Synaesthesia: supernormal integration? Catherine M. Mulvenna and Vincent Walsh Institute of Cognitive Neuroscience & Department of Psychology, University College London, 17 Queen Square, London, WC1N 3AR, UK Synaesthesia has been known to scientific research for over 100 years but has undergone something of a renais- sance recently as new investigations begin to uncover its neurological basis. Rather than being an anomaly, it might offer beneficial insights into the basis of normal perception. A new study by Esterman et al. epitomises this current trend and claims to show that the posterior parietal cortex is a crucial locus of synaesthetic experi- ence. As the posterior parietal cortex is commonly linked to normal sensory integration, Esterman et al.’s finding might lend support to the claim that synaesthesia is an extension of the normal perceptual processes assumed to occur in binding. Synaesthesia occurs when an individual experiences sen- sations of attributes triggered reliably by specific external stimuli but not normally associated with those stimuli. It ranges in vividness and can occur between different senses, for example the sound of notes can trigger tastes or shapes. However it commonly occurs within a sense, for example, reading letters, words or numbers may trigger the sensa- tion of colours [1]. Synaesthesia affects about 4% of the population [2], and understanding it requires the wider principles of cortical development, sensory organization, sensory integration and the generation of qualia to be addressed. A new study by Esterman et al. [3] marks an important step in understanding the neural basis of synaesthesia, and also speaks to these wider issues. In recent years the efforts to understand the sensory cross-activation in synaesthesia have paralleled broader neuroscientific trends. For example, the identification of functionally specialized modules in the brain has been followed by a realization that understanding the interaction between modules in an extended network is as valuable as fractionating that network into its component parts [4]. Moreover, it is possible that different sensory mechanisms use common algorithms [5] that underpin cross-modal asso- ciations and synaesthesia; indeed, it might make them inevitable. Esterman et al. address the idea that synaesthesia repre- sents one end of a spectrum of sensory connectivity and focus on the role of the posterior parietal cortex in the synaesthetic process. Their study reflects a growing recognition that synaesthesia presents a means to investigate variability in modularity and interconnection between human sensory systems. In normal cross-modal integration, the ability to combine multiple sources of information yields the best estimate of the external properties of a stimulus [4], and reduces perceptual uncertainty [6]. It is therefore important to investigate whether synaesthetes’ bimodal experiences (word–colour, tone–colour, smell–touch, etc.) are an exten- sion of normal cross-modal integration [7]. Do synaesthetes bind? Esterman et al. adapted the Stroop task (Figure 1) and presented two grapheme–colour synaesthetes with letters, which elicited a synaesthetic colour, and symbols, which did not. The letters were presented in a colour that was either congruent or incongruent with the synaesthetic colour normally triggered by that letter. Response time for identifying the physical colour of the letter was measured. A significant delay in naming incongruently coloured letters has been demonstrated in synaesthetes [8], and is used to indicate the presence and interference of synaesthetic colours. In their experiment Esterman et al. applied repetitive Transcranial Magnetic Stimulation (rTMS) to the left and right angular gyri at the junction of the posterior intrapar- ietal and transverse occipital sulci (IPS/TOS), and also to V1 as a control site. rTMS (480 pulses at 1 Hz for 8 min) was applied immediately before two blocks of 120 trials, counterbalanced with sham TMS. An increase in response latency would demonstrate interference with the synaes- thetic process. Indeed, with sham TMS the synaesthetes displayed their usual response latency but after rTMS over 350 Update TRENDS in Cognitive Sciences Vol.10 No.8 Figure 1. Adapting the Stroop task for synaesthesia. The Stroop Effect [10] desc- ribes the delay in reaction time when naming the ink colour of an incongruent colour-name (e.g. blue).This can be adapted for grapheme–colour synaesthesia [8]. When presented with a grapheme printed in a colour that is incongruent with its synaesthetic colour (e.g. printed in red when it triggers a synaesthetic blue) syn- aesthetes have a significant reaction-time delay in identifying the ink colour, which is not present for congruent (printed in green when it triggers a synaesthetic green) or control conditions (symbols that do not trigger a synaesthetic colour). Corresponding author: Mulvenna, C.M. (c.mulvenna@ucl.ac.uk) www.sciencedirect.com