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Behavioural testing indicated that the tactile-vision synaesthetes had an enhanced discrimination of spatial texture but not orientation or tactile-visual integration relative to controls. The neural correlates of discrimination on the texture task indicate that only the right angular gyrus (RAG) is more active then when compared to the orientation task (Zhang et al., 2005). The RAG is functionally involved in discriminating distances between locations on the skin (Spitoni et al., 2013). Lesions to the RAG can result in deficits for body image (location of limbs) but not body schema (number and type of limbs) in reporting tactile localisation (Anema, Kessels, de Haan, Kappelle & Leijten, 2008). Similarly it has been suggested that the RAG is crucial in transforming co-ordinates from motor reference frames to visual reference frames (Muggleton, Cowey & Walsh, 2008). As such, if the improved discrimination is primarily due to the RAG, we might also expect superior tactile localisation on the body as well. Further underlining the use of the RAG for tactile-visual synaesthesia is the tight mapping between the location of human touch and location of colours seen in the photism-illustration task. It might be expected that the visual concurrents may be able to

assist in spatial discrimination similar to the enhanced temporal discrimination seen from auditory concurrents (Saenz & Koch, 2008). However during discrimination tasks CS and ES denied any influence of the task on their photisms, with MLS noting that only the orientation task influenced the orientation of her photisms (other tasks had no effect). MLS is unique here in that the presence of mirror-touch synaesthesia is already known to raise tactile sensitivity (Banissy et al., 2009), this in combination of touch-colour synaesthesia might mean that given a certain level of tactile sensitivity orientation information specifically from inanimate objects might be enough to have the POC influence subsequent photisms, marking it as a viable route in her case of touch-colour synaesthesia. Whether this is applicable to the other touch-colour synaesthetes might only be evident if their tactile sensitivity were raised, such as was previously seen using tDCS (Ragaert et al., 2008). Despite using the RAG, the texture task appears not to automatically induce informative photisms to the task, so more powerful inducers like human touch may only be able to 'break through' into consciousness. An alternative possibility is the recruitment of new processes well suited to spatial discrimination as seen with V1 recruitment for tactile texture discriminations in visually deprived populations (Merabet et al., 2004, 2007, 2008). In support of this is that tasks that already use visual processing for controls are not enhanced for synaesthetes, but the texture task that does not normally have visual processing is enhanced.

In examining other regions used in the JVP texture task, unfortunately there is no neuro- imaging study directly showing a comparison between the JVP texture task and a baseline of no tactile stimulation, however Zhang et al.'s (2005) data features texture, orientation and no- stimulation conditions. From this it is possible to gauge which regions are used in the JVP texture task relative to baseline by identifying regions that are significantly related to the ‘orientation > no tactile stimulation’ condition and are no longer significant in the ‘orientation > texture’ condition. Regions that meet these criteria include: Left post-central sulcus, left parietal operculum, bilateral frontal operculums, inferior frontal gyrus, right cerebellum and dorsal pre-motor regions. The primary and secondary somatosensory cortices are within this post-hoc comparison (Keysers et al., 2010) and are likely to be used in both the JVP texture and orientation tasks. Supporting this suggestion is the knowledge that other forms of tactile texture activate both the primary and secondary somatosensory cortices (Burton, Macleod, Videen & Raichle, 1997; Stilla & Sathian, 2008) and that inhibition of the primary somatosensory cortex using TMS can revert texture and orientation discrimination performance to chance levels (Zangaladze et al., 1999). Conversely lowering the neural firing threshold for S1 through rTMS or tDCS can enhance discrimination for orientation and spatial texture tasks (Ragert et al., 2008; Tegenthoff et al., 2005). As such tactile-

vision synaesthetes are unlikely to show functional changes to S1 and S2 relative to controls, otherwise we would see enhancements to discrimination for both the texture and orientation tasks, not just texture.

In a questionnaire given to 21 tactile-vision synaesthetes, the most commonly reported inducers include emotional human touch, itchiness and sexual experiences, in addition to this, variations in texture are reported to influence the subsequent photism. Common to all of these points is the involvement of the insula (Deshpande et al., 2008; Herde et al., 2007; Hole et al., 2012; Komisaruk et al., 2004; Morrison et al., 2011; Olausson et al., 2002; Valet et al., 2008). The insula also is involved in temperature and pain processing (McGlone, Vallbo, Olausson, Loken & Wessberg, 2007; Singer, Critchley & Preuschoff, 2009), which are inducers for certain forms of synaesthesia which have a high co-morbidity with tactile-vision synaesthesia (Novich et al., 2011). As such the insula is likely to be a strong modulator of touch for inducing tactile-vision synaesthesia.

If the insula is indeed involved in the specific mapping of touch-colour synaesthesia, we might also expect similar mappings between visual and emotional dimensions as might be seen in ‘emotion-colour’ synaesthesia. Currently emotion-colour synaesthesia is known to more reliably invoke colour for words with strong emotional content and that the type of emotion (positive / negative) also has some influence on the specific colours selected (Ward, 2004). The colours chosen for positive words tended towards highly saturated colours such as yellow, while for negative words black was overwhelmingly selected. These results follow trends between emotions and colours for non-synaesthetes (Collier, 1996; D’Andrade & Egan, 1974; Palmer et al., 2013). The preference order for emotions and colours independent of one another are closely related to the association between emotions and colours, especially for children (Terwogt & Hoeksma, 1995), since synaesthesia is a developmental condition, the prevalence of these links in childhood may become more firmly rooted as synaesthesia manifests, maintaining the association further into adulthood (Ludwig & Simner, 2013; Simner & Ludwig, 2012). Specific colours have been reported to co-inside with the level of familiarity to the person triggering the synaesthesia and these colours become fixed when the personality of the inducing person is firmly known (Collins, 1929; Cytowic, 1989; Ramachandran, Miller, Livingstone & Brang, 2012; Riggs & Karwoski, 1934). In some other individuals, thinking of an emotion either as a concept or as a word can elicit congruently emotional colours (Raines, 1909) while unexpected swathes of emotion can tint their visual perspective (Cutforth, 1925). Furthermore in breaking down a link between ‘touch to emotion to colour’ are cases of tactile-emotion synaesthesia, where specific textures (e.g. silk) evoke specific emotional states such as relief

(Ramachandran & Brang, 2008). It was speculated by Ramachandran and Brang (2008) that this process might involve additional connections between the insula and somatosensory cortices. If emotion-colour forms the basis of touch-colour associations, we would expect that touch-emotion, emotion-colour and touch-colour associations would all be congruent with one another, and so positive variations of touch would be more likely to evoke yellowish colours, while negative sensations are more likely to be black. Whether emotion is the true elicitor of the colour sensations or whether emotion plays a guiding role in the formation of touch-colour synaesthesia remains to be determined.