Why vicarious experience is not an instance of synesthesia

A vicarious experience is an empathetic state in response to the observation of others’ sensations, emotions, and actions (Keysers and Gazzola, 2009). Vicarious experiences in response to social stimuli are quite common in the general healthy population and they may even constitute an important basis for social behavior. Interestingly, vicarious experiences recruit similar neural processes as the primary experience of a certain sensation, emotion, or action, and it is assumed that the mirror neuron system is involved in these vicarious neural processes (e.g., Morrison et al., 2004; Singer et al., 2004; Jackson et al., 2005). Synesthesia denotes a condition that leads to specific experiences in response to normal sensory input that is not experienced by non-synesthetes. Synesthetic experiences are characterized as idiosyncratic, involuntarily elicited, and consistent over time (Grossenbacher and Lovelace, 2001; Ward, 2013; but see, Simner, 2011). Synesthesia tends to run in families suggesting a genetic component and has a neural basis (Asher et al., 2006, 2009; Barnett et al., 2008). The best studied and most accepted form of synesthesia is grapheme-color synesthesia. People affected by this type of synesthesia experience colors for numbers and letters printed in black on a white background (e.g., Rothen et al., 2012). On a neural basis, it has been suggested that brain regions concerned with binding processes, the modality of the inducing stimulus, and the modality of the respective sensory experience are involved (e.g., Hubbard et al., 2011; Rouw et al., 2011). Recently, it has been suggested that also vicarious experiences represent an instance of synesthesia. In particular, the term mirror-sensory synesthesia has been introduced in the scientific literature to describe instances of overt phenomenological experiences reflecting the actual state of an observed sensation and/or emotion (i.e., a phenomenologically overt vicarious experience, Fitzgibbon et al., 2012b). It has been suggested that people who report to have explicit and consciously accessible experiences of touch and/or pain upon the observation of other people being touched or in pain may be called mirror-touch and mirror-pain synesthetes, respectively (Fitzgibbon et al., 2010, 2012b). However, other mirror-sensory forms, such as for example mirror-disgust experiences, seem possible. Here, we argue that the label synesthesia should be reserved for canonical cases of synesthesia (such as graphemecolor or lexical-gustatory) and we outline similarities and differences between synesthesia and vicarious experiences (Table 1) (for the use of the term synesthesia see also, Deroy and Spence, 2013). By using the term mirrored sensory experiences, we focus on phenomenologically open instances of vicarious experiences because as by the definition of “mirror-sensory synesthesia” phenomenologically less overt forms are not to be regarded as instances of synesthesia.