In this project, we aim at demonstrating a causal relationship between cortical oscillations and cognitive processes. In order to achieve this goal, we plan to modulate brain oscillations via transcranial alternating current stimulation (tACS). In humans, we will investigate the influence of tACS on perception of visual stimuli. Concurrently, EEG signals will be recorded to test, after removal of stimulation artefacts, whether tACS is able to modulate ongoing brain activity and stimulus-evoked oscillatory brain responses in the human brain. An animal model (anesthetized ferrets) will be used to study the effects of tACS on neuronal oscillations and synchrony in invasive recordings from visual cortical areas. This combined procedure will allow to draw the desired conclusions. In ferrets, we will be able to demonstrate the modulation of spike activity by tACS, since spike signals are not confounded by the low-frequency (1-100 Hz) tACS. In humans, we will be able to demonstrate the influence of tACS on perception. Taken together, these findings will shed new light onto the relationship of brain oscillations and cognitive functions. In addition, we will develop new tools for the simulation of electrical stimulation. using a finite element model (FEM), we will predict intracranial current densities from electrode positions and stimulation parameters. Furthermore, we plan to allow an inverse direction of simulating tACS, i.e. the user decides in which cortex region maximal/minimal current densities are desired and the software determines the appropriate electrode layout and stimulation parameters to achieve the desired stimulation.