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

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'

Exploring the relationship between grapheme colour-picking consistency and mental imagery

Previous research has indicated a potential link between mental imagery and synaesthesia. However, these findings are mainly based on imagery self-report measures and recruitment of self-selected synaesthetes. To avoid issues of self-selection and demand effects we recruited participants from the general population, rather than synaesthetes specifically, and used colour-picking consistency tests for letters and numbers to assess a "synaesthete-like" experience. Mental imagery ability and mental rotation ability were assessed using both self-report measures and behavioural assessments. Consistency in colour-picking for letters (but not numbers) was predicted by performance on the visual mental imagery task, but not by a mental rotation task or self-report measures. Using the consistency score as a proxy measure of grapheme-colour synaesthesia, we provide more evidence for the suggestion that synaesthetic experience is associated with enhanced mental imagery, even when participants are naïve to the research topic

The prevalence and cognitive profile of sequence-space synaesthesia

People with sequence-space synaesthesia visualize sequential concepts such as numbers and time as an ordered pattern extending through space. Unlike other types of synaesthesia, there is no generally agreed objective method for diagnosing this variant or separating it from potentially related aspects of cognition. We use a recently-developed spatial consistency test together with a novel questionnaire on naïve samples and estimate the prevalence of sequence-space synaesthesia to be around 8.1% (Study 1) to 12.8% (Study 2). We validate our test by showing that participants classified as having sequence-space synaesthesia perform differently on lab-based tasks. They show a spatial Stroop-like interference response, they show enhanced detection of low visibility Gabor stimuli, they report more use of visual imagery, and improved memory for certain types of public events. We suggest that sequence-space synaesthesia develops from a particular neurocognitive profile linked both to greater visual imagery and enhanced visual perception

Better Together? The Cognitive Advantages of Synaesthesia for Time, Numbers and Space

Synaesthesia for time, numbers and space (TNS synaesthesia) is thought to have costs and benefits for recalling and manipulating time and number. There are two competing theories about how TNS synaesthesia affects cognition. The ‘magnitude’ account predicts TNS synaesthesia may affect cardinal magnitude judgements, whereas the ‘sequence’ account suggests it may affect ordinal sequence judgements and could rely on visuospatial working memory. We aimed to comprehensively assess the cognitive consequences of TNS synaesthesia and distinguish between these two accounts. TNS synaesthetes, grapheme-colour synaesthetes and non-synaesthetes completed a behavioural task battery. Three tasks involved cardinal and ordinal comparisons of temporal, numerical and spatial stimuli; we also examined visuospatial working memory. TNS synaesthetes were significantly more accurate than non-synaesthetes in making ordinal judgements about space. This difference was explained by significantly higher visuospatial working memory accuracy. Our findings demonstrate an advantage of TNS synaesthesia which is more in line with the sequence account

Beyond visual imagery: How modality-specific is enhanced mental imagery in synesthesia?

Synesthesia based in visual modalities has been associated with reports of vivid visual imagery. We extend this finding to consider whether other forms of synesthesia are also associated with enhanced imagery, and whether this enhancement reflects the modality of synesthesia. We used self‐report imagery measures across multiple sensory modalities, comparing synesthetes’ responses (with a variety of forms of synesthesia) to those of nonsynesthete matched controls. Synesthetes reported higher levels of visual, auditory, gustatory, olfactory and tactile imagery and a greater level of imagery use. Furthermore, their reported enhanced imagery is restricted to the modalities involved in the individual’s synesthesia. There was also a relationship between the number of forms of synesthesia an individual has, and the reported vividness of their imagery, highlighting the need for future research to consider the impact of multiple forms of synesthesia. We also recommend the use of behavioral measures to validate these self‐report findings

Do sequence-space synaesthetes have better spatial imagery skills? Yes, but there are individual differences

People with sequence-space synaesthesia perceive sequences (e.g. numbers, months, letters) as spatially extended forms. Here, we ask whether sequence-space synaesthetes have advantages in visuo-spatial skills such as mental rotation. Previous studies addressing this question have produced mixed results with some showing mental rotation advantages (Simner et al. in Cortex 45:1246–1260, 2009; Brang et al. in Cogn Process, 2013), but one that did not (Rizza and Price in Cogn Process 13:299–303, 2012). We tested this hypothesis again with a new group of sequence-space synaesthetes, and we also tested a range of individual differences that might have caused this conflict across previous studies. Specifically, we tested: years of education, visual imagery ability, nature of forms (2D or 3D representation of sequences), number of forms (e.g. for months, days, numbers), and tendency to project sequences into external space versus the mind’s eye. We found yet again that synaesthetes had enhanced abilities in mental rotation compared to controls, but that one individual difference in synaesthetes (the ability to project forms into space) was especially linked to performance. We also found that synaesthetes self-reported higher visual imagery than controls (Price in Cortex 45:1229–1245, 2009; Mann et al. in Conscious Cognit 18:619–627, 2009; Rizza and Price 2012). Overall, our data support previous studies showing superior imagery reports (Price 2009) and mental rotation (Simner et al. 2009; Brang et al. 2013) in sequence-space synaesthetes, and we suggest that one previous failure to replicate (Rizza and Price 2012) might be explained by individual differences among synaesthetes recruited for testing

Synaesthesia in children

Synaesthesia is a developmental condition that triggers phantom sensations (e.g., colours or tastes) when exposed to everyday stimuli such as graphemes, music, and pain. Yet, despite synaesthesia being a developmental condition, there is very little work in children to date. In this thesis, I explore two types of synaesthesia in children aged 6-10 years old; grapheme-colour synaesthesia (letters and numbers elicit colours) and grapheme-personality synaesthesia (letters and numbers elicit personalities). I first use tests designed specifically for children to identify individuals with these types of synaesthesia. Here I tested children with and without synaesthesia who had been identified from a very large screening endeavour, called MULTISENSE (funded by the European Research Council; I played a central role in this project, but my thesis focuses on the children identified by this process, rather than the screening itself). Then once this cohort was identified I looked at group differences between synaesthetes and non-synaesthetes in two domains: personality and cognition (specifically, numerical cognition). Throughout the thesis I use tests targeted specifically for our child population. Where these did not already exist in Chapter 2 (e.g., suitable self-report personality measures for children) we created and validated them independently. In Chapter 3 I use some of these measures to identify whether synaesthetes have a different personality profile to non-synaesthetes. In the second half of the thesis I tested synaesthetes’ numerical cognition, and looked, too, at ‘synaesthesia-like’ phenomena in the general population. Here in Chapter 4 I explored whether a widely implemented maths tool that pairs numbers with colours aids non-synaesthete children in their numerical cognition. I then finally return to synaesthetes in Chapter 5 using the same tests of numerical cognition to determine if grapheme-colour synaesthetes show advantages in this domain. Overall, this thesis shows that child synaesthetes have a distinct personality profile, and show a pattern of differences in numerical cognition found also in ‘synaesthesia-like’ phenomena such as the educational colour-coding of numbers

Exceptional Abilities in the Spatial Representation of Numbers and Time: Insights from Synesthesia

In the study of basic and high-level cognitive functions, neuroscientists, psychologists, and philosophers have tended to focus on normal psychological processes and on deficits in these processes, whereas the study of exceptional abilities has been largely neglected. Here the authors emphasize the value of researching exceptional abilities. They make the case that studies of exceptional representations, such as of time, number, and space in synesthesia, can provide us with insights regarding the nature of the neurocognitive mechanisms of these dimensions, as well as their developmental, evolutionary, and cultural origins