Extended case study on the phenomenology of spatial form synaesthesia
Synesthesia has many sub-types and shows large inter-individual variation. At the level of phenomenology, our understanding of these subtypes remains rudimentary. We report an extended phenomenological investigation of spatial-form synaesthesia in a single case (BC). We used the ‘Explicitation Interview’ method, which facilitates the reliving of a particular experience by inducing an ‘evocation state’ within which the subject’s attention can be selectively guided by the interviewer (Vermesch, 1994; Petitmengin, 2006). In a first application to synesthesia, BC was guided to explore phenomenological details of his spatial-form synesthesia. Detailed analysis of the resulting 11 hours of interview transcripts provided a comprehensive description of BC’s synesthetic experience, including several novel observations. The basic phenomenology of BC’s spatial-form involves the appearance of numbers and letters in definite visuo-spatial configurations. Although the appearance of synesthetic concurrents is usually described as automatic, BC reports to engage in various cognitive acts in order for concurrents to be fully visible, suggesting an important role for attention and motor intention. BC’s concurrents also appear with within specific contexts of a ‘white page’, in a ‘corridor’ or in a ‘cave’; he also describes a ‘mental room’ in which his percepts are experienced and asserts an ability to voluntarily switch attention between ‘mental’ and ‘physical’ rooms. These descriptions move beyond existing subtype categorisations based on individual phenomenology (e.g., projector versus associator synesthesia), providing new targets for neurobehavioral analysis. Strikingly, some aspects of BCs synesthesia were previously unknown even to him.