Synesthesia is more prevalent in people with autism

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People with autism often have enhanced sensory sensitivity.

They are, for example, much more likely to be affected by bright light and loud noises. They also have a better eye for detail.

In a new paper, which was published earlier this week in the journal Philosophical Transactions of the Royal Society B, researchers at Radboud University show that synesthetes also often have enhanced sensory sensitivity and that they have similar social skills to individuals with autism.

Around 2 to 4 percent of people have a condition called synesthesia; this means they mix their senses. For example, a synesthete can see a color while looking at a letter or experience a taste while listening to music.

Synesthesia and autism

Synesthesia is more prevalent in people with autism: 20 percent also have synesthesia, a much higher figure than average.

“We therefore asked ourselves whether there are perhaps commonalities between synesthesia and autism,” says cognitive neuroscientist Tessa van Leeuwen, first author of the publication.

Visual tests showed that synesthetes, just like individuals with autism, pay more attention to details.

In one of the tests the synesthetes had to find a small figure embedded in pictures with a complex background.

If you pay a lot of attention to details, you can pick out these small figures more easily against the background. On average the synesthetes made fewer mistakes in this test, just like people with autism, but only when the test became very difficult.

Previous studies in England showed that synesthetes score higher in questionnaires about autistic characteristics, but only for the questions that relate to the senses.

This was a first indication that the link between synesthesia and autism may possibly be found on the sensory level.

What this new publication demonstrates is that synesthetes and individuals with autism also have similar social skills: the synesthetes also scored higher than average on this aspect in an autism questionnaire.

In this questionnaire people report themselves on how easy and pleasant social contacts are for them: such as doing things with someone else or going to the library.

Van Leeuwen intends to carry out a follow-up study into the link between synesthesia and autism. “The commonalities between synesthesia and autism can help us obtain a better understanding of autism.

If the senses of individuals with synesthesia and individuals with autism work the same, this could, for example, tell us something about how autism works in the brain and how synesthesia with autism is related to other problems that people with autism come up against.”

Synesthesia may also help us to define subtypes of people with autism, for example with regard to sensory sensitivity and oversensitivity.


Synaesthesia is a mixing of the senses: specific sensory stimuli evoke unusual, additional experiences. For instance, ‘A’ evokes the colour red; music elicits colours; or the word ‘parents’ tastes like apple. Synaesthesia is elicited automatically, is stable over time and the prevalence is estimated at 2–4% of the population [1]. The prevalence of synaesthesia is substantially higher (approx. 20%) among individuals with autism spectrum disorder (ASD) [2,3]. ASD is a neurodevelopmental condition affecting approximately 1% of the population [4] and characterized by deficits in language and social interaction, repetitive behaviour and restricted interests [5]. The remarkably high co-occurrence of synaesthesia and autism—both relatively rare conditions—suggests that the two conditions are related, but the exact nature of the relationship is unknown.

An important hypothesis states that synaesthesia and autism share atypical sensory sensitivity and altered sensory perception [6,7]. In autism research, increased attention to sensory abnormalities (e.g. [810]) has resulted in the addition of sensory dysregulation as a diagnostic criterion in the recent DSM-5 [5]. Individuals with ASD frequently experience hypersensitivity or hyposensitivity to the environment, e.g. hypersensitivity to bright lights or strong colours or, to the contrary, seek stimulation by engaging in repetitive behaviours (e.g. fluttering the arms) [5].

Sensory atypicalities can negatively impact daily functioning, as in sensory overload [9], but can also entail enhanced perceptual skills [1113] or savant abilities (e.g. [14]). Enhanced perception of details (a ‘local bias’) is widespread in autism (e.g. [12,15,16]) and features centrally in several autism theories, e.g. the weak central coherence theory [11] and the enhanced perceptual functioning model [13].

A recent formal meta-analysis on local–global visual processing in ASD, however, revealed no structurally enhanced local visual processing in autism, but concluded that global visual processing requires more time and effort in individuals with autism, especially when incongruent low-level information is present [17].

