Perceptual learning may increase the strength of connections between cortical neurons. Transcranial magnetic stimulation (TMS), which transiently perturbs visual perception, was used to test whether learning a line orientation task would strengthen visual circuits. Nine subjects detected the presence of an odd element, a line which differed from distractor lines in orientation. In Experiment 1, performance on the task improved over time, demonstrating that the task was learnable. In Experiments 2 and 3, during randomly selected trials, TMS was delivered at various delays and intensities to occipital cortex. In Experiment 2, performance in TMS trials did not significantly change as subjects learned the task, suggesting that learning does not strengthen visual circuits so that they are more resistant to TMS. In Experiment 3, when feedback was given in TMS trials, performance did not change after several weeks of training, indicating that subjects cannot learn to overcome TMS.
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