A fascinating case study from 1907 describes the self-reported sensory “taste” experiences evoked by hearing specific words, names, or sounds (Pierce, 1907). The subject was a young woman about to graduate from college. As far as she could tell, she's always had these experiences, and for most of her life she didn't know they were unusual. This surprise upon discovering the uniqueness of one's one internal experience is similar to what is reported by many contemporary individuals with less typical phenomenology, such as aphantasia (the inability to generate visual images).
Pierce noted that the subject was anosmic (had a loss of smell), although this was not formally tested. Nonetheless, he observed:
Coffee burning upon a stove is not noticed, though she may be close by. Camphor placed in the nostrils gives only a stinging sensation. Ammonia can be sniffed without discomfort. And, as with all anosmics, foods are discriminated on the basis of the pure taste qualities, or by the characters of texture, astringency, and so on, which any food-complex may possess. These sensory defects are worth noting for their possible significance in connection with the theory of this special case of synæsthesia.
Some aspects of the report were rather quaint by modern standards (e.g., the first person narration), but others were quite prescient.
But what evidence have we that an actual case of synæsthesia is here being reported, and not a case of artificial association due to a lively dramatic fancy? This is a point that must be raised, for it is very easy to entertain a suspicion that these phenomena are essentially ungenuine. Now, of course, in matters of this kind general impressions and personal knowledge of the subject count heavily. And on both these grounds I have no hesitation in asserting my conviction that the above-cited equivalents are the expression of a genuine synæsthesia.
One key piece of evidence was the consistency of gustatory experiences associated with the same words at a later date:
...the equivalents possess a constancy which would hardly be possible apart from a true synæsthesia. After an interval of six months a number of words were given at random from the original list, with the result that the identical equivalents were described in almost precisely the same language.
Also, there was a somatotopic-like arrangement for some of the gustatory experiences:
Many of the experiences are given quite definite locations in the mouth. Thus the equivalents of Ethel and Hall (tactual) are felt at the tip of the tongue; of lox (irritation) at the back of the throat; of Judith (salt) at the sides of the tongue; of Sarah (cold) on the lips; of amethyst (bitter) "at the back of the mouth, on the roof, where the root of the tongue seems to hit it" ...
In attempting to decide whether the above-cited experiences are to be explained by the physiological or by the psychological theory, we are, apparently, in no better and in no worse case than in respect to all varieties of synæsthesia. No decisive facts are at hand.
An interesting commentary in JAMA predated MRI connectivity studies of synesthesia by nearly a century in its speculation about alternative brain wiring:
There is little evidence one way or the other as to whether or not this phenomenon is to be explained on purely physiologic grounds, depending on a cross circuiting of the association fibers between different brain centers.
Pierce concludes his paper with a pithy aphorism that acknowledges the inconclusiveness of the study:
Still, here no less than in all known cases of synaesthesis, we can only regret that our theory is so lame while our facts are so secure.
References
Pierce AH. (1907). Gustatory audition; a hitherto undescribed variety of synæsthesia. The American Journal of Psychology. Jul 1:341-52.
Commentary (1907). GUSTATORY AUDITION. JAMA. XLIX(10):857-858.
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