Synaesthesia is a sensory condition characterized by unusual perception. Sensory atypicalities have been reported for synaesthesia beyond the synaesthetic experiences themselves. These include hypersensitivity of the parvocellular visual system [18], enhanced perception of and sensitivity to colours in colour synaesthetes [1921] and enhanced tactile sensitivity in tactile synaesthetes [19]. Enhanced colour processing in synaesthetes may come at the cost of reduced motion-processing abilities [22], an example of sensory impairment in synaesthesia which parallels reduced motion processing in autism (e.g. [23,24]).

Synaesthesia occurs more frequently among ASD individuals with savant abilities compared to those without these abilities [25] and savant skills often involve synaesthesia [14,26]. Enhanced perception of details is reported for synaesthetes, e.g. for perception of detailed facial features [27] and on visual tasks [7]. Altogether, sensory atypicalities in both synaesthesia and autism form a reason to explore whether atypical sensory processing is shared between the conditions.

A better understanding of the relationship between synaesthesia, autism and sensory atypicalities may elucidate the role of sensory processing in the mechanisms of autism. Deficits in early (multi)sensory processing in autism are suggested to lead to social symptoms [28,29] as these complex cognitive functions rely on proper integration of (multi)sensory cues, for instance, when interpreting speech and simultaneously integrating facial expressions. Care for individuals with autism may benefit from better understanding of (synaesthesia-related) sensory symptoms.

Autism and synaesthesia may share other mechanisms: implicit learning, for instance, is altered in both populations [30,31]. Here, however, we focus on sensory processing and explicitly test whether sensory sensitivity and perception in synaesthesia resemble sensory processing in autism.

Several studies have used self-report questionnaires and visual experiments to directly assess autism-related sensory atypicalities in synaesthetes [6,7,32,33]. Ward et al. [6] deployed the Glasgow Sensory Questionnaire (GSQ) which indexes sensory hypersensitivity and hyposensitivity in seven modalities and typically correlates positively with autistic traits [3436].

Synaesthetes scored intermediate between controls and ASD individuals, qualitatively resembling the ASD pattern. On the autism spectrum quotient (AQ) [37], a self-report questionnaire about autistic traits, synaesthetes scored within the ASD range on the Attention-to-detail subscale but not on other AQ subscales (e.g. social skills).

Ward et al. [7] replicated the elevated GSQ and AQ-Attention-to-detail scores for synaesthetes and found dose effects of synaesthesia: having more types of synaesthesia was associated with higher scores supporting a common mechanism of synaesthesia and autism.

In Burghoorn et al. [32], autistic traits (AQ-Total) and synaesthesia consistency scores correlated positively in neurotypical individuals extending the autism–synaesthesia relation to neurotypicals. In Mealor et al. [33], synaesthetes scored higher on AQ-Attention-to-detail-derived questions. Hence, self-report questionnaire findings suggest that the synaesthesia–ASD relationship involves perceptual and sensory atypicalities.

Experimental findings from the same studies [7,32] are somewhat mixed. Individuals with autism tend to focus more easily on local than on global visual elements (although they are not necessarily impaired at the global level as reported, for example, by Van der Hallen et al. [15,17]). Ward et al. [7] assessed local/global perceptual abilities in synaesthetes using an embedded figures test and a change blindness test on which individuals with ASD have been reported to perform better [3842].

The embedded figures test involves identifying a local target shape in a larger complex figure, while the change blindness test involves finding the difference between two alternating pictures (e.g. removal of a small object). On both tests, synaesthetes were more accurate than controls without response time differences, supporting an interpretation of enhanced perception of detail in synaesthetes similar to ASD.

In Burghoorn et al. [32], neurotypical individuals performed three visual experiments on local/global perception and were assessed for autistic traits (AQ) and the degree of synaesthesia. Given that AQ-Total and synaesthesia consistency scores were correlated, both measures were expected to relate to enhanced perception of details—individuals with high AQ resemble individuals with ASD [43]. High AQ-Attention-to-detail correlated positively with performance on an embedded figures task and a visual illusions task (two ‘local’ tasks), but not on a motion coherence task (a ‘global’ task). The degree of synaesthesia, however, did not correlate with the performance: only a trend in the visual illusions task resembled the AQ results.

