Genuine and drug-induced synesthesia: a comparison

Introduction

Synesthesia is defined as a crossover of sensory perception in which stimulation in one sensory modality (the inducer) evokes an internally generated experience in another modality (the concurrent). Sinke and colleagues distinguish three principal forms: genuine (constitutional) synesthesia, present from early life and typically lifelong; acquired synesthesia following brain damage or sensory loss, often involving phosphenes; and drug-induced synesthesia that emerges transiently during the acute effects of hallucinogens. Previous literature has focused heavily on genuine synesthesia, while acquired and drug-induced variants have been less systematically compared despite their frequent separate treatment in the literature. This paper aims to provide a comprehensive phenomenological and etiological comparison of genuine, acquired and drug-induced synesthesia. The study sets out to compare these forms across multiple dimensions (for example, consistency, automaticity, inducers and concurrents, localisation, dynamics and affectivity) and to assess how the phenomenology bears on competing etiological models. The authors also consider what is known about how hallucinogens influence genuine synesthesia and indicate directions for further research.