Lexical-Gustatory Synesthesia

Lexical-Gustatory Synesthesia
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Lexical gustatory synesthesia is a type of synesthesia in which people involuntarily experience food tastes when they hear, read or say a word; words acts as an inducer while taste is the concurrent. Lexical-gustatory synesthesia can be more accurately named as lexical-flavour as gustatory implies that the experience of a synesthetic being limited to sweetness, sourness, bitterness etc. whereas studies show that the triggered concurrent involves a complex array of flavours, much like one would find when eating a meal [1]. 
Studies regarding lexical-gustatory synesthesia have ranged from the factors, such as the structure of a word, that elicit a specific taste to the brain regions and subsequent mechanisms involved in this particular type of synesthesia. Moreover, it has been greatly reiterated, through various tests, that lexical-gustatory synesthesia is a genuine experience and not a purely psychological one however, concurrents have been shown to be shaped by experience, and food eaten during childhood have a higher likelihood of being concurrents. Furthermore, studies have shown that genes also play a role in this phenomenon, thus synesthesia could be hereditary [11]. (See Genetics)
Studies of lexical-gustatory synesthesia have resurfaced as the neural models of this type of synesthesia can be used to understand integration and separation of different cortical regions and the rules that apply to the wiring and interaction of these different regions.

A Case of Lexical-Gustatory Synesthesia
James prefers his girlfriends' to have a nice tasting name 
original video source: http://www.bbc.co.uk/news/health-21060207

1. Consistent Results

To separate and draw a line between psychology and neuroscience, studies have shown that lexical-gustatory synesthesia is a neural phenomenon rather than a perceptual one.
Firstly, most attempts at teaching synesthesia through making associations and repeating them over and over again have failed in the past[6], this can imply that the association might be learned but the neural connection behind the phenomenon are not. 
In a study to show the consistency of reported concurrents, a synesthetes response had stayed 100% consistent across three tests over 27 and 23 year [7]. Imagining results build upon this consistency as studies have shown that activation of a synesthetic brain differs, in response to an inducer, when compared to the brain of a non-synesthetic [8].
Moreover, test of genuineness or TOG has been applied and revised to test the genuineness of synesthesia, and an advantage regarding this test is that it can be applied to a variety of types of synesthesia as the diagnosis is not dependent on the type of stimulus [15] In test of genuineness a detailed description of concurrent is required and it’s stability is tested over a period of time. It has proven successful as a case study shows that a subject is 94% consistent compared to control [4]. 
However, TOG fails to provide a gateway into the personal experience of other people so more methods have been adopted to test the genuineness. As triggered concurrents appear to be related to phonemes, a phonemic awareness test is conducted to establish that the synesthetes phonemic awareness is not superior; studies have shown that the awareness falls in a normal range [4]. 
The last thing that supports the authenticity of this synesthesia is the activation of different brain regions when concurrent is triggered when compared to people with no synesthesia [4].

2. Factors Involved

Studies have shown that if the structure of a word and significant components are shared with other words, they can all induce similar concurrents; similarly sharing specific components increases the likelihood of a said taste being elicited. However, predicting the kind of taste a word can elicit has proven to be hard as conceptual factors can outweigh phonological ones [4]. 
It is also interesting to note that the way words affect grapheme-colour synesthesia do not have the same effect on the concurrent of lexical-gustatory synesthesia; for lexical-gustatory synesthesia the taste is unrelated to spellings which is not the case in grapheme-color synesthesia [9][18]. Moreover, lexical gustatory also differs from color-taste synesthesia in the sense that the former elicits a complex array of tastes while the latter elicits tastes that differ from normal tastes [5]. In addition to this, unlike colour in word colouring, taste is sensitive to the repetition and lexically of the word [16]

Phonemes and Taste
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Phonemes that elicit that same taste in a case study.
Image source: Ward and Simner, 2003

2.1 Phonemes

Studies have shown that people with lexical-gustatory synesthesia experience similar taste when words are alike and share a phoneme. The partial similarity of the words may induce a fractional activation and induce similar taste. Therefore, a presence of a phoneme in a word increases the likelihood of generating a particular taste. For example, for synesthete JIW the presence of /k/ in a word tends to induce the flavour of egg or yoke; accept, fax, chuck and York. This trait has been extended to more synesthetes and it shows that phonological units are a general trait of this condition [2].

2.2 Non-words

Extending from the phoneme theory, there has been a study comparing concurrents triggered with non-words as inducers compared to real words; it has been shown that non-words that are orthogonally close to real words, like peeple, trigger more taste when compared to those that are distant, like baybee. Therefore, words that are more closely spelled as compared to those that are more likely to trigger taste. These findings are extended to non-words that do not sound like real words, and it has been found that non-words that have neighbouring real words tend to generate more taste than those with no neighbouring real words (neighbouring words: different from real words by one letter only). However, the non-words with no neighbouring real words did trigger taste but the hypothesis is that they may be perpetuating distant neighbourhoods. Moreover, the consistency and intensity is stronger with real words [2]. In addition to this, non-words may or may not fail to activate real word neighbours on different occasions [3].

