Misophonia may be a form of synesthesia

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Nevertheless, misophonia displays similarities to a genetic condition known as synesthesia. In synesthesia, as in misophonia, particular sensory stimuli evoke particular and consistent, additional sensations and associations. Well-known forms of synesthesia include letters evoking a particular color, or sounds/music evoking colors (Cytowic, 1989; Baron-Cohen et al., 1996; Simner et al., 2006) but there are in fact many different subtypes of synesthesia, with a variety of “inducers” (e.g., music, taste, words, sequences) evoking certain “concurrents” (e.g., color, shapes, taste). While most synesthesia research has examined the perceptual sensations related to synesthesia, the condition seems to have an affective component as well. First, synesthetic congruency (e.g., when a grapheme-color synesthete sees a letter in the “correct” color) is related to positive affect (e.g., Callejas et al., 2007). Furthermore, both inducers (Ward, 2004; Ramachandran et al., 2012) and concurrents (Simner and Holenstein, 2007) can be of emotional rather than perceptual nature. Interestingly, the latter indicates that for certain subtypes of synesthesia, similar to misophonia, inducers evoke a particular feeling or emotion rather than a pure perceptual sensation. This has been studied in tactile-emotion synesthesia (e.g., feeling sandpaper evokes a feeling of jealousy; Ramachandran and Brang, 2008). Synesthetic associations, like misophonic experiences, are automatic (in the sense that they do not take effort or conscious deliberation), are consistent within an individual and persist throughout life, and seem to run in families (Asher et al., 2009; Tomson et al., 2011; for a review see Brang and Ramachandran, 2011). Given these similarities, neuroimaging findings in synesthetes may provide us with hypotheses on the neural basis of misophonia. First, associated sensations in synesthesia are found to be associated with co-activation in relevant (associated) brain areas (Nunn et al., 2002; Hubbard et al., 2005; Rouw et al., 2011). Furthermore, previous studies support a direct linking of relevant sensory regions in synesthesia (Hubbard and Ramachandran, 2001), mediated by an actual increase of anatomical connectivity (Rouw and Scholte, 2007; Zamm et al., 2013). Similarly, altered connections from a lesioned thalamus to the cerebral cortex (Ro et al., 2007; Beauchamp and Ro, 2008) led to a type of acquired synesthesia in which auditory stimuli produced tactile percepts. Differing in the level of specificity and complexity of evoked responses observed in synesthetes, individuals with misophonia display basic and non-elaborated responses to triggering stimuli, varying largely in the intensity of the response. Nevertheless, the underlying neurological cause of this condition may be similar to that of synesthesia in terms of enhanced connectivity between relevant brain regions. In short, a pathological distortion of connections between the auditory cortex and limbic structures could cause a form of sound-emotion synesthesia.