Acquiring synaesthesia: insights from training studies

Abstract

Synaesthesia denotes a condition of remarkable individual differences in experience characterized by specific additional experiences in response to normal sensory input. Synaesthesia seems to (i) run in families which suggests a genetic component, (ii) is associated with marked structural and functional neural differences, and (iii) is usually reported to exist from early childhood. Hence, synaesthesia is generally regarded as a congenital phenomenon. However, most synaesthetic experiences are triggered by cultural artifacts (e.g., letters, musical sounds). Evidence exists to suggest that synaesthetic experiences are triggered by the conceptual representation of their inducer stimuli. Cases were identified for which the specific synaesthetic associations are related to prior experiences and large scale studies show that grapheme-color associations in synaesthesia are not completely random. Hence, a learning component is inherently involved in the development of specific synaesthetic associations. Researchers have hypothesized that associative learning is the critical mechanism. Recently, it has become of scientific and public interest if synaesthetic experiences may be acquired by means of associative training procedures and whether the gains of these trainings are associated with similar cognitive benefits as genuine synaesthetic experiences. In order to shed light on these issues and inform synaesthesia researchers and the general interested public alike, we provide a comprehensive literature review on developmental aspects of synaesthesia and specific training procedures in non-synaesthetes. Under the light of a clear working definition of synaesthesia, we come to the conclusion that synaesthesia can potentially be learned by the appropriate training.

Keywords: acquiring; control; definition; development; learning; synaesthesia; training.

Figure 1

Number of publications per year in the field of synaesthesia research. The figure represents the hits for the search terms “synaesthesia OR synaesthesia” in the title, abstract, or keywords. Source: Scopus (17/09/2013).

Figure 2

Number of publications per year in the scientific field of cognitive training. The figure represents the hits for the search term “cognitive training” in the title, abstract, or keywords. Source: Scopus (17/09/2013).

Figure 3

Synaesthetic conditioning. In this particular example, the letter D was associated with blue during the training. That is, blue squares not followed by the startling sound acted as CS color and the letters as CS letter. The remaining colors were neutral. Each square represents one trial with the first trial in the upper left corner and the last trial in the lower right corner (moving from left to right). Adopted from Meier and Rothen (2009).

Figure 4

Results from the synaesthetic conditioning paradigm showing a conditioning response for the CS color, but not the CS letter and neutral color stimuli for trained non-synaesthetes. Error bars represent standard errors. Adopted from Meier and Rothen (2009).

Figure 5

Adaptive training. Exemplary depiction of a trial. Adopted from Rothen et al. (2011).

Figure 6

Visual search task. Two exemplary trials. The two target letters of interest (i.e., H and U) appeared more often in a specific associated color. The other letters appeared in each color equally often. Adopted from Kusnir and Thut (2012).

Figure 7

Implicit numerical magnitudes. Exemplary depiction of experimental conditions: Facilitation and interference effects due to activation of implicit numerical magnitudes via color information. The red horizontal arrows represent the virtual numerical magnitude as indicated by the colors. Adopted from Cohen Kadosh et al. (2005).