3. Neuroanatomy

Currently, not a lot is known about the neural basis of lexical-gustatory synesthesia but it has been shown that words that induce concurrents activate additional neural linkages in people with synesthesia, as compared to people with no synesthesia.

Activated Insula
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Words activating the left anterior insula in a synesthete
Image source: Jones et al., 2011

3.1 Insula

(Also see Self-images, non-self images, and the insula)
The primary gustatory cortex lies in the insula [17] and studies have shown that any damage to the insula affects the perception of taste [12]. Furthermore, a higher activation of insula has been shown in the presence of food odors when compared to non-food odors [13]. Therefore, insula has shown to be an important region when it comes to taste, hence, it is important in lexical-gustatory synesthesia as studies have a shown a bilateral activation of insula in people with this type of synesthesia as compared to people with no synesthesia [18]. Additionally the left insula has shown to play a role in language. A study that used Functional Magnetic Resonance Imaging (fMRI) showed that JIW, a synesthete, had a greater activation of left anterior insula to valent words, and the activation of insula seems to be related word that elicit a more unpleasant experience [17]. 

3.2 Activation of Medial Parietal Lobe

(Also See Parietal Lobe)
This area of the brain seems to play an important role in the subjective intensity of synesthesia experienced. In synesthetes, BW, the greatest activation was in the precuneus while in JW, the highest activation was of the inferior occipital gyrus and precuneus. Additionally, this region is not known to be involved in the perception of flavour and the activity of this region was positively related intensity [17].

3.2.1 Role of Precuneus

The precuneus, in studies, have been shown to be involved in polymodal imagery like visual and auditory. Furthermore it has also been shown that this area is involved in 'default mode network' – the area gets activated when at rest. Interestingly, the opposite phenomenon is seen to happen in people with synesthesia, that it is activated when a task is presented [17].
Therefore, its role in lexical-gustatory synesthesia could be that it might be involved in producing the content of synesthesia, or it could be integrating information related to synesthetic experience. However different fMRI studies regarding synesthesia have shown that the role of precuneus is not exclusive to lexical-gustatory synesthesia [9]. Moreover, in grapheme-color synesthetes show a larger surface area in the cortex in this region when structural imaging is done [17].

4. Neural Mechanisms

4.1 Proximity of Anatomical Regions

Phoneme and taste areas in the brain reside close to each other in the brain, this could promote direct connectivity between these regions; this concept is extended from the same idea proposed for the grapheme-colour synesthesia [4].  This idea is supported by the fact that phonemes play an important role in lexical-gustatory synesthesia [12].

4.2 Hyperconnectivity

Diffusion Tensor Imaging (DTI) has shown that there might be hyperconnectivity between auditory and gustatory cortex (in the insula and frontal operfulum) in people with synesthesia. This is important due to the aforemtioned reason that the insula is shown to be involved in gustatory processing and auditory and phonology processing as well [14].

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"Switchboard of Information"
Image Source: Wikipedia

4.3 Involvement of Thalamus

Even though modern theories of synesthesia argue that this phenomenon involves the cortical region primary, studies have shown that thalamic routing might also be involved in preventing synesthetic experience. Attention related mechanism directed by thalamus and direct cortical access may allow odors to generate experiences of a synesthete [12].

Theories of Synesthesia
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(i) Crosstalk Theory (ii) Disinhibition Feedback
Image Source: Ward and Simner, 2003

4.4 Crosstalk and Disinhibited Feedback

(See Prominent Theories)
Crosstalk theory could be an explanation for lexical gustatory synesthesia as neural connections between insular regions and somatosensory cortex in parietal lobe may have arisen and not disintegrated with age [10]. These connections could have been further molded by experience to give rise to specific associations between food and words[4].
However, disinhibited feedback provides another level of understanding of lexical-gustatory synesthesia as connection between taste and words can be mediated by lexical semantics. This theory involved the connection of conceptual knowledge of food and names of the foods, to areas that produce taste[4].

5. Bi-directionality

Majority of the studies have shown that this type of synesthesia is unidirectional, that is that words elicit taste but taste do not elicit words. However one study of a woman, PS, showed the first case of bi-directionality in lexical-gustatory synesthesia, as she reported that specific tastes made her think of certain words. And the study showed that 30% of pseudo-words, derived from inducers, induced a concurrent; some proved to be more effective than the words they were related to. Furthermore, the same study shows the limitations in stability of synesthesia over time as PS reported plasticity in learned associations. [14].

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