The Induction of Synaesthesia in Non-Synaesthetes

Abstract

In this chapter we review research examining the induction of synaesthesia with training, posthypnotic suggestion, and pharmacological agents in non-synaesthetes. Each of these methods has been shown to produce different aspects of synaesthesia, but none have produced experiences that have been corroborated using neuroimaging assays. Nevertheless, the close parallels between induced and congenital synaesthesias have the potential to illuminate different facets of this condition. We argue that training may be a valuable model for studying the learning mechanisms underlying congenital synaesthesia, posthypnotic suggestion may have greater utility in the experimental manipulation of this condition, and the administration of pharmacological agents may serve as a useful tool for studying the development of synaesthesia or for large-scale studies of induced synaesthesia. Induced synaesthesias also raise important questions regarding espoused criteria for demarcating synaesthesia from other phenomena.

The chemical induction of synaesthesia

“…This is consistent with visual cortex hyperexcitability in developmental synaesthesia (Terhune et al, 2011; and in trained synaesthesia (Rothen et al, 2018) (see also Lungu et al, 2020). Elsewhere, we have argued that these effects are alone unlikely to produce the hallmark behavioural features of developmental synaesthesia (e.g., automaticity and inducerconcurrent consistency) (Terhune et al, 2016;Terhune et al, 2017). Rather, we maintain that these features arise from a consolidation process in which inducer-concurrent associations are driven by statistical regularities in one's environment (Witthoft & Winawer, 2013), resulting in the consolidation of inducer-concurrent associations over time (Simner et al, 2008).…”

“…A point of controversy is whether synaesthesia can be induced in non-synaesthetes (Luke & Terhune, 2013). Multiple lines of evidence suggest that the behavioural and phenomenological characteristics of synaesthesia can be temporarily triggered through cognitive training, verbal suggestion, and drugs (for a review, see Terhune, Luke, & Kadosh, 2017). Drug-induced synaesthesia is of particular interest, as it can broaden our knowledge about the neurochemical basis of synaesthesia.…”

Anomalous Psychedelic Experiences: At the Neurochemical Juncture of the Humanistic and Parapsychological

“…There may well be multiple etiologies for congenital synesthesia, as there are many types, however determining the nature of the relationship between Luke 5 induced and congenital synesthesia is crucial, though there is no common agreement about whether psychedelic experiences are genuinely synesthetic or just synesthesia-like (for a review see, Terhune et al, 2017). The four defining features of congenital synesthesia are its consistency over time within individuals (e.g., Tuesday is always green), automaticity (occurs without effort), specificity (e.g., B minor is chartreuse), and access to consciousness.…”

Mixing up the Senses: Sensory Substitution Is Not a Form of Artificially Induced Synaesthesia

“…Four fundamental criteria are used to characterise synaesthesia: (i) an inducer-concurrent pairing; (ii) the relative idiosyncrasy of the pairings; (iii) the automaticity of the process; and (iv) the consistency over time. These criteria have been used in the past to evaluate the validity of a synaesthetic view of different processes (see details in Auvray and Farina, 2017 1 ; Grossenbacher and Lovelace, 2001;Terhune et al, 2017; see also Deroy and Spence, 2013;Ward, 2013;Ward and Mattingley, 2006). Next, we evaluate whether sensory substitution adheres to each of these criteria in order to establish whether an 'artificial synaesthesia' view of sensory substitution is tenable.…”

“…The extent to which these phenomena can and should be described as 'artificial synaesthesias' is also controversial. Indeed, many of these examples do not meet the essential criteria that characterise synaesthesia (for reviews see Auvray and Farina, 2017;Deroy and Spence, 2013;Luke and Terhune, 2013;Terhune et al, 2016Terhune et al, , 2017. The over-generalisation of synaesthesia to seemingly related phenomena could risk fundamental misunderstandings of the mechanisms underlying these phenomena.…”