Differences in color memory between grapheme-color synesthesia associators and projectors
Hanna Brekelmans University of Amsterdam
Abstract
Grapheme-color synesthesia is a phenomenon where seeing particular letters always evoke specific colors (e.g. the letter A is red). Based on the experience of the synesthetic color, two subtypes can be distinguished. These are associators, who experience the synesthetic color “in the mind’s eye”, and projectors, who experience the synesthetic color in the outside world. Studies on grapheme-color synesthesia provide information about memory differences between grapheme-color synesthetes and non-synesthetes. For instance, grapheme-color synesthetes have enhanced verbal memory for word pairs and enhanced visual memory for colored matrixes. However, no research has been conducted on memory differences between associators and projectors. The current study investigated whether there is a difference in short term visual memory of color between associators and projectors. In addition, differences in perceptual sensitivity of color was measured, since this could affect the interpretation of color memory differences. It was hypothesized that associators would have enhanced color memory compared to projectors, since previous research has shown that associators have increased activity and gray matter volume of the hippocampus. However, results showed no significant difference in color memory, and no significant difference in perceptual sensitivity. Furthermore, the current study investigated whether grapheme-color synesthetes have
enhanced color memory and perceptual sensitivity compared to non-synesthetes, since previous research indicated this. Indeed, results showed enhanced color memory for
grapheme-color synesthetes, however, no significant difference in perceptual sensitivity was found. In conclusion, the current study indicates that there is no significant difference in visual memory of color between associators and projectors.
Keywords: Grapheme-color synesthesia, projectors, associators, visual memory, perceptual sensitivity
Differences in color memory between grapheme-color synesthesia associators and projectors
Synesthesia is a phenomenon in which a sensory stimulus evokes the experience of an additional sensation. For instance, different days of the week evoke a specific color, typical sounds evoke specific colors (e.g. musical notes), or ordinal sequences are experienced as specific points in space (e.g. months). The most studied type of synesthesia, with a prevalence of 1%, is grapheme-color synesthesia (Simner et al., 2006). Grapheme-color synesthetes experience specific colors with particular letters (e.g. the letter ‘A’ is red).
Studies on the differences between grapheme-color synesthetes and non-synesthetes show neurobiological differences. Both Weiss and Fink (2009) and Rouw and Scholte (2010) used voxel-based morphometry to investigate the neuro-anatomical differences between grapheme-color synesthetes and non-synesthetes. Results showed increased gray matter of the superior posterior parietal lobe and intraparietal cortices. However, not all neurobiological studies show similar results. For instance, Jäncke et al. (2009) investigated neuroanatomical and structural connectivity differences between grapheme-color synesthetes and
non-synesthetes using magnetic resonance imaging (MRI) and fractional anisotropy (FA)
respectively. Results showed increased cortical thickness, volume and surface area in the left and right fusiform gyrus of grapheme-color synesthetes compared to non-synesthetes.
Furthermore, their results revealed no difference in FA connectivity of the fusiform gyrus. Similarly, Rouw and Scholte (2007) investigated structural connectivity differences between grapheme-color synesthetes and non-synesthetes. However, their results did show increased connectivity near the fusiform gyrus in grapheme-color synesthetes compared to non-synesthetes. Several studies investigated whether activation of the human color area (V4) underlies the synesthetic experience using functional MRI (Hubbard et al., 2005; Sperling et al., 2006). Results showed increased activation of V4 as a reaction to letters which evoke
synesthetic experiences compared to letters which do not evoke a synesthetic experience. On the contrary, Hupé et al. (2012) did not find enhanced activation of V4 as a reaction to synesthetic inducing letters. Thus, previous studies show conflicting results regarding neurobiological differences between synesthetes and non-synesthetes.
