Review
Can synaesthesia research inform cognitive science?

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The renaissance of synaesthesia research has produced many insights regarding the aetiology and mechanisms that might underlie this intriguing phenomenon, which abnormally binds features between and within modalities. Synaesthesia is interesting in its own right, but whether it contributes to our knowledge of neurocognitive systems that underlie non-synaesthete experience is an open question. In this review, we show that results from the field of synaesthesia can constrain cognitive theories in numerical cognition, automaticity, crossmodal interaction and awareness. Therefore, research of synaesthesia provides a unique window into other domains of cognitive neuroscience. We conclude that the study of synaesthesia could advance our understanding of the normal and abnormal human brain and cognition.

Introduction

‘Vermillion has a sound like a tuba and a parallel can be drawn with a loud drum beat.’

Vassily Kandinsky (1866–1944)

As the quote by the famous artist and music–colour synaesthete Kandinsky [1] demonstrates, synaesthesia is a fascinating phenomenon in which sensory experiences (e.g. sound or taste) or concepts (e.g. word, number or time) automatically evoke additional precepts (e.g. colour) [2].

The majority of experimental work that deals with synaesthesia, as well as review articles 3, 4, 5, has focused on understanding the phenomenon in isolation. For example, research has attempted to reveal the mechanism(s) that underlies synaesthesia 5, 6, 7, or the stage(s) of processing on which the synaesthetic experience depends 8, 9, 10, 11. However, independent of this line of research, the study of synaesthesia might help improve our understanding of the non-synaesthetic mind. Understanding the phenomenon requires forays into fields such as perception, awareness, representation, development and neuroanatomy and, therefore, it provides a good testing ground for many ideas and theories about different areas of cognitive science. The rapid growth of the field of synaesthesia in recent years enables us to examine the possible contribution of synaesthesia outside the field of synaesthesia per se.

Strictly speaking, synaesthesia is not a normal phenomenon because it exists in ∼4% of the population [12]. However, it should be noted that, aside from their exceptional crossmodal experience, synaesthetes have normal cognitive abilities and brain activation. For example, brain imaging of synaesthetes shows elevated activation in areas that correspond to the synaesthetic experience but not in other brain areas 13, 14. In addition, in a variety of cognitive tasks and domains, independent of the synaesthetic experience, synaesthetes show effects that are comparable with those of non-synaesthete participants 8, 9, 15, 16, and the incidence of mental illnesses or neurological deficits among synaesthetes is the same as in the normal population [17].

Many studies have shown that the synaesthetic experience is triggered involuntarily (e.g. Refs 6, 8, 10, 11, 15, 17). One particular aspect of synaesthetic experience, which involves numbers – whether digit–colour (Figure 1) or number–form synaesthesia (Figure 2) – has been widely investigated 9, 18, 19, 20, 21, 22, 23. In what follows, we present several examples that are related to numerical cognition, automaticity and crossmodal interaction. We present themes that are of interest to each field of study, and we show how research into synaesthesia can advance our understanding of these subjects. Other topics that will be discussed are awareness (Box 1), synaesthetic-like experience such as hallucinations, and sensory deprivation (Box 2).

We hope that the current review will encourage researchers in the field of cognitive sciences to use synaesthesia as an additional tool for studying the human brain and cognition.

Section snippets

The mental number line

Findings in the study of numerical cognition are frequently interpreted in terms of analogue operations in which the representation and comparison of numerical magnitude exploit a number line [24]. On this line, numbers that are numerically close are represented by points that are spatially close to each other and numbers that are numerically far apart are represented by points that are spatially far apart from each other [25] (see also Refs 26, 27). However, certain experimental effects,

Potential underlying mechanism for automaticity

Much discussion and research in cognitive psychology revolves around the concept of automaticity. Many researchers distinguish between two modes of human operation: one is automatic–reflexive and the other is controlled–voluntary. This distinction characterises various areas of cognitive functioning, such as language [37], memory [38] and visual-spatial orienting [39]. A given process might be automatic owing to a specialized neuronal mechanism(s) 40, 41. Such a dedicated operation works as a

Synaesthesia and crossmodal interaction

Several studies showed that the principle organization behind pitch–colour (hearing tone induces a colour) [48] and even letter–colour [49] connections in synaesthetes and non-synaesthetes might be, at least partly, shared. This evidence suggests that synaesthetes recruit the same mechanism as non-synaesthetes [48], but this use might be quantitatively different. However, it should be examined whether this holds true for all types of synaesthesia. As was suggested in the previous section,

Concluding remarks

It is clear that the knowledge gained from research on synaesthesia is not confined to the understanding of synaesthesia per se; rather, it can be used to constrain psychological theories in other areas. Moreover, we believe that the study of synaesthesia can contribute to additional areas that are not covered in the current article, such as language [57], emotion, imagery and attention. An integrative approach to perception and cognition requires an understanding that, by studying subjects

Acknowledgements

We wish to thank Kathrin Cohen Kadosh, Desiree Meloul, Riel Meloul, Guilherme Wood and especially David E.J. Linden, Noam Sagiv, Vincent Walsh and the anonymous reviewers for their very helpful suggestions. This work was partly supported by a research fellowship to R.C.K. from the International Brain Research Organization and by a grant to A.H. from the Israel Science Foundation (grant 431/05).

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