Cerebral Cortex Advance Access originally published online on January 15, 2009
Cerebral Cortex 2009 19(10):2321-2325; doi:10.1093/cercor/bhn249
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TMS-Adaptation Reveals Abstract Letter Selectivity in the Left Posterior Parietal Cortex
1 Department of Psychology, University of Pavia, Piazza Botta, 6, Pavia 27100, Italy, 2 Institute of Cognitive Neuroscience, University College London, London WC1N 3AR, UK, 3 Department of Psychology, University of Essex, Colchester CO4 3SQ, UK
Address correspondence to Juha Silvanto, University of Essex, Department of Psychology, Wivenhoe Park, Colchester CO4 3SQ, UK. Email: juha{at}essex.ac.uk.
Activation of the left posterior parietal cortex (PPC) has been associated with the encoding of letters independent of visual form. Here we used transcranial magnetic stimulation (TMS)-adaptation to investigate whether this abstract letter selectivity plays a causal role in letter processing. Visual adaptation was used to manipulate the initial activation state of neurons tuned to different letters prior to the application of TMS, after which subjects performed a detection task on letters that were presented in a different case from the adapting letter. After adaptation, TMS applied over the left PPC facilitated the detection of the adapted letter, whereas it had no impact on the detection of nonadapted letters. TMS applied over the right PPC had no significant effect on either type of letter. This interaction between adaptation and the effects of left PPC TMS demonstrates that adaptation modulated neural activity in the left PPC and thus demonstrates abstract letter selectivity in this region. Importantly, as the adapted letter and the target letters were presented in different cases, this finding demonstrates that the left PPC plays a causal role in letter processing independent of visual form.
Key Words: adaptation • letter processing • posterior parietal cortex • transcranial magnetic stimulation