In addition, previous research showed cognitive differences between grapheme-color synesthetes and non-synesthetes. For instance, synesthetes performed better on verbal and visual free recall word list tests and on cued recall tests in which they were asked to remember word pairs compared to non-synesthetes (Gross et al., 2011; Radvansky et al., 2011; Rothen and Meier, 2010a). Furthermore, synesthetes performed better on tasks where they were asked to remember stimuli which induce synesthesia (e.g. word lists) as well as on tasks with stimuli which do not induce synesthesia (e.g. colors associated with line drawings and simple visual figures) (Rothen and Meier, 2010a; Yaro and Ward, 2007). Yaro and Ward (2007)
investigated differences in verbal memory and visual memory of color between synesthetes and non-synesthetes. Participants were asked to perform the Rey Auditory –Verbal Learning Test, a task where participants learn a word list and recall this in a retention phase. Results showed enhanced verbal memory for synesthetes compared to non-synesthetes. Furthermore, participants were asked to learn a matrix containing 27 squares printed in 9 different colors, and to recall this matrix after a time delay. No significant difference was found between synesthetes and non-synesthetes in the learning phase. However, in the retention phase, synesthetes performed significantly better than non-synesthetes. Thus, these results support the idea that synesthetes have enhanced verbal memory and visual memory of color compared to non-synesthetes.
Moreover, Banissy et al. (2009) investigated whether synesthetes who experience color, including grapheme-color synesthetes, perform better on the color discrimination Farnsworth–Munsell Colour Hue Test than non-synesthetes. In this task, participants were
asked to arrange different hues of a color sequence in a tray, while the two end points of the sequence remained in the tray (e.g. purple and pink). An error score was calculated based on how far each hue deviated from the correct location in the color sequence. Results showed that synesthetes had a lower error score than non-synesthetes, and thus were better in color discrimination.
Besides research on memory tasks and color discrimination, the Stroop task is a common task to investigate cognitive differences between grapheme-color synesthetes and non-synesthetes. In a standard Stroop task participants are presented several color words (e.g. RED) printed in the congruent or incongruent color (e.g. green). Participants are asked to name the color of the color words as accurate and fast as possible. Color words printed in incongruent colors lead to a slower reaction times and more errors compared to color words printed in the congruent color (Stroop, 1935). This is called the interference effect. In a synesthetic Stroop task, letters are printed in the congruent or incongruent synesthetic color of the letter. For instance, when a grapheme-color synesthete experiences the letter A as red, the letter is printed in red or green. The interference effect occurs when the letter is printed in the incongruent color of the synesthetic color (Mattingley et al., 2001; Nikolić et al., 2007).
Furthermore, previous research showed differences in the synesthetic Stroop task among grapheme-color synesthetes. Synesthetes can be distinguished based on the experience of the synesthetic color. Projectors experience the synesthetic color in the outside world, and associators experience the synesthetic color “in the mind’s eye” (Dixon et al., 2004). Dixon et al. (2004) and Ward et al. (2007) studied the differences in the synesthetic Stroop interference effect between associators and projectors. In the first condition, participants were asked to recall the colors of the letters and to ignore the synesthetic colors of the letters. In the second condition, the instructions were reversed such that participants were asked to recall the synesthetic color of the letters and to ignore the real color. Results showed that associators
were faster in the first condition compared to projectors, and that projectors were faster in the second condition compared to associators.
Rouw and Scholte (2010) investigated the structural and functional neurobiological differences between associators and projectors. Gray matter (GM) volume and density were measured with voxel-based morphometry for each grapheme-color synesthete. In their research, the GM volumes and densities were contrasted for projectors with associators. Functional neurobiological differences were measured with functional MRI. All grapheme-color synesthetes were presented a personalized individualized set of stimuli with graphemes that elicited a strong, weak or no synesthetic experience. Meanwhile, the BOLD signal was measured. For each grapheme-color synesthete, the BOLD signal caused by the condition where graphemes elicited a strong and weak synesthetic experience, was contrasted with the condition where graphemes did not elicit a synesthetic experience. Then, these BOLD signals were compared between associators and projectors. Associators showed increased activation and gray matter volume in and near the hippocampus, including the parahippocampal gyrus, compared to projectors. The hippocampus and parahippocampal gyrus interact with memory encoding and retrieval (Maguire and Mummery, 1999; Olton et al., 1979), which mediates an experience similar to retrieving a memory. This leads to the synesthetic ‘in the mind’s eye’ experience of associators. Projectors showed structural differences in brain areas involved in perceiving (visual cortex, Heschl’s gyrus, superior precuneus cortex) and in brain areas involved in acting in the outside world (precentral gyrus). The synesthetic outside-world experiences are related to these brain areas.
