Does synaesthesia lead to better memory capacities?

One can read in many books and newspaper articles that synaesthetes have superior memory abilities [2]. Do they really? Yaro and Ward [3] surveyed 46 synaesthetes: persons had to estimate, if their memory abilities are a) better than the average, b) like the average or c) worse than the average. 70% of them believed that their memory abilities were better than the mean. In a second task 16 synaesthetes and 16 matched controls had to pass for several memory tests: for example they had to recall words they heard remember digits from colored matrices and copy freely complex figures. Synaesthetes only showed a "minimal" better memory performance in recalling words but not in memorizing digits or copying complex figures. Similar studies did not reveal any difference between synaesthetes and non-synaesthetes in episodic memory. The conclusion they draw is that synaesthesia per se does not lead to a better memory. They explained individual differences by using synaesthesia as mnemonic strategy [4]. Yaro and Ward stated that "synaesthesia plus training may lead to truly exceptional memory".
The myth that synaesthetes have generally a better memory may come from several single-case studies. Luria [5] describes synaesthete S. which was able to learn a matrix with 50 digits in a few minutes and to recall it 15 years later. Daniel Tammet [6] is also a very special case: He is an autistic synaesthete. He broke the European record for reciting 22´514 digits of Pi without error. He also learned the islandic language within a week. There are further descriptions of synaesthetes with superior memory abilities [7, 8] but one never knows to which degree synaesthesia is responsible for that. These single-case studies also reflect a selection-bias: people with an exceptional memory are more likely to attract the attention of a researcher [4]. Thus, it is quite delicate to generalize these extraordinary memory skills to all people with synaesthesia.

positive influence of synaesthesia on learning
I suggest that there are several processes important for long-term storage which could be influenced beneficially by synaesthesia. In learning theory it is proposed that learning processes can be supported by using different modalities: Something only heard is not stored that easy as when it is seen, too [9]. synaesthesia provides this opportunity: several senses are activated at the same time. Synaesthetes do not only perceive historic dates like 1492 as a number. Trough the vivid nature of synaesthesia they also can perceive colors, genders or personalities as additional information. They automatically make several kinds of facts meaningful [10]. I remember when I was a child my mind was full of mathematical synaesthetic perceptions. One example is the equation 4 * 6 = 24: this mathematical term I perceived as the ‘story of the 4 who was obligated to marry 6'. These personifications were elicited involuntary and were not created actively by myself. It supported me in dealing with numbers. Adding meaning to information and rely it onto prior knowledge is an aid in learning processes. Synaesthetic perception exactly does that, one only has to be aware of it.
Synaesthetes experience more vivid visual imagery than non-synaesthetes. Those images arise automatically and are consistent over time [11, 12]. Making calculations, some synaesthetes see blocks piling up when calculating ‘7 - 5 = 2' (figure 4) or reading historic texts they see movies on their mind's eye. Additionally some see temporal concepts like weeks and decades spatially visualized [13] or they perceive the alphabet and the numbers arranged on a spatial line [14]. Ormrod [9]explains that "forming visual images can be a powerful means of storing information". Therefore, synaesthesia offers different beneficial opportunities to act as a mental aid in learning.But synaesthetic awareness is required and an reasonable implementation of the synaesthetic perception.

negative influence of synaesthesia on learning
There are a great number of studies [8, 15], that claim that synaesthetes confronted with ‘wrongly' colored digits can lead to a distracting effect. Synaesthetes report "feeling uneasy". They may claim that incongruent colors just do not match and sometimes they describe this as ‘ugly'. Perceiving incongruently colored graphemes also leads to affected judgments of emotional valence [16]. One study, examining synaesthete C., showed that she was faster to name an equation like 2 + 3 when it was followed by the color that matched the result. Vice versa she was slower in naming the solution when there was an incongruent color presented [17].
The synaesthetic Stroop-Task works in a similar way like the ordinary Stroop-Task: colored digits are shown to testing subjects consisting of synaesthetes and non-synaesthetes. The color of a digit was either a) neutral, b) congruent (subjectively right to the synaesthetes perception) or c) incongruent. For example, synaesthetes had more difficulties in naming the showed letter when it was presented in his subjective wrong color [18]. Further the incongruity-effect caused an enlargement of synaesthetes pupils [19] .
C. provides superior memory and has no problems to learn a matrix with 50 digits. In a second experiment she had to memorize 50 digits which were in a "wrong" color. She performed "considerably poorer" in this task [8]. Contemporary, other studies did not found this incongruency effect in learning tasks with adults. This may be based on the fact that some synaesthetes report to mentally "translate" incongruent colors into congruent ones [3, 4] or because the task to learn 50 digits in general is very difficult.
Green et al. [20] conducted experiments with children between 7 and 15 years. In one task they had to memorize digits in matrices. The synaesthetic children showed a significant decrease in recall when they had to learn the incongruent colored digits. Green reported that incongruency has a "detrimental effect" on the performance of synaesthetic children. Maybe incongruent colors might have a stronger negative effect on memory in children than for adults, but further studies are needed to say more about this.
I remember an email of a 13-year-old boy. He claimed that his colored letters do confuse him and with this explication he reasoned his weakness in math. But blaming synaesthesia for own weaknesses is just too simple.
Ward [21] describes a survey conducted among synaesthetes which had to rate their strengths and their weaknesses. Interestingly, mathematics was claimed equally as strength and as weakness. Synaesthetes are individually different. One can't generalize their abilities neither their interests. Colored numbers do not automatically lead to a better comprehension of mathematical terms. For some synaesthetes colored numbers are more confusing than beneficial. But I believe that assistance in dealing with these confusing effects of letters can be deleted through helping synaesthetes in arranging it, integrating it and thus learning to use the colors in a beneficial way.

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references
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