Personality and cognitive profiles of a general synesthetic trait

Highlights

• Synesthesia prevalence of 24% in a large semi-representative sample.

• Six different types of synesthesia were measured.

• Increased Intelligence in Synesthetes.

• Increased Openness, Fantasizing, and Emotionality in synesthetes.

• Effects are related to the general synesthetic ‘Trait’ rather than separate ‘Types’.

Abstract

The recent sharp increase in studies on synesthesia has taught us a lot about this fascinating condition. Still, while we define synesthesia as ‘the mixing of senses’, the great majority of synesthesia studies focus on only one synesthesia type (in particular grapheme-color synesthesia). In this study, a large group of subjects are tested on the presence or absence of different types of synesthesia. Efforts to recruit a representative sample of the Dutch population, not related to or aware of synesthesia as a research topic, helped counter a selection bias or a self-report bias in our subject group. A sharp increase in synesthesia prevalence was found, at least partially due to including many different types of synesthesia in the synesthesia ‘diagnoses’. The five synesthesia types reported in the Novich et al (2011) study were obtained; Colored Sequences, Colored Music, Colored Sensations, Spatial Sequences, Non-Visual Sequelae, as well as an additional synesthesia type, Sequence-Personality. No differences were found between synesthetes and non-synesthetes in education level, handedness, age, and sex. The synesthetes showed increased intelligence as compared with matched non-synesthetes. This was a general effect rather than bound to a specific cognitive domain or to a specific (synesthesia-type to stimulus-material) relationship. The expected effect of increased “Openness” in synesthetes was obtained, as well as two unexpected effects in personality traits (increased “Neuroticism” and decreased “Conscientiousness”). We also found increased “Emotionality” (experiencing emotions) and increased “Fantasizing”, but synesthetes did not differ in cognitive appraisal of emotions (identifying/analyzing/verbalizing of emotions). The personality and cognitive characteristics were found related to having synesthesia (in general) rather then to particular synesthesia subtypes. This supports the existence of a general synesthetic ‘trait’, over the notion of relatively independent ‘types’ of synesthesia. In further support, exploratory analyses showed that a measurement of synesthetic strength (number of subtypes of synesthesia) correlates with stronger findings (increased “Openness”, “Fantasizing”, and “Emotionality”, and decreased “Conscientiousness”). In conclusion, results are in line with the notion of a general synesthetic ‘trait’, and this synesthetic trait is associated with particular personality traits and cognitive characteristics.

Introduction

The past two decades have seen a steep increase in synesthesia literature. Accordingly, a lot has been learned about this fascinating condition. Yet, it would be just as valid to state that we hardly know anything at all about synesthesia. Almost all synesthesia research includes only one type of synesthesia, and the great majority of these studies are on one particular type, namely ‘grapheme-color’ (linguistic-color) synesthesia. Yet the definition of the condition stresses its broadness: the ‘mixing of senses’, and an important characteristic of synesthesia is that it includes a range of different subtypes (e.g., Cytowic and Eagleman, 2009; Novich et al., 2011; Niccolai, Jennes, Stoerig, & van Leeuwen, 2012). Currently, we do not know the relationship between the general condition of synesthesia (the “Synesthetic Trait”), and the plurality of different subtypes (“Synesthetic Types”). We also know very little about the scope of the synesthetic characteristics we have acquired so far; are they general characteristics that apply equally to all different types, or are part of these findings in fact specific to the studied type (grapheme-color synesthesia) only? In order to learn about the trait ‘synesthesia’, larger-scale studies are needed that include many different types of synesthesia, and compare this diverse group of synesthetes with a group of controlled non-synesthetic subjects. In this project, we examine the ‘trait’ synesthesia by including a larger set with different synesthesia types. The current study was set up to find different types of synesthesia in a large sample of subjects. The second goal is to examine the general personality and cognitive characteristics of these synesthetes, as compared with non-synesthetes.

