Opposite impact on <Superscript>14</Superscript>C-2-deoxyglucose brain metabolism following patterns of high and low frequency repetitive transcranial magnetic stimulation in the posterior parietal cortex

Repetitive transcranial magnetic stimulation (rTMS) appears capable of modulating human cortical excitability beyond the duration of the stimulation train. However, the basis and extent of this “off-line” modulation remains unknown. In a group of anesthetized cats, we applied patterns of real or sham focal rTMS to the visuo-parietal cortex (VP) at high (HF) or low (LF) frequency and recorded brain glucose uptake during (on-line), immediately after (off-line), or 1 h after (late) stimulation. During the on-line period LF and HF rTMS induced a significant relative reduction of ¹⁴C-2DG uptake in the stimulated VP cortex and tightly linked cortical and subcortical structures (e.g. the superficial superior colliculus, the pulvinar, and the LPl nucleus) with respect to homologue areas in the unstimulated hemisphere. During the off-line period HF rTMS induced a significant relative increase in ¹⁴C-2DG uptake in the targeted VP cortex, whereas LF rTMS generated the opposite effect, with only mild network impact. Moderate distributed effects were only recorded after LF rTMS in the posterior thalamic structures. No long lasting cortical or subcortical effects were detected during the late period. Our findings demonstrate opposite modulation of rTMS on local and distant effects along a specific network, depending on the pattern of stimulation. Such effects are demonstrated in the anesthetized animal, ruling out behavioral and non-specific reasons for the differential impact of the stimulation. The findings are consistent with previous differential electrophysiological and behavioral effects of low and high frequency rTMS patterns and provide support to uses of rTMS in neuromodulation. ^†Prof. Payne passed away in May 2004. This article is submitted in his memory.     

Interhemispheric transfer of phosphenes generated by occipital versus parietal transcranial magnetic stimulation

Phosphenes represent a perceptual effect of transcranial magnetic stimulation (TMS) or electric stimulation of visual cortical areas. One likely neural basis for the generation of static phosphenes is the primary visual cortex (V1) although evidence is controversial. A peculiar feature of V1 is that it has sparse callosal connections with the exception of a central portion of visual field representation. In contrast, visually responsive cortical areas in the parietal lobe have widespread callosal connections. Thus, interhemispheric transfer (IT) time of off-centre phosphenes should be slower when generated by V1 than by visual parietal areas. To verify this possibility, in Exp. 1 we measured IT of phosphenes generated by TMS applied to V1 and in Exp. 2 we measured IT of phosphenes obtained by TMS applied to posterior parietal cortex. In both experiments, we obtained static bright circular phosphenes appearing in the contralateral hemifield. We measured IT time behaviorally by comparing unimanual simple reaction time to the onset of a phosphene under crossed or uncrossed hemifield-hand condition (Poffenberger paradigm). In keeping with our prediction, we found a substantially longer IT time for V1 than for parietal phosphenes. Additionally, an IT similar to that obtained with V1 stimulation was found when participants were asked to imagine the phosphenes previously experienced during TMS. In conclusion, the present results suggest that IT of phosphenes either generated by V1 TMS or imagined is subserved by slower callosal channels than those of real visual stimuli or parietal phosphenes.     

The gap effect and express saccades in the auditory modality

Latencies of eye movements to peripheral targets are reduced when there is a short delay (typically 200 ms) between the offset of a central visual fixation point and the target onset. This has been termed the gap effect. In addition, some subjects, usually with practice, exhibit a separate population of very short latency saccades, called express saccades. Both these phenomena have been attributed to disengagement of visual attention when the fixation point is extinguished. A competing theory of the gap effect attributes it to disengagement of oculomotor fixation during the temporal gap. It is known that auditory targets are effective in eliciting saccadic eye movements, and also that covert attention operates in the auditory modality. If the gap effect and express saccades are due to disengagement of spatial attention, both should persist in the auditory modality. However, fixation of gaze is largely under visual control. If the gap effect results from disengagement of fixation, then at least a reduced effect should be seen in the auditory modality. Human subjects performed the gap task and a control task in the dark, using auditory fixation points and saccadic targets, on five successive days. Despite this practice, express saccades were not observed. There was a reliable gap effect, but the reduction in saccadic latency was only 17 ms, compared with 32 ms for the same subjects in the visual modality. This suggests that about half the gap effect is due to disengagement of visual fixation. The remainder was not due to non-specific warning effects and could be attributed to offset of the auditory fixation stimulus. Received: 1 March 1996 / Accepted: 11 July 1997     

