Event-related potentials in silent reading of Japanese katakana letters

Event related potentials (ERPs) were recorded from seven subjects, using twelve scalp electrodes and three other electrodes, monitoring eye and throat movements. The experiments were divided into two blocks. In one block Japanese katakana letters (phonograms) or unreadable pseudo letters that were made by deforming letters were visually presented at random. The subject was instructed to silently read when readable letters were presented and to withhold it when pseudo letters were presented (control state). Fifty letters and fifty pseudo letters were presented one at a time in random order. In the other block, meaningless words having three katakana letters, or words having three pseudo letters were presented. Each subject carried out three or four of each block. The average ERPs with silent reading minus those without silent reading showed five peaks: a negative peak at a 400 ms latency in the left posterior temporal scalp area, a positive peak at a 400 ms latency in the frontal scalp area, a positive peak at a 520 ms latency in the parietal scalp area, a negative peak at a 620 ms latency in the frontal, parietal, and temporal scalp area and a negative peak at a 700-ms latency in the frontal scalp area. These peaks except for the 520 ms positive peak, were larger for words than for letters. The present silent reading task included mental processes involving recognition of letters judgement of whether the stimulus is for silent speech and silent speech itself, but the control task did not. The negative difference in the left posterior temporal scalp area was not observed in the previous ERP experiments using silent speech prompted by light emitting diodes, and is considered as related to recognize the presented letters. The positive difference in the parietal scalp area was observed in the previous experiments and is considered as related to the P3 component for target detection. The frontal positive and negative differences were also observed in the previous ERP experiments, and are considered to be related to silent speech.