Not all synaesthetes are the same: cognitive and personality differences in different types of synaesthetes

Abstract

Synaesthesia is a perceptual condition in which stimulation of one sensory or cognitive pathway leads to automatic and involuntary experiences in a secondary sensory or cognitive pathway (e.g. seeing music or tasting words). Despite the fact that synaesthetes constantly perceive additional information during these inducer-concurrent associations, they are relatively unaffected by this irrelevant information. Chapter II investigates whether different samples of -visual synaesthetes (i.e. those experiencing synaesthesia types involving visual concurrents such as colours for letters or numbers – grapheme-colour synaesthesia – or sequence-space synaesthesias like calendar-forms) are better than-non synaesthetes at filtering out task-irrelevant stimuli in different conflict tasks. Synaesthetes were more efficient than controls at ignoring visual irrelevant stimuli presented together with tactile targets, but no group differences were observed when they had to perform the same visuo-tactile task with reversed instructions (i.e. attend visual and ignore tactile information) or in unimodal visual tasks (Studies 1 and 2). However, these results were not replicated in Study 3, which assessed a new sample of participants with the two versions of the same visuo-tactile tasks. This study also evaluated a) whether the observed synaesthetic attentional advantage was consistent across different sensory modalities combinations by introducing audio-visual modalities of the same tasks, and b) whether different types of -visual synaesthetes showed the same attentional advantages or not by comparing groups of colour-synaesthetes (i.e. those experiencing synaesthesias involving -colour as the concurrent) and sequence-synaesthetes (i.e. those experiencing sequence-space synaesthesias). Results revealed that sequence-synaesthetes were better than non-synaesthetes and colour-synaesthetes at filtering tactile irrelevant distractors presented with visual targets; no other group differences were observed. This suggests that the specific types of synaesthesias, together with other factors discussed, might play a relevant role in shaping the cognitive abilities of synaesthetes. In order to explore the extent of the influence of synaesthetic individual differences, the second part on of the thesis examines differences in personality in individuals with different types of synaesthesia (Chapter III – Study 4). Synaesthetes have a distinct personality profile compared to non-synaesthetes, but there are inconsistencies in the literature with respect to the personality traits that differ. Most studies have focused on grapheme-colour synaesthetes, ignoring other types of synaesthesia. Here, we compare matched groups of colour-synaesthetes, sequence-synaesthetes, and non-synaesthetes on the Big Five personality traits and on specific empathy and positive schizotypy subscales. We replicated previous findings that synaesthetes experienced higher rates of Openness to Experience, Fantasising (a dimension of empathy), and Unusual Experiences (positive schizotypy) compared to non-synaesthetes. Importantly, some of these differences were only observed for sequence-synaesthetes, with higher rates of Openness to Experience compared to non-synaesthetes and colour-synaesthetes. However, no differences between synaesthetes and non-synaesthetes or between the two types of synaesthetes were found in a second sample assessed. We discuss several possible limitations affecting subject recruitment and assessment administration methods that could explain the different sample results. The last section of the thesis addresses synaesthetic heterogeneity from a methodological point of view. The need to screen and classify synaesthetes led to the development and validation of a screening questionnaire, the Edinburgh Synaesthesia Screening Assessment or ESSA (Chapter IV – Study 5). Although synaesthetic tests of genuineness or consistency tests are considered the ‘gold standard’ of synaesthesia assessment, they are only available for a few synaesthesia types. The ESSA is a self-report questionnaire developed to cover an exhaustive range of synaesthesia types (108) and designed to assess both synaesthetes and non-synaesthetes by asking responders to rate how much each synaesthetic experience applies to them (5-point Likert scale). Sensitivity and specificity analyses were carried out on ESSA scores obtained from a sample of over 150 (synaesthete and non-synaesthete) participants who also completed synaesthetic consistency tests for -colour and sequence-space synaesthesias. Synaesthetes obtained significantly higher scores than non-synaesthetes, and the analyses showed acceptable rates of sensitivity and specificity (±85.5 and ±75.8, respectively). These results were validated internally and externally (in a new sample of 275 participants) yielding some modest values. We consider different detected bias and other factors that might reduce ESSA’s performance and propose ways to address them in future studies. In sum, converging evidence seems to indicate that synaesthetes are not a homogeneous category of individuals. Different cognitive and personality profiles are associated with different synaesthesia types. These findings have wider implications for the synaesthetic research area, as they suggest that grapheme-colour synaesthetes, predominantly assessed in in synaesthesia studies, might not be representative of all synaesthetes. These observations might at least in part explain contrasting results reported in the literature.