Lateralized contribution of prefrontal cortex in controlling task-irrelevant information during verbal and spatial working memory tasks: rTMS evidence

The functional organization of working memory (WM) in the human prefrontal cortex remains unclear. The present study used repetitive transcranial magnetic stimulation (rTMS) to clarify the role of the dorsolateral prefrontal cortex (dlPFC) both in the types of information (verbal vs. spatial), and the types of processes (maintenance vs. manipulation). Subjects performed three independent experiments (1-back and 2-back tasks) while rTMS was applied over dlPFC for 500 ms in the last period of the delay. In two experiments (1 and 2) physically identical stimuli (letters shown at different locations on a screen) under different domain conditions (letters or locations) were employed. Under these conditions, we discovered a double dissociation only in the 2-back task: during the letter condition, when applied to the right dlPFC, rTMS significantly delayed task performance, whereas, the same result was present during the location condition, but only when rTMS was applied to the left dlPFC. The other 2-back task (experiment 3), in which we had eliminated the task-irrelevant information (i.e. we used stimuli that varied only in one domain), did not show significant results. We propose that the functional dichotomy of the hemispheres may be due to mechanisms of cognitive control on interference, which resolve conflict through the inhibition of task-irrelevant information only during high WM load. In conclusion, these findings confirm the role of dlPFC in implementing top-down attentional control, and provide evidence for the theoretical suggestion that working memory serves to control selective attention in the normal human brain.