Event-related alpha oscillations in task processing Vasil Kolev a, * , Juliana Yordanova a , Martin Schu Èrmann b , Erol Bat Ëar b, c a Institute of Physiology, Bulgarian Academy of Sciences, Acad. G. Bonchev str., bl. 23, 1113, So®a, Bulgaria b Institute of Physiology, Medical University of Lu Èbeck, D-23538 Lu Èbeck, Germany c Brain Dynamics Research Unit, TU È BITAK, Ankara, Turkey Accepted 19 April 1999 Abstract Objectives: Recent ®ndings substantiate the view that electroencephalographic (EEG) alpha rhythm (7±13 Hz) is functionally involved in information processing. However, the association of alpha rhythms with cognitive brain processes is less well understood because both augmentation and suppression of alpha oscillations have been observed to accompany task performance. The present study evaluates the effect of task processing on event-related alpha oscillations at the level of single-sweep analysis. Methods: EEG was recorded from Fz, Cz and Pz electrodes in 10 subjects participating in two experimental sessions, in which auditory stimuli with equal physical parameters were presented under different instructions (passive and task). Separate measurements of single- sweep amplitude and phase-locking were performed and statistically analyzed for consecutive time windows in the poststimulus epoch. Results: Major results show that, during the cognitive task, the phase-locking of alpha oscillations at the frontal site is signi®cantly increased for the time window of 500±1000 ms after stimulation. Conclusions: The involvement of enhanced and synchronized frontal alpha activity in higher brain processes is strongly emphasized. q 1999 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Brain oscillations; Event-related potentials; Single-sweep analysis; Phase-locking; Frontal alpha rhythm; EEG; Cognitive 1. Introduction New results obtained by means of different approaches and paradigms substantiate the notion that the electroence- phalographic (EEG) alpha rhythm (7±13 Hz) is more than just a spontaneous or `idling' rhythm of the brain (Mulhol- land, 1995; Bat Ëar et al., 1997b; Niedermeyer, 1997). The EEG alpha activity recorded at various scalp locations and in different conditions is now referred to as the prototype of a dynamic process related to a large ensemble of integrative brain functions, among them sensory, motor and cognitive processes. The EEG alpha activity has been proposed to re¯ect the functioning of diffuse and selectively distributed alpha systems in the brain, giving rise to multiple types of alpha (Bat Ëar et al., 1991 1997b; Lopes da Silva, 1993; Klimesch, 1996; Lopes da Silva et al., 1997). Alpha systems are suggested to generate and control the spontaneous alpha rhythms (Bat Ëar et al., 1997a). Upon stimulation, they are able to reset their activity so that enhanced and phase-reor- dered alpha oscillations are produced in the poststimulus epoch. Upon further processing demands, alpha systems are also capable of dynamically reorganizing their patterns of functional activation. The association of EEG alpha activity with sensory processing has been ®rst inferred by the observation that external sensory stimuli of different modalities (auditory, visual, somatosensory) produce enhanced alpha oscillations called the alpha response (Adrian, 1941), while visual stimuli may also block the occipital alpha rhythm (Berger, 1929). Because the alpha response can be recorded from a variety of cortical and subcortical structures like auditory and visual cortices, speci®c thalamic nuclei, hippocampus, cerebellum, reticular formation, etc., it is proposed to be one of the most fundamental and invariant brain responses to sensory events (Lopes da Silva et al., 1973; Bat Ëar, 1980; Bat Ëar and Schu Èrmann, 1994; Schu Èrmann and Bat Ëar, 1994; Schu Èrmann et al., 1997). The involvement of EEG alpha frequency in the regulation of motor behaviour has also been well documented (e.g. Pfurtscheller et al., 1997, 1988; Pfurtscheller and Neuper, 1992;). In addition, changes in the ongoing EEG alpha activity have been observed to accompany attention, memory, and lexical processes (Pfurtscheller and Klimesch, 1991, 1992; Klimesch, 1996, 1997; Klimesch et al., 1994; Krause et al., 1995; Hari and Clinical Neurophysiology 110 (1999) 1784±1792 CLINPH 98706 1388-2457/99/$ - see front matter q 1999 Elsevier Science Ireland Ltd. All rights reserved. PII: S1388-2457(99)00105-4 www.elsevier.nl/locate/ifcn * Corresponding author. Tel.: 1 359-2-979-3749; fax: 1 359-2-738- 469. E-mail address: kolev@iph.bio.bas.bg (V. Kolev)