To summarize, previous research shows neurobiological and cognitive differences between grapheme-color synesthetes and non-synesthetes as well as among grapheme-color synesthetes. However, neurobiological research on grapheme-color synesthetes show conflicting findings. Moreover, the findings of the synesthetic Stroop task among
grapheme-color synesthetes provides more in-depth information about the synesthetic experiences. Considering the evidence that supports the fact that projectors experience the synesthetic color in the outside world, and associators experience the synesthetic color “in the mind’s eye”, it is relevant to distinguish associators and projectors. As mentioned before, Rothen and Meier (2010a) and Yaro and Ward (2007) investigated differences in visual memory between synesthetes and non-synesthetes. However, no research has been done in visual memory differences between associators and projectors.
The current study aimed to provide insight into differences in short term visual memory of color between associators and projectors, since previous research indicated that grapheme-color synesthetes have a better memory for colors than non-synesthetes (Yaro and Ward, 2007). Differences in color memory between associators and projectors were measured with a memory task which required color matching. Since associators have increased activity of the hippocampus compared to projectors (Rouw and Scholte, 2010), and the hippocampus is involved in memory (Olton et al., 1979), it was hypothesized that associators have
enhanced color memory compared to projectors, and thus that associators perform better in color matching in the memory task than projectors. Furthermore, perceptual sensitivity of color matching was measured for associators and projectors. This was relevant to investigate, since differences in perceptual sensitivity would affect the interpretation of color memory differences between associators and projectors. Furthermore, the current study was the first study which investigates differences in perceptual sensitivity between projectors and associators.
Besides differences between associators and projectors, the current study investigated whether there are differences in short term visual memory of color and perceptual sensitivity of color between grapheme-color synesthetes and non-synesthetes. This was done to
studies, since previous research already investigated differences in visual memory performance and color discrimination between grapheme-color synesthetes and non-synesthetes (Banissy et al., 2009; Rothen and Meier, 2010a; Yaro and Ward, 2007). It was hypothesized that grapheme-color synesthetes have a better color memory than
non-synesthetes, since, as mentioned before, synesthetes performed better at recalling a colored matrix in the retention phase than non-synesthetes (Yaro and Ward, 2007). Furthermore, it was hypothesized that grapheme-color synesthetes have enhanced perceptual sensitivity, since previous research has shown that synesthetes are better at color discrimination than non-synesthetes (Banissy et al., 2009).
The findings of the current study are of interest since it would give more insight to differences between associators and projectors. In addition, previous research investigated visual memory differences between synesthetes and non-synesthetes (Rothen and Meier, 2010a; Yaro and Ward, 2007), however, this has not been done for associators and projectors. In addition, this is the first study which investigated differences in perceptual sensitivity between associators and projectors. Furthermore, this study emphasizes the importance of research on cognitive differences between associators and projectors, since previous research already found neurobiological differences (Rouw and Scholte, 2010).
Method Participants
122 fluent Dutch-speaking participants were recruited via the University of Amsterdam, these were psychology and psychobiology students, and via a formal advertisement for participation in our research posted on various social media platforms (Facebook, Twitter, Instagram, Reddit). This advertisement was posted in synesthesia-, art- and creativity-related groups, since synesthesia is more prevalent in fine art and art students
compared to the general population (Domino, 1989; Rothen and Meier, 2010b). In addition, the database of Professor Rouw, containing 200 grapheme-color synesthetes, was contacted.
A color picker consistency task was performed based on the internal consistency task of the Synesthesia Battery test of Eagleman et al. (2007). Participants were instructed to match the color of the letter with their synesthetic experience of the letter, using a color wheel. This color wheel allowed subjects to change the hue, color and saturation which allowed them to choose between approximately 16.5 million colors (250x250x250). Each letter was presented three times, printed in black and projected on a white background to create a high contrast, since a low contrast can affect or even eliminate the synesthetic experience (Eagleman et al., 2007; Hubbard et al., 2006). The independent variable in the color picker task was being synesthete or non-synesthete, and the dependent variable was the mean color distance between the three selected colors for each letter. This color distance is perceptual distance, since we used CIELUV color space for our analysis (Judd, 1940). Perceptual distance is defined as colors that are located further away from each other in the color space are perceived as less alike, and colors that are located closer to each other in the color space are perceived as more alike, and thus harder to discriminate.
