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Assessment of the hemispheric lateralization of grapheme-color synesthesia with Stroop-type tests.

Ruiz MJ, Hupé JM - PLoS ONE (2015)

Bottom Line: Overall, the lateralization effect was 7 ms (the 95% confidence interval was [1.5 12] ms), a delay compatible with an additional callosal transfer for stimuli presented in the left hemifield.Though weak, this effect suggests that the association of synesthetic colors to graphemes may be preferentially processed in the left hemisphere.We speculate that this left-lateralization could be a landmark of synesthetic grapheme-color associations, if not found for color associations learnt by non-synesthete adults.

View Article: PubMed Central - PubMed

Affiliation: Grenoble Institut des Neurosciences, Institut National de la Santé et de la Recherche Médicale U836 & Université Grenoble Alpes, 38000 Grenoble, France; Centre de Recherche Cerveau et Cognition, Université de Toulouse & Centre National de la Recherche Scientifique, 31300 Toulouse, France.

ABSTRACT
Grapheme-color synesthesia, the idiosyncratic, arbitrary association of colors to letters or numbers, develops in childhood once reading is mastered. Because language processing is strongly left-lateralized in most individuals, we hypothesized that grapheme-color synesthesia could be left-lateralized as well. We used synesthetic versions of the Stroop test with colored letters and numbers presented either in the right or the left visual field of thirty-four synesthetes. Interference by synesthetic colors was stronger for stimuli in the right hemifield (first experiment, color naming task). Synesthetes were also faster in the right hemifield when naming the synesthetic color of graphemes (second experiment). Overall, the lateralization effect was 7 ms (the 95% confidence interval was [1.5 12] ms), a delay compatible with an additional callosal transfer for stimuli presented in the left hemifield. Though weak, this effect suggests that the association of synesthetic colors to graphemes may be preferentially processed in the left hemisphere. We speculate that this left-lateralization could be a landmark of synesthetic grapheme-color associations, if not found for color associations learnt by non-synesthete adults.

No MeSH data available.


Related in: MedlinePlus

Individual variability for the strength of synesthetic associations.Left: Interference for the Color Naming task (x-axis) is negatively correlated with the speed to name synesthetic colors relative to real colors (y-axis). Right: The distribution of the index of photism strengh is unimodal, with a few subjects with extreme values. The subject with the largest photism strength could be unambigously characterized as a “projector” based on questionnaires and phenomenological reports, while the subject with the second largest value was characterized as an associator. Several other subjects, with lower values, were tentatively categorized as projectors based on questionnaires. The average effect size of the congruency effect in the color naming task (x-axis of the left panel) is 65 ms (CI = [34 96] ms). If excluding the two large values (above 3 standard deviations), it is 46 ms (CI = [34 59] ms).
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pone.0119377.g001: Individual variability for the strength of synesthetic associations.Left: Interference for the Color Naming task (x-axis) is negatively correlated with the speed to name synesthetic colors relative to real colors (y-axis). Right: The distribution of the index of photism strengh is unimodal, with a few subjects with extreme values. The subject with the largest photism strength could be unambigously characterized as a “projector” based on questionnaires and phenomenological reports, while the subject with the second largest value was characterized as an associator. Several other subjects, with lower values, were tentatively categorized as projectors based on questionnaires. The average effect size of the congruency effect in the color naming task (x-axis of the left panel) is 65 ms (CI = [34 96] ms). If excluding the two large values (above 3 standard deviations), it is 46 ms (CI = [34 59] ms).

Mentions: Synesthetes form a heterogeneous group: there is much variability in the way they report their synesthetic experiences. It has been proposed that synesthetes should be classified as either “projectors” or “associators” based on their first-person reports, but we and others have found this classification ambiguous and unreliable [16, 34]. Following Flournoy [35], we characterized synesthetes according to the strength of their synesthetic associations. We used a “photism strength” measure similar to that designed in our previous study ([16]; see their Figure 4 for an example). This measure was constructed based on the results of Dixon et al. [25] and Ward et al. [26] (Rothen et al.[36] further showed correlations between the responses to questionnaires and performances to Stroop tasks). These authors had observed that, at the group level, so-called “projectors” and “associators” exhibited a different pattern of results when comparing response times and interference in the photism and color synesthetic tasks. Projectors had stronger interference in the color task than in the photism task (meaning that they had more difficulty ignoring the synesthetic color than the real color), contrary to associators. Moreover, projectors tended to name synesthetic colors faster than real colors. We did not expect individuals to differ for color interference in the photism task (we supposed that color processing was similar), so we did not include in our index the interference in the photism task (mean interference was 103 ms, CI = [86 119] ms). Indeed, interferences in both tasks were not correlated (non-parametric Spearman R = -0.22, N = 34, p = 0.22; Pearson correlation r = -0.26, CI = [-0.55 0.08]; we had previoulsy suggested that this interference term could equalize “for volitional control and speed” [16], which may differ between subjects but be equivalent for different tasks; the absence of correlation shows little support for this assumption). Interference in the color naming task was negatively correlated to the RT to name synesthetic colors, in accord with the results by Dixon et al. [25] and Ward et al. [26] (Fig. 1; Spearman R = -0.49, N = 34, p = 0.003; Pearson correlation r = -0.61, CI = [-0.79–0.34]). Our index of synesthetic strength (‘photism strength’, ps) was:ps=(Incongruent−Congruent)color−(Photism−Color)congruent(1)Subtracting the term (Incongruent—Congruent)photism or using effect sizes instead of the central tendency, like we had done in our previous study [16], gave very similar results. A positive ps index indicates a strong association between graphemes and synesthetic colors (strong interference by photisms) and an easier task when naming photisms. Since the Color task was always performed before the Photism task the index could be compared between subjects.


