Transcranial direct-current stimulation (tDCS) on temporal lobe selectively affects the encoding of visual long-term memory
Classical views of human visual long-term memory propose that people first encode the visual stimuli into a long-term store, and then retrieve the visual information during task period. In examining the temporal dynamics of visual memory encoding, previous human electroencephalogram (EEG) recordings have shown that an increase in the amplitude of a positive frontal event-related potential (ERP) and the suppression of posterior alpha band (8-12 Hz) oscillations both occur during the successful encoding of a memory. However, we do not know whether these two different signals are functionally independent as EEG neural signatures. In this study, we used transcranial direct current stimulation (tDCS) to dissociate these two neural signatures in human subjects during a recognition memory task. We found that the parietal-occipital alpha suppression, but not the time-domain frontal positivity, followed the improvement in memory under anodal stimulation relative to when the same subjects were given sham stimulation. Meanwhile, our experiments also show that the improvement of recognition memory was not due to the attentional arousal of participants or a better retrieval quality. Collectively, our findings show that the time-voltage ERP measure of memory encoding and the alpha oscillations clearly index independent mechanisms that contribute to how well we later remember a stimulus.