The Role of Meaning in Grapheme-Colour Synaesthesia

doi:10.1016/S0010-9452(08)70349-6

Cortex

Volume 42, Issue 2, 2006, Pages 243-252

https://doi.org/10.1016/S0010-9452(08)70349-6 Get rights and content

Abstract

When the synaesthete, J, is shown black graphemes, in addition to perceiving the black digits or letters she also experiences highly specific colours that overlay the graphemes (e.g., 5 is pink, S is green). We used ambiguous graphemes in a Stroop-type task to show that the exact same forms (e.g., 5) can elicit different synaesthetic colours depending on whether they are interpreted as digits or letters. J was shown strings of black digits (e.g., 3 4 5 6 7) or words (e.g., MUSIC) for 1 sec. All but one of the graphemes then disappeared and the remaining grapheme changed to a colour that J had to name as quickly as possible. The key trials involved coloured graphemes that were ambiguous (e.g., the 5 in the strings above could be interpreted either as a digit or as a letter). On congruent trials, the colour of the ambiguous target grapheme was the same as J's photism for the digit or letter interpretations of the grapheme. On incongruent trials, the colours of the ambiguous target graphemes were different than the colours of J's photisms for the digit or letter interpretations of the graphemes. On digit-context incongruent trials, the ambiguous graphemes were presented in J's colour for the letter-interpretations of the graphemes, whereas on letter-context incongruent trials, the ambiguous graphemes were presented in J's colours for the digit-interpretations of the graphemes. Thus the same ambiguous grapheme (e.g., a pink 5) served as a congruent stimulus in one context and an incongruent stimulus in another context. J's response times showed that ambiguous graphemes elicited different photisms depending on whether they were interpreted as digits or letters. This finding suggests that it is not the form but the meaning of graphemes (whether they are interpreted as digits or letters) that determines the colours of synaesthetic photisms.

