Which memory does brain stimulation improve? The priming problem

Background: Transcranial magnetic stimulation (TMS) of cortical sites connected with the hippocampus improves scores on episodic memory tests and is being tested for memory decline, including in Alzheimer's disease. These gains are read as gains in intentional retrieval. Objective: To examine whether standard memory outcomes can support this reading and how the contribution of repetition priming can be measured. Methods: Narrative review of hippocampal-network stimulation studies, cognitive and electrophysiological research on priming and processing fluency, stimulation studies of priming and parietal cortex, and clinical trials of TMS for memory decline. Results: Old/new recognition and cued recall present studied items or cues again at test. Repetition priming makes these items easier to process, and this fluency can raise a memory score with intentional retrieval unchanged. The fluency behind word-recognition decisions is largely conceptual, so it persists after a change of surface form. TMS can also alter priming directly, and stimulating the standard parietal target changes the subjective side of recollection. Meta-analytic evidence shows that the gains are specific to episodic memory and largest for recollection tests after stimulation before encoding, but the priming contribution to these gains has not been estimated. Conclusions: Process-level claims about stimulation-related memory gains require the priming contribution to be measured. Five measurement steps, centred on matched indirect tests and process-dissociation procedures, make it measurable in the same participants, including in clinical trials.