In these visual illusions (Müller-Lyer and Ebbinghaus), relatively more focus on local elements reduces susceptibility to the illusion, which is induced by the global Gestalt context of the display. Higher AQ-Attention-to-detail was associated with more veridical perception in the Müller-Lyer illusion in Burghoorn et al. [32], in line with the reduced susceptibility to these illusions in ASD (e.g. [44]). The absence of any correlations of performance with the degree of synaesthesia suggested that the relationship between synaesthesia and perceptual abilities is weaker than the relationship between autistic traits and perception.

Here, we assessed autistic traits, sensory sensitivity and local/global visual perceptual abilities of synaesthetes and related those characteristics to known features of ASD. The same tasks were performed by a large heterogeneous online cohort of synaesthetes (Study 1) and a smaller laboratory cohort of sequence-space synaesthetes (Study 2). Sequence-space synaesthetes perceive sequences such as days of the week, months and/or numbers in a spatial arrangement. We assessed autistic traits (AQ [37]) and sensory sensitivity (Dutch Glasgow Sensory Questionnaire (GSQ) [34,35]). We hypothesized that synaesthetes would score high on AQ-Attention-to-detail and would score more extreme on the GSQ.

Two visual tasks were used to test local/global visual perception in synaesthetes. In a Motion Coherence task, we assessed global motion processing: participants indicate the global direction of motion of a dot display in which a limited number of dots move in a coherent direction (electronic supplementary material, figure S2).

Attending to individual dots impairs performance. Individuals with autism generally need approximately 10% more dots to move coherently before perceiving the global motion direction (e.g. [12,15,23,24,45]), although several studies have reported better performance for ASD individuals [46], mixed results depending on whether individuals with ASD or Asperger syndrome were tested [47] or no difference between ASD individuals and controls [48]. On the basis of the majority of studies, however, we expected a similarly decreased performance (higher motion coherence thresholds (MCTs)) for synaesthetes.

We replicate one previous study with N= 10 synaesthetes [22] that reported increased MCTs in synaesthetes, and one study with N = 34 synaesthetes in which sequence-space synaesthetes (N = 22) displayed lower MCTs than grapheme-colour synaesthetes (N = 12) and controls (N = 34) [49].

The second experiment was the Leuven Embedded Figures Test [50,51] on which we expected synaesthetes to do better because focusing on local elements is beneficial in this task (electronic supplementary material, figure S3). We explored correlations of performance on these visual tasks with AQ and GSQ scores, and dose effects of synaesthesia.

Our overall approach expands upon Ward et al. [7] in three ways: we added a motion task on which decreased performance of synaesthetes was expected, to rule out stronger motivation of synaesthetes as a potential confounding factor; we added a difficulty manipulation in the embedded figures task, to examine the performance of synaesthetes on this task in more detail, and we used Dutch versions of the AQ and GSQ, replicating earlier studies in another language.

2. Methods Study 1: heterogeneous cohort of online synaesthetes

(a) Participants

Participants were recruited via a nationally advertised crowd-sourcing website about sensory perception and synaesthesia (gno.mpi.nl, [52,53]) and the institute’s online recruitment system. One hundred and fifty-nine participants (109 self-reported synaesthetes) completed the online synaesthesia screening questionnaire. Thirty-three people (26 synaesthetes) were excluded because of neuropsychiatric disorders (e.g. ASD, depression), six synaesthetes stopped after the first questionnaire and one synaesthete did not meet our synaesthesia cut-off (consistency tests and classification of synaesthesia types according to Novich et al. [54] are described in the electronic supplementary material). In total, 76 synaesthetes and 43 non-synaesthetes completed the study online. Participants were compensated by entering a raffle for a tablet and/or course credits. The study was approved by the Ethics Committee of the Faculty of Social Sciences (ECSS) at Radboud University, Nijmegen.

Thirty-nine individuals, recruited on campus and via the institute’s participant recruitment system, completed the study in the laboratory as part of a perception study [32]. Three individuals were excluded because of neuropsychiatric conditions, and three laboratory participants were synaesthetes, resulting in 33 non-synaesthetes and 3 synaesthetes. Participants were compensated with 12.50 euros or 1.5 course credits. The study was approved by the ECSS at Radboud University, Nijmegen.