Participants were defined as grapheme-color synesthetes when they both reported that seeing letters evokes a color experience, and when they scored below the cut-off value of 135 on the color picker consistency task. This cut-off value was based on Rothen et al. (2013). A lower value implies greater consistency. 94 participants reported experiencing colors when seeing letters, however, 25 of these participants scored above the cut-off value on the
consistency task and were omitted from the data analysis. One synesthete only completed the questionnaires and not the tasks since lack of time, and thus was omitted from the data. This led to 68 grapheme-color synesthetes (58 women, 9 men and 2 non-specified, mean age = 36, range =19-66). 28 of the 122 participants were non-synesthetes, however, one participant did
not complete the tasks since lack of time and thus was omitted from the data analysis. This led to 27 controls (22 women, 4 men and 1 non-specified, mean age = 23, range = 19-53). Associators versus projectors
To distinguish projectors and associators, grapheme-color synesthetes filled out the projector-associator (PA) questionnaire (Rouw and Scholte, 2007) (see Appendix A). This questionnaire consists of six projector- (e.g. “When I look at a certain letter or number, I see a specific color”), and six associator-related questions (e.g. “The color is not projected on the paper but floats in space”). Each question was rated on 5-point Likert-scale (1 strongly
disagree, 2 moderately disagree, 3 neutral, 4 moderately agree, 5 strongly agree). To calculate the PA score for each participant, the mean score of the six associator questions were
subtracted from the mean score of the six projector questions.
Procedure
The experiment used a between-groups design (associators and projectors, and grapheme-color synesthetes and non-synesthetes). Participants received a link leading to the study, which allowed them to conduct the study from home. Furthermore, they were
instructed to perform the test in a non-distracting environment. The color picker memory and color picker sensitivity task were part of a larger experiment, which took 1.5 hour. After giving consent, participants were asked to complete the color picker task. Several questions about the color picker task followed, a questionnaire about demographic information, the PA questionnaire, questions about the strength of the synesthetic experiences, the Coloured Letters and Numbers questionnaire of Rothen et al. (2013) and questions whether grapheme-color synesthetes experience other types of synesthesia. Then, participants were asked to complete a synesthetic Stroop task, a color picker sensitivity task, a color memory natural scenes task and a color picker memory task. After participation, participants received a
financial compensation. The tasks of this study were designed in Qualtrics version 2019 (Qualtrics, Provo, UT) using Javascript. The color wheel tool used in the color picker, color picker memory and color picker sensitivity task was an improved version of the color consistency tool of Eagleman et al. (2007).
Materials
Color picker memory task
Participants completed the color picker memory task to measure short term visual memory of color. A stimulus square was presented for five seconds. The color of this stimulus was based on one of the four randomly presented conditions: six primary colors (red, yellow, green, blue, cyan and magenta); six fully-saturated non-primary colors (halfway in between each of the primary colors); six darker versions of the primary colors; six darker versions of the non-primary colors. Thus, in total there were 24 trials. Then, a multi-colored mask was presented for 1000 ms. The mask contained 150 different colors, based on the red green blue (RGB) palette. A new square followed the mask, which was presented in a random color but different from the stimulus square color. Participants were instructed to remember the exact color of the stimulus square, to ignore the mask, and to match the color of the second square with the color of the stimulus square, using the same color wheel tool as used in the color picker task (Fig. 1). There was no time limit. The independent variable was defined as being associator or projector, and the dependent variable was defined as the color distance between the color of the stimulus square and the selected color of the second square.
Figure 1
Design color picker memory task
Note. Example of one trial of the color picker memory task.
Color picker sensitivity task
The color picker sensitivity task was performed to measure perceptual sensitivity of color. The design was similar to the design of the color picker memory task, however, the memory component was omitted. On the left side of the screen, participants were presented a mono-colored stimulus square projected on a multi-colored background. This background was the same as used as a mask in the color picker memory task, and the color of the stimulus square was based on the same four conditions as used in the color picker memory task. Thus, in total there were 24 trials. Participants were instructed to match the color of a square presented on the right side of the screen, projected on the same background, with the color of the stimulus square on the left side (Fig.2). Again, participants used the color wheel. There was no time limit. The independent variable of this task was being associator or projector, and the dependent variable was the color distance between the color of the stimulus square and the selected color of the right square.
Figure 2
Design of color picker sensitivity task
Note. Example of one trial of the color picker sensitivity task.
