The sensitivity and specificity of a diagnostic test of sequence-space synesthesia

synr: An R package for handling synesthesia consistency test data

Synesthesia is a phenomenon where sensory stimuli or cognitive concepts elicit additional perceptual experiences. For instance, in a commonly studied type of synesthesia, stimuli such as words written in black font elicit experiences of other colors, e.g., red. In order to objectively verify synesthesia, participants are asked to choose colors for repeatedly presented stimuli and the consistency of their choices is evaluated (consistency test). Previously, there has been no publicly available and easy-to-use tool for analyzing consistency test results. Here, the R package synr is introduced, which provides an efficient interface for exploring consistency test data and applying common procedures for analyzing them. Importantly, synr also implements a novel method enabling identification of participants whose scores cannot be interpreted, e.g., who only give black or red color responses. To this end, density-based spatial clustering of applications with noise (DBSCAN) is applied in conjunction with a measure of spread in 3D space. An application of synr with pre-existing openly accessible data illustrating how synr is used in practice is presented. Also included is a comparison of synr's data validation procedure and human ratings, which found that synr had high correspondence with human ratings and outperformed human raters in situations where human raters were easily mislead. Challenges for widespread adoption of synr as well as suggestions for using synr within the field of synesthesia and other areas of psychological research are discussed.

Consistency and strength of grapheme-color associations are separable aspects of synesthetic experience

Consistency of synesthetic associations over time is a widely used test of synesthesia. Since many studies suggest that consistency is not a completely reliable feature, we compared the consistency and strength of synesthetes' grapheme-color associations. Consistency was measured by scores on the Synesthesia Battery and by the Euclidean distance in color space for the specific graphemes tested for each participant. Strength was measured by congruency magnitudes on the Implicit Association Test. The strength of associations was substantially greater for synesthetes than non-synesthetes, suggesting that this is a novel, objective marker of synesthesia. Although, intuitively, strong associations should also be consistent, consistency and strength were uncorrelated, indicating that they are likely independent, at least for grapheme-color synesthesia. These findings have implications for our understanding of synesthesia and for estimates of its prevalence since synesthetes who experience strong, but inconsistent, associations may not be identified by tests that focus solely on consistency.

Idiosyncratic spatial representations of the days of the week in individuals without synesthesia

People with spatial sequence synesthesia have clear and stable mental images of spatial arrays of numbers, days of the week, and months. The shapes of the arrays vary by individual. We conducted an experiment to estimate the shapes of spatial arrays in people without synesthesia. We presented a pair of Japanese characters that represent the days of the week on a display, and asked participants without synesthesia to state which day they thought came later by pressing a key that was congruent with the spatial position of the day shown. We then estimated the shape of the spatial array in the participants' mental space (mental weekday lines) from the distribution of the reaction times. We found that these 'estimated mental weekday lines' differed by individual, and that the shapes (straight or bent lines, U-shapes, zigzags, or triangles) were similar to the spatial arrays found in individuals with synesthesia. We then conducted simulations using a self-organizing map and found that differences in the shapes of the spatial arrays compared with those in individuals with synesthesia could be explained by the inputs to the network. These results support the hypothesis that interactions between spatial and ordinal/cardinal representations in both synesthetes and non-synesthetes develop under the influence of self-organizing learning.

Enhanced spatial navigation skills in sequence-space synesthetes

Individuals with sequence-space synesthesia (SSS) perceive sequences like months, days and numbers in certain spatial arrangements. Several cognitive benefits have been associated with SSS, such as enhanced mental rotation, more vivid visual imagery and an advantage in spatial processing. The current study aimed to further investigate these cognitive benefits, focusing on spatial navigation skills, to explore if their enhanced sensitivity to spatial relations is reflected in enhanced navigational performance. Synesthetes were distinguished from controls by means of a questionnaire, a consistency test and drawings. A virtual Morris Water Maze (MWM) task with two allocentric and two egocentric navigation conditions was used to assess spatial navigation abilities. For the allocentric tasks, participants had to use object cues to find a hidden platform and for the egocentric tasks, they had to use their own position as a reference. Results showed that synesthetes performed significantly better compared to controls on the allocentric and egocentric tasks that reflected real life situations more accurately. However, this significant result was only found for the time taken to find the platform and not for the length of the path that was taken. In exploratory analyses, no significant relations were found between task performance and the specific features of the manifestation of each individual's synesthesia. Our hypothesis that synesthetes with the ability to mentally rotate their spatial arrangements would perform better on the allocentric task was not confirmed. Results add to the growing body of literature concerning the cognitive benefits of SSS and are consistent with the possibility that enhanced spatial navigation skills emerge from generally enhanced visuospatial abilities in SSS.