Combining online and laboratory participants,1 the total sample included 79 synaesthetes and 76 non-synaesthetes. The groups differed in age (synaesthetes of 36.2 ± 15.1 years; range, 18–72 years; non-synaesthetes of 23.3 ± 6.7 years; range, 18–61 years; t153 = −6.83, p < 0.001). Age was included as a covariate in all group analyses. Gender distribution was similar across groups (synaesthetes, 8M/71F; non-synaesthetes, 15M/61F; χ21,155=2.83χ1,1552=2.83, p = 0.092). Synaesthetes predominantly experienced grapheme-colour synaesthesia (N= 59) and sequence-space synaesthesia (SSS) (N = 21); 36 synaesthetes experienced one type of synaesthesia, 26 synaesthetes experienced two types and the remainder (N = 17) three or more types (see electronic supplementary material, figure S1 and table S1 for details). We created three subgroups of synaesthetes to explore whether synaesthesia type influenced the results (e.g. [49]): only grapheme-colour synaesthesia (N = 29), synaesthesias including SSS (N = 21) and ‘other’ types (N = 29). All but three synaesthetes in the sequence-space group had more than one type of synaesthesia (see electronic supplementary material, figure S1).

(b) General procedure

Online participants completed the study in LimeSurvey (https://www.limesurvey.org/). The opening webpage informed participants about the study’s purpose and duration and invited them to a synaesthesia consistency test (see electronic supplementary material). Next, participants gave online informed consent. Subsequently, a synaesthesia screening questionnaire (5 min), the AQ (10 min) and the GSQ (10 min) were completed. After completing the GSQ, participants received feedback and clarifications on their AQ and GSQ scores, including a general explanation about the questionnaires, comparisons of their own scores to normative data and the information that the AQ was not a diagnostic instrument. Instructions and weblinks to the motion coherence task (5 min) and embedded figures task (10 min) were provided. Feedback on the performance was given after each task. The entire experiment took approximately 50 min.

Participants in the laboratory had a slightly different task order. They started with the two experimental tasks (and an added visual illusions task reported elsewhere [32]). Instructions were provided on paper and by the researcher. Next, the synaesthesia consistency test, AQ and GSQ were completed. If the allotted laboratory time was insufficient, the GSQ was completed on a voluntary basis at home. At the end of the laboratory session, participants were debriefed on the research purpose and hypothesis.

(c) Questionnaires

(i) Synaesthesia screening questionnaire

Participants reported types of synaesthesia by ticking pre-set boxes and free typing. If synaesthesia was reported, detailed questions on synaesthesia characteristics were presented (e.g. ‘Since when have you experienced synaesthesia?’). Demographic and health-related questions were used to exclude individuals with poor eye-sight, ASD/psychiatric conditions, etc.

(ii) Autism Spectrum Quotient

The AQ assesses autistic traits in non-clinical populations [37] and consists of 50 statements to which individuals agree or disagree on a four-point Likert scale (e.g. ‘I tend to notice details that others do not’, ‘I find social situations easy’). The Dutch version (AQ-NL, [55]) uses the full Likert scale (slightly (dis)agree to definitely (dis)agree) resulting in a minimum score of 50 and a maximum score of 200; a score above 145 is within the ASD range. Aside from the cumulative score (AQ-Total), the AQ has five subscales: Attention to detail, Social skills, Communication, Attention switching and Fantasy. We specifically hypothesized that synaesthetes would score higher on AQ-Attention-to-detail [6,7] and AQ-Total [32].

Data analysis. AQ-Total and the five subscores were subjected to ANCOVAs including Group as a between-subjects factor and age as a covariate of no interest.

(iii) Glasgow Sensory Questionnaire

The GSQ consists of 42 questions (e.g. ‘Do you find certain sounds and/or pitches annoying?’) assessing hypersensitivity and hyposensitivity across seven sensory modalities (Visual, Auditory, Olfactory, Proprioceptive, Gustatory, Tactile and Vestibular). Individuals with ASD or a high AQ typically score higher [35]. We used the Dutch version recently validated by Kuiper et al. [34]. We hypothesized higher total GSQ scores for synaesthetes [6,7].

