Timing Differences in Neural Responses Related to Perception of Synesthetic Colors
Post by Lincoln Tracy
What's the science?
Grapheme-color synesthesia is a neurological phenomenon in which symbols, such as letters or numbers, are seen in colors. While scientists know that synesthetic experiences are not driven by direct sensory input, the neural mechanisms underlying synesthetic color experiences remain unknown. This week in PNAS, Teichmann and colleagues determined whether the neural activity associated with direct color perception also occurs during synesthetic color perception.
How did they do it?
The authors recruited 18 (14 females) people with synesthesia. Initially, Online questionnaires were used to identify letters and numbers that each individual perceived as being green or red, as synesthesia-inducing symbols can be different for each synesthete. Synesthetes then completed a target-detection task while in a magnetoencephalography (MEG) scanner, which records the magnetic fields produced by the brains’ electrical activity. The target-detection task had two conditions: In the colored shapes condition, synesthetes viewed three different shapes which were red or green. Then, in the synesthetic-inducing symbol condition, synesthetes were presented with their six unique symbols that evoked red or green synesthetic colors in three different fonts with black text. The authors used linear discriminant classification models and time generalization methods to test for similar neural activity during the colored shapes and synesthetic-inducing symbol conditions. First, classification models were trained to distinguish between the red- and green-colored shapes in the colored shape trials then tested for prediction accuracy in the colored shape and synesthetic-inducing symbol trials.
What did they find?
The authors found that neural activity from perceiving the red or green shapes could be identified ~80 to 200ms after stimulus presentation. This activity was found to be the same in both the training and testing data sets for the colored shape conditions. This trained model was able to generalize patterns evoked by the induced synesthetic color at ~300 to 400ms, meaning that the neural similarities between color representations evoked by synesthesia occurred at a later time. In other words, the neural response to the red and green shapes and synesthetic-inducing symbols were similar but occurred later when the synesthetic-inducing symbols were presented.
What's the impact?
Teichmann and colleagues provide objective verification of synesthetic colors and highlight the value of time-resolved decoding methods for studying such phenomena. These findings indicate that synesthesia-inducing stimuli require considerable processing time before the synesthetic color experience is generated. These findings provide a unique insight into the timeframe of the influence of knowledge on visual perception, demonstrate a neural signature for this phenomenon, and support the role of higher-level brain processing in synesthesia.
Teichmann et al. Temporal dissociation of neural activity underlying synesthetic and perceptual colors. PNAS (2021). Access the original scientific publication here.