Do synaesthesia and mental imagery tap into similar cross-modal processes?

Abstract

Synaesthesia has previously been linked with imagery abilities, although an understanding of a causal role for mental imagery in broader synaesthetic experiences remains elusive. This can be partly attributed to our relatively poor understanding of imagery in sensory domains beyond vision. Investigations into the neural and behavioural underpinnings of mental imagery have nevertheless identified an important role for imagery in perception, particularly in mediating cross-modal interactions. However, the phenomenology of synaesthesia gives rise to the assumption that associated cross-modal interactions may be encapsulated and specific to synaesthesia. As such, evidence for a link between imagery and perception may not generalize to synaesthesia. Here, we present results that challenge this idea: first, we found enhanced somatosensory imagery evoked by visual stimuli of body parts in mirror-touch synaesthetes, relative to other synaesthetes or controls. Moreover, this enhanced imagery generalized to tactile object properties not directly linked to their synaesthetic associations. Second, we report evidence that concurrent experience evoked in grapheme—colour synaesthesia was sufficient to trigger visual-to-tactile correspondences that are common to all. Together, these findings show that enhanced mental imagery is a consistent hallmark of synaesthesia, and suggest the intriguing possibility that imagery may facilitate the cross-modal interactions that underpin synaesthesic experiences. This article is part of a discussion meeting issue ‘Bridging senses: novel insights from synaesthesia’.

Language evolution: examining the link between cross-modality and aggression through the lens of disorders

“…It is thus not surprising that synaesthetes would have superior understanding of unfamiliar sound-symbolic foreign words, as found in Bankieris & Simner's [128] experiment, suggesting that sound symbolism relies on the type of cross-modal integration that characterizes synaesthesia to an exaggerated degree. Overall, the enhanced cross-modality in synaesthesia has been ascribed to the hyper-connectivity of a synaesthete's brain ([114,115,117,129]; more on this in §5).…”

Bridging senses: novel insights from synaesthesia

“…A second experiment, involving 16 English-speaking grapheme-colour synaesthetes and 22 controls, found that induced colours in synaesthesia evoked cross-modal correspondences of weight, considered a tactile property of objects, in a manner that was consistent with that experienced for veridical colours (red/ blue perceived as heaviest and yellow as lightest). Thus, the authors provide further evidence of enhanced imagery as one of the hallmarks of synaesthesia and stress the importance of a cross-modal perspective on this topic [15].…”

“…Indeed, two of the articles of this special issue are specifically concerned with what synaesthesia can tell us about imagery, and vice versa. O'Dowd et al [15] argue that understanding of these relationships has been limited by a lack of knowledge about the nature of imagery for royalsocietypublishing.org/journal/rstb Phil. Trans.…”

“…In one experiment, they investigated three groups of English-speaking participants (n = 10 for each): mirror-touch synaesthetes (see also §6), individuals with other forms of synaesthesia, and non-synaesthetic controls. For the mirror-touch synaesthesia group, there were higher ratings of imagined touch when viewing regions of the body, as well as enhanced tactile imagery of object properties and materials that were not directly implicated in the synaesthetic experience (for example, on being asked to imagine squeezing a wet sponge, synaesthetes reported higher ratings for the vividness of the experience) [15]. A second experiment, involving 16 English-speaking grapheme-colour synaesthetes and 22 controls, found that induced colours in synaesthesia evoked cross-modal correspondences of weight, considered a tactile property of objects, in a manner that was consistent with that experienced for veridical colours (red/ blue perceived as heaviest and yellow as lightest).…”

Neuroplasticity and crossmodal connectivity in the normal, healthy brain.

“…Even more interestingly, sights of touching evoke activity in somatosensory cortex (Hansson et al, 2009), visual cues to an object's surface properties evoke activity in secondary somatosensory cortex (Sun, Welchman, Chang, & Luca, 2016), and the mere sight of photographs (Proverbio, D'Aniello, Adorni, & Zani, 2011), and lipreading (watching a speaker's lips during face-to-face conversation; Calvert et al, 1997) activate auditory cortex. These particular findings together suggest the role of crossmodal mental imagery, the formation of mental image in a sensory modality from the stimulation or experience in another sensory modality, in perception and neuroplasticity: what people imagine in their minds can affect how they perceive the world, and how the sensory cortex reorganizes in absence of actual stimulation (e.g., Berger & Ehrsson, 2018;Nanay, 2018;O'Dowd, Cooney, McGovern, & Newell, 2019;Schmidt & Blankenburg, 2019;Spence & Deroy, 2013;Wise, Frangos, & Komisaruk, 2016).…”