Effects of W-CDMA 1950 MHz EMF emitted by mobile phones on regional cerebral blood flow in humans

Use of the third generation mobile phone system is increasing worldwide. This is the first study to investigate the effects of the third generation system on regional cerebral blood flow (rCBF) in humans. We compared effects of the electromagnetic field (EMF) emitted from the Wideband Code Division Multiple Access (W-CDMA) cellular system versus sham control exposure on rCBF in humans. Nine healthy male volunteers participated in this study. Positron emission tomography (PET) scans were obtained before, during, and after unilateral 30 min EMF exposure. The subtraction analysis revealed no significant rCBF changes caused by the EMF conditions compared with the sham exposure, suggesting that EMF emitted by a third generation mobile phone does not affect rCBF in humans.

Primary motor cortical metaplasticity induced by priming over the supplementary motor area

Motor cortical plasticity induced by repetitive transcranial magnetic stimulation (rTMS) sometimes depends on the prior history of neuronal activity. These effects of preceding stimulation on subsequent rTMS-induced plasticity have been suggested to share a similar mechanism to that of metaplasticity, a homeostatic regulation of synaptic plasticity. To explore metaplasticity in humans, many investigations have used designs in which both priming and conditioning are applied over the primary motor cortex (M1), but the effects of priming stimulation over other motor-related cortical areas have not been well documented. Since the supplementary motor area (SMA) has anatomical and functional cortico-cortical connections with M1, here we studied the homeostatic effects of priming stimulation over the SMA on subsequent rTMS-induced plasticity of M1. For priming and subsequent conditioning, we employed a new rTMS protocol, quadripulse stimulation (QPS), which produces a broad range of motor cortical plasticity depending on the interval of the pulses within a burst. The plastic changes induced by QPS at various intervals were altered by priming stimulation over the SMA, which did not change motor-evoked potential sizes on its own but specifically modulated the excitatory I-wave circuits. The data support the view that the homeostatic changes are mediated via mechanisms of metaplasticity and highlight an important interplay between M1 and SMA regarding homeostatic plasticity in humans.

Forty-hertz triple-pulse stimulation induces motor cortical facilitation in humans

A single pulse of transcranial magnetic stimulation (TMS) can reset the 15- to 30-Hz beta-band oscillations in the motor cortex. These oscillations are known to influence the amplitude of corticospinal activity evoked by TMS. To garner further evidence for this resetting, we tested how electromyographic responses to motor cortex TMS were modulated by a preceding series of TMS pulses. We used a triad of conditioning TMS pulses at various interstimulus intervals (ISIs) in an attempt to drive cortical activity at the corresponding frequency. We then analyzed how the amplitude of motor-evoked potentials (MEPs) to a test pulse varied at different intervals after the conditioning triad. When conditioning pulses were given at an ISI of 25 ms, responses to the fourth (test) pulse were facilitated 25 ms later. Neither a single conditioning pulse nor triad of conditioning pulses separated by other ISIs enhanced responses to the test pulse at the expected timings. Triads of pulses at an ISI of 25 ms did not enhance subsequent MEPs to brainstem stimulation. Based on the intensity of the conditioning stimuli necessary to produce this effect and on the effective interval, we conclude that the facilitation at 25 ms differs from intracortical facilitation at 7-10 ms seen in the paired-pulse experiment originally reported by Kujirai et al. These results suggest that a triad of TMS pulses can enhance an intrinsic oscillatory rhythm of the motor cortex (40 Hz) and facilitate cortical activity at an ISI corresponding to the frequency of that rhythm.

