Grapheme-color synesthesia is a perceptual experience where graphemes, letters or words evoke a specific color, which are experienced either as spatially coincident with the grapheme inducer (projector sub-type) or elsewhere, perhaps without a definite spatial location (associator sub-type). Here, we address the question of whether synesthesia can be rapidly produced using a hypnotic color suggestion to examine the possibility of “hypnotic synesthesia”, i.e., subjectively experienced color hallucinations similar to those experienced by projector synesthetes. We assess the efficacy of this intervention using an “embedded figures” test, in which participants are required to detect a shape (e.g., a square) composed of local graphemic elements. For grapheme-color synesthetes, better performance on the task has been linked to a higher proportion of graphemes perceived as colored. We found no performance benefits on this test when using a hypnotic suggestion, as compared to a no-suggestion control condition. The same result was found when participants were separated according to the degree to which they were susceptible to the suggestion (number of colored trials perceived). However, we found a relationship between accuracy and subjective reports of color in those participants who reported a large proportion of colored trials: trials in which the embedded figure was accurately recognized (relative to trials in which it was not) were associated with reports of more intense colors occupying a greater spatial extent. Collectively, this implies that hypnotic color was only perceived after shape detection rather than aiding in shape detection via color-based perceptual grouping. The results suggest that hypnotically induced colors are not directly comparable to synesthetic ones.

There is increasing evidence from case reports that synesthesia is more common in individuals with autism spectrum conditions (ASC). Further, genes related to synesthesia have also been found to be linked to ASC and, similar to synaesthetes, individuals with ASC show altered brain connectivity and unusual brain activation during sensory processing. However, up to now a systematic investigation of whether synesthesia is more common in ASC patients is missing. The aim of the current pilot study was to test this hypothesis by investigating a group of patients diagnosed with Asperger Syndrome (AS) using questionnaires and standard consistency tests in order to classify them as grapheme-color synaesthetes. The results indicate that there are indeed many more grapheme-color synaesthetes among AS patients. This finding is discussed in relation to different theories regarding the development of synesthesia as well as altered sensory processing in autism.

Grapheme-color synesthesia is a condition characterized by enduring and consistent associations between letter/digits and colors. This study is the continuation of longitudinal research begun by Simner et al. (2009) which aimed to explore the development of this condition in real time within a childhood population. In that earlier study we randomly sampled over 600 children and tested them aged 6/7 and 7/8 years. We identified the child synesthetes within that cohort and measured their development over 1 year, in comparison to a group of non-synesthetic children with both average and superior memories. We were able to show the beginnings of a developmental progression in which synesthetic associations (e.g., A = red) mature over time from relatively chaotic pairings into a system of fixed consistent associations. In the current study we return to this same population three years later when participants are now 10/11 years. We used the same paired-association memory task to determine the synesthetic status of our participants and to also establish synesthetes' inventories of grapheme-color associations. We compared their inventories to those from age 6/7 and 7/8 years to examine how synesthesia matures over time. Together with earlier findings, our study shows that grapheme-color synesthesia emerges with a protracted trajectory, with 34% of letters/digits fixed at age 6/7 years, 48% fixed at 7/8 years and 71% fixed at 10/11 years. We also show several cases where synesthesia is not developing in the same time-frame as peers, either because it has died out at an older age, or because it was slower to develop than other cases. Our study paints the first picture of the emergence of synesthesia in real-time over four years within a randomly sampled population of child synesthetes.