Neurophenomenology of induced and natural synaesthesia

Abstract

People with synaesthesia have additional perceptual experiences, which are automatically and consistently triggered by specific inducing stimuli. Synaesthesia therefore offers a unique window into the neurocognitive mechanisms underlying conscious perception. A long-standing question in synaesthesia research is whether it is possible to artificially induce non-synaesthetic individuals to have synaesthesia-like experiences. Although synaesthesia is widely considered a congenital condition, increasing evidence points to the potential of a variety of approaches to induce synaesthesia-like experiences, even in adulthood. Here, we summarize a range of methods for artificially inducing synaesthesia-like experiences, comparing the resulting experiences to the key hallmarks of natural synaesthesia which include consistency, automaticity and a lack of ‘perceptual presence’. We conclude that a number of aspects of synaesthesia can be artificially induced in non-synaesthetes. These data suggest the involvement of developmental and/or learning components in the acquisition of synaesthesia, and they extend previous reports of perceptual plasticity leading to dramatic changes in perceptual phenomenology in adults.This article is part of a discussion meeting issue ‘Bridging senses: novel insights from synaesthesia’.

Difficult Turned Easy: Suggestion Renders a Challenging Visual Task Simple

“…Research into consciousness deals with a similar conundrum: Reports of awareness may sometimes follow from a response bias (Peters, Lau, & Ro, 2016). Some researchers have attempted to address this particular issue in the context of hypnotic hallucinations by inducing synesthesia-like experiences through posthypnotic suggestions and then validating the effect with a challenging perceptual task (Anderson, Seth, Dienes, & Ward, 2014; Cohen Kadosh, Henik, Catena, Walsh, & Fuentes, 2009; Kallio, Koivisto, & Kaakinen, 2017)—so far, however, with mixed results (Schwartzman, Bor, Rothen, & Seth, 2019).…”

Bridging senses: novel insights from synaesthesia

“…As highlighted by the above overview papers [5][6][7][8], the majority of our knowledge about synaesthesia has so far come from investigations of grapheme-colour and related types, and this forms the primary focus of four research articles of the special issue. Studies of adults suggest that these types of synaesthesia are sometimes associated with higher cognitive skills (see [5] in this issue for discussion).…”

“…The issue begins with several leading voices laying out their distinctive perspectives on synaesthesia [5][6][7][8]. Jamie Ward presents a view of synaesthesia as a special condition with causal underpinnings distinct from neurotypical cognition, but at the same time one that encompasses a considerable range of profiles [5].…”

“…Expanding on this theme of unified explanations for developmental and induced synaesthesia, Schwartzman et al [7] review a range of methods that have been claimed to artificially induce synaesthesia in non-synaesthetic individuals, including intense cognitive training on letter-colour associations, psychedelic drugs, hypnosis, sensory deprivation and brain injury. This work demonstrates how perceptual plasticity can persist even into adulthood.…”

“…This work demonstrates how perceptual plasticity can persist even into adulthood. The authors highlight phenomenological, behavioural and, in some cases, neural parallels between developmental and induced synaesthesia, while arguing that the variable degree of 'perceptual presence' should be a major focus for further investigations in this area [7].…”