Biol. Cybern. 76, 471—480 (1997) Is the alpha rhythm a control parameter for brain responses ? * Erol Bas ,ar , Juliana Yordanova , Vasil Kolev , Canan Bas ,ar-Eroglu   Institute of Physiology, Medical University of Lu¨beck, D-23538 Lu¨beck, Germany  Institute of Physiology, Bulgarian Academy of Sciences, BG-1113 Sofia, Bulgaria  Institute of Psychology and Cognition Research, University of Bremen, Bremen, Germany  Brain Dynamics Research Unit, TU BITAK, Ankara, Turkey Received: 6 November 1995 / Accepted in revised form: 13 March 1997 Abstract. The main goal of the present study is to develop a conceptual analysis of alpha response in the brain based on single sweep evaluation. A new method was employed to estimate a set of single-sweep para- meters and quantify the oscillatory behaviour of single, electroencephalograph (EEG) sweeps. It was aimed to demonstrate that brain alpha responses are governed by spontaneous alpha activity and to validate the principle of brain response excitability. Because the spon- taneous alpha activity depends on both the topology of recording and the subject’s age, topology and age models were used. Spontaneous and evoked alpha activ- ity were recorded at frontal and occipital sites in three groups of subjects: 3-year-old children, young adults and middle-aged subjects. Amplitude, enhancement and phase-locking of single alpha responses to visual stimuli were analysed. Major results showed that: (1) visual al- pha responses could be recorded only if the alpha rhythm was developed in the spontaneous EEG independent of electrode location; (2) middle-aged adults showed more expressed frontal spontaneous alpha activity in compari- son with young adults; (3) accordingly, alpha responses with higher amplitude and stronger phase-locking were produced over the frontal brain area in middle- aged than young adults. These results validate the prin- ciple of brain response excitability and demonstrate that a shift towards frontal brain areas for both the spontan- eous and evoked alpha activity occurs with increasing age in adults. The results are discussed in the context of the diffuse and distributed alpha system of the brain. Age-dependent changes in frontal alpha activity are sug- gested to be related to frontal brain functioning during aging. Correspondence to: E. Bas ,ar, Institute of Physiology, Medical Univer- sity of Lu¨beck, Ratzeburger Allee 160, D-23538 Lu¨beck, Germany (Tel.: ( # 49) 451 500 4170, Fax: ( # 49) 451 500 4171, e-mail: ebasar @physio-mu.luebeck.de) * Supported by Deutsche Forschungsgemeinschaft (DFG), Project 436- BUL-113/76 1 Introduction The conceptual analysis of alpha rhythm in electro- encephalography (EEG) was one of the major problems in cybernetics put forward by Norbert Wiener (e.g. Wiener 1955, 1956, 1957). Wiener has proposed that the alpha rhythm may reflect the mutual entrainment of an ensemble of non-linear alpha rhythm generators such that the frequencies of individual members of the en- semble would have a tendency to be pulled synchronous- ly toward a central frequency, thus serving a gating function. Hence, studies of the functional correlates of alpha rhythm have gained increasing importance (Bas ,ar et al. 1997). However, although a large number of corre- lates and models have been proposed, the genesis and functional meaning of the 10-Hz EEG activity is not exactly known. In the present study, a new conceptual analysis re- lated to the biocybernetical relevance of the alpha rhythm in the brain is proposed. It is based on demon- strating the causality within the alpha frequency channel as reflected by the relations between the spontaneous and externally evoked alpha activity. The major assumption is that if upon input (stimulus) application to the system (brain) a causality exists between the intrinsic (spontaneous) and output (response) characteristics of alpha activity, then alpha frequency would be an active functional oper- ator for signal transmission and processing rather than a passive background rhythm. Firstly, an efficient method for single-trial analysis was applied in order to study the spontaneous alpha together with evoked alpha responses (Kolev and Daskalova 1990; Kolev and Yor- danova 1997). Secondly, the effects of aging and topology were explored to test the hypothesis of brain response excitability, which states that brain alpha responses are governed by the spontaneous alpha activity (Bas ,ar 1980, 1992), with the following theoretical background taken into consideration. To explain the role of the oscillatory behaviour of single neurons and/or neuronal populations in the entire brain, the existence of a diffuse and distributed alpha system has been proposed (Bas ,ar et al. 1991). Recent