Transcranial magnetic stimulation (TMS) uses rapidly changing magnetic fields to induce electric currents in targeted cortical regions, temporarily disrupting or modulating neural activity. Unlike correlational neuroimaging methods (fMRI, EEG), TMS can establish causal relationships between brain regions and cognitive functions: if disrupting region X impairs function Y, then X is necessary for Y. This makes TMS a uniquely powerful tool in cognitive neuroscience.
Applications
Single-pulse TMS can create "virtual lesions" — brief disruptions of cortical processing that reveal the timing and necessity of specific regions for cognitive tasks. Repetitive TMS (rTMS) can produce longer-lasting effects: low-frequency rTMS inhibits cortical excitability, while high-frequency rTMS enhances it. TMS has been used to demonstrate the necessity of specific brain regions for language production (Broca's area), visual perception (V1, V5), attention (parietal cortex), and working memory (DLPFC). Clinically, rTMS is approved for treatment of depression and is being investigated for other psychiatric and neurological conditions.
TMS has important limitations: it can only reach cortical surface regions (not deep structures), its spatial precision is limited (~1 cm), the exact neural mechanism is uncertain (it may affect passing fibers as well as the targeted region), and participant discomfort varies. Despite these limitations, TMS provides causal evidence that complements the correlational evidence from fMRI and EEG.