Executive control over response priming and conflict: a transcranial magnetic stimulation study

In the present study repetitive transcranial magnetic stimulation (rTMS) was utilised to interrupt neural activity in selected cortical areas at several different time periods while participants performed a stimulus-response correspondence (SRC) task. Responses are usually faster and less error-prone when stimulus (S) and response (R) features correspond than when they do not. Dual-route models of response preparation account for such SRC effects by postulating an indirect route performing S-R selection and a parallel direct route where S features prime their corresponding responses. SRC effects have recently been shown to depend on the preceding trial type, that is, SRC effects are largely reduced when preceded by a non-corresponding trial as compared to a preceding corresponding trial. Present results show that this context dependency of the SRC effect was hindered when rTMS was applied to the left dorsolateral prefrontal cortex (DLPFC) 500-300 ms before the onset of the next trial. Moreover, the SRC effect was reduced overall when applying rTMS volleys to the right posterior parietal cortex (PPC) for 200 ms with the onset of the visual stimulus. We conclude that the left DLPFC is involved in the context-dependent control of response conflicts, whereas the right PPC serves early visuomotor transformations and is, therefore, related to direct route priming.     

Cathodal transcranial direct current stimulation on posterior parietal cortex disrupts visuo-spatial processing in the contralateral visual field

Transcranial direct current stimulation (tDCS) has recently undergone a resurgence in popularity as a powerful tool to non-invasively manipulate brain activity. While tDCS has been used to alter functions tied to primary motor and visual cortices, its impact on extrastriate visual areas involved in visuo-spatial processing has not yet been examined. In the current study, we applied tDCS to the cat visuoparietal (VP) cortex and assayed performance in a paradigm designed to assess the capacity to detect, localize and orient to static targets appearing at different spatial eccentricities within the visual field. Real or sham cathodal tDCS was unilaterally applied to the VP cortex, and orienting performance was assessed during (online), immediately after (offline; Experiments 1 and 2), and 1 or 24 h after the end of the tDCS stimulation (Experiment 2). Performance was compared to baseline data collected immediately prior to stimulation. Real, but not sham, tDCS induced significant decreases in performance for static visual targets presented in the contrastimulated visual hemifield. The behavioral impact of tDCS was most apparent during the online and immediate offline periods. The tDCS effect decayed progressively over time and performance returned to baseline levels ∼60 min after stimulation. These results are consistent with the effects of both invasive and non-invasive deactivation methods applied to the same brain region, and indicate that tDCS has the potential to modify neuronal activity in extrastriate visual regions and to sculpt brain activity and behavior in normal and neurologically impaired subjects. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Transcranial magnetic stimulation to right parietal cortex modifies the attentional blink

The attentional blink (AB) reflects a limitation in the ability to identify multiple items in a stream of rapidly presented information. Repetitive transcranial magnetic stimulation (rTMS), applied to a site over the right posterior parietal cortex, reduced the magnitude of the AB to visual stimuli, whilst no effect of rTMS was found when stimulation took place at a control site. The data confirm that the posterior parietal cortex may play a critical role in temporal as well as spatial aspects of visual attention.     

Low frequency transcranial magnetic stimulation on the posterior parietal cortex induces visuotopically specific neglect-like syndrome

The visuo-parietal (VP) region of the cerebral cortex is critically involved in the generation of orienting responses towards visual stimuli. In this study we use repetitive transcranial magnetic stimulation (rTMS) to unilaterally and non-invasively deactivate the VP cortex during a simple spatial visual detection task tested in real space. Adult cats were intensively trained over 4 months on a task requiring them to detect and orient to a peripheral punctuate static LED presented at a peripheral location between 0° and 90°, to the right or left of a 0° fixation target. In 16 different interleaved sessions, real or sham low frequency (1 Hz) rTMS was unilaterally applied during 20 min (1,200 pulses) to the VP cortex. The percentage of mistakes detecting and orienting to contralateral visual targets increased significantly during the 15–20 min immediately following real but not sham rTMS. Behavioral deficits were most marked in peripheral eccentricities, whereas more central locations were largely unaffected. Performance returned to baseline (pre-TMS) levels when animals were tested 45 min later and remained in pre-TMS levels 24 h after the end of the stimulation. Our results confirm that the VP cortex of the cat is critical for successful detection and orienting to visual stimuli presented in the corresponding contralateral visual field. In addition, we show that rTMS disrupts a robust behavioral task known to depend on VP cortex and does so for the far periphery of the visual field, but not for more central targets.Prof. Payne passed away May 2004. This article is submitted in his memory.     