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Studies have suggested that GCS is a perceptual phenomenon by showing that synesthetes perform faster and more accurately than nonsynesthetes on visual search tasks (Kim & Blake, 2013; Ramachandran & Hubbard, 2001a, 2001b). However, other studies (Chiou & Rich, 2014; Dixon, Smilek, Duffy, Zanna, & Merikle, 2006; Mroczko-Wasowicz & Nikolic, 2014) have argued that GCS arises from semantic mechanisms after digit and letter recognition. Although many early studies focused on the reality of synesthesia, more recent investigations have turned to examining the consequences of synesthesia.
Synesthesia is a benign neurodevelopmental condition in which stimulation of one sensory modality evokes experiences in a second, unstimulated modality (Simner and Hubbard, 2013). In grapheme-color synesthesia (GCS), which is experienced by 1–2% of adults, synesthetes reliably and involuntarily experience specific colors when viewing blackand-white graphemes. Previous case-studies have identified synesthetes with spectacular memory (Luria, 1968; Smilek, Dixon, Cudahy, & Merikle, 2001) and group studies have found advantages for synesthetes compared to nonsynesthetes in long-term memory (Rothen, Meier, & Ward, 2012). Here, we tested whether similar advantages are also present in earlier stages of memory. We tested visual iconic memory—the sensory store for vision—which has a very large capacity, but decays approximately 1000 ms after stimulus offset (Chow, 1985; Sergent et al., 2013; Sperling, 1960). We tested 20 synesthetes and 20 nonsynesthetes in a direct replication of the Sperling (1960) Partial Report Paradigm using letters (Experiment 1) and non-alphanumeric symbols (Experiment 2) as stimuli. Overall, synesthetes had a greater iconic memory capacity than nonsynesthetes when presented with synesthesia-inducing letter stimuli. This advantage was reduced when they were presented with non-synesthesia inducing symbol stimuli. Critically this advantage was most prominent when memory was stressed by asking participants to remember large arrays. Our results demonstrate that synesthetic memory advantages extend to the earliest stages of memory, and suggest that advantages in later stages of memory may arise from these earlier advantages.
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Individuals with grapheme-colour synaesthesia experience anomalous colours when reading achromatic text. These unusual experiences have been said to resemble ‘normal’ colour perception or colour imagery, but studying the nature of synaesthesia remains difficult. In the present study, we report novel evidence that synaesthetic colour impacts conscious vision in a way that is different from both colour perception and imagery. Presenting ‘normal’ colour prior to binocular rivalry induces a location-dependent suppressive bias reflecting local habituation. By contrast, a grapheme that evokes synaesthetic colour induces a facilitatory bias reflecting priming that is not constrained to the inducing grapheme’s location. This priming does not occur in non-synaesthetes and does not result from response bias. It is sensitive to diversion of visual attention away from the grapheme, but resistant to sensory perturbation, reflecting a reliance on cognitive rather than sensory mechanisms. Whereas colour imagery in non-synaesthetes causes local priming that relies on the locus of imagined colour, imagery in synaesthetes caused global priming not dependent on the locus of imagery. These data suggest a unique psychophysical profile of high-level colour processing in synaesthetes. Our novel findings and method will be critical to testing theories of synaesthesia and visual awareness.
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This review provides a commentary on coloured-hearing arising on hearing music: music-colour synaesthesia. Although traditionally explained by the hyperconnectivity theory (Ramachandran & Hubbard, 2001a) and the disinhibited feedback theory (Grossenbacher & Lovelace, 2001) as a purely perceptual phenomenon, the review of eight coloured-hearing neuroimaging studies shows that it may not be assumed that these explanations are directly translatable to music-colour synaesthesia. The concept of ‘ideaesthesia’ (Nikolić, 2009) and the role of conceptual and semantic inducers challenge the likelihood of a single mechanism underlying the cause of synaesthesia and argue for a move away from a purely sensory to sensory explanation. Finally, music-colour synaesthesia forms a challenge for established philosophical theories and the position of synaesthesia is considered within the larger context of musical qualia.
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Recent research into synaesthesia has highlighted the role of learning, yet synaesthesia is clearly a genetic condition. Here we ask how can the idea that synaesthesia reflects innate, genetic differences be reconciled with models that suggest it is driven by learning. A number of lines of evidence suggest that synaesthesia relies on, or at least interacts with, processes of multisensory integration that are common across all people. These include multisensory activations that arise in early regions of the brain as well as feedback from longer-term cross-modal associations generated in memory. These cognitive processes may interact independently to influence the phenomenology of the synaesthetic experience, as well as the individual differences within particular types of synaesthesia. The theoretical framework presented here is consistent with both an innate difference as the fundamental driver of the condition of synaesthesia, and with experiential and semantic influences on the eventual phenotype that emerges. In particular, it proposes that the internally generated synaesthetic percepts are treated similarly to other sensory information as the brain is learning the multisensory attributes of objects and developing cross-modal associations that merge in the concept of the object.
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This study used grapheme-colour synaesthesia, a neurological condition where letters evoke a strong and consistent impression of colour, as a tool to investigate normal language processing. For two sets of compound words varying by lexical frequency (e.g., football vs lifevest) or semantic transparency (e.g., flagpole vs magpie), we asked 19 grapheme-colour synaesthetes to choose their dominant synaesthetic colour using an online colour palette. Synaesthetes could then select a second synaesthetic colour for each word if they experienced one. For each word, we measured the number of elicited synaesthetic colours (zero, one, or two) and the nature of those colours (in terms of their saturation and luminance values). In the first analysis, we found that the number of colours was significantly influenced by compound frequency, such that the probability of a one-colour response increased with frequency. However, semantic transparency did not influence the number of synaesthetic colours. In the second analysis, we found that the luminance of the dominant colour was predicted by the frequency of the first constituent (e.g. rain in rainbow). We also found that the dominant colour was significantly more luminant than the secondary colour. Our results show the influence of implicit linguistic measures on synaesthetic colours, and support multiple/dual-route models of compound processing.
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Studies have shown that both serotonin and glutamate receptor systems play a crucial role in the mechanisms underlying drug-induced synesthesia. The specific nature of these mechanisms, however, continues to remain elusive. Here we propose two distinct hypotheses for how synesthesia triggered by hallucinogens in the serotonin agonist family may occur. One hypothesis is that the drug-induced destabilization of thalamic projections via γ-aminobutyric acidergic neuronal circuits from sensory areas leads to a disruption of low-level, spontaneous integration of multisensory stimuli. This sort of integration regularly occurs when spatial and temporal attributes match. Destabilization of feedback loops, however, can result in incongruent experiences or binding of random thalamus activation with sensory input in a particular sensory modality. The second hypothesis builds on embodied cognition, cases in which visual images of external stimuli activate task-related neural regions. In this proposal, binding processes that do not normally generate awareness become accessible to consciousness as a result of decreased attentional discrimination among incoming stimuli.

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CognitionThe Role of Meaning in Grapheme-Colour Synaesthesia

Abstract

Trends in Cognitive Sciences

Brain and Cognition

Brain and Cognition

Coloured speech perception: Is synaesthesia what happens when modularity breaks down?

Perception

Five plus two equals yellow

Nature

Not all synaesthetes are created equal: Projector vs. associator synaesthetes

Cognitive, Affective and Behavioral Neuroscience

Grapheme-Colour Synaesthesia: When 7 is Yellow and D is Blue

Unconscious priming eliminates automatic binding of colour and alphanumeric form in synaesthesia

Nature

Digit synaesthesia: A case study using a Stroop-type test

Cognitive Neuropsychology

Cognition
The Role of Meaning in Grapheme-Colour Synaesthesia