emgm t~ '~ IETTHIS E LS EV l E R Neuroscience Letters 168 (1994) 126-129 .... Transient phase-locking of 40 Hz electrical oscillations in prefrontal and parietal human cortex reflects the process of conscious somatic perception John E. Desmedt*, Claude Tomberg Brain Research, University of Brussels Faculty of Medicine, 20 rue Evers, B-IO00 Brussels, Belgium Received 14 December 1993; Accepted 4 January 1994 A~tract Electrical potential oscillations in the range of 35-45 Hz (gamma waves) have recently been shown to occur rather ubiquitously in the brain of awake humans. During selective somatic attention, we demonstrate a transient phase-locking of the gamma waves generated in the contralateral prefrontal and parietal cortical areas that we had previously shown to be involved in such selective attention tasks. In line with other microphysiological evidence obtained on mammalian visual cortex, this selective functional synchronization between critical human brain areas (as far as about 9 cm apart) is proposed to reflect the transient 'binding' of discrete cognitive features that are processed in distributed neuronal assemblies of the brain whereby the conscious perception of an object or event can be achieved. On this basis we emphasize that the conscious function of the brain is neither epiphenomenal nor delayed, but operates transiently to integrate relevant perceptual features at the time of target object identification and of conscious behavioural decision. Key words." Working memory; Consciousness; Cognitive brain potential; Selective attention; 40 Hz neuronal oscillations; Electro- encephalogram Recent microphysiological studies of the visual cortex in cat and monkey disclosed phase-locked spike dis- charges at 30-50 Hz in single neurons up to 7 mm apart [5]. Because the different features of a visual object are analyzed by distinct cortical columns distributed over many visual subareas [7,18], such transient synchroniza- tions may reveal a functional 'binding' [17] that inte- grates pertinent features into a coherent percept. In hu- mans, electroencephalograms rhythms at 35-45 Hz (gamma waves) appear during waking and paradoxical sleep [9]. Such oscillations occur in widespread brain territories and in thalamocortical circuits, and they re- flect a condition of increased vigilance [14]. They are also found in vitro in cortical inhibitory interneurons in which they are generated by activation of a TTX-sensi- tive persistent sodium conductance and a delayed recti- fier [8]. This ubiquity of brain gamma waves appears to exclude interpretations considering them to subserve dis- *Corresponding author. Fax: (32) 2-538 29 05. 0304-3940/94/$7,00 © 1994 Elsevier Science Ireland Ltd. All rights reserved SSDI 0304-3940(94)00026-7 crete cognitive functions. The present paper attempts to resolve this problem and discloses a remarkable transient 40 Hz phase-locking in the course of a somatic selective attention task in humans. Electrical potentials were recorded and digitized on- line from 16-32 scalp sites in 4 healthy adult subjects (25-34 years) under informed consent and ethical Com- mittee approval. One earlobe served as reference [16]. Amplifier bandpass was 0. I to 500 Hz and analog-to- digital conversion was done at 1 kHz. Samples with eye movement artifacts were edited out. During each of 15 experimental runs of 5-10 min, brief electrical stimuli were delivered to several fingers of the hands in a ran- domly intermixed sequence. The intensity was 2 times the subjective threshold as estimated by the method of limits. One of the fingers was designated as 'target' that the subject had to identify by pressing a key with the right big toe. Coded pulses for reaction times (RT) and time of delivery of stimuli were recorded on a technical chan- nel (Fig. 1C). All data were stored on hard disk and processed off line on Next workstations under UNIX.