Analysis
To investigate whether there was a significant difference in color memory between grapheme-color synesthetes and non-synesthetes, and between associators and projectors, the color distances between the color of the stimulus squares and the selected color of the second squares for the color picker memory task were calculated for each participant via CIELUV analysis. After that, the total mean of these color distances was calculated for each participant. Then, differences in the total color distance mean between associators and projectors, and between grapheme-color synesthetes and non-synesthetes were calculated with an
independent two sample t-test.
In a second set of analysis, the total mean color distance between the colors of the stimulus squares and the selected colors of the right squares for the color picker sensitivity task was calculated for each participant via CIELUV analysis. Again, an independent two
sample t-test was conducted to investigate differences in the total color distance means between grapheme-color synesthetes and non-synesthetes, and between associators and projectors. Data-analysis was performed in R version 3.5.1. For statistical analyses alpha was set to 0.05.
Results Distinguishing associators and projectors
To distinguish associators and projectors, grapheme-color synesthetes completed the projector-associator questionnaire. For each participant, the PA score was calculated by subtracting the mean score on the six associator questions from the mean score on the six projector questions (Rouw and Scholte, 2007). The mean PA score for all grapheme-color synesthetes was -1.29 (SD = 1.60). However, the PA score ranged from -3.67 to 4.00. One synesthete scored PA = 0, and could not be defined as an associator or projector. This participant was omitted from further analysis. Based on the PA values, grapheme-color synesthetes could be distinguished in 59 associators (50 women, 7 men and 2 non-specified , mean age = 36, range = 19-66) and 9 projectors (8 women and 1 man, mean age = 35, range = 29-55). Table 1 represents a frequency table of PA values of all grapheme-color synesthetes. Based on the data analysis of the PA scores used in the study of Rouw and Scholte (2010), a one sample Kolmogorov–Smirnov test was performed to test the distribution of the PA scores on normality. The results showed that this distribution was not normal (D = 0.55, p < 0.01). For the current study, the PA scores and the results of the Kolmogorov-Smirnov test are sufficient to distinguish projectors and associators.
Table 1
Frequency table of the PA scores PA value Range from-to Frequency -4 ≤, <-3 5 -3≤, <-2 17 -2≤, <-1 21 -1≤, < -0 16 0≤, <1 4 1≤, <2 1 2≤, <3 2 3≤, <4 2 Total 68
Note Frequency table of 68 grapheme-color synesthetes on the Projector Associator questionnaire of Rouw and Scholte (2007).
aCut-off between associators and projectors
Grapheme-color synesthetes versus non-synesthetes
To investigate whether there was a significant difference in color memory between grapheme-color synesthetes and non-synesthetes, the results of the color picker memory task of the grapheme-color synesthetes were compared with the results of the non-synesthetes by conducting an independent two samples t-test, since this test is robust against violation of the normality assumption (normality assumption was tested with the Shapiro-Wilk’s and equality of variances with the Levene's test) (Venables and Ripley, 2002). The assumption of
normality for both grapheme-color synesthetes (W = 0.94, p < 0.01) and non-synesthetes (W = 0.83, p < 0.01) was violated. The data met the assumption of equal variances (F = 0.91, p =
0.34). The independent two sample t-test showed a significant difference in the mean color distance between the color of the stimulus square that had to be remembered and the matched color of the second square between grapheme-color synesthetes (M = 19.66, SD = 5.44) and non-synesthetes (M = 23.07, SD = 7.38); t (93)= 2.48, p = 0.01 (Fig. 3).
Figure 3
Results of short term color memory of grapheme-color synesthetes and non-synesthetes
Note Grapheme-color synesthetes had a better color memory than non-synesthetes, since the mean color distance of the color picker memory task was significantly lower for grapheme-color synesthetes than for non-synesthetes.
Furthermore, the results of the color picker sensitivity task were compared between grapheme-color synesthetes and non-synesthetes to investigate whether there was a difference in perceptual sensitivity. The assumption of normality for both grapheme-color synesthetes (W = 0.92, p < 0.01) and for non-synesthetes (W = 0.56, p < 0.01), and the assumption of equal variances (F = 8.35, p < 0.01) were violated. Therefore, the Welch’s two sample t-test was conducted, which indicated no significant difference in the mean color distance between the color of the stimulus square and the matched color of the second square between
grapheme-color synesthetes (M = 9.50, SD = 3.61) and non-synesthetes (M = 13.63, SD = 10.82); t (28.33)= 1.94, p = 0.06 (Fig. 4).
