| Petal Photism |
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| Figure 1. Example of a visual percept seen by an auditory-visual synesthete |
Auditory-Visual synesthesia, a particular type of synesthesia, is a perceptual phenomenon in which the perception of an auditory stimuli (aka. the inducer) elicits a simultaneous visual percept (aka. the concurrent) or vice versa.[1] This usually occurs unilaterally (ie. Auditory → Visual or Visual → Auditory), although there have been few documented cases of synesthesthetes with bilateral sensory stimulation.[2] The auditory and visual percepts experienced by the synesthete may vary from individual to individual but the inducer-concurrent percept combinations are marked by consistency and automaticity. This means that if an auditory-visual synesthete sees the colour blue whenever he/she hears a middle C on the piano, this same sensation will be automatically perceived consistently over time.[3] Within auditory-visual synesthesia there are various types of auditory and visual percepts experienced, out of which the most common ones are musical tones and color percepts.[4] An example of a coloured visual percept (Fig.1) can be seen on the right.[5]
Particular chromosomes, genetic linkages and structural differences in neural pathways have been implicated in auditory-visual synesthetes. Through investigating the suggested genetic and neural mechanisms that underlie this phenomenon, one may hope to further understand the workings of cross-modal interactions of both synesthetes and non-synesthetes.
Genetics
Several chromosomes have been suggested to be linked to auditory-visual synesthesia. Asher and colleagues performed the first whole genome scan and mapping for possible genes and chromosomes that could be linked to auditory-visual synesthesia. Their results suggested auditory-visual synesthesia to be an oligogenic condition with multiple modes of strong familial inheritance. They found 4 suggested significant chromosome linkages that met the criteria, namely, chromosome 2q24, 5q33, 6p12 and 12p12. Due to the predominant number of females with auditory-visual synesthesia, auditory-visual synesthesia has also previously been thought to be X-linked, however this particular study successfully refuted the notion with evidence of male-to-male transmission of the condition (Fig.2). [6]
| Male-to-Male Transmission Pedigree |
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| Figure 2. Pedigree chart depicting male-to-male transmission of auditory-visual synesthesia |
The 4 suggested linkage chromosomes are each associated with different aspects of auditory-visual synesthesia. For example, chromosome 2q24 is linked to autism, which in turn is closely associated with synesthesia. (synesthesias-links-with-other-mental-disorders) Many individuals with autistic-spectrum disorders have reported synesthesia as a symptom of the disorder, and clinical prevalence of the individuals with both autism and synesthesia is high. On the other hand, a gene (DPYSL3) that is involved with neuronal growth and plasticity is coded on chromosome 5. [6] This is related to neuronal growth and plasticity, which may partly account for the phenomenon as some have found evidence for neural connectivity and density in relation to auditory-visual synesthesia. [7]
Chromosome 6p12 is associated with dyslexia and so may relate to linguistic types of stimuli in some forms of synesthesia. (personification-based-synesthesia) It also codes of genes related to apoptosis, where a mutation in the gene will reduce apoptotic effects, leading back to increased neuronal growth in relation to auditory-visual synesthesia. And lastly, chromosome 12p12 codes for a gene responsible for NMDA receptors. NMDA receptors are known to be crucial for memory consolidation, learning and long-term potentiation. (synesthesia-and-memory) This relates to synesthesia as enhanced memory may lead to enhanced learning, and inducer-concurrent pairs may be developed due to enhanced learned associations during early childhood. [6]
Neural Mechanisms
There have been countless studies aiming to discover the neural underpinnings of synesthesia, as the study of these cross-modal interactions may shed light on multisensory integration on non-synesthetes as well. In the plethora of synesthesia-related studies, there are a few studies in relation to the neural correlates and temporal characteristics that may be specific to auditory-visual synesthesia that are worth noting. As well, evidence supporting each of the two competing models seeking to explain this neural phenomenon will be explored.
