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Anomalous perception in synaesthesia: A cognitive neuroscience perspective

Nature Reviews Neuroscience volume 3, pages 43–52 (2002)Cite this article

Key Points

  • Synaesthesia is a rare phenomenon in which a sensory stimulus in one modality can trigger sensory experience in other modalities. For example, hearing a word (the 'inducer') might cause a synaesthete to see a particular colour (the 'concurrent'). Although there are many forms of synaesthesia involving different combinations of sensory inducer and concurrent, the most common consists of specific colours being elicited by written or spoken digits, letters or words.

  • Although early explanations for synaesthesia suggested that the associations between inducer and concurrent were learned, more recent data showing that synaesthesia is automatic and involuntary have led researchers to propose that it might arise from anomalous connectivity between brain areas, perhaps as a result of the failure of pruning during the development of the nervous system. Alternative explanations invoke abnormal sprouting of connections, or disinhibition of feedback within existing pathways.

  • Functional imaging studies have shown that visual synaesthetic experiences involve activation of extrastriate visual cortices, but not early visual areas, and that this activation seems to occur later than activation associated with the sensory modality of the inducer. Future imaging studies will need to be carefully designed to eliminate possible confounds and to provide further information on the neural bases of synaesthesia.

  • Objective measures of synaesthesia have shown that it is consistent over time, involuntary and automatic: for example, synaesthetes show interference when asked to name the colour in which a letter or digit is printed if that colour is incongruent with the synaesthetic colour induced by the character. Studies of priming by synaesthetic inducers seem to show that the inducing stimulus must be processed to conscious levels by the visual system in order for the synaesthetic effect to occur. However, other evidence relating to the stage of perceptual processing at which synaesthesia occurs indicates that synaesthetic colours arise before the inducer is identified.

  • A neurocognitive model of colour–graphemic synaesthesia is presented, based on the assumption that there are multiple routes for processing symbolic form and colour in the brain. According to the model, the experience of colour in synaesthesia might arise at different levels in the processing hierarchy. This model could provide a framework for further investigations of the neurocognitive bases of synaesthesia.

Abstract

An enduring question in cognitive neuroscience is how the physical properties of the world are represented in the brain to yield conscious perception. In most people, a particular physical stimulus gives rise to a unitary, unimodal perceptual experience. So, light energy leads to the sensation of seeing, whereas sound waves produce the experience of hearing. However, for individuals with the rare phenomenon of synaesthesia, specific physical stimuli consistently induce more than one perceptual experience. For example, hearing particular sounds might induce vivid experiences of colour, taste or odour, as might the sight of visual symbols, such as letters or digits. Here we review the latest findings on synaesthesia, and consider its possible genetic, neural and cognitive bases. We also propose a neurocognitive framework for understanding such anomalous perceptual experiences.

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Figure 1: Brain activation during colour vision, imagery and synaesthesia.
Figure 2: Example displays used in studies of the synaesthetic Stroop effect.
Figure 3: Displays used to show perceptual grouping effects in colour–graphemic synaesthesia.
Figure 4: Tasks used to examine the influence of synaesthetic colour on perception of digit inducers.
Figure 5: A cognitive model of colour–graphemic and colour–phonemic synaesthesia.

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Acknowledgements

We thank J. L. Bradshaw, M. Howard, M. O'Boyle and M. A. Williams for their helpful comments on the manuscript. A.N.R. is supported by an Australian Postgraduate Award. J.B.M. is supported by grants from the Australian Research Council, the National Health and Medical Research Council and Unilever (UK).

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Authors and Affiliations

  1. Department of Psychology, Cognitive Neuroscience Laboratory, University of Melbourne, Victoria, 3010, Australia

    Anina N. Rich & Jason B. Mattingley

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Correspondence to Jason B. Mattingley.

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FURTHER INFORMATION

American Synesthesia Association

The International Synaesthesia Association

MIT Encyclopedia of Cognitive Sciences

electrophysiology, electric and magnetic evoked fields

positron emission tomography

magnetic resonance imaging

modularity of mind

Synaesthesia

Glossary

ISHIHARA COLOUR PLATES

A series of plates used for the diagnosis of colour blindness. They are designed so that individuals with normal colour vision can see shapes or characters, but colour-blind people cannot.

FEEDFORWARD INFORMATION FLOW

Information processing that proceeds in a single direction from sensory input, through perceptual analysis, to motor output, without involving feedback information flowing backwards from 'higher' centres to 'lower' centres.

FEEDBACK INFORMATION FLOW

The flow of information from 'higher' to 'lower' centres.

AREA MT/V5

A region of visual association cortex in the middle temporal extrastriate area that is important for the analysis of visual motion.

PRIMARY VISUAL CORTEX

The cortical area that is the main recipient of visual information from the retinae (by way of the lateral geniculate nucleus); also known as V1 or striate cortex.

AREA V4

A region of the posterior fusiform gyrus that has been implicated in colour perception.

EVENT-RELATED POTENTIALS

Electrical potentials generated in the brain as a consequence of synchronized activation of neuronal networks by external stimuli. These evoked potentials are recorded at the scalp and consist of precisely timed sequences of waves or 'components'.

NEGATIVE PRIMING

The increased time taken to respond to a target item that was an ignored distractor in a preceding trial. The effect is thought to occur because inhibition associated with the ignored distractor carries over to processing of the target.

POP OUT

In displays composed of identical distractor stimuli (for example, red 'X's), a stimulus with a unique feature (for example, a blue 'X') can be detected rapidly and effortlessly, with little or no increase in reaction time as the number of distractor stimuli increases.

TRANSCRANIAL MAGNETIC STIMULATION

A technique used to induce a transient interruption of normal activity in a relatively restricted area of the brain. It is based on the generation of a strong magnetic field near the area of interest, which, if changed rapidly enough, will induce an electric field sufficient to stimulate neurons.

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Rich, A., Mattingley, J. Anomalous perception in synaesthesia: A cognitive neuroscience perspective. Nat Rev Neurosci 3, 43–52 (2002). https://doi.org/10.1038/nrn702

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