Intracerebral Electrical Stimulation of the Left Occipito-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 lesion studies, is now widely accepted. Although this brain region has been extensively studied with functional magnetic resonance imaging (fMRI), its causal role as a critical node of a cortical network for reading remains unclear. Here we report a comprehensive case of pure alexia during direct electrical stimulation (DES) of the left VOTC (patient SV, female, 38 yr old, implanted with intracerebral electrodes for refractory epilepsy). During DES of the left posterior occipito-temporal sulcus, but not of neighboring and remote cortical sites, SV was transiently impaired at reading single words while being able to slowly read letter-by-letter. However, SV was impaired when presenting a single letter in a rapid serial visual presentation, which showed that their letter reading is not entirely preserved. In contrast, DES to the same critical sites left performance for oral naming, auditory naming, reading numbers, writing, auditory lexical decision, and semantic matching of pictures unaffected. Intracerebral electrophysiological frequency-tagging investigations showed highly word-selective neural responses at the critical sites. These functional responses were abolished by concurrent DES, which also affected remote word-selective neural activity in the left VOTC. Altogether, these observations provide original evidence for word-selective representations of the left VOTC as a critical node of the cortical reading network.