Unfortunately, relatively little is known about the precise mechanisms of TMS activation of neural tissue in vivo. One study utilizing extracellular recordings in the visual cortex of anesthetized cats assessed the effects of single-pulse TMS on neuronal activity (Moliadze et al., 2003) and demonstrated that a single TMS pulse was associated with a strong facilitation of spontaneous and visual-evoked spiking activity during the first 500ms after the TMS pulse. This was followed by a subsequent long-lasting (several second) suppression of activity, the duration of which increased with increasing stimulus strength. In another study utilizing different TMS pulse trains (1 to 4 seconds, 1 to 8 Hz), TMS increased the spontaneous activity for up to sixty seconds; in contrast, visual evoked responses were significantly decreased for approximately five minutes (Allen et al., 2007). A number of recent studies have evaluated the effect of TMS on motor cortex during epidural recordings from human patients with electrodes implanted in the spinal cord for treatment of chronic pain (see Di Lazzaro (2008) for review). These studies have demonstrated that the various TMS protocols all
