Synaesthesia, multisensory integration and imagery: associated or independent processes to bridge the senses?
Professor Fiona Newell, Trinity College Dublin, Ireland
Although cross-modal interactions in the brain are often assumed to mediate synaesthesia, relatively few investigations have attempted to elucidate the nature of these interactions. Synaesthesia can be driven by integrated, multisensory rather than unisensory inputs, which is consistent with growing evidence that multisensory interactions are the norm, both within the brain and on behaviour. Moreover, evidence for cross-modal, synaesthetic-like associations in non-synaesthetes as well as enhanced multisensory processes in synaesthetes suggests that synaesthesia may be scaffolded onto multisensory processes which are common to all. However, this line of research has primarily focussed on cross-modal interactions that lie outside of synaesthetic experiences and has yet to address whether synaesthesia is associated with differences in how related sensory information is processed. In particular, the phenomenology and prevalence of synaesthesia suggests that cross-modal interactions associated with synaesthesia may be discrete, and not cognitively penetrable. However, evidence that synaesthesia can also be triggered by a mental image of the inducing stimulus challenges the idea that synaesthesia relies on discrete processes. Mental imagery itself has also been shown to induce multisensory illusions, but its role in broader synaesthetic experiences is mainly unknown. This can largely be attributed to our relatively poor understanding of mental imagery in sensory domains beyond vision. Current investigations are aimed at exploring individual differences in the ability to imagine stimulation across other sensory domains. Such abilities may, in turn, be linked with specific types of synaesthesia thus providing further insight into the cross-modal mechanisms underpinning synaesthesia.
Synesthesia evoked through mild sensory deprivation
Assistant Professor David Brang, University of Michigan, USA
The natural environment contains auditory and visual cues, conveying both redundant and unique information about multisensory objects and events. To facilitate the processing of this information, the brain integrates auditory and visual signals, leading to better detection and response. While these multisensory benefits can result from one sensory system influencing another (eg, the spatial position of a tone modulates neural activity in areas of visual cortex that process similar regions of space), tones presented in isolation do not typically evoke conscious visual experiences. In individuals with synesthesia, however, these multisensory interactions do lead to qualitatively different experiences such as tones evoking flashes of light. Why, if multisensory interactions are present in all individuals, do only synesthetes experience multisensory hallucinations? Models of synesthesia propose that this difference is due to either reduced inhibition or increased connectivity between associated modalities in synesthetes. Case reports suggest that non-synesthetes can experience these sensations through drug ingestion, raising the possibility that synesthesia exists as a latent feature in all individuals, manifesting only when the balance of activity across the senses has been altered. This talk with presents data from a series of studies showing that aspects of generic multisensory interactions present in all individuals operate at early sensory levels, and that perturbation of these networks can lead to auditory-visual hallucinations (synesthesia) in the general population, providing tentative links between these conventionally distinct processes.
Using mirror-sensory synaesthesia to examine how we perceive and understand others
Professor Michael Banissy, Goldsmiths, University of London, UK
Our capacity to share the experiences of others is a critical part of human behaviour. One process thought to be important for this is vicarious perception - the ability to co-represent the experiences of other people by matching the observed state onto representations of our own first-hand experience. For example, observing pain in other people activates some of the same network of brain regions as the first-hand experience of pain. The degree of vicarious perception is contextually and socially embedded. It has thus been used as a model system for exploring the broader mechanisms that underpin inter-personal representations and complex phenomena such as empathy. For most of us vicarious perception is implicit (i.e. unconscious), but for some individuals viewing another person’s state results in them literally experiencing a conscious sensation of the observed event (known as mirror-sensory synaesthesia). This talk will discuss what factors contribute to mirror-sensory synaesthesia and ways in which mirror-sensory synaesthesia can be used to enhance our knowledge of how we understand the experiences of others.
Neurophenomenological investigations of natural and trained synaesthesia
Professor Anil Seth, University of Sussex, UK
Synaesthesia offers a unique window into the neurocognitive mechanisms underlying conscious perception. Intriguingly, recent evidence suggests that extensive perceptual and cognitive training can induce synesthesia-like experiences in non-synaesthetic adult volunteers (Bor et al, Scientific Reports, 2014, 4:7089). On the other hand, neuroimaging findings of colour-selective responses during natural (grapheme-colour) synaesthesia have been inconsistently reported. Focusing on grapheme-colour synaesthesia, I will describe a series of studies linking neural responses to phenomenology in both natural and ‘trained’ synaesthesia. For natural synaesthetes, our results show that colour-specific specific brain responses can be predicted by individual differences in synaesthetic phenomenology captured by ‘localisation’ and ‘automaticity’. For trained non-synaesthetes, we find coordinated phenomenological, behavioural and neurophysiological changes following a battery of adaptive cognitive training, revealing an unexpectedly powerful capability for perceptual plasticity even in adults. Finally, I will highlight an overlooked property of synaesthesia, which is that synaesthetic concurrents usually lack perceptual ‘presence’; that is, they are not experienced as being part of the external world. A theory based on counterfactual predictive processing suggests why this might be so.
Professor Simon E Fisher, Max Planck Institute for Psycholinguistics, The Netherlands