The beneficial effects of rTMS on tinnitus are independent of mood changes. Moreover, they appear in the context of an original placebo stimulation designed to more closely replicate the somatic sensation of active stimulation. Because of the limited temporal duration of the clinical benefit, these neuromodulatory effects could be mediated by transient functional changes taking place in the neural circuits underlying tinnitus processing.

Event-related repetitive transcranial magnetic stimulation (rTMS) can dynamically interfere with the memory encoding of complex visual scenes. Here, we investigated the critical time elapsing from stimulus presentation to the formation of an effective memory trace by delivering rTMS (900 ms at 20 Hz) during the encoding of visual scenes at different poststimulus delays (from 100 to 500 ms) in 28 healthy volunteers. The stimulation delay showed a robust inverse correlation with the correct retrieval of encoded images. In particular, rTMS stimulation delivered with a delay of 500 ms and lasting for 400 ms after stimulus offset resulted in a huge drop in retrieval accuracy. Such a timing suggests that rTMS affects the formation of long-term memory through interference with postperceptual executive processes, rather than with perceptual analysis of the stimuli. The effect was specific for stimulation of the left dorsolateral prefrontal cortex (DLPFC), whereas rTMS applied to the right DLPFC, vertex (active control site), as well as sham stimulation (placebo) did not affect accuracy. These results confirm the crucial role of the left DLPFC in encoding and provide novel information about the critical timing of its engagement in the formation, consolidation, and maintenance of the memory trace.

BackgroundConverging evidence indicates that action observation and action-related sounds activate cross-modally the human motor system. Since olfaction, the most ancestral sense, may have behavioural consequences on human activities, we causally investigated by transcranial magnetic stimulation (TMS) whether food odour could additionally facilitate the human motor system during the observation of grasping objects with alimentary valence, and the degree of specificity of these effects.Methodology/Principal FindingsIn a repeated-measure block design, carried out on 24 healthy individuals participating to three different experiments, we show that sniffing alimentary odorants immediately increases the motor potentials evoked in hand muscles by TMS of the motor cortex. This effect was odorant-specific and was absent when subjects were presented with odorants including a potentially noxious trigeminal component.The smell-induced corticospinal facilitation of hand muscles during observation of grasping was an additive effect which superimposed to that induced by the mere observation of grasping actions for food or non-food objects. The odour-induced motor facilitation took place only in case of congruence between the sniffed odour and the observed grasped food, and specifically involved the muscle acting as prime mover for hand/fingers shaping in the observed action.Conclusions/SignificanceComplex olfactory cross-modal effects on the human corticospinal system are physiologically demonstrable. They are odorant-specific and, depending on the experimental context, muscle- and action-specific as well. This finding implies potential new diagnostic and rehabilitative applications.

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Somatosensory inputs to the primary sensory cortex (S1) after median nerve stimulation include temporally overlapping parallel processing, as reflected by standard low-frequency somatosensory-evoked potentials (LF-SEPs) and high-frequency SEPs (HF-SEPs), the latter being more sensitive to arousal and to other rapid adaptive changes. Experimental data suggest that cortical HF-SEPs are formed by two successive pre- and postsynaptic components, respectively, generated in the terminal part of thalamo-cortical radiation (early burst) and in specialized neuronal pools within S1 (later burst). In eight healthy subjects, slow (1 Hz) or rapid (10 Hz) repetitive transcranial magnetic stimulations (rTMS), which are known to induce opposite changes on cortical excitability, applied on S1 did not modify LF-SEPs, while HF-SEPs showed a series of dissociate changes in the early and later high-frequency burst, moreover occurring with a different time-course. Slow rTMS caused an immediate and lasting decrease of the later burst activity, coupled with an immediate increase of the earlier part of the burst, suggesting that inhibition of cortical excitability triggered opposite, compensatory effects at subcortical levels; rapid rTMS induced a delayed increase of later HF-SEPs, leaving unaltered the earlier subcortical burst. Findings causally demonstrate that LF- and HF-SEPs reflect two distinct functional pathways for somatosensory input processing, and that early and late high-frequency burst do actually reflect the activity of different generators, as suggested by experimental data. Possible underlying neurophysiological phenomena are discussed.