Assessment of the hemispheric lateralization of grapheme-color synesthesia with Stroop-type tests.

Ruiz MJ, Hupé JM - PLoS ONE (2015)

Individual variability for the strength of synesthetic associations.Left: Interference for the Color Naming task (x-axis) is negatively correlated with the speed to name synesthetic colors relative to real colors (y-axis). Right: The distribution of the index of photism strengh is unimodal, with a few subjects with extreme values. The subject with the largest photism strength could be unambigously characterized as a “projector” based on questionnaires and phenomenological reports, while the subject with the second largest value was characterized as an associator. Several other subjects, with lower values, were tentatively categorized as projectors based on questionnaires. The average effect size of the congruency effect in the color naming task (x-axis of the left panel) is 65 ms (CI = [34 96] ms). If excluding the two large values (above 3 standard deviations), it is 46 ms (CI = [34 59] ms).
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4368719&req=5

pone.0119377.g001: Individual variability for the strength of synesthetic associations.Left: Interference for the Color Naming task (x-axis) is negatively correlated with the speed to name synesthetic colors relative to real colors (y-axis). Right: The distribution of the index of photism strengh is unimodal, with a few subjects with extreme values. The subject with the largest photism strength could be unambigously characterized as a “projector” based on questionnaires and phenomenological reports, while the subject with the second largest value was characterized as an associator. Several other subjects, with lower values, were tentatively categorized as projectors based on questionnaires. The average effect size of the congruency effect in the color naming task (x-axis of the left panel) is 65 ms (CI = [34 96] ms). If excluding the two large values (above 3 standard deviations), it is 46 ms (CI = [34 59] ms).
Mentions: Synesthetes form a heterogeneous group: there is much variability in the way they report their synesthetic experiences. It has been proposed that synesthetes should be classified as either “projectors” or “associators” based on their first-person reports, but we and others have found this classification ambiguous and unreliable [16, 34]. Following Flournoy [35], we characterized synesthetes according to the strength of their synesthetic associations. We used a “photism strength” measure similar to that designed in our previous study ([16]; see their Figure 4 for an example). This measure was constructed based on the results of Dixon et al. [25] and Ward et al. [26] (Rothen et al.[36] further showed correlations between the responses to questionnaires and performances to Stroop tasks). These authors had observed that, at the group level, so-called “projectors” and “associators” exhibited a different pattern of results when comparing response times and interference in the photism and color synesthetic tasks. Projectors had stronger interference in the color task than in the photism task (meaning that they had more difficulty ignoring the synesthetic color than the real color), contrary to associators. Moreover, projectors tended to name synesthetic colors faster than real colors. We did not expect individuals to differ for color interference in the photism task (we supposed that color processing was similar), so we did not include in our index the interference in the photism task (mean interference was 103 ms, CI = [86 119] ms). Indeed, interferences in both tasks were not correlated (non-parametric Spearman R = -0.22, N = 34, p = 0.22; Pearson correlation r = -0.26, CI = [-0.55 0.08]; we had previoulsy suggested that this interference term could equalize “for volitional control and speed” [16], which may differ between subjects but be equivalent for different tasks; the absence of correlation shows little support for this assumption). Interference in the color naming task was negatively correlated to the RT to name synesthetic colors, in accord with the results by Dixon et al. [25] and Ward et al. [26] (Fig. 1; Spearman R = -0.49, N = 34, p = 0.003; Pearson correlation r = -0.61, CI = [-0.79–0.34]). Our index of synesthetic strength (‘photism strength’, ps) was:ps=(Incongruent−Congruent)color−(Photism−Color)congruent(1)Subtracting the term (Incongruent—Congruent)photism or using effect sizes instead of the central tendency, like we had done in our previous study [16], gave very similar results. A positive ps index indicates a strong association between graphemes and synesthetic colors (strong interference by photisms) and an easier task when naming photisms. Since the Color task was always performed before the Photism task the index could be compared between subjects.

Bottom Line: Overall, the lateralization effect was 7 ms (the 95% confidence interval was [1.5 12] ms), a delay compatible with an additional callosal transfer for stimuli presented in the left hemifield.Though weak, this effect suggests that the association of synesthetic colors to graphemes may be preferentially processed in the left hemisphere.We speculate that this left-lateralization could be a landmark of synesthetic grapheme-color associations, if not found for color associations learnt by non-synesthete adults.

View Article: PubMed Central - PubMed

Affiliation: Grenoble Institut des Neurosciences, Institut National de la Santé et de la Recherche Médicale U836 & Université Grenoble Alpes, 38000 Grenoble, France; Centre de Recherche Cerveau et Cognition, Université de Toulouse & Centre National de la Recherche Scientifique, 31300 Toulouse, France.

ABSTRACT
Grapheme-color synesthesia, the idiosyncratic, arbitrary association of colors to letters or numbers, develops in childhood once reading is mastered. Because language processing is strongly left-lateralized in most individuals, we hypothesized that grapheme-color synesthesia could be left-lateralized as well. We used synesthetic versions of the Stroop test with colored letters and numbers presented either in the right or the left visual field of thirty-four synesthetes. Interference by synesthetic colors was stronger for stimuli in the right hemifield (first experiment, color naming task). Synesthetes were also faster in the right hemifield when naming the synesthetic color of graphemes (second experiment). Overall, the lateralization effect was 7 ms (the 95% confidence interval was [1.5 12] ms), a delay compatible with an additional callosal transfer for stimuli presented in the left hemifield. Though weak, this effect suggests that the association of synesthetic colors to graphemes may be preferentially processed in the left hemisphere. We speculate that this left-lateralization could be a landmark of synesthetic grapheme-color associations, if not found for color associations learnt by non-synesthete adults.

No MeSH data available.


Related in: MedlinePlus