Numerous studies have demonstrated that the power of ongoing alpha oscillations is related to neural excitability, as reflected in spike-firing rate, multi-unit activity, or the hemodynamic fMRI signal. Furthermore, alpha oscillations also affect performance in perceptual tasks. However, the psychophysical mechanisms linking neural excitability and perceptual performance are not well understood. For example, an open question is whether excitability fluctuations induced by alpha oscillations affect an observer’s acuity or perceptual bias. I will present a series of experiments that aim to clarify this link. In short, a survey of the literature and our own experiments show that ongoing alpha oscillations affect performance by reducing visual response gain predominantly in detection experiments. These results are in line with a signal detection framework in which spontaneous fluctuations of neural excitability are modeled as a change in the observer’s criterion without a change in sensitivity.