Intracerebral electrical stimulation of the left word-selective temporal cortex induces pure alexia

The ability to read relies on the rapid mapping of perceived visual letters and their combinations (i.e., visual word forms) to phonology and meaning. The central role of the left ventral occipito-temporal cortex (VOTC) in processing letter strings, initially suggested by pure alexia in lesion studies, is now widely accepted. While this region has been intensely studied with functional MRI, direct electrical stimulation (DES) has rarely been used, although it allows a more direct assessment of causality between region(s) and function(s). Moreover, the few DES case studies reported did not provide stringent evaluation of the stimulation effect on reading performance. Here we report a comprehensive case of pure alexia during DES of the left VOTC (subject LV, female, 38 y.o, implanted with SEEG electrodes for refractory epilepsy). During DES of the left posterior occipito-temporal sulcus, but not of other sites, LV was transiently impaired at reading single words (performance on paper without time constraint: 99% correct before and after stimulation vs. 71% during stimulation) but was still able to slowly read letter-by-letter. LV was also impaired at making lexical decision on written words/pseudo-words (performance on paper: 100% vs. 75%; on computer screen: 92% vs. 72%), showing that she had impaired access to the lexico-semantic representation of the words. By contrast, performance was intact during stimulation for oral naming, auditory naming, reading numbers, writing, auditory lexical decision or semantic matching of pictures. Independent functional mapping using a frequency-tagging approach in SEEG showed that the stimulated site was located in a highly word-selective region. Altogether, our results show that DES of the word-selective left VOTC induced pure alexia remarkably selective to words reading