Event-related desynchronization (ERD) is the short-lasting attenuation or blocking of rhythms within the alpha (beta) band. ERD is found during but also before visual stimulation. Two different types of ERD can be differentiated: one short-lasting, localized to occipital areas and involving upper alpha components; the other longer lasting, more widespread, most prominent over parietal areas and maximal for lower alpha components. The former most likely reflects primary visual processing and feature extraction, the latter is more related to cognitive processing and mechanisms of attention.