Abstract
BackgroundLong-term potentiation (LTP) and long-term depression (LTD) play important roles in mediating activity-dependent changes in synaptic transmission and are believed to be crucial mechanisms underlying learning and cortical plasticity. In human subjects, however, the lack of adequate input stimuli for the induction of LTP and LTD makes it difficult to study directly the impact of such protocols on behavior.ResultsUsing tactile high- and low-frequency stimulation protocols in humans, we explored the potential of such protocols for the induction of perceptual changes. We delivered tactile high-frequency and low-frequency stimuli (t-HFS, t-LFS) to skin sites of approximately 50 mm2 on the tip of the index finger. As assessed by 2-point discrimination, we demonstrate that 20 minutes of t-HFS improved tactile discrimination, while t-LFS impaired performance. T-HFS-effects were stable for at least 24 hours whereas t-LFS-induced changes recovered faster. While t-HFS changes were spatially very specific with no changes on the neighboring fingers, impaired tactile performance after t-LFS was also observed on the right middle-finger. A central finding was that for both t-LFS and t-HFS perceptual changes were dependent on the size of the stimulated skin area. No changes were observed when the stimulated area was very small (< 1 mm2) indicating special requirements for spatial summation.ConclusionOur results demonstrate differential effects of such protocols in a frequency specific manner that might be related to LTP- and LTD-like changes in human subjects.
Highlights
Long-term potentiation (LTP) and long-term depression (LTD) play important roles in mediating activity-dependent changes in synaptic transmission and are believed to be crucial mechanisms underlying learning and cortical plasticity
Tactile high-frequency stimulation (t-HFS) Effect of large-field tactile HFS on discrimination thresholds of the right d2 During the four initial training sessions (s1–s4 = pre) all subjects achieved a stable baseline of discrimination performance on their right d2 as estimated by repeated measures ANOVA with factor SESSION (F(3,39) = 0.759; p = 0.524; n = 14)
Linear correlation analysis (Pearson correlation coefficient) revealed no relation between baseline performance before stimulation was applied and the individual gain in performance found after 20 min of t-HFS (r = -0.090; p = 0.761; n = 14), indicating that baseline performance is not a predictor for perceptual changes evoked by t-HFS
Summary
Long-term potentiation (LTP) and long-term depression (LTD) play important roles in mediating activity-dependent changes in synaptic transmission and are believed to be crucial mechanisms underlying learning and cortical plasticity. We demonstrate that brief periods (20 minutes) of intermittent high-frequency (HFS @ 20 Hz) and continuous low-frequency (LFS @ 1 Hz) protocols of tactile stimulation applied to the index-finger (d2) of the right hand evokes significant and long-lasting changes in tactile discrimination behavior. Bidirectional synaptic modifications such as LTP and LTD are believed to be crucial mechanisms underlying learning-induced cortical plasticity. It is difficult to study the outcome of synaptic modifications on behavioral changes induced by stimuli that drive LTP or LTD-like processes in vivo. High-frequency stimulation of cutaneous afferents of the proximal forearm resulted in a long-lasting increase in perceived pain to electrical test stimuli. Low-frequency stimulation decreased the individual pain perception [5]
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