We also aim to address another issue in synesthesia research, which is sampling bias. In order to include as many synesthetes as possible, researchers are often forced to recruit synesthetes based on the subject’s knowledge and recognition of this condition. This means that the tested subjects are most likely not a representative sample. For example, synesthetes have been proposed to show increased cognitive abilities (Brang and Ramachandran, 2011, Rothen et al., 2012). Yet testing this hypothesis might be problematic if a sampling bias in synesthesia studies favors subjects (e.g. with increased cognitive or meta-cognitive abilities) who find it easier to recognize their own condition. Furthermore, synesthetic subjects with a connection to a university (e.g. students) are contacted more easily by synesthesia researchers. Similarly, the female bias obtained in synesthesia has been explained as increased tendency in females, as compared with males, to self-report (Simner et al., 2006; but see Rich, Bradshaw and Mattingley, 2005). This means that some of the found characteristics of synesthesia might be influenced by this gender bias. Studies that recruit subjects on other characteristics and subsequently test these subjects on synesthesia show different prevalence and synesthesia characteristics as compared with the studies where subjects are recruited based on self-referral (Simner et al., 2006). Simner and colleagues (2006) found in a student population higher prevalence, and absence of the skewed male-to-female ratio, if biases in self-report were avoided.

In this study, we address two questions. First, which types of synesthesia are found and what is the prevalence of these synesthesia types in the normal population? We recruited a large (semi-) representative sample of the Dutch population. Recruiting of subjects was done without referring to (or knowledge of) synesthesia. These subjects were then presented with a set of questions probing for the (possible) presence of synesthesia. As we explain in the methods section, the five types of synesthesia obtained in the Novich et al. (2011) study were taken as a starting point, and we included additional questions to explore the possible presence of other types of synesthesia. The second question is whether synesthetes differ from non-synesthetes in their general personality or cognitive profiles. As far as we know this is the first study where different types of synesthesia are searched in a large group of representative subjects (diminishing self-reference or sampling bias), and subsequently tested with a set of personality questionnaires and cognitive tests. We tested three different domains: cognitive ability (intelligence), personality characteristics (‘big five’), and emotional/cognitive style.

Previous reports have shown increased IQ in the synesthete subjects (Simner et al., 2009, Paulesu et al., 1995). Zamm et al., (2013) found increased connectivity in white matter tracts related to color-music synesthesia, and proposed a possible link between synesthesia and other populations characterized by enhanced local white matter connectivity, such as individuals with absolute pitch, high cognitive intelligence, high emotional intelligence, and high creativity, but also patients with hallucinations and subjects with autistic spectrum disorders (see also Ramachandran and Hubbard, 2001). The current project tests the hypothesis of generally increased intelligence in synesthetes as compared with non-synesthetes (Experiment 1).

Synesthetes have also been found to differ from non-synesthetes in having superior memory ability (though normally not as extraordinary as the famous case of Solomon Shereshevsky from Luria in 1968, for a review see Rothen et al., 2012). Superior memory for word lists has been found in grapheme-color synesthetes, where the graphemes in the words are assumed to elicit colors that subsequently aid memory performance (Radvansky et al., 2011, Mills et al., 2006, Smilek et al., 2002, Yaro and Ward, 2007). Grapheme-color synesthetes have also been found to score above average (but still within normal range) in the concurrent domain (visual as compared to verbal memory). Such advantage in visual memory performance has been found for grapheme-color synesthetes, in different types of memory tasks and even extending to abstract (thus not inducing synesthesia) visual stimuli, (Terhune et al., 2013, Pfeifer et al., 2014, Pritchard et al., 2013, Rothen and Meier, 2010a, Ward et al., 2013). However, superior memory was not always obtained in grapheme-color synesthesia, and case-study findings may be influenced by selection bias or the particular strategic use of synesthesia as a mnemotechnique (see Rothen and Meier, 2009). There are few studies on the memory abilities of other types of synesthesia. Ward et al. (2013) showed that the increased visual memory performance obtained in grapheme-color synesthesia was not found in a non-visual synesthesia type, namely lexical gustatory synesthesia. Sequence-space has a visuospatial component (the concurrent is a spatial configuration). Accordingly, enhanced visual or visuospatial memory has been obtained (Hale et al., 2014, Simner et al., 2009). However, Rothen et al., (2013) found no memory advantage in sequence-space synesthetes for memory of letters or symbols.