Impact of repetitive transcranial magnetic stimulation of the parietal cortex on metabolic brain activity: a<Superscript> 14</Superscript>C-2DG tracing study in the cat

Transcranial magnetic stimulation (TMS) is increasingly utilized in clinical neurology and neuroscience. However, detailed knowledge of the impact and specificity of the effects of TMS on brain activity remains unresolved. We have used ¹⁴C-labeled deoxyglucose (¹⁴C-2DG) mapping during repetitive TMS (rTMS) of the posterior and inferior parietal cortex in anesthetized cats to study, with exquisite spatial resolution, the local and distant effects of rTMS on brain activity. High-frequency rTMS decreases metabolic activity at the primary site of stimulation with respect to homologue areas in the unstimulated hemisphere. In addition, rTMS induces specific distant effects on cortical and subcortical regions known to receive substantial efferent projections from the stimulated cortex. The magnitude of this distal impact is correlated with the strength of the anatomical projections. Thus, in the anesthetized animal, the impact of rTMS is upon a distributed network of structures connected to the primary site of application.B.R. Payne has passed away since this paper was completed.     

Effects of repetitive transcranial magnetic stimulation over dorsolateral prefrontal and posterior parietal cortex on memory-guided saccades

We investigated the role of the dorsolateral prefrontal cortex (DLPFC) and the posterior parietal cortex (PPC) in a visuospatial delayed-response task in humans. Repetitive transcranial magnetic stimulation (20 Hz, 0.5 s) was used to interfere temporarily with cortical activity in the DLPFC and PPC during the delay period. Omnidirectional memory-guided saccades with a 3-s delay were used as a quantifiable motor response to a visuospatial cue. The question addressed was whether repetitive transcranial magnetic stimulation (rTMS) over the DLPFC or PPC during the sensory of memory phase affects accuracy of memory-guided saccades. Stimulation over the primary motor cortex served as control. Stimulation over the DLPFC significantly impaired accuracy of memory-guided saccades in amplitude and direction. Stimulation over the PPC impaired accuracy of memory-guided saccades only when applied within the sensory phase (50 ms after cue offset), but not during the memory phase (500 ms after cue offset). These results provide further evidence for a parieto-frontal network controlling performance of visuospatial delayed-response tasks in humans. It can be concluded that within this network the DLPFC is mainly concerned with the mnemonic respresentation and the PPC with the sensory representation of spatially defined perceptual information. Received: 22 April 1996/Accepted: 16 June 1997     

Horizontal and vertical dimensions of visual extinction: a theta burst stimulation study

After a lesion of the posterior parietal cortex (PPC), the perception of a contra-lesional stimulus in presence of a simultaneous, ipsilesional stimulus may be impaired, a phenomenon referred to as visual extinction. In the present study, visual extinction was transiently induced in healthy subjects by interfering with the function of the right PPC by means of continuous theta burst stimulation (TBS). We investigated to which extent the horizontal and vertical position of visual stimuli influenced the extinction rate. A single TBS train over the right PPC induced a significant increase of left visual extinctions of at least 30 min. Left visual extinction rate was higher when the left sided visual stimulus was presented at a more eccentric position on the horizontal axis (irrespective of right sided visual stimulus position) and in the lower part of the visual field. The results are discussed within the framework of current explanatory models and of putative inter- and intrahemispheric mechanisms directing visuospatial attention.     

Laterality effects in selective attention to threat after repetitive transcranial magnetic stimulation at the prefrontal cortex in female subjects

Recently, several experiments have indicated that the left and right prefrontal cortex (PFC) are differently involved in emotional processing. The aim of this study was to investigate the role of the left and right PFC in selective attention to angry faces by using a pictorial emotional Stroop task. Slow repetitive transcranial magnetic stimulation (rTMS) was applied to the left and right PFC of 10 female subjects for 15 min on separate days. Results showed a significant effect of stimulation position: right PFC rTMS resulted in selective attention towards angry faces, whereas left PFC rTMS resulted in selective attention away from angry faces. This finding is in accordance with theoretical accounts of the neural implementation of approach and withdrawal systems.     

