| Working memory refers to the ability to hold on to information after it has been removed from the environment and to use that information in order to perform a task. Prior studies using various neuroimaging techniques have produced conflicting results in regard to the neuroanatomical organization of the systems involved in working memory. The set of studies described in this dissertation use repetitive transcranial magnetic stimulation (rTMS) to alter brain activity during tasks of visual working memory, and electroencephalography (EEG) to observe the changes in brain activity caused by rTMS in order to provide a more clear understanding of the mechanisms involved in storage.;The results of delay-period rTMS establish that storage of spatial information preferentially relies on dorsal visual stream areas in the posterior cortex, whereas, the result of response-period rTMS suggests that the prefrontal cortex plays a dominant role in memory-guided response.;Investigation of the effect of delay-period rTMS on brain activity with simultaneous EEG revealed a complex interaction between delivery of rTMS, oscillatory activity and task performance. When applied to a task-relevant brain area, the effect of rTMS on alpha-band power (8.5-14 Hz) predicts its effect on working memory performance. Specifically, the observed changes suggest that rTMS affects working memory by altering alpha-band induced inhibition of cognitive processes. rTMS was also found to affect cross-frequency [specifically, alpha:gamma (> 40 Hz)] phase synchrony in a task-relevant manner. The direction of this effect suggests that cross-frequency synchrony supports storage of information in working memory.;These studies emphasize that, contrary to popular belief, rTMS does not induce a "virtual lesion" in the targeted brain area. Rather, it has complex, task-specific effects on oscillatory amplitude and phase dynamics across a wide range of frequencies and over many cortical areas. Additionally, the results described in this dissertation provide an example of the potential of combining rTMS with neuroimaging techniques to better understand the processes underlying cognitive functions. |
