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Some synesthetes report seeing bright flashes when hearing a loud noise. However, in the same person, bright flashes of light are not reported as being loud. I've read of other examples like this; for instance, the number 1 may always be yellow, but yellow is not the number 1.

Has there been any research as to the neurological reasons why synesthesia is asymmetric? Alternately, is my anecdotal assumption that it is asymmetric incorrect and there are actually frequent symmetric presentations of the condition?

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    $\begingroup$ @AliceD, I understand the intention of the edit, but I'd like to add that the term 'associated' understates the quality of the perception that sudden loud noises are actually bright. The expression seems ill-suited in a similar sense as saying "seeing certain apples is associated with seeing green." One would typically say "he saw green apples" rather than "he saw apples and then also saw green". Likewise, a synesthete may report he saw a "bright noise". This distinction may be relevant in tracking down the point in processing at which the senses are conflated. $\endgroup$ – Dan Bryant Aug 12 '15 at 14:28
  • $\begingroup$ Feel free to roll back any edits. Just trying to improve clarity and draw more answers in :) How about now? English isn't my native language. Perhaps I should refrain editing native-English posts! Sorry. Again, feel free to roll back any or all edits. PS: I flagged Seanny's comment for removal because it's obsolete. $\endgroup$ – AliceD♦ Aug 12 '15 at 14:30
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Generally spoken, synesthesia is unidirectional. For example, grapheme–color synesthesia (i.e., letter–color and digit–color synesthesia) is the most prevalent type of synesthesia. The presentation of a grapheme leads to an additional synesthetic color percept. Although grapheme–color synesthetes are strongly influenced by the synesthetic color perception triggered by a specific grapheme, they usually do not report that a color triggers the perception of a specific grapheme.

However, while explicit synesthesia is unidirectional, there is evidence that it can be implicitly bidirectional. To show this phenomenon, clever experiments need to be devised. For example, the left-hand responses of digit-color synesthetes were faster for colors representing a small number, whereas their right-hand responses were faster to colors representing large numbers. Hence, apparently there was an implicit association of the color with numbers.

Reference
Weiss et al., J Cognitive Neurosci (2008); 21(10): 2019–26

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    $\begingroup$ Looks like an interesting study. An implicit association makes sense, much like one can begin to associate a particular scent of perfume with a particular person or a color (red/green) with an action (stop/go); this might involve associative memory and learning. It would interesting to see a study with some form of classical association training of non-synesthetes prior to the experiment to see if their performance is improved by a similar magnitude. $\endgroup$ – Dan Bryant Aug 12 '15 at 14:41

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