Effects of electrical stimulation of the vagus nerve on the development of visual habituation in the cat

The vagus nerve participates in the control and regulation of important autonomous functions, emotional tasks, and neural activity. Electrical vagus nerve stimulation (VNS) is an approved procedure for the treatment of refractory epilepsy in humans. VNS has also been shown to improve mood complaints and cognitive function in both human patients and animals. Thus, the purpose of this study was to analyse and describe the effects of VNS on the development and establishment of sensory habituation and electrographic activity of the visual pathway in freely moving cats. Six cats had implants placed in the optic chiasm (OC), lateral geniculate body (LGB), mesencephalic reticular formation (MRF), primary visual cortex (VC) of the left hemisphere, and left vagus nerve. Immediately after surgery, all cats presented anisocoria and relaxation of the left nictitant membrane. Also showed vegetative-type responses such as myosis, licking, and swallowing during VNS. Animals were then subjected to repeated luminous stimuli at intervals of 1 and 3 s to cause habituation. The effect of VNS on the frequency and latency of the habituation episodes and the electrographic changes in the registered brain structures were analysed. Latency analysis showed that VNS delayed the first habituation episode. VNS had transitory effects on the neural activity of the primary visual pathway structures, which caused a small but measurable delay in the establishment of habituation. In conclusion, VNS interferes with the development and establishment of visual habituation, an elementary form of non-associative learning, in freely moving cats.