Investigations of Heterogeneity, Directionality, and Involvement of Associative Memory in Synaesthesia

Abstract

Abstract

The perceptual phenomenon of synaesthesia involves perception of additional, unrelated sensory experience (concurrent) when a stimulus is perceived (inducer). For grapheme-colour synaesthetes, perceptions of letters, numbers, and/or shapes (inducers) automatically produce perceptions of synaesthetic colour (concurrents) specific to each grapheme. Typically synaesthetes experience unidirectional forward synaesthesia where only perceptions of inducers produce perceptions of concurrents. However some synaesthetes also experience implicit reverse synaesthesia where presentations of synaesthetic colours bias their responses towards the associated graphemes without conscious perception of said graphemes. In very rare cases, synaesthetes experience explicit reverse synaesthesia where synaesthetic colours produce perceptions of associated graphemes. The current thesis explores the heterogeneity of bidirectional synaesthesia and involvement of associative memory in it.

In Chapter 2, the hypothesis that synaesthetic experiences were heterogeneous but consistent and automatic was supported. Consistency of synaesthetes’ grapheme-colour pairings was calculated and compared to those reported by 10 respective controls in colour-matching tasks. Synaesthetes consistently matched graphemes to their synaesthetic colours while controls did not. Synaesthete S1 and S2’s automatic synaesthetic experiences were confirmed using synaesthetic Stroop paradigms, where they identified font colours of presented graphemes (Stroop1) and synaesthetic colours experienced (Stroop2). Synaesthetes identified relevant colours faster when graphemes were presented in their synaesthetic colour compared to when graphemes were presented in another grapheme’s synaesthetic colour.

In Chapter 3 synaesthetes S1, S2, and their control named colours experienced or associated in response to visual and auditory graphemes presented in a 1.5T magnetic resonance imaging (MRI) scanner. It was hypothesised that neural correlate similarities accompanied behavioural similarities shown in Chapter 2. Post-hoc region of interest analyses supported the hypothesis as the right inferior frontal gyrus (RIFG) was commonly activated in synaesthetes only.

S3 reported that he experiences the very rare explicit reverse synaesthesia. Neural correlates which underlie his explicit reverse synaesthesia were hypothesised to be different from those of the forward synaesthesia (Chapter 4). When presented with his inducers, S3’s RIFG was activated; however when presented with his synaesthetic colours results indicated left IFG activation. Different IFG involvements in forward and reverse synaesthesia suggested that S3’s explicit reverse synaesthesia might be a learnt phenomenon, involving higher processes such as associative memory retrieval. This was supported by behavioural results as S3 performed faster than trained controls when matching colours to graphemes (forward-matching paradigm) and vice-versa (reverse-matching paradigm), but was slower in the reverse-matching paradigm compared to the forward paradigm.

Chapter 5 investigated further the involvement of associative memory in synaesthesia. Trained controls and synaesthetes with implicit reverse synaesthesia (S4, S5, and S6) responded to target stimuli in forward and backward associative-priming paradigms following letter and colour-patch primes presentations. It was hypothesised that synaesthetes would outperform controls indicating enhanced associative memory access is involved in a bidirectional manner. Synaesthetes were faster than controls when they accessed forward letter-colour pairings. When synaesthetes had to retrieve reverse colour-letter associations, varying levels of facilitation were shown. This indicated that a spectrum exists based on the degree synaesthetes use associative memory to access bidirectional grapheme-colour associations.

Taken together this thesis indicates that heterogeneity in bidirectional synaesthesia also exists due to the varying degrees of bidirectional inducer-concurrent access through associative memory retrieval. The findings provide further evidence that learning and memory play a role in synaesthesia.