Figure 4
Results of perceptual sensitivity of grapheme-color synesthetes and non-synesthetes
Note No significant difference in perceptual sensitivity of color was found between
grapheme-color synesthetes and non-synesthetes, since there was no significant difference in mean color distance of the color picker sensitivity task.
Associators versus projectors
Both results on the color picker memory and color picker sensitivity task were compared between associators and projectors to investigate differences in color memory and perceptual sensitivity respectively. For the color picker memory task, the assumption of normality was violated for associators (W = 0.95, p = 0.02), however, not for projectors (W = 0.88, p = 0.14). Furthermore, the assumption for equal variances was met (F = 0.91, p = 0.34). Results of the independent two samples t-test showed no significant difference in the mean color distance between the color of the stimulus square that had to be remembered and the matched color of the second square between associators (M = 19.82, SD = 5.44) and projectors (M = 17.25, SD = 3.44); t (65)= -1.37, p = 0.17 (Fig. 5).
Figure 5
Results of color memory of associators and projectors
Note No significant difference was found in color memory, since there was no significant difference in the mean color distance of the color picker memory task between associators and projectors.
For the analysis of the color picker sensitivity task, the Welch’s two sample t-test was conducted, since the assumption of equal variances was violated (F = 8.35, p < 0.01).
Furthermore, the assumption of normality was violated for associators (W = 0.94, p = 0.01), however, not for projectors (W = 0.92, p = 0.39). Results of the Welch’s two sample t-test indicated no significant difference in the mean color distance between the color of the stimulus square and the matched color of the second square between associators (M = 9.38, SD = 3.20) and projectors (M = 8.84, SD = 3.65); t (10.01)= -0.42, p = 0.68 (Fig. 6).
Figure 6
Results of perceptual sensitivity of associators and projectors
Note No significant difference was found in perceptual sensitivity of color, since there was no significant difference in the mean color distance of the color picker sensitivity task between associators and projectors.
Discussion
The current study is the first study which investigated whether there are differences in short term visual memory of color between associators and projectors, by comparing their results of the color picker memory task. This task required matching colors, therefore, perceptual sensitivity of color was measured for associators and projectors, since differences in perceptual sensitivity could affect the interpretation of the results of the color picker memory task. Furthermore, non-synesthetes conducted the same protocol, and these results were compared with all grapheme-color synesthetes. This was done to investigate whether the results of the current study could replicate findings of previous studies, since previous
research already investigated that synesthetes have better visual memory and are better in perceptual differentiation of colors than non-synesthetes (Rothen and Meier, 2010a; Yaro and Ward, 2007).
Associators versus projectors
Contrary to the hypothesis that associators have enhanced color memory compared to projectors, since the study of Rouw and Scholte (2010) concluded that associators have increased activation and gray matter volume of the hippocampus compared to projectors, the current results show no significant difference in color memory between associators and projectors. This can be explained by the fact that Rouw and Scholte (2010) only found
increased BOLD signal of the hippocampus as a reaction to the presentation of a personalized individualized set of stimuli with graphemes that elicited a strong and weak synesthetic experience, and not as a reaction to a set of stimuli which did not elicit a synesthetic
experience. In the current study, the colors of the stimulus squares of the color picker memory task were not based on synesthetic colors, however, the colors were predetermined. This could indicate that there is no difference in visual memory for general colors between associators and projectors, and perhaps only for synesthetic colors.
Furthermore, this was the first study which investigated differences in perceptual sensitivity between associators and projectors, by comparing the results of the color picker sensitivity task. Moreover, the results on this task indicated if perceptual sensitivity would have an influence on the interpretation of the current results of the color picker memory tasks. The current study showed no significant difference in perceptual sensitivity between
associators and projectors, which means that perceptual sensitivity does not affect the current results of the color picker memory task significantly. Thus, according to the findings of the color picker memory task, there is no difference in color memory between associators and projectors.