Similarly to these ‘memory’ findings, Meier and Rothen (2013) examined ‘cognitive style’ and found both effects specifically for certain combinations of stimuli types-to-synesthesia type, as well as general effects. In particular, a large sample of synesthetes containing four different types (grapheme-color, sound-color, lexical-gustatory, and sequence-space) was examined with the VVQ Verbalizer-Visualizer-Questionnaire (Richardson, 1977; Kirby, Moore, and Schofield, (1988). The ‘verbalizer’ style preference (a preference for verbal representations and an enhanced ability to work with verbal materials) was found related to grapheme-color synesthesia, but not to the other types of synesthesia. In contrast, vivid imagery visualizer style (a preference to let the mind wander and the ability to generate vivid mental images, particularly related to dream imagery) was marginally significant for sequence-space synesthesia, and significantly related to all other types of synesthesia.

Thus, some studies have shown generally increased cognitive performance associated with synesthesia. However, other studies obtained specific effects related to the specific inducer or concurrent of the examined synesthesia type. In the current study, we test the hypothesis that synesthesia is related to increased intelligence. Furthermore, based on the premise that there is such a thing as a general trait of synesthesia, rather than only a collection of independent different types of synesthesia, we hypothesize that general effects can be obtained. In particular, we expect to find a general increase of intelligence, regardless of synesthesia type. In addition to such a general factor, possible particular advantages related to congruency between stimulus type and synesthesia type (e.g. grapheme-color to verbal stimulus) are also explored.

Banissy et al., (2013) tested if the atypical experiences of synesthesia are associated with atypical personality profiles. Personality was assessed with the “Big Five Inventory” (BFI, John, Donahue, and Kentle, 1991). Respondents indicated, on a five point Likert scale, the extend to which statements related to the ‘big Five’ personality traits (Extraversion, Agreeableness, Conscientiousness, Neuroticism, and Openness) best describe their own characteristics. Two measures of empathy were also administered, the Inter-Personal Reactivity Index, (IRI; Davis, 1980) and the empathy quotient (EQ; Baron-Cohen and Wheelwright, 2004). A large group (N=81) of synesthetes who all had (minimally) grapheme-color synesthesia was compared with age- and sex matched controls (N=112). The synesthetes were recruited from a database of volunteers at the University of Sussex, while the controls were recruited from the student population and via acquaintances. Banissy and colleagues predicted increased scores for synesthetes on “Openness to Experience”, based on findings of increased artistic inclination in synesthetes (Rich et al., 2005, Ward et al., 2008, Rothen and Meier, 2010b). As ‘Fantasizing’ (a sub scale of the IRI empathy questionnaire) can be construed as conceptually related to Openness to Experience, the authors also predicted increased Fantasizing in synesthetes as compared with non-synesthetes. Banissy et al. (2013) did indeed obtain differences in the personality characteristics of synesthetes and non-synesthetes. As predicted, synesthetes showed increased “Openness to Experience”. In addition, a non-expected decrease in ‘Agreeableness’ was obtained. Furthermore, synesthetes reported higher levels of “Fantasizing”. No general increase in empathy was found.

In the current study, we examine the ‘big five’ personality traits in synesthetes as compared with non-synesthetes. In line with the theoretical arguments put forward by Banissy and colleagues, we predict increased “Openness” and increased “Fantasizing” in the synesthetes. One important difference with the previous study is that several different types of synesthesia are included. However we expect that the same findings are obtained in this group of synesthetes. Openness has been found to correlate with intelligence measurements (Ackerman and Heggestad, 1997, , DeYoung, 2011, Farsides and Woodfield, 2003), thus the prediction of increased openness is not unrelated to the prediction of increased intelligence. For the other four personality traits (Extraversion, Agreeableness, Conscientiousness, Neuroticism) we have not found clear theoretical connections with synesthesia, and therefore these characteristics will be examined in an exploratory analysis. This analysis also explores if Banissy’s unexpected finding of decreased “Agreeableness” is replicated. As we will explain in the next section, the expected increased ‘Fantasizing’ is measured with a different test (see “Emotional Style”).

The current study also investigates the link between emotional experiences and synesthesia. Previous studies have shown such association directly (emotions are specific synesthetic inducers or concurrents; Cytowic and Eagleman, 2009; Ramachandran and Brang, 2008; Schweizer et al., 2013; Ward, 2004; for a review see Dael et al., 2013). But the emotional association can also be present in an indirect manner, where the synesthetic experience is related to positive or negative emotions (e.g. because their concurrent is congruent or incongruent with the sensory stimulus; Callejas et al., 2007; Hochel et al., 2009; Perry and Henik, 2013). A neurological basis for this relationship is proposed in theories on hyperconnectivity. Ramachandran and Hubbard (2001) suggested that a mutation that causes hyperconnectivity in synesthesia would also imply increased connectivity between fusiform gyrus (and other sensory cortices) and the limbic system (especially the amygdala and nucleus accumbens). This hyperconnectivity would enhance the pleasurable or aversive associations through limbic reinforcement of concordant or discordant inputs.

