Low-level direct currents (DC) are thought to polarize brain tissue and to affect brain processes. The present experiments investigated the effects of low-level transcortical DC-currents on response speed and physiological variables in humans. The DC-currents were applied during the warning interval within a constant foreperiod reaction time paradigm. Currents of less than 0.3 mA were applied between a vertex electrode and a noncephalic reference. The polarity of the electrical source varied randomly across trials within subjects. The first two studies showed that subjects respond fastest when the positive pole is applied to the vertex. Under the same conditions higher skin conductance responses were observed as compared to vertex negative conditions. The dynamics of the observed responses suggest that the brain "learns" to respond differentially to the different current polarities. This was confirmed by the third study, which documented different conditioned electroencephalic responses subsequent to current application.
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