State-dependent TMS effects in the visual cortex after visual adaptation: A combined TMS-EEG study.

OBJECTIVE: The impact of transcranial magnetic stimulation (TMS) has been shown to depend on the initial brain state of the stimulated cortical region. This observation has led to the development of paradigms that aim to enhance the specificity of TMS effects by using visual/luminance adaptation to modulate brain state prior to the application of TMS. However, the neural basis of interactions between TMS and adaptation is unknown. Here, we examined these interactions by using electroencephalography (EEG) to measure the impact of TMS over the visual cortex after luminance adaptation. METHODS: Single-pulses of neuronavigated TMS (nTMS) were applied at two different intensities over the left visual cortex after adaptation to either high or low luminance. We then analyzed the effects of adaptation on the global and local cortical excitability. RESULTS: The analysis revealed a significant interaction between the TMS-evoked responses and the adaptation condition. In particular, when nTMS was applied with high intensity, the evoked responses were larger after adaptation to high than low luminance. CONCLUSION: This result provides the first neural evidence on the interaction between TMS with visual adaptation. SIGNIFICANCE: TMS can activate neurons differentially as a function of their adaptation state.

Effects of postural and voluntary muscle contraction on modulation of the soleus H reflex by transcranial magnetic stimulation.

Modulation of spinal reflexes depends largely on the integrity of the corticospinal tract. A useful method to document the influence of descending tracts on reflexes is to examine the effects of transcranial magnetic stimulation (TMS) on the soleus H reflex elicited by posterior tibial nerve electrical stimuli (PTS). In 12 healthy volunteers, we investigated how postural or voluntary muscle contraction modified such descending modulation. We first characterized the effects of TMS at 95 % of motor threshold for leg responses on the H reflex elicited by a preceding PTS at inter-stimuli intervals (ISIs) between 0 and 120 ms at rest and, then, during voluntary plantar flexion (pf), dorsal flexion (df), and standing still (ss). During pf, there was an increase in the facilitation of the H reflex at ISIs 0-20 ms. During df, there were no effects of TMS on the H reflex. During ss, there was inhibition at ISIs 40-60 ms. Our observations suggest that muscle contraction prevails over the baseline effects of TMS on the soleus H reflex. While contraction of the antagonist (df) suppressed most of the effects, contraction of the agonist had different effects depending on the type of activity (pf or ss). The characterization of the interaction between descending corticospinal volleys and segmental peripheral inputs provides useful information on motor control for physiological research and further understanding of the effects of spinal cord lesions.

A segregated neural pathway for prefrontal top-down control of tactile discrimination.

It has proven difficult to separate functional areas in the prefrontal cortex (PFC), an area implicated in attention, memory, and distraction handling. Here, we assessed in healthy human subjects whether PFC subareas have different roles in top-down regulation of sensory functions by determining how the neural links between the PFC and the primary somatosensory cortex (S1) modulate tactile perceptions. Anatomical connections between the S1 representation area of the cutaneous test site and the PFC were determined using probabilistic tractography. Single-pulse navigated transcranial magnetic stimulation of the middle frontal gyrus-S1 link, but not that of the superior frontal gyrus-S1 link, impaired the ability to discriminate between single and twin tactile pulses. The impairment occurred within a restricted time window and skin area. The spatially and temporally organized top-down control of tactile discrimination through a segregated PFC-S1 pathway suggests functional specialization of PFC subareas in fine-tuned regulation of information processing.

Two-point tactile discrimination ability is influenced by temporal features of stimulation.

Two-point discrimination threshold is commonly used for assessing tactile spatial resolution. Since the effect of temporal features of cutaneous test stimulation on spatial discrimination ability is not yet well known, we determined whether the ability to discriminate between two stimulus locations varies with the interstimulus interval (ISI) of sequentially presented tactile stimuli or the length of the stimulus train. Electrotactile stimuli were applied to one or two locations on the skin of the thenar eminence of the hand in healthy human subjects. Tactile discrimination ability was determined using methods based on the signal detection theory allowing the assessment of sensory performance, independent of the subject's response criterion. With stimulus pairs, the ability to discriminate spatial features of stimulation (one location vs. two stimulus locations 4 cm apart) was improved when the ISI was equal to or longer than that required for tactile temporal discrimination. With stimulus trains, the ability to discriminate spatial features of stimulation was significantly improved with an increase in the stimulus train (from 3 to 11 pulses corresponding to train lengths from 40 to 200 ms). These results indicate that temporal features of tactile stimulation significantly influence sensory performance in a tactile spatial discrimination task. Precise control of temporal stimulus parameters should help to reduce variations in results on the two-point discrimination threshold.

