Why study atypical multisensory development?
The relation between unisensory and multisensory processing impairments in developmental disorders
While the majority of the chapters in this volume examine the typical development of multisensory processes, here we will review the literature pointing to multisensory processing abnormalities in atypically developing individuals. We will consider developmental disorders that are, for the most part, diagnosed on the basis of behavioural characteristics and are characterized by specific cognitive profiles. We will not consider neurological or genetic markers, or a sensory receptor impairment—for coverage of atypical multisensory processing in individuals with congenital and late blindness see Chapter 13 by Roder. We will focus specifically on three developmental disorders, namely developmental coordination disorder (DCD), autism spectrum disorder (ASD), and developmental dyslexia (DD) in which abnormalities point most clearly towards a potential multisensory impairment. Because other disorders, such as attention deficit hyperactivity disorder (ADHD), are also suggestive of sensory processing impairments, we will refer to these where appropriate throughout.
As already stated, there have been relatively few studies of multisensory impairments in developmental disorders. We believe that there are now a number of converging reasons to pursue the question of multisensory impairment in atypical development more seriously. The broad literature on multisensory perceptual processes (cf. Calvert et al. 2004; Spence and Driver 2004; Stein and Meredith 1993) demonstrates quite conclusively that our senses do not function in isolation, either at the level of basic perceptual processes or attention. Consequently, the growing literature investigating sensory processing impairments across various disorders will have to acknowledge, sooner or later, that, even if the ontogeny of a sensory processing problem is a basic unisensory deficit, difficulties with one sense will have important consequences for the way the other senses function, and importantly how multisensory development proceeds. Strong evidence for the developmental impact of a unisensory deficit on the development of multisensory processes can be seen from the literatures investigating sensory deprivation in developing animals (Chapter 14 by Wallace et al.) and in human infants and children with sensory loss (Chapter 13 by Roder). Similarly, a deficit with multisensory integration can have consequences for sensory processing when that is measured with respect to a single sensory modality.
-  We have not devoted an entire section to ADHD as it is as yet unclear whether reported atypical responsesto sensory information arise from sensory processing abnormalities per se or from difficulties with thetop-down control processes implicated in the hyperactive and inattentive difficulties found in this disorder(Friedman-Hill et al. 2010).