Posted by Deepak H. Singh, MS IV Drexel University College of Medicine
Putting my intense desire to describe the mauve affect of a patient, or the loud shirt that a colleague is wearing aside, synesthesia is a fascinating phenomenon in which two or more senses in certain individuals are overlapped, meaning that the experience of both senses is connected in someway.
This has been described in terms of various different senses including forms such as grapheme-color synesthesia in which letters and numbers are perceived as colors, chromesthesia in which sounds are perceived as color, or lexical-gustatory synesthesia in which individual words are perceived as taste sensations in the mouth as alluded to in the title of this blog post.
Click here to find out more about word-taste synesthesia from the BBC.
This condition has long perplexed neuroscientists who are only touching the surface of the unique neural pathways that may account for the various experiences described by synesthetes.
One school of thought that has gained some traction is the concept of cross-activation, which is made possible by a failure of the physiological process of “synaptic pruning” that occurs in all of our brains during the initial developmental stages. Synaptic pruning refers to a series of regulatory processes during which various axonal networks that were functional in one stage of development are outcompeted and subsequently eliminated as other synaptic connections become more frequently used as maturation occurs. In synesthetes it is hypothesized that certain of these connections fail to regress leading to atypical connections between two sensory regions of the brain, thereby opening the door for some pretty vivid sensorial experiences. This, however, has only really held up for sensory regions directly adjacent to one another, as was demonstrated by fMRI studies showing significant brain activity in both the auditory cortex and the fusiform gyrus (responsible for color perception) in synesthetes while no such congruous activity was seen in age-matched controls. Similarly, in one study on lexical-gustatory synesthesia, the lateral sulcus (responsible for taste processing) was activated simultaneously with the auditory cortex in synesthetes.
Another prevailing hypothesis is the concept of “disinhibited feedback“. Normally, signals are travelling in both directions between the primary sensory regions of the brain and those that are involved in organizing that information, and feedback (both positive and negative) is constantly occurring to reconcile all of the different sensory input. If this balance were disrupted, however, it would be possible for signals encountered in the later stage of processing to influence those that were encountered earlier, resulting in the overlapping sensations that are perceived by synesthetes. This, too, may make more sense of the case reports in which individuals with temporal lobe epilepsy or individuals who have just experienced head trauma or stroke to “acquire” a synesthesia-like experience due to some disruption in those pathways, though no concrete studies have been done to test that theory.
Perhaps we’ll never know what is truly occurring at a neuroanatomical level that causes such a curious phenotype. In the meantime, looking at the accomplished list of puported “sufferers” of this condition, it may be worthwhile to pose the following question: which came first the synesthesia or the visionary?