Disinhibited feedback as a cause of synesthesia: Evidence from a functional connectivity study on auditory-visual synesthetes

A deafening flash! Visual interference of auditory signal detection

“…In terms of brain mechanisms, it is currently debated to what extent synaesthesias reflect genotypically unusual cross-wiring between sensory areas (Brang et al, 2012;Tomson et al, 2011), versus physiological disinhibition of normally-occurring connections (Cohen Kadosh et al, 2009;Cohen Kadosh & Walsh, 2006;Grossenbacher & Lovelace, 2001;Neufeld et al, 2012). This debate might be informed by whether the behavioural measures have a discontinuous or continuous distribution (Cohen Kadosh, 2013;Deroy & Spence, 2015;Martino & Marks, 2001;Simner, 2012).…”

“…Such connections may function to improve spatial localisation of sound or amplify the auditory response to visually identifiable sources (Schroeder & Foxe, 2005). Given this normal connectivity, the additional emergence of a conscious visually-evoked auditory concurrent might thus be more readily explained by individual physiological variations, which might influence whether existing connections, or their interactions with higher areas, are inhibited or unmasked (Cohen Kadosh et al, 2009;Cohen Kadosh & Walsh, 2006;Grossenbacher & Lovelace, 2001;Neufeld et al, 2012), to greater or lesser degree.…”

Developing Synaesthesia

“…The pharmacological evidence thus lends some support to a disinhibited feedback mechanism, which suggests that synesthesia is not a result of altered structural connectivity but arises from altered functional feedback connections. This type of mechanism has received prior support from psychophysical and neuromagining studies of non-autistic synesthetes Neufeld et al, 2012). A recent imaging study of 14 auditory-visual non-autistic synesthetes, for example, found increased functional connectivity of the left inferior parietal cortex with the left primary auditory and right primary visual cortex , suggesting that the aberrant synesthetic binding takes place in parietal cortex.…”

“…It is widely believed that most forms of the condition involve functional or structural aberrant brain connectivity. The proposed mechanisms include direct or indirect projection through increased structural connectivity (Ramachandran and Hubbard, 2001a,b;Jancke et al, 2009;Zamm et al, 2013), functionally driven disinhibited-feedback mechanisms Neufeld et al, 2012), and mixed models . However, it is unclear what causes the onset of the condition and whether the different types of synesthesia have different causes.…”

Do Synesthetic Colors Grab Attention in Visual Search?

“…(Armel and Ramachandran 1999;Grossenbacher 1997;Grossenbacher and Lovelace 2001;Smilek et al 2001;Myles et al 2003;Gaschler-Markefski et al 2011;Neufeld et al 2012). The indirect model has received support from studies of how visual context and meaning can influence which synesthetic colors a grapheme gives rise to (Armel and Ramachandran 1999;Myles et al 2003;Dixon and Smilek 2005;Cytowic and Eagleman 2009: 75).…”