Investigation of the primary visual cortex using short-interval paired-pulse transcranial magnetic stimulation (TMS)

Previous studies using short-interval paired-pulse TMS have provided valuable insights into physiology of human motor cortex. Depending on the interstimulus interval (ISI) between the two pulses intra-cortical facilitation (ICF) or intra-cortical inhibition (ICI) can be observed. Similar patterns of inhibition and facilitation have also been demonstrated in prefrontal and parietal cortices. In order to prove whether principles that govern cortical excitability in the motor system also extend to the visual system and to further characterize possible neural correlates of phosphene generation, we applied short-interval paired-pulse TMS to the occipital cortex. In addition, we examined the effect of different coil orientations on perception of phosphenes induced by paired-pulse TMS. In all of 10 healthy subjects, a general facilitation of phosphene perception could be observed for interstimulus intervals of 2-12 ms (conditioning stimulus (CS) 90% and test stimulus (TS) 100% of subject's phosphene threshold) compared to TS alone. With CS intensity decreasing to 80% or less, the effect diminished. No significant changes occurred when TS intensity was increased to 110%. Phosphene perception was enhanced with an induced current direction from lateral to medial at an ISI of 12 ms. Inhibition was not observed in any condition. Our results indicate that the mechanisms underlying phosphene induction in the visual cortex are different from those underlying intracortical inhibition and facilitation in the motor cortex.