In a previous event-related fMRI study, we showed that the left occipitotemporal cortex was specifically involved in the abstract categorization of visually presented alphabetic symbols. Here, we duplicate the fMRI study by measuring visual event-related potentials to verify whether the left posterior cortex supports the processing of graphemic representations at a perceptual, prelexical level. N170 amplitudes elicited by the categorically ambiguous stimulus 'O' were about twice as small in the left as in the right occipitotemporal region, and comparable to that of other letters on the left, and to that of other geometric figures on the right. The side asymmetry suggests that a graphemic module is unilaterally implemented in the left hemisphere, where it automatically processes heteromorphic representations that do not depend on the physical characteristics of the signal, whereas the difference in amplitude suggests that this left graphemic module is in series with, precedes and does not transmit information to, or inhibits a general shape-analysis module.