Electrocorticography Analysis in Patients With Dual Neurostimulators Supports Desynchronization as a Mechanism of Action for Acute Vagal Nerve Stimulator Stimulation.

PURPOSE: Both vagal nerve stimulation (VNS) and responsive neurostimulation (RNS System) are treatment options for medically refractory focal epilepsy. The mechanism of action of both devices remains poorly understood. Limited prior evidence suggests that acute VNS stimulation may reduce epileptiform activity and cause EEG desynchronization on electrocorticography (ECoG). Our study aims to isolate effects of VNS on ECoG as recorded by RNS System in patients who have both devices, by comparing ECoG samples with and without acute VNS stimulation. METHODS: Ten 60-second ECoGs each from 22 individuals at 3 epilepsy centers were obtained-5 ECoGs with VNS "off" and 5 ECoGs with VNS "on." Electrocorticograps containing seizures or loss of telemetry connection artifact were excluded from analysis (total of 169 ECoGs were included). Electrocorticographs were analyzed for differences in spectral content by generating average spectrograms for "on" and "off" states and using a linear mixed-effects model to isolate effects of VNS stimulation. RESULTS: Acute VNS stimulation reduced average power in the theta band by 4.9%, beta band by 3.8%, and alpha band by 2.5%. The reduction in theta power reached statistical significance with a P value of <0.05. CONCLUSIONS: Our results provide evidence that acute VNS stimulation results in desynchronization of specific frequency bands (salient decrease in theta and beta bands, smaller decrease in alpha band) in ECoGs recorded by the RNS device in patients with dual (VNS and RNS) neurostimulators. This finding offers support for desynchronization as a theorized mechanism of action of VNS. Further research may lead to future improved neurostimulator efficacy by informing optimal stimulation programming parameters.

Vagus nerve stimulation induces widespread cortical and behavioral activation.

Vagus nerve stimulation (VNS) is used for management of a variety of neurological conditions, although the therapeutic mechanisms are not fully understood. Accumulating evidence suggests that VNS may modulate cortical state and plasticity through activation of broadly projecting neuromodulatory systems. Using a mouse model, we compared arousal-linked behaviors with dorsal cortical activity obtained with widefield and two-photon GCaMP6s calcium imaging and electrophysiological recordings. We observed robust and reliable cortical and behavioral dose-dependent activation in waking mice to VNS, including pupil dilation and, frequently, whisker movements and locomotion. Widefield calcium imaging and multiunit recording during VNS revealed that this observed increase in arousal state is coupled with a rapid and widespread increase in excitatory activity, including, but not limited to, activation of somatosensory, visual, motor, retrosplenial, and auditory cortical regions. Two-photon GCaMP6s calcium imaging of cholinergic and noradrenergic cortical axons revealed that VNS strongly activates these neuromodulatory systems. Importantly, VNS-evoked activation of neuromodulatory axons and excitatory neurons in the cortex persisted in mice under light anesthesia, in the absence of overt movement. Arousal state changes were abolished by vagus nerve transection, confirming that observed VNS effects were specific to nerve stimulation and triggered widespread activity above that which can be explained by motor activity. Taken together, our results support a model of VNS in which activation of subcortical structures leads to widespread activation of cortex and an increase in arousal state, at least partially due to the activation of cholinergic and noradrenergic modulatory pathways.

Distinct Waking States for Strong Evoked Responses in Primary Visual Cortex and Optimal Visual Detection Performance.

Variability in cortical neuronal responses to sensory stimuli and in perceptual decision making performance is substantial. Moment-to-moment fluctuations in waking state or arousal can account for much of this variability. Yet, this variability is rarely characterized across the full spectrum of waking states, leaving the characteristics of the optimal state for sensory processing unresolved. Using pupillometry in concert with extracellular multiunit and intracellular whole-cell recordings, we found that the magnitude and reliability of visually evoked responses in primary visual cortex (V1) of awake, passively behaving male mice increase as a function of arousal and are largest during sustained locomotion periods. During these high-arousal, sustained locomotion periods, cortical neuronal membrane potential was at its most depolarized and least variable. Contrastingly, behavioral performance of mice on two distinct visual detection tasks was generally best at a range of intermediate arousal levels, but worst during high arousal with locomotion. These results suggest that large, reliable responses to visual stimuli in V1 occur at a distinct arousal level from that associated with optimal visual detection performance. Our results clarify the relation between neuronal responsiveness and the continuum of waking states, and suggest new complexities in the relation between primary sensory cortical activity and behavior.SIGNIFICANCE STATEMENT Cortical sensory processing strongly depends on arousal. In the mouse visual system, locomotion (associated with high arousal) has previously been shown to enhance the sensory responses of neurons in primary visual cortex (V1). Yet, arousal fluctuates on a moment-to-moment basis, even during quiescent periods. The characteristics of V1 sensory processing across the continuum of arousal are unclear. Furthermore, the arousal level corresponding to optimal visual detection performance is unknown. We show that the magnitude and reliability of sensory-evoked V1 responses are monotonic increasing functions of arousal, and largest during locomotion. Visual detection behavior, however, is suboptimal during high arousal with locomotion, and usually best during intermediate arousal. Our study provides a more complete picture of the dependence of V1 sensory processing on arousal.