This 2018 UK review subject was colored-hearing experiences from music:
“Music-colour synaesthesia has a broad scope encompassing not only tone-colour synaesthesia elicited on hearing individual tones, but a complex and idiosyncratic mixture of phenomenological experiences often mediated by timbre, tempo, emotion and differing musical style.
Possession of synaesthesia or absolute pitch was shown to have very little effect on the actual colours chosen for each of the musical excerpts. But it might be reasonable to expect that music that elicits a strong emotional response may be more likely to induce synaesthesia than music that does not.
Examination of eight neuroimaging studies were found to be largely inconclusive in respect of confirming the perceptual nature of music-colour synaesthesia. Neither the hyperconnectivity nor the disinhibited feedback theory currently holds as a single categorical explanation for synaesthesia.
Theories promoting the notion of ‘ideaesthesia’ have highlighted the importance of role of concept and meaning in understanding of synaesthesia..and a replacement definition: Synaesthesia is a phenomenon in which a mental activation of a certain concept or idea is associated consistently with a certain perception-like experience.”
https://www.sciencedirect.com/science/article/pii/S1053810017305883 “Music-colour synaesthesia: Concept, context and qualia” (not freely available)
Much of the review was philosophizing and casting around for clues. The review cited interesting studies and reviews, including The Merit of Synesthesia for Consciousness Research.
One relevant element missed by the underlying research and review was critical periods of human development. A cited reference in How brains mature during critical periods was Sensitive periods in human development: Evidence from musical training (not freely available) illuminated some aspects:
“In contrast to a critical period, where a function cannot be acquired outside the specific developmental window, a sensitive period denotes a time where sensory experience has a relatively greater influence on behavioral and cortical development. Sensitive periods may also be times when exposure to specific stimuli stimulates plasticity, enhancing changes at neuronal and behavioral levels.
The developmental window for absolute pitch may be more similar to a critical than a sensitive period.
The auditory cortex appears to have an unusually long period of developmental plasticity compared with other sensory systems; changes in its cellular organization and connectivity continue into late childhood.
Effects of musical training have been shown to impact auditory processing in the brainstem as well.”
Let’s say that a researcher wanted – as one cited study did – to examine absolute pitch, a rare trait, present in a subset of synesthetes – music-color, another rare trait. The study as designed would probably be underpowered due to an insufficient number of subjects, and it would subsequently find “very little effect.”
Let’s say another researcher focused on cerebral brain areas – and like eight cited studies – ignored brainstem pons nuclei which are the first brain recipients of sound and equilibrium information from the inner ear via the eighth cranial nerve. Like those studies, the researcher was also biased against including limbic brain areas that would indicate “a strong emotional response.”
A study design that combined leaving out important brain-area participants in the synesthesia process with a few number of synesthetes would be unlikely to find conclusive evidence.
The reviewer viewed a lack of evidence from “eight neuroimaging studies” as indicating something about the “perceptual nature of music-colour synaesthesia.” An alternative view is that “inconclusive” evidence had more to do with study designs that:
- Had a small number of subjects;
- Omitted brain areas relevant to the music-color synesthesia process;
- Didn’t investigate likely music-color synesthesia development periods; and
- Didn’t investigate associations of music-color synesthesia with epigenetic states.
Consider the magnitude of omitting the thalamus brain area from synesthesia studies as one “perceptual nature of music-colour synaesthesia” example. Just background information of Thalamus gating and control of the limbic system and cerebrum is a form of memory indicated its relevance to synesthesia:
“Despite fundamental differences between visual, auditory and somatosensory signals, basic layouts of thalamocortical systems for each modality are quite similar.
For a given stimulus, output neural response will not be static, but will depend on recent stimulus and response history.
Sensory signals en route to the cortex undergo profound signal transformations in the thalamus. A key thalamic transformation is sensory adaptation in which neural output adjusts to statistics and dynamics of past stimuli.”
One of this study’s researchers described ways that an individual’s “stimulus and response history” became unconscious memories with the thalamus. Including the thalamus in synesthesia studies may also have findings that involve reliving or re-experiencing a memory, possibly an emotional memory.
In such future research, it could be a design element to ask synesthetes before and after an experiment to identify feelings and memories accompanying synesthesia experiences.
It shouldn’t be a requirement, however, to insist that memories and emotions be consciously identified in order to be included in findings. Human studies such as Unconscious stimuli have a pervasive effect on our brain function and behavior have found:
“Pain responses can be shaped by learning that takes place outside conscious awareness.
Our results support the notion that nonconscious stimuli have a pervasive effect on human brain function and behavior and may affect learning of complex cognitive processes such as psychologically mediated analgesic and hyperalgesic responses.”
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