Prospective memory and working memory: Asymmetrical effectsduring frontal lobe TMS stimulation

The role of working memory (WM) for the realization of an intended action (prospective
memory, PM) is a debated issue in recent neuropsychological literature. The present study aimed to
assess whether WM and PM share resources or are, alternatively, two distinct mechanisms. A verbal
task was used, which manipulated the cognitive demand of both WM and PM dimensions on an eventbased
prospective task. Transcranial Magnetic Stimulation (TMS) was also employed to clarify the
matter by disentangling the causal contribution of the frontal areas (related to WM) to the PM process.
The prospective task required the subject to respond whenever a word appeared which had been
presented before the beginning of the task. Two ongoing tasks were administered: an Updating WM
task (in two conditions of medium and high WM demands) and a Lexical Decision task (representing a
low WM demand). In the first two experiments, ongoing accuracy was affected by higher PM and WM
loads, and ongoing speed (RTs) by higher WM load; conversely, both RTs and accuracy in the
prospective performance were only affected by PM load. In the third experiment, single pulse TMS was
applied to either left or right dorsolateral prefrontal cortices. Results showed higher error rates when
TMS was applied to the experimental sites both in the ongoing and prospective tasks, without
differences due to lateralization. These findings demonstrated, from both behavioural and
neurofunctional perspectives, that WM and PM processes are not based on the same memory system,
but share resources at high demands.