Evaluation of cortical excitability by motor and phosphene thresholds in transcranial magnetic stimulation

Introduction: Motor threshold (MT) as determined by transcranial magnetic stimulation JMS) is used as an index for cortex excitability. In TMS stimulus intensity is usually referred to the individual MT. It is unclear whether MT also reflects the excitability of non-motor cortical areas such as the visual cortex or correlates with the phosphene threshold (PT) as demonstrated by TMS over the occipital skull. Methods: In 22 healthy volunteers TMS with single pulses was applied over the motor and visual cortex with a figure-of-eight shaped coil connected to a Dantec MagPro stimulator. Biphasic pulses were used to avoid influence Of Current direction on excitatory effects. MT and PT were individually measured (% of maximum stimulator output). Results: There was no correlation between MT and PT. Compared to mean MT (40.5 +/- 6.6%) PT was significantly higher (58.5 +/- 12.1%) (p = 0.01). Discussion: MT does not reflect the excitability of the visual cortex. In TMS of the visual cortex PT should be used to define the stimulus intensity to compare stimulus effects interindividually.