Double-Pulse Magnetic Brain Stem Stimulation: Mimicking Successive Descending Volleys

Magnetic stimulation with a double-cone-coil over the back of the head activates the motor tracts at the level of pyramidal decussation (brain stem stimulation [BST]). However, single-pulse BST (single BST) sometimes cannot elicit motor-evoked potentials (MEPs) in patients with corticospinal tract involvement. We developed a technique using double-pulse BST (double BST) to elicit MEPs even in patients whose threshold is abnormally elevated. Subjects were 11 healthy volunteers and 12 patients with corticospinal tract involvement in whom single BST evoked no discernible MEP. Double BST was performed at the intensities of resting and active motor threshold for single BST; MEPs were recorded from the first dorsal interosseous muscle. Interstimulus intervals (ISIs) between two pulses were 1.5, 2, 3, 5, and 10 ms in healthy subjects. Double BST enlarged MEPs at ISIs of 1.5–5 ms with a peak at 2 ms in the relaxed condition, but not in the active condition. At an ISI of 2 ms in the relaxed condition, the MEP amplitude was 15 times as large as that to single BST in relaxed muscles. The onset latency of the enlarged MEP from the second pulse in relaxed muscles was the same as that by single BST in active muscles. Double BST at a 2-ms interval elicited MEPs in eight patients. Double BST can enhance MEPs probably by temporal summations of excitatory postsynaptic potentials at the spinal motoneurons. Using this new technique, we can obtain more information about the central motor conduction even when single BST fails to elicit any MEP.

Gaze behavior when reaching to remembered targets

People naturally direct their gaze to visible hand movement goals. Doing so improves reach accuracy through use of signals related to gaze position and visual feedback of the hand. Here, we studied where people naturally look when acting on remembered target locations. Four targets were presented on a screen, in peripheral vision, while participants fixed a central cross (encoding phase). Four seconds later, participants used a pen to mark the remembered locations while free to look wherever they wished (recall phase). Visual references, including the screen and the cross, were present throughout. During recall, participants neither looked at the marked locations nor prevented eye movements. Instead, gaze behavior was erratic and was comprised of gaze shifts loosely coupled in time and space with hand movements. To examine whether eye and hand movements during encoding affected gaze behavior during recall, in additional encoding conditions, participants marked the visible targets with either free gaze or with central cross fixation or just looked at the targets. All encoding conditions yielded similar erratic gaze behavior during recall. Furthermore, encoding mode did not influence recall performance, suggesting that participants, during recall, did not exploit sensorimotor memories related to hand and gaze movements during encoding. Finally, we recorded a similar lose coupling between hand and eye movements during an object manipulation task performed in darkness after participants had viewed the task environment. We conclude that acting on remembered versus visible targets can engage fundamentally different control strategies, with gaze largely decoupled from movement goals during memory-guided actions.

Bidirectional long-term motor cortical plasticity and metaplasticity induced by quadripulse transcranial magnetic stimulation

Repetitive transcranial magnetic stimulation (rTMS) has emerged as a promising tool to induce plastic changes that are thought in some cases to reflect N-methyl-d-aspartate-sensitive changes in synaptic efficacy. As in animal experiments, there is some evidence that the sign of rTMS-induced plasticity depends on the prior history of cortical activity, conforming to the Bienenstock-Cooper-Munro (BCM) theory. However, experiments exploring these plastic changes have only examined priming-induced effects on a limited number of rTMS protocols, often using designs in which the priming alone had a larger effect than the principle conditioning protocol. The aim of this study was to introduce a new rTMS protocol that gives a broad range of after-effects from suppression to facilitation and then test how each of these is affected by a priming protocol that on its own has no effect on motor cortical excitability, as indexed by motor-evoked potential (MEP). Repeated trains of four monophasic TMS pulses (quadripulse stimulation: QPS) separated by interstimulus intervals of 1.5-1250 ms produced a range of after-effects that were compatible with changes in synaptic plasticity. Thus, QPS at short intervals facilitated MEPs for more than 75 min, whereas QPS at long intervals suppressed MEPs for more than 75 min. Paired-pulse TMS experiments exploring intracortical inhibition and facilitation after QPS revealed effects on excitatory but not inhibitory circuits of the primary motor cortex. Finally, the effect of priming protocols on QPS-induced plasticity was consistent with a BCM-like model of priming that shifts the crossover point at which synaptic plasticity reverses from depression to potentiation. The broad range of after-effects produced by the new rTMS protocol opens up new possibilities for detailed examination of theories of metaplasticity in humans.