Neuroimaging studies have shown functional and structural brain differences in synesthetes, as compared with non-synesthetes, in brain regions functionally related to emotion (e.g., retrosplenial cortex and insula; Nunn et al., 2002; Weiss Shah et al., 2001; Niccolai et al., 2012; Sperling et al., 2006; Specht and Laeng, 2011, Melero et al., 2013). These findings relate synesthesia to differences in experiencing emotions (emotionality). This does not necessarily imply increased cognitive ability to monitor, assess or reflect on emotions in oneself or in another person (Vorst and Bermond, 2001). In fact, Amin et al. (2011) found, in a small group of linguistic-personality synesthetes, both increased and decreased scores on an Empathy Quotient (EQ) measurement, in grapheme-personification synesthetes as compared with controls. The current project will test the hypothesis of increased (experiencing of) emotionality in synesthetes as compared with non-synesthetes. Furthermore, the cognitive component of assessing of/ reflecting on emotions is also examined, but in an exploratory fashion.

One of the scales in the test used to examine Emotional Style is a ‘Fantasizing’ scale. This scale measures Fantasizing about virtual matters: “the degree to which someone is inclined to fantasize, imagine, day-dream, etc.”. An example of a (positive) item in this scale is: ``Before I fall asleep, I make up all kinds of events, encounters and conversations''. The “Fantasy” factor in the IRI (Davis, 1980) that was found related to synesthesia by Banissy et al. (2013), is not exactly the same, as the IRI describes this factor as: “respondents' tendencies to transpose themselves imaginatively into the feelings and actions of fictitious characters in books, movies, and plays”. However, there is overlap between the concepts measured in these questionnaires. In the IRI, the Fantasy scale consists of six questions; one on the frequency of daydreaming and fantasizing, two about degree of involvement in a book or movie, and three questions on identification with the main characters in a book or movie. The fantasizing scale of the BVAQ has eight questions; five on frequency of daydreaming and fantasizing, two on interest in fairytales and bizarre stories, and one question asks whether fantasizing is a waste of time. Thus, there is overlap in the type of examined behaviors. Furthermore, both questionnaires seem to ask about behaviors related to (visual) mental imagery ability.

Given these similarities, we predict increased “Fantasy” scores on the BVAQ in synesthetes, as compared with non-synesthetes. However, there are also differences between the two questionnaires. Thus, obtaining a relationship between ‘Fantasizing’ and synesthesia again in the current study, will support the notion of a more general relationship between fantasizing/daydreaming and synesthesia, rather than only a specific relationship with a particular questionnaire.

Increased (visual) mental imagery has been found related to the synesthesia trait (e.g., Barnett and Newell, 2008; Price, 2009). This has been more consistently found in imagery tests relying on self-report than in imagery test using objective measurements (Spiller and Jansari, 2008). The increased (self-reported) visual imagery experiences have been proposed to be a critical aspect of visual synesthesia (Galton, 1880, Price, 2009). Simner (2013) suggested that the trends toward high imagery might arise from a recruitment bias, where increased mental imagery leads to increased awareness of the condition and therefore increased likelihood to self-refer. In the current study, recruitment of subjects was performed by a recruitment agency, to obtain a ‘semi-representative’ sample of the Dutch population. In our study, a relationship between the factor “Fantasy” and the synesthetic trait is predicted. Obtaining this relationship in the current project would not easily be explained by self-report bias.

All predictions on personality and cognitive characteristics are tested by contrasting a diverse group of synesthetes with a group of non-synesthetes that were carefully controlled for a possible (weak) presence of synesthesia. This comparison of synesthetes versus these non-synesthetes is the basic comparison in this project, informing us about personality and cognitive characteristics related to having synesthesia (note that both are subgroups of the larger subject group and thus were recruited in identical manner). Second, we also examine if obtained effects are trait-specific or type-specific. As far as we know, this is the first study where prevalence of different types of synesthesia are explored in a large group of a semi- representative sample (diminishing self-reference or sampling bias), and subsequently tested with personality questionnaires and cognitive tests.