However, the generalizability of the results of this study on associators and projectors is limited by the unequal sample sizes between associators and projectors, and the small number of projectors. This is probably due to the fact that approximately 10% of the
synesthetes are projectors (Dixon and Smilek, 2005). This leads to less power to detect differences in color memory and perceptual sensitivity. Moreover, the design of the color picker memory task can be seen as a limitation of the study. Rouw and Scholte (2010) measured the BOLD signal of associators and projectors as a reaction to a personalized individualized set of graphemes which elicited a synesthetic experience, and a set of graphemes which did not elicit a synesthetic experience. The BOLD signals of the two sets were contrasted against each other, and the results were compared between associators and projectors. Associators showed enhanced BOLD signal of the hippocampus as a reaction to stimuli which induce synesthesia compared to projectors. However, the stimulus squares in the color picker memory task were printed in predetermined colors and were not based on the personalized synesthetic colors indicated in the color picker task. Thus, further research should replicate this study with higher sample sizes to gain more power to detect possible differences in color memory. Moreover, further research should use two different types of the color picker memory task. The first one contains the same design of the current study, with stimulus squares printed in predetermined colors. This would measure differences in visual memory of color in general. The second one contains stimulus squares printed in personalized individualized synesthetic colors to investigate whether there is a difference in visual memory of synesthetic colors. Besides the fact that this will investigate differences in visual memory between associators and projectors, it is interesting in a different way. Studies to differences in memory namely use stimuli which induce synesthesia, instead of stimuli of what is experienced, the synesthetic colors. For instance, Yaro and Ward (2007) investigated that synesthetes have enhanced verbal memory for word lists compared to non-synesthetes. Thus, the color picker memory task approaches differences in memory among synesthetes in the opposite way. Moreover, the current study emphasizes the importance of more research on different types of memory between associators and projectors, such as verbal memory.
Since this is the first study on differences in perceptual sensitivity between associators and projectors, further research is required to ensure the current finding that there is no difference in perceptual sensitivity. This can be investigated by replicating the current study with higher sample sizes, and to replicate the study of Banissy et al. (2009), where
participants were asked to arrange different hues of a color in a specific order. This will measure differences in perceptual sensitivity in terms of color discrimination between associators and projectors.
Grapheme-color synesthetes versus non-synesthetes
Non-synesthetes performed the color picker memory and sensitivity task to compare differences in short term visual memory and perceptual sensitivity of color between all grapheme-color synesthetes and non-synesthetes. This was done to investigate whether the current study could replicate previous findings, since previous research concluded that
grapheme-color synesthetes have a better visual memory and are better in color discrimination than non-synesthetes (Banissy et al., 2009; Rothen and Meier, 2010a; Yaro and Ward, 2007). The current study showed enhanced color for grapheme-color synesthetes compared to non-synesthetes, however, no difference in perceptual sensitivity was found. The results of the color picker memory task give new insight in the study on differences in visual memory between grapheme-color synesthetes and non-synesthetes, since Yaro and Ward (2007) only found enhanced visual memory for grapheme-color synesthetes in the retention phase when participants were asked to remember a colored matrix, and not in the learning phase. The design of the color picker memory task only included immediately matching the color of the stimulus square after a short time delay, and no learning and retention phase. The fact that grapheme-color synesthetes have enhanced visual memory compared to non-synesthetes in
the current study, could indicate that grapheme-color synesthetes have enhanced short term visual memory of colors in general compared to non-synesthetes.
Furthermore, the results of the color picker sensitivity task are contrary to the
hypothesis that grapheme-color synesthetes would be better in color discrimination than non-synesthetes. Results of the current study showed no significant difference in perceptual sensitivity. This can be explained by the fact that Banissy et al. (2009) asked participants to arrange different color hues of a color sequence in the right order. This tested perceptual sensitivity in terms of color discrimination. However, in the color picker sensitivity task, participants were asked to match the color of a square with the color of a stimulus square which was presented next to it. This tested perceptual sensitivity in terms of matching colors. Thus, compared to Banissy et al. (2009), the current study investigated a different type of perceptual sensitivity, which gives more insight to the study on perceptual sensitivity differences between grapheme-color synesthetes and non-synesthetes.
