Synesthesia is a condition in which sensory input from one cognitive stream gives rise to experience in another unstimulated cognitive stream. The most common forms of synesthesia include grapheme-color synesthesia and week-color synesthesia. In grapheme-color synesthesia, viewing or thinking about a letter or number written in black ink induces the experience of a certain color. Week-color synesthetes experience a particular color associated with days of the week. For example, Monday might be brown, Tuesday red and Wednesday green.
How is synesthesia distinguished from other types of learned associations? This is starting to become a tricky question. Genuine synesthesia is thought to exhibit idiosyncrasy, test-retest reliability and automaticity. For example, in grapheme-color synesthesia, it appears that colors are assigned to letters in an arbitrary manner (idiosyncrasy). But once these colors are assigned, the become “married” to the graphemes, that is, the color associations remain stable over time (test-retest reliability) and the grapheme-color synesthete cannot help but see the associated color when thinking of the letter (automaticity).
The extent to which synesthesia idiosyncratic has been a subject of much debate. It is possible that grapheme-color associations are learned during childhood. For example, some authors have suggested that grapheme-color associations often match the colors of the alphabetic refrigerator magnets found in the homes of many children. Many researchers have responded that the evidence is nothing more than anecdotal and thus does not support the theory that these associations can be learned.
However, a recent study provides the evidence proponents of a memory model have been looking for. Researchers looked at the letter-color associations of 11 grapheme-color synesthetes that reported growing up with the same Fisher-Price refrigerator magnets. Using a standardized synesthesia battery that measures differences in color associations, the team found a striking similarity of letter-color associations among the synesthetes and nearly all color associations matched the colors of the Fisher-Price refrigerator magnets. This is an amazing result! The fewest matches among the 11 subjects was 14 letters. The chance that 11 synesthetes would choose the same color for 14 out of 26 letters of the alphabet is less than 1 in a billion! Such a striking result suggests that memory plays a crucial role in grapheme-color synesthesia.
But how exactly is memory involved in forming grapheme-color associations? Our lab has proposed that the automatic association between graphemes and colors in grapheme-color synesthesia is much like the automatic association between smell and memories. For example, the smell of chlorine may automatically induce visual images of a particular pool party. In the case of smell, the tight association presumably is formed immediately as a result of the negative value of the event.
According to a recent model of memory, which we might call the ‘reactivation model’, the hippocampus is not a storage space for information but a region of the brain in charge of maintaining connections between neural networks located in different areas of the brain. Working memory in the prefrontal cortex and hippocampus work in tandem. The hippocampus guides the depositing of proteins at the synapses of neurons in areas that originally processed the information to be remembered. Together with neighboring hippocampal areas it also keeps track of the relative order of events and binds together events that belong together. Memory retrieval by working memory reactivates the original areas of information processing by interaction with the executive hippocampus.
On the memory model, synesthesia is the result of an indirect mechanism. The hippocampus would at some point have bound together long-term potentiation (LTP) in visual color areas as well as in grapheme areas. Exposure to achromatic grapheme-stimuli would trigger both recognition of the grapheme as a particular grapheme (e.g., the numeral ‘2’) and memory retrieval of synesthetic colors to executive areas of the brain. The renewed activity in the color areas taking place in order for memory retrieval of synesthetic color information to occur may simultaneously give rise to a conscious projection of synesthetic color from visual color areas.
In synesthetes in which graphemes and colors truly are bound together to the extent that graphemes literally are seen as having colors, hippocampus may be treating the distinct neural networks the way it normally would with form and color that belong together (e.g., tomato and red). In cases in which grapheme and color are not tightly bound together, the hippocampus must be treating the neural networks more like involuntary quick associations, such as the association between the striking of a match and its being lit.