sites within the brain, and that the product of this convergence is often a response that looks very different from the sum of the responses to the individual senses. Recent strides within this area have extended our understanding of the neural correlates of multisensory integration from the single cell to more distributed neural representations, have begun to provide evidence on the role of dynamic binding mechanisms in multisensory processing (e.g. Cappe et al., 2009, 2012; Carriere et al., 2008; Maier et al., 2008; Royal et al., 2009; Senkowski et al., 2007) and started to establish strong functional links between neural activity profiles and changes in behavior and perception (e.g. Murray et al., 2016). It is such a context that research into synaesthesia has become a focus for some interested in multisensory processes. With regard to relating the fields of multisensory processing and synaesthesia, one of the important issues that is being discussed is whether synaesthesia can be regarded as an extreme