Again, the generalizability of the results of this study is limited by the difference in sample size between grapheme-color synesthetes and non-synesthetes. This leads to less power to detect a difference in color memory and perceptual sensitivity. Further research should replicate this study with higher sample sizes, to increase the power to find differences between grapheme-color synesthetes and non-synesthetes. Furthermore, to investigate whether grapheme-color synesthetes indeed have enhanced visual memory of colors in general than non-synesthetes, as the color picker memory task indicates, further research should design tasks which test visual memory in a different way. For instance, a design can be used were individual colors are presented one after the other. Participants will be asked to remember these colors in the right order. In the first condition, the colors are based on personalized synesthetic colors. In the second condition, the colors are predetermined. Then, the results of grapheme-color synesthetes will be compared to the results of non-synesthetes.
Results of the first condition will indicate whether there is a difference in visual memory of synesthetic colors. Results of the second condition will indicate whether there is a difference in visual memory of general colors. In addition, the results of grapheme-color synesthetes can be compared between the two tasks, to investigate whether there is a difference in visual memory of colors in general and for synesthetic colors. This is interesting to investigate, since it will test the hypothesis of Yaro and Ward (2007) about why grapheme-color synesthetes perform better at the colored matrix memory task than non-synesthetes. Namely, they suggest that the results of their study can be explained by the idea that synesthetes have enhanced memory due to enhanced retention of color in both non-synesthetic and synesthetic situations. This can be investigated by testing the color picker memory task in synesthetic and non-synesthetic colors as explained above. To create more power for this improved design, it is of great importance to insert the color picker sensitivity task, since the current findings of the color picker sensitivity task are not generalizability. To have a clear interpretation of the improved designs of the color picker memory task, differences in perceptual sensitivity should be investigated.
To summarize, the current study provides more insight into the cognitive differences between associators and projectors, and emphasized the importance of more research on this research field. Results indicate no differences in short term visual memory of color and perceptual sensitivity between associators and projectors. In addition, grapheme-color synesthetes showed enhanced color memory compared to non-synesthetes, however, no difference in perceptual sensitivity was found.
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Appendix A
Projector-Associator (PA) questionnaire (Rouw and Scholte, 2007)
Statement (Dutch) Statement (English) Categorized as projector or associator question Wanneer ik naar een
bepaalde letter kijk, dan zie ik een specifieke kleur
When I look at a certain letter, I see a specific color
Projector
Wanneer ik naar een bepaalde letter kijk, verschijnt de bijbehorende kleur alleen in mijn gedachten en niet ergens buiten mijn hoofd (zoals op het papier)
When I look at a certain letter, the corresponding color appears only in my thoughts and not somewhere outside my head (such as on the paper)
Associator
Wanneer ik naar een bepaalde letter kijk, komt daarvan de bijbehorende synesthetische kleur in mijn gedachten maar op het papier verschijnt enkel de kleur waarin de letter gedrukt is (bijvoorbeeld een zwarte letter tegen een witte achtergrond)
When I look at a certain letter, the corresponding synesthetic color appears in my thoughts, but on the paper appears only the color in which the letter is printed (for example, a letter printed in black presented on a white background)
Associator
Het is alsof de kleur zich daadwerkelijk op het papier bevindt waarop de letter gedrukt staat
It seems that the color is actually on the paper on which the letter is printed.
Projector
De figuur zelf heeft geen kleur maar ik ben ervan bewust dat deze
geassocieerd is met een specifieke kleur
The figure itself has no color, but I am aware that it is associated with a specific color
Associator
De kleur is als het ware geprojecteerd op de letter
The color is as it were projected on the letter
Projector Ik zie letters niet letterlijk in
een kleur maar heb een sterk gevoel dat ik weet welke kleur bij een bepaalde letter hoort
I do not literally see letters in a color, but I have a strong feeling that I know which color belongs to a certain letter
Associator
De kleur bevindt zich niet op het papier maar zweeft in de ruimte
The color is not on the paper but floats in space
Associator
De kleur heeft dezelfde vorm als de letter
The color has the same shape as the letter
Projector Ik zie de kleur van een letter
alleen in mijn hoofd
I only see the color of a letter in my head
Ik zie de synesthetische kleur heel duidelijk in nabijheid van de stimulus (b.v. erop of erachter of er overheen)
I see the synesthetic color very clearly in the vicinity of the stimulus (e.g. on or behind it or over it)
Projector
Wanneer ik naar een bepaalde letter kijk, verschijnt de bijbehorende kleur ergens buiten mijn hoofd (zoals op het papier)
When I look at a certain letter, the corresponding color appears somewhere outside my head (like on the paper)
