Brain rhythms in the alpha frequency range [8Hz-12Hz] are present in idling conditions and sleep. For instance, a prominent occipital alpha wave emerges in mammals when the subjects closes eyes (alpha generation) and it is blocked when opening eyes (alpha blocking). This switch in alpha power is accompanied by an inverse switch in gamma activity [1]. Considering input as random input fluctuations, this switch may result from an alteration in exogenous fluctuations. In a first part, the work proposes a mathematical mesoscopic neural mass model for the cortico-thalamic feedback (CTF) loop derived from a microscopic neuron network model. This model describes this fluctuations-induced switch as a phase transition induced by additive noise [2]. The results (Fig. 1(A)) correspond qualitatively well to previous published experimental data [1].A related emergence of alpha-activity has been also observed in the frontal lobe under general anaesthesia [3]. Here, low doses of GABAergic anaesthetics induce sedation accompanied by enhanced frontal EEG in the alpha- and beta-frequency range. Considering a similar hypothesis as in the previous study, we assume that the anaesthetics affect endogenous brain fluctuations and the local GABAergic synaptic time scale and efficacy [4]. We have implemented a CTF model and find that a level decrease of endogenous fluctuations modulates EEG-spectra from beta activity to alpha activity (see Figure 1(B)).[1] Geller et al. (2014). Clin.Neurophysiol. 125(9): 1764–1773.[2] Hutt and Lefebvre (2021). Brain Topography 35:108-120.[3] Purdon et al. (2012). PNAS 110: E1142-1150.[4] Hutt et al. (2018). Neuroimage 179: 414-428.