Diagnosing Synesthesia

Diagnosis:

At first, to definitively or objectively diagnose synesthesia may seem an impossible goal. It is, after all, a condition defined by subjective sensory experiences rather than a more evident physical condition like a broken bone or an ulcer with clearly observable symptoms. However, synesthesia is a real condition, and effective sets of objective diagnostic criteria, as well as objective tests, have been developed for its diagnosis. Grapheme – color synesthesia is the type for which these tests are most fully realized and most comprehensive.

Clinical Features:

A number of defining and distinguishing features of synesthesia have been outlined over the years, and, in 1995, as part of a symposium on synesthesia, Richard Cytowic, a leading synesthesia researcher, outlined a set of clinical features of synesthesia to be used for its diagnosis. Firstly, one of the most fundamental aspects of synesthesia is its involuntary, passive nature, meaning that synesthetes have no control over the presence or nature of synesthetic sensations – they will always occur with the proper stimulus or context.

These sensations are “projected” externally, near to the synesthete, generally often “over” the standard perceptual field. They are not imagined, nor are they perceived delusionally as a part of the surroundings, like the external hallucinations of other, unrelated, conditions.

Synesthetic perceptions are also “durable and generic, never pictorial or elaborated.(Cytowic 1995)” In other words, a synesthetic sensation is simple and objective – visual synesthetic sensations would consist of a geometric pattern or a color, rather than a complex imagining of a scene.

Synesthetes often have enhanced memory due to their synesthesia, particularly in the case of grapheme-color synesthesia, where a synesthete would more easily remember a sequence of numbers because of their colors. Cytowic defines this memory of synesthetic perceptions, too, as a distinguishing diagnostic feature. Synesthetic attributes of an object or word are remembered particularly well, often better than the object or word itself. Cytowic gives the example that a synesthete might remember that a person's name is green, but not which green name it is.

We can see from these criteria that, as synesthesia is an individual experience, it is important to acknowledge the value and validity of an individual's subjective reports and descriptions of their synesthesia in order to both diagnose and learn more about different types and subtypes of synesthesia.

Objective Tests:

Tests based on these more general features of synesthesia, as well as more specific features of grapheme-color synesthesia, can objectively diagnose the presence of synesthesia. synesthetes score 70-90% on the Test of Genuineness (TOG), which relies mainly on testing the consistency of synesthetic response, while control subjects without synesthesia score 20-38%. This test, however, cannot characterize different subtypes or individual characteristics of synesthesia.

Other tests have been developed, notably a battery of tests developed by Dr. David M. Eagleman's laboratory (available at http://synesthete.org). This battery originally included only tests relating to grapheme-color synesthesia, but it has since been expanded to include attempts at diagnosing other forms of synesthesia, such as sound-color synesthesia. These tests represent an effort to provide a standardized, quantifiable test that allows for comparisons across different individuals and different variants of synesthesia, facilitating not only individual diagnosis, but also further research with larger sets of comparable data than previously compiled.

The tests for grapheme color synesthesia are the most accurate and complete. They begin with a color consistency test, wherein a grapheme is presented and the subject is prompted to provide the color of the grapheme on an interactive color palette allowing the selection of 16.7 million different colors. Additionally, as some graphemes may not provoke a synesthetic response in all synesthetes, an option for “no color” is provided. The variation in color responses for each grapheme is analyzed within the testing program, and assigned a color variation value. Values below 1.0 indicate very low color variation, and thus synesthesia, while control subjects without synesthesia score well above this, generally around 2.0.

The second test in the grapheme-color battery is the 'speeded congruency test,' wherein a colored grapheme is presented for one second, and the subject must respond as quickly as possible as to whether or not the color matches their synesthetic association. Half of the colors match the subject's reported synesthetic color from the previous section, while half are colors distinguishably different from the reported synesthetic response that were previously reported as associated with other graphemes. Synesthetes respond with an average of 94% accuracy to this test, with reaction times averaging 0.64 seconds, plus or minus 0.78 seconds. Subjects without synesthesia respond in an average of 0.91 seconds, plus or minus 0.87 seconds, with an average accuracy of 67%.. While it would be difficult, a non-synesthetic subject could theoretically create a cheat sheet for themselves to assure consistency, and fool the first test, especially given that it provides an unlimited amount of time to most accurately find the correct color match. The second timed test, however, provides good confirmation of the presence (or lack) of synesthesia, as it makes this deception impossible.

The second test in the grapheme-color battery is the 'speeded congruency test,' wherein a colored grapheme is presented for one second, and the subject must respond as quickly as possible as to whether or not the color matches their synesthetic association. Half of the colors match the subject's reported synesthetic color from the previous section, while half are colors distinguishably different from the reported synesthetic response that were previously reported as associated with other graphemes. Synesthetes respond with an average of 94% accuracy to this test, with reaction times averaging 0.64 seconds, plus or minus 0.78 seconds. Subjects without synesthesia respond in an average of 0.91 seconds, plus or minus 0.87 seconds, with an average accuracy of 67%.. While it would be difficult, a non-synesthetic subject could theoretically create a cheat sheet for themselves to assure consistency, and fool the first test, especially given that it provides an unlimited amount of time to most accurately find the correct color match. The second timed test, however, provides good confirmation of the presence (or lack) of synesthesia, as it makes this deception impossible.

This set of tests also includes questions about specific qualities of synesthetic sensations, allowing for the categorization by subtype of the subject, as well as collection of data related to the various subtypes of synesthesia. In particular, there is a set of questions presenting graphemes at different contrast levels, in order to attempt to distinguish between hypothesized subtypes of grapheme-color synesthesia that may be distinguished by a change in or elimination of synesthetic response with reduced contrast. This is a potential feature, though not the only one, of the proposed distinction between 'higher' synesthesia, where color associations are made with concepts of letters and 'lower' synesthesia, where it is the simpler details of letters that define synesthetic responses. Questions about the subject's psychological/neurological history and individual experiences are also included. Perhaps one of the most notable additions of this test, which addresses concerns about dismissing individual experience, is the inclusion of fields allowing subjective elaboration and notes about other types of synesthetic experience not tested for in this battery. Much of this data will likely prove useful not only to individual diagnosis and research, but also to understanding and further guiding research into synesthesia and its variations and their subtypes.