Influence of transcranial magnetic stimulation on the execution of memorised sequences of saccades in man

Memorised sequences of saccades are cortically controlled by the supplementary motor area (SMA), as shown in animal experiments and in humans with isolated SMA lesions. We applied transcranial magnetic stimulation (TMS) in eight healthy subjects executing memorised sequences of saccades. Sequences of three targets were presented. Then, upon a go-signal, the subjects had to execute the appropriate sequences. Ten to fifteen sequences were performed in each experiment, and the number of errors were counted. The number of errors increased significantly if TMS was given 80 ms before or 60 ms after the go-signal, with the stimulation coil overlying the SMA. There was no significant increase in errors if different stimulation intervals were chosen (160ms and 120ms before the go-signal; 100 ms, 140 ms or 240 ms after the go-signal), if the coil was positioned inappropriately (e.g. over the occipital cortex), or if the stimulator output was too low. We conclude that TMS can interfere specifically with the function of the SMA during a critical time interval close to the go-signal.     

The effect on corticospinal volleys of reversing the direction of current induced in the motor cortex by transcranial magnetic stimulation

Descending corticospinal volleys were recorded from a bipolar electrode inserted into the cervical epidural space of four conscious human subjects after monophasic transcranial magnetic stimulation over the motor cortex with a figure-of-eight coil. We examined the effect of reversing the direction of the induced current in the brain from the usual posterior-anterior (PA) direction to an anterior-posterior (AP) direction. The volleys were compared with D waves evoked by anodal electrical stimulation (two subjects) or medio-lateral magnetic stimulation (two subjects). As reported previously, PA stimulation preferentially recruited I1 waves, with later I waves appearing at higher stimulus intensities. AP stimulation tended to recruit later I waves (I3 waves) in one of the subjects, but, in the other three, I1 or D waves were seen. Unexpectedly, the descending volleys evoked by AP stimulation often had slightly different peak latencies and/or longer duration than those seen after PA stimulation. In addition the relationship between the size of the descending volleys and the subsequent EMG response was often different for AP and PA stimulation. These findings suggest that AP stimulation does not simply activate a subset of the sites activated by PA stimulation. Some sites or neurones that are relatively inaccessible to PA stimulation may be the low-threshold targets of AP stimulation.     

The effects of repetitive transcranial magnetic stimulation on cortical inhibition in healthy human subjects

It has been suggested that the therapeutic effects of repetitive transcranial magnetic stimulation (rTMS) are mediated through changes in cortical inhibition (CI). However, in healthy human subjects the effects of rTMS on CI have been inconsistent. Therefore, this study sought to improve on the methodological limitations of previous studies by exploring several different rTMS-stimulus conditions on inhibition in the human motor cortex. In the first experiment, 12 healthy control subjects were randomly assigned to receive regular 1, 10 or 20 Hz rTMS in a counterbalanced order with sessions separated by at least 1 week. In the second experiment, 10 of these 12 subjects received priming rTMS (600 stimuli at 6 Hz followed by 600 stimuli at 1 Hz). Cortical inhibition was indexed using short-interval intracortical inhibition (SICI) and cortical silent period (CSP). Corticospinal excitability was indexed using motor threshold and MEP amplitude. We found no significant overall change in SICI, although there was a significant correlation between changes in SICI with baseline SICI. Subjects with greater SICI at baseline tended to have reduction in SICI post-rTMS, whereas subjects with less SICI tended to have increase in SICI post-rTMS. There was also a significant lengthening of the CSP with higher stimulation frequencies compared to lower stimulation frequencies. These findings suggest that rTMS increases CI, particularly in subjects with reduced baseline inhibition, a finding consistent with the concept of homeostatic plasticity. Baseline physiological characteristics may be further explored as a method to select patients who may benefit from rTMS treatment.     