Disinhibited Feedback Model vs. Cross-Modal/Hyper-Binding Model
There are two main hypothesized models that seek to explain the neural underpinnings of synesthesia; namely, the disinhibited feedback model and the cross-modal/hyper-binding model. (grapheme-colour-synesthesia) The first suggests synesthesia as a result of “leaky” neuronal feedback passed from the specific inducer area to concurrent area that should otherwise be normally inhibited, resulting in consecutive activation of inducer area then concurrent area. [3] The latter is a two-stage model that proposes the binding of cross-modal percepts due to the proximity of two sensory association areas. This should then result in simultaneous neural activation of both inducer and concurrent associated areas. [7]
Neural Correlates
Neural correlates specifically associated with auditory-visual synesthetes have been suggested through neuroimaging techniques. Notably, an fMRI study conducted by Zamm et al., have found asymmetrical rightward hemispheric patterns in the brains of auditory-visual synesthetes but not non-synesthetes – specifically increased white-matter connectivity within the right inferior fronto-occipital fasciculus (IFOF) (Fig.3). This pathway primarily connects the visual and auditory association areas in the frontal and temporal lobes respectively, to the frontal lobes. Implicated in the processing of visual information, speech, language and multimodal integration, it suggests enhanced top-down, cross-modal interactions that are evident only in synesthetes. [7]
| Colored-Hearing Photisms |
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| Figure 4. Example of colored percepts experienced by colored-hearing synesthetes |
| Rightward Hemispheric Patterns |
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| Figure 3. Increased white-matter integrity found in IFOF of auditory-visual synesthetes |
Another fMRI study conducted by Gaschler-Markefski et al. revealed significantly higher acoustic activations present in brains of colored hearing synesthetes (fig.4) [8] when compared to non-synesthetes. These significant activations were observed in a number of visual association areas – namely, the right occipital lobe, fusiform gyrus, left middle temporal gyrus and lastly, auditory associated area – left superior temporal gyrus This observation is especially significant, because the acoustic presented to the synesthetes did not reliably generate synesthetic percepts yet the stimuli were able to evoke strong activation in visual association areas in the absence of visual sensations. This finding then extends support for the disinhibited feedback model, as auditory stimuli presented to synesthetes were able to generate general feedback activation to visual association areas as well as auditory areas. [3]
Two fMRI studies, conducted by Neufled et al., similarly shows support for the disinhibited feedback model, in that the processing of auditory stimuli elicited stronger connectivity between the left inferior parietal cortex (IPC) to the left primary auditory cortex and right primary visual cortex. The left IPC is an important convergence region implicated in a multitude of functions – multisensory integration, mental imagery, spatio-dynamic processing, non-synesthetic feature binding as well as the processing of synesthetic stimuli (therefore, not a direct pathway between primary visual and auditory cortices). This pattern of activation was only evident in auditory-visual synesthetes and not controls suggesting that general auditory stimulation “leaked” to concurrent primary cortices at low-level stages of processing. [8-9]
Temporal Characteristics
Understanding the temporal characteristics displayed by auditory-visual synesthetes is important as it occurring at early or late stages will provide one with additional information on how the phenomenon works, and may even shed light on the possible neuronal pathways/correlates it adopts.
Temporal characteristics of the brain can be adequately investigated through the use of event-related potentials. Two particular studies adopted this method in analyzing the time course of processes in auditory-visual synesthetes. Both studies found increased activation by auditory-visual synesthetes at left posterior inferior temporal regions as early as 100-122ms after the onset of auditory stimuli [10-11], while the second study also additionally observed increased activation at color processing area (V4) and orbitofrontal brain regions. [10] These associated neural correlates coincide with areas mentioned above, demonstrating converging evidence for auditory-visual synesthetes. Such early differences arising between synesthetes and controls suggest that the onset of inducer stimuli and concurrent percepts may be simultaneous, pre-attentive and automatic [2] as well as driven by bottom-up associated regions like the median geniculate nucleus.11 A third study employed the mismatch negativity (MMN) paradigm to dissect the time course of inducer-concurrent stimulation in colored-hearing synesthetes. Larger the MMN deviations, the more it reflects automatic and pre-attentive processing, which is precisely what the authors observed. [2]
Types of Auditory-Visual Synesthesia
There are various combinations of inducer-concurrent pairs that classify under auditory-visual synesthesia. The most prominently documented with auditory-visual synesthetes is when musical tones elicit color percepts. This particular type is called chromesthesia. A case study of a female chromesthetic called ‘D’ describes her experience with musical tones and reports her color associations: “A, lavender; B, orange; C, red; D, blue; E, green; F, brown; and G, black” but she “never see[s] yellow much. I don't paint with yellow either, but it is my favorite color." [12]
Other types of auditory-visual synesthesia include possible voice-induced associations with color and texture percepts [13], tonal stimuli inducing colored geometric shapes in space [4] and bilaterally, visually-induced auditory stimuli [14] to list a few. Chiou et al. conducted a study with a group of synesthetes that perceived colored geometric objects spatially when they heard specific sounds, and was the first to provide evidence that synesthetes were able to consciously select which feature of perceptual stimuli to purposely attend to despite them being internally experienced. [4]
Multisensory Integration
Through furthering our understanding of the workings of cross-modal interactions in synesthesia, one may simultaneously shed light on multisensory integration in non-synesthetes. A study conducted by Ward et al. suggested that synesthetes and non-synesthetes may not be distinctly different from each other; rather we may merely be different ends of a spectrum. Similarly, maybe some of us just lost the synesthetic hardware during the natural course of growth. [15] The authors observed that both auditory-visual synesthetes and non-synesthetes adopted identical mapping strategies when dealing with pitch and lightness of color (fig.5). We naturally associate darker colors with lower pitched sounds and lighter, pastel colors with higher pitched sounds. This similar mapping of auditory-visual percepts may indicate that synesthetes and non-synesthetes may be adopting the same neural processes when deciphering multi-modal information – except that synesthetes are processing the information with more precision, whereas non-synesthetes experience a noisier type of mapping. [15]
| Monotonic Increase of Color with Pitch |
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| Figure 5. Identical pattern of monotonic increase in the lightness of color with increasing pitch. |
I love your topic - always was fascinated by synesthesia - and how well you covered your topic in a limited word count!
Great content and excellent explanation of a fascinating topic!