RESUMO
Short-term synaptic plasticity is found in many areas of the central nervous system. In the inhibitory half-center central pattern generators involved in locomotion, synaptic depression is believed to act as a burst termination mechanism, allowing networks to generate anti-phase bursting patterns of varying periods. To better understand burst generation in these central pattern generators, we study a minimal network of two neurons coupled through depressing synapses. Depending on the strength of the synaptic conductance between the two neurons, this network can produce symmetric n : n anti-phase bursts, where neurons fire n spikes in alternation, with the period of such solutions increasing with the strength of the synaptic conductance. Relying on the timescale disparity in the model, we reduce the eight-dimensional network equations to a fully-explicit scalar Poincaré burst map. This map tracks the state of synaptic depression from one burst to the next and captures the complex bursting dynamics of the network. Fixed points of this map are associated with stable burst solutions of the full network model, and are created through fold bifurcations of maps. We derive conditions that predict the bifurcations between n : n and (n + 1) : (n + 1) solutions, producing a full bifurcation diagram of the burst cycle period. Predictions of the Poincaré map fit excellently with numerical simulations of the full network model and allow the study of parameter sensitivity for rhythm generation.
RESUMO
BACKGROUND: Electrical brain stimulation can elicit neurosensory side effects that are unrelated to the intended stimulation effects. This presents a challenge when designing studies with blinded control conditions. OBJECTIVE: The aim of this research was to investigate the role of different transcranial alternating current stimulation (tACS) parameters, i.e. intensity, frequency, and electrode montage, on the probability, duration and intensity of elicited neurosensory side effects. METHODS: In a first study, we examined the influence of tACS on sensations of phosphenes, dizziness, pressure, and skin sensation in fifteen healthy subjects, during 8 s of stimulation with different amplitudes (1500 µA, 1000 µA, 500 µA, 250 µA), frequencies (2 Hz, 4 Hz, 8 Hz, 16 Hz, 32 Hz, 64 Hz), and montages (F3/F4, F3/C4, F3/P4, P3/F4, P3/C4, P3/P4). In a second study, ten healthy subjects were exposed to 60 s of tACS (1000 µA, 2 Hz versus 16 Hz, F3/F4 versus P3/P4) and were asked to rate the intensity of sensations every 12 s. RESULTS: The first study showed that all stimulation parameters had an influence on the probability and intensity of sensations. Phosphenes were most likely and strongest for frontal montages and higher frequencies. Dizziness was most likely and strongest for parietal montages and at stimulation frequency of 4 Hz. Skin sensations and pressure was more likely when stimulation was performed across central regions and at posterior montages, respectively. The second study also revealed that the probability and the intensity of sensations were neither modified during more extended periods of stimulation nor affected by carry-over effects. CONCLUSION: We demonstrated that the strength and the likelihood of sensations elicited by tACS were specifically modulated by the stimulation parameters. The present work may therefore be instrumental in establishing effective blinding conditions for studies with tACS.
