Neuroscience and Behavioral Physiology, Vol. 28, No. 6. 1998 EEG FREQUENCY RANGES DURING PERCEPTION AND MENTAL ROTATION OF TWO- AND THREE- DIMENSIONAL OBJECTS A. R. Nikolaev and A. P. Anokhin UDC 612.821.3+612.825+612.822.3 Spectral EEG powers were compared in 4 frequency ranges (8-13, 15-25, 25-35, and 35-45 Hz) in a group of 20 subjects during the performance of tasks requiring mental rotation of two- and three-dimensional objects. Only those EEG segments corresponding to tasks with identical solution times were analyzed. The spectral powers of oscillations in the alpha range were higher in control conditions than during task performance. Power in the frequency range 15-45 Hz was greater during task performance than in control conditions; this supports the concept that alpha rhythm desynchronization accompanies the synchronization of higher-frequency EEG rhythms. Frequency power during task performance with two-dimensional objects was greater than that during tasks with three-dimensional objects. Since the angle of rotation between two-dimensional objects was greater than that between three-dimensional objects, this factor, rather than the depth of the perceived space, increased the level of cortical activation. In all experimental situations, power at frequencies of 15-45 Hz was significantly greater in the occipital regions than any other regions, reflecting the visual modality of the stimulus. Particular changes were noted in the gamma range (35-45 Hz), where power in the first second of task performance was significantly higher than in the second second; this may provide evidence that this range is mare closely associated with perception and recognition processes than with mental transformation of the image. Attempts to link EEG rhythms with the process of performing purposive activity have continued from the moment at which these rhythms could be recorded. It has been suggested that EEG rhythms arise because of the activity of neuronal oscillators - groups of neurons conjoined by systems of direct and reverse connections such that the output discharges at a particular frequency which is significantly lower than the discharge frequency of the individual neurons and corresponds to EEG rhythm frequencies. The functional importance of this rhythm is determined both by the cortical topography of the oscillator, which is especially strongly pronounced in the projection areas, and by properties acquired by oscillators as a result of learning processes [3]. Rhythms evoked by external or internal events reflect those brain operations which are located between the primary sensory processes and mental functions such as recognition, remembering, utilization of movement commands, etc. [8]. However, the identification of particular rhythms associated with particular functions is only possible in certain simple behavioral situations, in which the brain is in a state of high cooperation or synergy. Complex behavior is reflected in combinations of rh Ires which, having specific topographical and frequency characteristics, can be regarded as "structural units" for physiological and mental processes occurring in the brain [7, 18]. Spectral power levels for the various rhythms are not direct reflections of the activation of a given brain area. While the well-localized phenomenon of reduced alpha-rhythm power ("evoked desynchronization") can serve as a clear indicator of cortical activity specific for a given type of activity [24], there are no clear data on connections between power changes and regional activation for the other frequency bands. For example, data for the beta rhythm (13-30 Hz [17]) have been obtained indicating a reduction (as for the alpha rhythm) in power during mental activity [6, 10, 21, 24] and also indicating an increase in power [14, 20]. Activity at about 40 Hz (the gamma rhythm) usually increases during activation of areas relevant to the Institute of Higher Nervous Activity and Neurophysiology, Russian Academy of Sciences. Institute of Humans, Russian Academy of Sciences, Moscow. Translated from Zhurnal Vysshei Nervnoi Deyatel'nosti imeni I. P. Pavlova, Vol. 47, No. 5, pp. 908-917, September-October, 1997. Original article submitted December 9, 1996; revision submitted January 13, 1997. 670 0097-0549/98/2806-0670520.00 9 Plenum Publishing Corporation