1.1.3 The nature of linguistic synaesthesia

Linguistic synaesthesia, according to various studies, is a specific type of metaphor, and this view has become the “default” opinion in the literature （Strik Lievers 2017：87）. For instance, Osgood et al.（1978 [1957]）and Geeraerts （2010）suggested that linguistic synaesthesia shared a similar nature with other types of metaphor in terms of describing one concept through another concept based on similar features. Apart from the rhetorical framework for metaphors, the CMT-based framework （Lakoff and Johnson 1980; Lakoff and Johnson 1999）has also recognized the metaphorical nature of linguistic synaesthesia, which manifests itself as mappings of concepts from more embodied domains to less embodied ones （e.g., Shen 1997; Yu 2003; Popova 2005）. Strik Lievers （2017）also argued for linguistic synaesthesia as a particular type of metaphor, as there are conceptual conflicts between sensory concepts from the different modalities involved. However, as pointed out by Rakova （2003）and Peng and Bai （2008）, linguistic synaesthesia is not as the same as canonical metaphors that generally map from bodily domains to non-bodily domains （Steen 1999; Gibbs 2011）.

Following CMT, metaphor is not only a matter of words, but also a conceptual mechanism structuring human thoughts and actions （Lakoff and Johnson 1980; Johnson 1987）. Gibbs （2011）suggested that systematic metaphorical expressions using the concrete to describe the more abstract in different languages, such as LOVE IS A JOURNEY and ARGUMENT IS WAR, are crucial supporting evidence for metaphors not only in languages but also in conceptual systems. Nevertheless, linguistic synaesthesia has source domains and target domains that are both embodied and concrete sensory modalities. Thus, linguistic synaesthesia is different from canonical metaphors whose source domains exhibit an evident contrast with target domains concerning embodiment and concreteness.

Linguistic synaesthesia differs from canonical metaphors with respect to directional tendencies as well. Strik Lievers’（2015）corpus-based study has attested that linguistic synaesthesia does not follow absolute mapping directionality from the more embodied to the less embodied, as transfers violating this directional tendency can also be found in the language. For example, although transfers from touch to hearing occupy 23.2% of all English synaesthetic examples collected by Strik Lievers, there are still 0.2% of the total examples found to exhibit the transfer direction from hearing to touch （Strik Lievers 2015：80）. This pattern, however, has not been reported for typical metaphorical expressions mapping from the concrete to the abstract （see Lakoff and Johnson 1980; Sweetser 1990; Lakoff and Johnson 1999; among others）.

Theoretical explanations for mechanisms underlying linguistic synaesthesia and metaphors are also not identical. For instance, Rakova （2003：43）assumed that linguistic synaesthesia could not be explained by theories of metaphor, since both source and target domains of linguistic synaesthesia are bodily and concrete perceptions without evident contrasts on “experiential primacy” and “conceptual primacy”. Based on the corpus distribution of synaesthetic data, Zhao et al.（2018b）and Zhao and Huang （2018）have found that tendencies of linguistic synaesthesia cannot be accounted for or predicted completely by the embodiment mechanism, which has been widely recognized to underlie typical metaphors conceptualizing non-bodily experiences in terms of bodily experiences （Lakoff and Johnson 1980; Sweetser 1990; Gibbs 2011）.

Differences between linguistic synaesthesia and canonical metaphors can also be observed concerning research methodologies adopted. Studies on metaphorical expressions mainly begin with concepts expressed by lexical items （Steen 1999）, based on which conceptual domains and mapping directions are determined subsequently （e.g., Clausner and Croft 1999; Paradis 2001; Grady 2005; Lien 2005; Ou 2014; among many others）. Take the expression my mind just isn’t operating today for example （see Lakoff and Johnson 1980：27）. The contextual meaning of operating expresses a concept related to the mind. However, the lexical item operating has a literal meaning concerned with machines. Thus，operating can then be analyzed to illustrate the mapping from the conceptual domain of MACHINE to the conceptual domain of MIND in the example. Nevertheless, research on linguistic synaesthesia generally begins with the determination of sensory domains. Transfers of sensory items involving more than one sensory domain are then established （e.g., Ullmann 1957; Williams 1976; Shen 1997; Strik Lievers 2015; among many others）. For instance, Williams （1976：476）first distinguished six sensory domains, and then determined that the English adjective sweet had the original sensory domain of taste. As the adjective can have an auditory use in contemporary English, such as sweet voice, the study generalized the transfer direction from the sensory domain of taste to the sensory domain of hearing. Thus, unlike studies on metaphors that generally rely on a concept-based approach, studies on linguistic synaesthesia mainly employ a domain-based approach. It is intriguing that some canonical metaphor research, such as Ahrens （2002）and Chung et al.（2013）, has also adopted the domain-based, rather than the concept-based, approach to examine metaphors, i.e., to investigate the interaction between established ontological knowledge and conceptual mappings in metaphors.

In short, linguistic synaesthesia manifesting itself as a mapping from one bodily experience to another bodily experience exhibits distinct characteristics from canonical metaphors. These distinct characteristics include:（1）the contrast of embodiment between source domains and target domains;（2）directional tendencies of mappings;（3）theoretical explanations; and （4）research methods adopted.

Apart from the differences between linguistic synaesthesia and canonical metaphors, the metaphorical claim about linguistic synaesthesia has been challenged by the growing body of research on neurological synaesthesia. For instance, Ramachandran and Hubbard （2001）and Ronga et al.（2012）hypothesized a neurological basis for linguistic synaesthesia, based on the common origins and mechanisms between neurological synaesthesia and linguistic synaesthesia. Ronga et al.（2012）employed a corpus-based approach to show that linguistic synaesthesia has parallels with neurological synaesthesia concerning the patterns of sensory associations. Asona and Yokosawa （2012）and Hung et al.（2014）found that similar patterns could be observed in neurological synaesthesia among people speaking different languages from different cultures. This finding could provide a support for the similarity between linguistic synaesthesia and neurological synaesthesia, as Williams （1976）has suggested that transfer patterns of linguistic synaesthesia are cross-linguistically universal.

Winter （2019a，2019b）has claimed that linguistic synaesthesia is neither metaphorical nor neurological in nature, but instead it is literal, as sensory words have “highly multisensory or supramodal meanings” in language （Winter，2019a：107）. Take the English adjective sweet for example. Sweet fragrance was considered the literal usage of sweet, as the adjective is supramodal involving both taste and smell （Winter 2019b：125）. The assumption is similar to Rakova’s （2003）view, who suggested no distinction between literal and metaphorical meanings of sensory adjectives with no perceptual and conceptual priorities on any sensory modality. However, as pointed out by Popova （2005）, this literal claim on the nature of linguistic synaesthesia would be inconsistent with the directional patterns of transfers in linguistic synaesthesia.

Therefore, the nature of linguistic synaesthesia has not been decided. It is still arguable whether linguistic synaesthesia can be properly analyzed as a specific type of metaphor, given the differences between linguistic synaesthesia and canonical metaphors and alternative assumptions on the nature of linguistic synaesthesia.