Until relatively recently, researchers believed that synaesthetic sensations and their triggers were arbitrarily paired, and entirely idiosyncratic from one synaesthete to the next. Put differently, they believed that no two synaesthetes would have similar experiences from the same set of triggers, unless this had occurred by chance. This position likely arose because, prior to the internet, it was extremely difficult to recruit more than a small handful of synaesthete participants, and on the surface, synaesthetes do tend to disagree on their (...) associations when compared one-on-one. However, recent advances in communication have allowed researchers access to far larger groups of synaesthete participants, and this has changed our opinion about how similar synaesthetes might in fact be. A dedicated body of research has now shown that synaesthetic associations are built, to some extent, on a set of unconscious 'rules', shared across synaesthetes. These rules dictate the choice of sensation for certain triggers, and allow us to see patterns emerging which, although not absolute, provide a type of coherence across synaesthetes as a whole. I shall also show that some of these underlying rules operate in the population at large, to allow all individuals to make intuitive pairings across the senses. This chapter describes some of these rules, across a range of different modalities, and across different variants of synaesthesia. (shrink)

This chapter provides a review of the hypothesis that synesthetic-like perception is present in infants and toddlers. Infants and very young children exhibit evidence of functional hyperconnectivity between the senses, much of which is reminiscent of the cross-sensory associations observed in synaesthetic adults. As most of these cross-sensory correspondances cannot be easily explained by learning, it is likely that these represent natural associations between the senses. In average adults, these 'natural associations' are felt only intuitively rather than explicitly. These observations (...) have led to the proposal of the 'neonatal synaesthesia hypothesis', which purports that all individuals are born synaesthetic, with explicit conscious perception of these natural cross-modal associations dissipating over development in typical individuals. This dissipation is likely the result of experience-dependent synaptic pruning and/or inhibition of cross-sensory neural connections. At the same time, cross-modal associations matching those common in the environment might be assumed to be learned. This hypothesis is re-evaluated in light of recent research findings, and is examined in the context of current evolutionary models of neuronal recycling and emerging evidence of longitudinal changes in children with synaesthesia. (shrink)

(1) The induced colours led to perceptual grouping and pop-out, (2) a grapheme rendered invisible through ‘crowding’ or lateral masking induced synaesthetic colours — a form of blindsight — and (3) peripherally presented graphemes did not induce colours even when they were clearly visible. Taken collectively, these and other experiments prove conclusively that synaesthesia is a genuine percep- tual phenomenon, not an effect based on memory associations from childhood or on vague metaphorical speech. We identify different subtypes of number–colour synaesthesia (...) and propose that they are caused by hyperconnectivity between col- our and number areas at different stages in processing; lower synaesthetes may have cross-wiring (or cross-activation) within the fusiform gyrus, whereas higher synaesthetes may have cross-activation in the angular gyrus. This hyperconnec- tivity might be caused by a genetic mutation that causes defective pruning of con- nections between brain maps. The mutation may further be expressed selectively (due to transcription factors) in the fusiform or angular gyri, and this may explain the existence of different forms of synaesthesia. If expressed very diffusely, there may be extensive cross-wiring between brain regions that represent abstract concepts, which would explain the link between creativity, metaphor and synaesthesia (and the higher incidence of synaesthesia among artists and poets). Also, hyperconnectivity between the sensory cortex and amygdala would explain the heightened aversion synaesthetes experience when seeing numbers printed in the ‘wrong’ colour. Lastly, kindling (induced hyperconnectivity in the temporal lobes of temporal lobe epilepsy [TLE] patients) may explain the purported higher incidence of synaesthesia in these patients. We conclude with a synaesthesia-based theory. (shrink)