Modulatory effects of high-frequency repetitive transcranial magnetic stimulation on the ipsilateral silent period

In healthy subjects, suprathreshold repetitive transcranial magnetic stimulation (rTMS) at frequencies >2 Hz prolongs the cortical silent period (CSP) over the course of the train. This progressive lengthening probably reflects temporal summation of the inhibitory interneurons in the stimulated primary motor cortex (M1). In this study, we tested whether high-frequency rTMS also modulates the ipsilateral silent period (ISP). In nine normal subjects, suprathreshold 10-pulse rTMS trains were delivered to the right M1 at frequencies of 3, 5, and 10 Hz during maximal isometric contraction of both first dorsal interosseous muscles. At 10 Hz, the second pulse of the train increased the area of the ISP; the other stimuli did not increase it further. During rTMS at 3 and 5 Hz, the ISP remained significantly unchanged. Control experiments showed that 10-Hz rTMS delivered at subthreshold intensity also increased the ISP. rTMS over the hand motor area did not facilitate ISPs in the biceps muscles. Finally, rTMS-induced ISP facilitation did not outlast the 10-Hz rTMS train. These findings suggest that rTMS at a frequency of 10 Hz potentiates the interhemispheric inhibitory mechanisms responsible for the ISP, partly through temporal summation. The distinct changes in the ISP and CSP suggest that rTMS facilitates intrahemispheric and interhemispheric inhibitory phenomena through separate neural mechanisms. The ISP facilitation induced by high-frequency rTMS is a novel, promising tool to investigate pathophysiological abnormal interhemispheric inhibitory transfer in various neurological diseases.     

A functional magnetic resonance imaging navigated repetitive transcranial magnetic stimulation study of the posterior parietal cortex in normal pain and hyperalgesia

Noxious stimuli activate a complex cerebral network. During central sensitization to pain, activity in most of these areas is changed. One of these areas is the posterior parietal cortex (PPC). The role of the PPC during processing of acute pain as well as hyperalgesia and tactile allodynia remains elusive. Therefore, we performed a functional magnetic resonance imaging (fMRI) based, neuro-navigated, repetitive transcranial magnetic stimulation (rTMS) study in 10 healthy volunteers. Firstly, pin-prick hyperalgesia was provoked on the right volar forearm, using the model of electrically-induced secondary mechanical hyperalgesia. fMRI was performed during pin-prick stimulation inside and outside the hyperalgesic areas. Secondly, on four different experimental sessions, the left and right individual intraparietal BOLD peak-activations were used as targets for a sham-controlled 1 Hz rTMS paradigm of 10 min duration. We measured psychophysically the (i) electrical pain stimulus intensity on an 11-point numeric pain rating scale (NRS, 0–10), the (ii) area of hyperalgesia, and the (iii) area of dynamic mechanical allodynia. Sham stimulation or rTMS was performed 16 min after induction of pin-prick hyperalgesia and tactile allodynia. Compared to sham stimulation, no significant effect of rTMS was observed on pain stimulus intensity and the area of allodynia. However, a reduction of the hyperalgesic area was observed for rTMS of the left PPC (P<0.05). We discuss the role of the PPC in central sensitization to pain, in spatial discrimination of pain stimuli and in spatial-attention to pain stimuli.     

The neural response to transcranial magnetic stimulation of the human motor cortex. I. Intracortical and cortico-cortical contributions

We investigated the properties of the neural response to transcranial magnetic stimulation (TMS) applied over the human primary motor cortex. Consistent with our previous findings, single pulses of TMS induce a characteristic negative deflection at 45 ms (N45) and a transient oscillation in the beta frequency-range (15–30 Hz), as measured using electroencephalograpy (EEG). Here we show the relative specificity of the beta oscillation and the N45; both are stronger when elicited by stimulation applied over the primary motor cortex, as compared with stimulation over the dorsal premotor cortex. We also provide a quantitative analysis of the beta responses to single pulses of TMS and show that the responses are highly phaselocked to the TMS pulses within single subjects; this phaselocking is similar from subject to subject. A single pulse of TMS applied over the primary motor cortex thus appears to reset the ongoing oscillations of the neurons, bringing them transiently into synchrony. Finally, we examine the effect of local or distal modulation of the excitability of the primary motor cortex on the beta oscillation and the N45 in response to single-pulse TMS. We applied low-frequency subthreshold repetitive TMS either over the primary motor cortex (local modulation) or, on a separate day, over the dorsal premotor cortex (distal modulation). The modulation was evaluated with single suprathreshold test pulses of TMS applied over the primary motor cortex before and after the subthreshold low-frequency rTMS. We recorded the EEG response throughout the testing session, i.e. to both the subthreshold and the suprathreshold pulses. After repetitive TMS applied over the primary motor cortex, but not the dorsal premotor cortex, the amplitude of the N45 in response to suprathreshold pulses tended to decrease (not significant), and subsequently increased (significant); neither type of repetitive TMS affected the amplitude of the beta oscillation. We conclude that (1) the N45 depends on circuits intrinsic to the primary motor cortex; (2) the beta oscillation is specific to stimulation of the primary motor cortex, but is not affected by modulation of either cortical area and; (3) the beta oscillatory response to pulses of TMS arises from resetting of ongoing oscillations rather than their induction.The first author was funded by a fellowship from the Canadian Institute of Health Research (CIHR).     