Autistic traits in synaesthesia: atypical sensory sensitivity and enhanced perception of details

In synaesthetes, specific sensory stimuli (e.g. black letters) elicit additional experiences (e.g. colour). Synaesthesia is highly prevalent among individuals with autism spectrum disorder (ASD), but the mechanisms of this co-occurrence are not clear. We hypothesized autism and synaesthesia share atypical sensory sensitivity and perception. We assessed autistic traits, sensory sensitivity and visual perception in two synaesthete populations. In Study 1, synaesthetes (N = 79, of different types) scored higher than non-synaesthetes (N = 76) on the Attention-to-detail and Social skills subscales of the autism spectrum quotient indexing autistic traits, and on the Glasgow Sensory Questionnaire indexing sensory hypersensitivity and hyposensitivity which frequently occur in autism. Synaesthetes performed two local/global visual tasks because individuals with autism typically show a bias towards detail processing. In synaesthetes, elevated motion coherence thresholds (MCTs) suggested reduced global motion perception, and higher accuracy on an embedded figures task suggested enhanced local perception. In Study 2, sequence-space synaesthetes (N = 18) completed the same tasks. Questionnaire and embedded figures results qualitatively resembled Study 1 results, but no significant group differences with non-synaesthetes (N = 20) were obtained. Unexpectedly, sequence-space synaesthetes had reduced MCTs. Altogether, our studies suggest atypical sensory sensitivity and a bias towards detail processing are shared features of synaesthesia and ASD. This article is part of the discussion meeting issue 'Bridging senses: novel insights from synaesthesia'.

Synesthesia & autistic features in a large family: Evidence for spatial imagery as a common factor

BACKGROUND

Autism and synesthesia are neurodevelopmental conditions associated with variants of perceptual processing. They also share some genetic variants and include a large magnitude of intra-categorical variation: 60 types for synesthesia, as well as a spectrum for autism. In order to investigate the relationship between these two phenomena, we investigated the family of FC, an autistic individual who also possess savant abilities and synesthesia manifestations.

METHOD

Autistic symptoms were assessed for the entire sample of participants entering the study (39 individuals) using the SRS. Participants above threshold were evaluated with standardized diagnostic tools. Synesthesia was explored in the entire participating sample using a self-reported questionnaire. Consistency tests were used for participants who reported synesthetic manifestations.

RESULTS

In addition to FC, four individuals with ASD were detected. Fifteen participants self-reported synesthesia (15 sequence-space, 4 sound-shape, 4 day-color), among which nine sequence-space synesthetes satisfied the consistency criteria. Two participants possess both autism and synesthesia.

CONCLUSION

This family illustrates the co-segregation of autism and synesthesia. This co-segregation is in favour of a partially overlapping genetic predisposition for both conditions, but also authorizes a large variety of manifestations in both conditions. The high prevalence of sequence-space synesthesia in this family strengthens the previous assumption that this form of synesthesia may be linked to autism. We discuss the potential role of spatial imagery in the development of this form of synesthesia and savant abilities.

Synaesthetes show advantages in savant skill acquisition: Training calendar calculation in sequence-space synaesthesia

Previous research has suggested that synaesthetic experiences may create the foundation for superior skills to emerge of the type found in savant syndrome (e.g., Simner, Mayo, & Spiller, 2009). People with sequence-space synaesthesia experience units of time (e.g., days, months, years) as a pattern in space, either within the mind's eye or as a 3d projection outside of the body. Our study investigates whether sequence-space synaesthesia facilitates the learning of the savant skill known as 'calendar calculation' where an individual can give the correct day of the week for any given date (e.g., 18th September 1990 was a Tuesday). Using a novel experimental methodology, we trained a group of sequence-space synaesthetes as well as non-synaesthete controls how to calendar calculate over two weeks with a final calendar calculation test in the third week. We show for the first time that calendar calculation is relatively easy to acquire: following training sessions totalling 1 h participants could select a day, from a set of several thousand, within ∼10 sec and with ∼80% accuracy. Synaesthetes were not found to have improved abilities from the start, but they outperformed controls in our final calendar calculation test. We suggest that sequence-space synaesthesia may have provided an advantage in performing calendar calculation after the opportunity for initial learning had taken place. This supports the notion of synaesthesia as a foundation for superior, and perhaps sometimes savant-like, skills.

Creativity and involvement in art in different types of synaesthesia

The aim of the present study was to test the relationship between different types of synaesthesia and their involvement in art, creative, and visual abilities. We tested 20 grapheme-colour, 18 sound-colour, 19 grapheme-colour-and-sound-colour, 20 sequence-space synaesthetes, and the same number of controls matched by age, gender, and education. We assessed the number of artistic professions, involvement in art, and the performance in psychometric tests of divergent and convergent creativity, as well as visual and visuo-spatial abilities. Results show a higher prevalence of artists among synaesthetes, especially sound-colour synaesthetes. Sound-colour synaesthetes also showed a higher involvement in artistic activities overall while sequence-space synaesthetes showed higher involvement in visual art. Only grapheme-colour-and-sound-colour synaesthetes showed significantly higher divergent creativity compared to matched controls. Additionally, overall, synaesthetes scored higher in visuo-spatial abilities (i.e., mental rotation). For synaesthetes and controls, visuo-spatial abilities correlated with divergent creativity. We discuss that synaesthetes' higher involvement in art is not necessarily reflected in their basic creative abilities.