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Synesthetic-Like Experiences After Awakening From Sleep

Could dreaming induce cross-sensory experiences similar to synesthesia?

Synesthesia is a condition where people perceive one sensory stimulus in multi-sensory modalities, such as ‘seeing’ sounds as different geometric shapes, or ‘tasting’ color as different flavors. One explanation for synesthesia, proposed by Grossenbacher and Lovelace (2001), is that all of us experience cross-sensory activity when processing stimuli, but that in most cases the brain inhibits cross-sensory activation to isolate perception to the single appropriate sensory modality. Synesthetes, on the other hand, may have decreased inhibition across sensory areas of the brain, or rather, an increase in cross-talk among sensory areas. This explanation assumes that there is nothing anatomically different between the brains of synesthetes and non-synesthetes, but rather that non-synesthetes normally inhibit cross-sensory cortical activation.

Nevertheless, even non-synesthetes can experience synesthesia in certain conditions; for example, individuals on hallucinogens, or under hypnosis, may at times experience synesthesia, e.g. hearing sounds as shapes, seeing numbers as colors and so on. Another state in which we may experience synesthesia is during dreaming, and even in the transition state between wakefulness and sleep. Several examples of this were explored in a previous post here, where for example sound stimuli from the environment triggered images of motion during brief transitions into sleep (e.g. a ‘thud’ corresponds with the image of a door suddenly slamming).

It’s thought in fact that the dream state, similar to synesthetic experiences, is characterized by decreased cortical inhibition, and the associative activations occurring throughout the cortex create multi-sensory dream images. The authors of the current paper sought to explore similarities between synesthesia and sleep by probing individuals during the period immediately following awakening, a period of ‘sleep-inertia’ during which altered sleep-related cognition persists.

The authors hypothesized that subjects would report synesthetic-like experiences during sleep-inertia. The authors first conducted an at-home diary study, but I will focus here on their second study, which was conducted in the sleep laboratory with polysomnography.

In this study, 20 healthy participants (non-synesthetes) took part. Each participant in the study underwent an overnight polysomnographic assessment (see previous post for description of polysomnography) in a sleep laboratory from 10 p.m. until 6 a.m., using a standard recording of EEG, EOG, EMG, and ECG. All subjects were awakened at 6:00 a.m.

For the study, subjects were randomized into two groups: 10 subjects in the experimental group completed a measure of synesthesia immediately upon awakening in the morning, and 10 subjects in the control group completed the measure prior to sleep.

The measure assessed Synesthetic-Like Experiences

1 Sounds evoke a sensation of color

2 Touch evokes a sensation of smell

3 Sounds evoke a sensation of taste

4 Letters/numbers evoke a sensation of color

5 Words evoke a sensation of taste

6 Touch evokes a sensation of smell

7 Numerical sequences appear arranged in space

8 Smell evokes a sensation of color

9 Touch evokes a sensation of taste

10 Sounds evoke a sensation of vision

The authors also added four masking statements unrelated to cross-modal sensory experiences (“I like watching documentary films”; “I like watching reality shows;” “I like watching thriller shows;” “I like watching the news”).

Participants were instructed to respond to the statements according to their agreement on a scale of 1 (strongly disagree) to 7 (strongly agree). The authors calculated for each subject the average rating across the nine sensory statements, and across the four masking statements.

As expected, participants during sleep-inertia agreed more with cross-sensory statements (e.g., “musical tones trigger the experience of colors”) as compared to during wakefulness. There was no difference in agreement with the control statements, so the effect was not simply due to participants agreeing more after sleep, but rather specifically agreeing more with the synesthetic-like statements.

While preliminary, these findings suggest that sleep—related cognition is more similar to synesthesia than wakeful cognition is. It’s possible that dreaming triggers synesthetic experiences as well and could be a viable path toward better understanding the synesthetic brain.

The authors suggest the results may be attributed to ‘hyper-associative’ cognition occurring during sleep and immediately after awakening. Further research could look towards what areas of the brain may be triggering this cross-modal associativity; particularly, parts of the prefrontal cortex are less active during sleep, and may play a role in waking experiences of synesthesia as well.


Reznik, D., Gertner-saad, L., Even-furst, H., Henik, A., Mair, E. B., Shechter-amir, D., & Soffer-dudek, N. (2018). Oneiric Synesthesia: Preliminary Evidence for the Occurrence of Synesthetic-like Experiences During Sleep-inertia. Psychology of Consciousness: Theory, Research, and Practice, 5(4), 374-383.

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