The effect of transcranial magnetic stimulation and peripheral nerve stimulation on corticomuscular coherence in humans

Cortex and muscle show coupled oscillations in the 15–35 Hz frequency band during voluntary movements. To obtain evidence of the neuronal network responsible for this rhythmicity we investigated the effect of transcranial magnetic stimulation (TMS) and peripheral nerve stimulation on the coupling between eletcroencephalographic (EEG) activity recorded from the scalp over the motor cortex and electromyographic (EMG) activity recorded from the tibialis anterior (TA) muscle in 15 healthy human subjects. TMS over the leg area at intensities between 0.95 and 1.1 × threshold for a motor evoked potential (MEP) in the TA increased corticomuscular coherence in the 15–35 Hz frequency band. This effect lasted on average for 300 ms, but could last up to 600–800 ms in some subjects. Stimulation of motor nerves from the ankle muscles suppressed corticomuscular coherence in the 15–35 Hz frequency range between leg area EEG and TA EMG for a period up to 600–800 ms. In addition, increased coherence around 10 Hz was observed for a period up to 250 ms after the stimulation. Stimulation of motor nerves in the arm and motor nerves from the ankle muscles in the other leg had no effect. The findings indicate that TMS has direct access to the neuronal circuitry in the motor cortex, which generates the corticomuscular coherence. This effect was caused either by direct activation of corticospinal cells or by activation of local neuronal circuitries in the motor cortex. The effects of peripheral nerve stimulation suggest that an alternative rhythm generator may entrain the cortical cells into a lower 10 Hz rhythm and disrupt the 15–35 Hz rhythm.     

The effects of 10 Hz repetitive transcranial magnetic stimulation on resting EEG power spectrum in healthy subjects

10 Hz rTMS over the left prefrontal cortex may be useful in the treatment of depressive disorders. However, the effects of 10 Hz rTMS applied in potentially effective doses on electroencephalographic activity are not well studied. Using EEG, we aimed to investigate the neurobiological effects of the 10 Hz rTMS set of parameters currently used for depression treatment in a sample of healthy subjects. In 18 healthy subjects, either 10 Hz real rTMS or sham stimulation were given in a crossover design. Real rTMS stimulation was carried out with an intensity of 110% of motor threshold (MT) over the left dorsolateral prefrontal cortex. For the sham condition, the coil was angled over a parietotemporal position and the intensity was reduced to 90% of MT. EEG recordings were taken before and after a single rTMS session. EEG power spectrum was extracted using the complex demodulation method and changes in power were evaluated statistically. Real 10 Hz rTMS induced an overall increase in delta power. This increase prevailed throughout the sample, whereas effects on the power of the alpha, beta and theta EEG bands were highly variable. Sham stimulation had no substantial effects on the EEG power spectrum. Furthermore, no changes in EEG asymmetry were detected. Real 10 Hz rTMS applied at 2000 stimuli and 110% intensity may induce significant changes in resting EEG in healthy subjects.     

The effect of transcranial magnetic stimulation on reciprocal inhibition in the human leg.

The effect of magnetic stimulation on reciprocal Ia inhibition of the human leg was investigated. Stimulation of the common peroneal nerve at the fibula head at the threshold of the alpha motoneuron axons resulted in inhibition of the soleus (SOL) H reflex at a conditioning-test interval of 2 ms. Magnetic stimulation over the contralateral motor cortex resulted in complex modulations of the SOL H reflex, including a short latency facilitation followed by inhibition. This inhibition may have been conveyed by Ia inhibitory interneurons projecting to SOL motoneurons. To test for convergence, whether or not the magnetic stimulation was capable of facilitating disynaptic reciprocal Ia inhibition of the SOL H reflex induced by stimulation of the peroneal nerve, the two stimuli were given together or separately. We observed the inhibition significantly increased when the two stimuli were given together than separately. These results suggest that the Ia inhibitory interneurons projecting to SOL motoneurons in humans might receive convergent input from the motor area of the brain and from Ia afferents of the tibialis anterior (TA) muscle in humans as well as in other animals.