Data analysis. Total GSQ scores were analysed with an ANCOVA including Group (between-subjects) and age as a covariate of no interest. In an exploratory analysis, hypersensitivity and hyposensitivity subscales for the seven sensory modalities were subjected to a 2 × 7 repeated-measures ANOVA with Group as between-subject factor and age as a covariate of no interest.

(d) Visual tasks

(i) Motion Coherence task

On each 600-ms motion coherence trial, 200 white dots (diameter, 0.15°) moved with a speed of 6° s−1 across an 11.7 by 11.7 cm grey square background (see electronic supplementary material, figure S2).

A subset of dots moved in a coherent direction: participants indicated the direction of coherent motion using arrow keys (right, left, up and down). Individual dot lifetime was 60 ms, discouraging the ‘local’ strategy of tracking individual dots ([56]; see Simmons et al. [12] for an overview of MC parameters in ASD studies).

Three staircase runs of 60 trials began with 50% of dots moving coherently. After each correct trial, the coherence level for the next trial was reduced logarithmically, dividing by 100.1; if incorrect, the coherence level was multiplied by 100.1, making the task easier. The minimum motion coherence level was close to 0, the maximum 1.0 (100% coherent dots).

The experiment was programmed in HTML 5 (HTML/CSS/JavaScript) running in an Internet browser. The online participants used their home devices to complete the task. As arrow keys were needed for the response, we deduce online participants completed the experiment on a device with a keyboard. For the laboratory participants, the task was run in Google Chrome and displayed on a 24′ BenQ screen (1920 × 1080 resolution) controlled by a Windows 7 Dell computer.

Data analysis. The end scores of the three staircase runs (trials 60, 120, 180) were averaged to obtain an overall MCT for each participant, defined as the percentage of coherent dots necessary to detect the coherent motion.

(ii) Embedded figures task

We implemented the Leuven Embedded Figures Test online [50]. On each trial, a target stimulus was presented above three embedding stimulus contexts (electronic supplementary material, figure S3A), and the participants identified the embedding context containing the target as fast and accurately as possible using their mouse or touchpad. Error rates and reaction times were recorded.

The 16 target stimuli consisted of 3, 4, 6 or 8 lines (electronic supplementary material, figure S3B, see also De-Wit et al. [50], and see the Data Accessibility statement for how to access the complete stimulus set).

Half the targets were closed forms, half were open forms; half were symmetric and half asymmetric. The difficulty of target detection was manipulated by modifying the number of target lines that continued into the embedding context (0%, 34%, 64% and 100% of lines on average, electronic supplementary material, figure S3C [50]).

The 100% continuous condition was the hardest. Sixteen target stimuli appeared once at each difficulty level, resulting in 64 experimental trials. Participants started with 12 practice trials. Visual feedback was given on every trial (green or red border around chosen context), and after an incorrect response, participants re-tried until succeeding.

After trial completion, an arrow appeared allowing the participants to continue to the next trial. The task took approximately 10 min.

Targets and contexts consisted of dark grey line stimuli on a white background (electronic supplementary material, figure S3). The background screen was light grey. The experiment was programmed in HTML 5 (HTML/CSS/JavaScript) running in an Internet browser. The online participants used their home devices to complete the task, and for the laboratory participants, the task was run in Google Chrome, displayed on a 24′ BenQ screen (1920 × 1080 resolution) controlled by a Dell Windows 7 computer.

Data analysis. Error percentages and reaction times were calculated for each subject and stimulus condition. Reaction times to incorrect trials and reaction time outliers of ±2 s.d. from the subject and condition mean were removed. The participants performing more than 2 s.d. away from their group mean on overall error rates or reaction times (RTs) were removed prior to analysis.


More information: Tessa M. van Leeuwen et al. Autistic traits in synaesthesia: atypical sensory sensitivity and enhanced perception of details, Philosophical Transactions of the Royal Society B: Biological Sciences (2019). DOI: 10.1098/rstb.2019.0024

Journal information: Philosophical Transactions of the Royal Society B
Provided by Radboud University

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