This paper first briefly describes rhythmic bioelectric oscillations that can be recorded from the neocortex (particularly in cats). This descriptive section is followed by a review of the mechanisms whereby these rhythms are generated, and a section in which we try to go beyond the purely correlative aspect and discuss the possible functional role of synchronized oscillations in thalamo-neocortical channels. Based on the literature data, it seems that two distinct and opposite roles can be attributed to these oscillations: either to lower the level of awareness, such as in slow wave sleep, when these oscillations are widespread on the neocortex; or to contribute to perceptual processing, when these rhythms are more localized and possibly of a higher frequency. Electrocortical (ECoG) rhythmical activities have been known and described since the early days of electrophysiological explorations of the human and animal brain. Surprisingly though, little importance has generally been attached to these oscillatory activities, except as indicators of sleep stages in animals and humans and, in the case of the alpha rhythms (the first rhythm to be discovered in the thirties by Hans Berger), of a certain state of 'relaxation' in humans. Despite the well-accepted fact that the human alpha rhythm evidently occurs during waking (even if it is in a 'relaxed' state), the idea has curiously prevailed that the waking ('aroused') state in general is mostly characterized by low voltage fast ('desynchronized') activity. It is only more recently that some studies have suggested a functional importance of a variety of regular oscillatory activities that can be recorded from the neocortex and/or from a variety of thalamic nuclei not only in the sleeping but also on the waking, behaving animal. This review considers possible reasons indicating that these rhythmical activities are more than just epiphenomena. Our analysis is essentially restricted to the cat, with only a very quick glance at other species.