Vestibular activation differentially modulates human early visual cortex and V5/MT excitability and response entropy.

Head movement imposes the additional burdens on the visual system of maintaining visual acuity and determining the origin of retinal image motion (i.e., self-motion vs. object-motion). Although maintaining visual acuity during self-motion is effected by minimizing retinal slip via the brainstem vestibular-ocular reflex, higher order visuovestibular mechanisms also contribute. Disambiguating self-motion versus object-motion also invokes higher order mechanisms, and a cortical visuovestibular reciprocal antagonism is propounded. Hence, one prediction is of a vestibular modulation of visual cortical excitability and indirect measures have variously suggested none, focal or global effects of activation or suppression in human visual cortex. Using transcranial magnetic stimulation-induced phosphenes to probe cortical excitability, we observed decreased V5/MT excitability versus increased early visual cortex (EVC) excitability, during vestibular activation. In order to exclude nonspecific effects (e.g., arousal) on cortical excitability, response specificity was assessed using information theory, specifically response entropy. Vestibular activation significantly modulated phosphene response entropy for V5/MT but not EVC, implying a specific vestibular effect on V5/MT responses. This is the first demonstration that vestibular activation modulates human visual cortex excitability. Furthermore, using information theory, not previously used in phosphene response analysis, we could distinguish between a specific vestibular modulation of V5/MT excitability from a nonspecific effect at EVC.

The effects of transcranial magnetic stimulation on vibratory-induced presynaptic inhibition of the soleus H reflex.

A single-pulse transcranial magnetic stimulus (TMS) may induce contraction in many muscles of the body at the same time. This is specially the case when using the double-cone coil to obtain the motor evoked potentials in leg muscles. Even if intensity is kept below threshold for the soleus muscle, TMS induces facilitation of the soleus H reflex that is separated into two phases: the first, peaking at 10-20 ms and the second, peaking at 70-90 ms. We investigated the possibility that TMS-induced facilitation of the H reflex was related, at least in part, to the reafferentation volley reaching the alpha motoneuron after synchronized contraction of other muscles in the body. To test this hypothesis, we examined the effects of vibration on the TMS-induced facilitation of the soleus H reflex. As expected, vibration applied over the triceps tendon caused a significant reduction in H reflex amplitude: 42.4 ± 6.4 % of control values. When conditioned by TMS at intervals corresponding to the first phase, the H reflex was facilitated to the same extent in both conditions: with and without vibration. However, at intervals corresponding to the second facilitation phase, there was a significantly reduced facilitation with vibration. These differential effects of vibration on the two phases of the TMS-induced facilitation of the H reflex indicate a different mechanism for each facilitation phase. The first phase could result from direct corticospinal excitatory input, while the second phase might depend on inputs via Ia afferents from heteronymous muscles.

Probing V5/MT excitability with transcranial magnetic stimulation following visual motion adaptation to random and coherent motion.

The response to stimulating the visual cortex with transcranial magnetic stimulation (TMS) depends on its initial activation state, for example, visual motion adaptation biases perceived TMS-induced phosphene characteristics (e.g., color). We quantified this state dependence by assessing the probability of reporting a phosphene (P(λ) ) with "threshold" TMS (i.e., the TMS intensity producing P(λ) = 0.5 at baseline) following visual motion adaptation to a random dot motion display. Postadaptation, P(λ) was increased, and this effect was confined to the adapted neuronal population. We then adapted subjects using a population of moving dots of fixed average motion direction with standard deviations (SD) ranging from 1° to 128° (SD fixed for a given trial). P(λ) was significantly increased at all dot motion SDs except SD = 1°. Neuronal adaptation increases the susceptibility of the neuronal population to activation by threshold intensity TMS. Thus the process of neuronal adaption is not necessarily synonymous with a downmodulation of neuronal excitability.