Correlation of brain rhythms between frontal and left temporal (Wernicke's) cortical areas during verbal thinking.

The aim of this study was to investigate an interaction between frontal and left temporo-parietal cortices in tasks requiring word association. A new method was used to examine averaged event-related potentials in different frequency bands by calculating correlation coefficients between wavelet curves in distant cortical areas. This method was applied to previous event-related potentials recordings which found successive activation of frontal and left posterior areas [1]. Correlated activity at 17 Hz was observed between frontal and left temporal (Wernicke's) areas prior to full activation of Wernicke's area.

Cortical connectivity during word association search.

Cortical connectivity was studied in tasks of generating the use of words in comparison with reading aloud the same words. These tasks were used earlier in PET and high density ERP recordings studies (Posner and Raichle, 1997; Abdullaev and Posner, 1998), in which both the functional anatomy and the time course of cortical areas involved in word processing were described. The wavelet transforms of ERP records and the calculation of correlations between wavelet curves were used to reveal connections between cortical areas. Three stages of intracortical communications while task performance were found. These were: (1) the connections between right and left frontal and central areas which preceded stimulus delivery and persisted up to 180 ms after it; (2) the network connecting right and left frontal with left posterior temporal-parietal junction at 280-450 ms; and (3) communications between left and right temporal zones in 550-800 ms. The data are in good agreement with results of previous PET and ERP studies and supply the earlier findings with circuitry of cortical information transfer.

Investigation of the stages of the mental rotation of complex figures with the intracortical interaction mapping technique.

The stages of the mental rotation of complex three-dimensional figures were investigated in nine healthy subjects using the intracortical interaction mapping technique. It was established that mental rotation is accomplished with the participation of both parietal regions; the prerotation setup is associated with the involvement of the frontal, central, and right temporal divisions of the brain; decision-making regarding the result of the rotation and the response is accomplished through the conjunction of both frontal and temporoparietooccipital regions of the left hemisphere. In the case of the unsuccessful solution of the problem the process seems to stop at the stage of the prerotation setup. With verbal control the configuration of the zone of high-level association during the first three stages is reminiscent of that observed in the baseline activity.

EEG frequency ranges during perception and mental rotation of two- and three-dimensional objects.

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 more closely associated with perception and recognition processes than with mental transformation of the image.

Studies of cortical interactions over short periods of time during the search for verbal associations.

Interactions between cortical areas were studied during the search for verbal associations and reading of words. The functional anatomy and the sequence of involvement of cortical areas during the solution of these tasks have been described previously, in studies using positron emission tomography and multichannel recordings of evoked potentials combined with identification of the locations of dipole sources [8, 9, 19, 25]. Cortical interactions reflected in terms of the synchronization of EEG rhythms were studied by developing a method based on correlating curve wavelets, which allows the moments at which this synchronization occurs to be identified over short periods of time comparable with the speeds of individual thought operations (up to 100 msec). Three main stages were identified in the search for associations. During the first 200 msec after stimulus presentation, cortical connections were seen between the right and left frontal areas; at 200-500 msec, there were connections between the frontal and the temporal-parietal areas; finally, at 450-700 sec, there were connections between the left temporal and the right frontal-central-temporal areas. These results are in good agreement with data obtained previously using other methods and supplement them with mapping data on cortical connections. A number of differences in the mechanisms of information processing during the search for associations and reading were also identified.

[The effect of the duration of mental work on problem solving in mental rotation]. / Vliianie prodolzhitel'nosti umstvennoi raboty na reshenie zadach na myslennuiu rotatsiiu.

Effect of sustained mental work on the first stage of mental rotation was studied by the method of intracortical interaction. It was revealed that during sustained mental work the zone of the high level of connections in the alpha-band was considerably reduced due to an increase of specificity, rationality of thinking. In the beta-band the zone of the high connection level moved from the central and parietal areas to the anterior regions because of a switch of intention of the mental work from the spatial analysis to the quick verbalization of the response. Tired subjects in search for a word for the response did not fulfil the spatial analysis of the pictures. Subjects with high working ability searched for the verbal response on the basis of mental operations with images, that turned to be optimal for the mental rotation task solving.

[The stages in the mental rotation of complex figures studied by mapping intracortical interaction]. / Issledovanie étapov myslennoi rotatsii slozhnykh figur metodom kartirovaniia vnutrikorkovogo vzaimodeistviia.

The stages of mental rotation of complicated three-dimensional figures were studied using the intracortical interaction mapping. The role of the parietal areas in the mental rotation was shown. The frontal, central and the right temporal areas are involved in the prerotation setup. Both frontal areas and the left temporo-parieto-occipital areas participate in decision making and verbal response. In the case of unsuccessful task solving the process stops at the prerotation setup stage. During the verbal control the zones of the predominant connections at the first 3 stages are close to those in the background activity.

[The EEG frequency spectra during the perception and mental rotation of 2- and 3-dimensional objects]. / Chastotnye diapazony EEG pri vospriiatii i myslennoi rotatsii dvu- i trekhmernykh ob''ektov.

The EEG spectral power in 4 frequency ranges (8-13, 15-25, 25-35, and 35-45 Hz) was studied during mental rotation of two- and three-dimensional objects. Only those EEG segments were analyzed which corresponded to the tasks with similar time of solving. The EEG spectral power of the alpha range was higher under control conditions than during task solving. The spectral power in the range of 15-45 Hz was higher under the operation than under the control conditions. This finding confirms the assumption that EEG desynchronization in the alpha range is accompanied by the high-frequency synchronization. The EEG high-frequency power was higher during mental rotation of two-dimensional objects than three-dimensional ones. Since the angular difference between the two-dimensional objects was larger than between the three-dimensional ones, the rotation angle rather than the "depth" of space increased cortical activation. Under experimental conditions, EEG spectral power in the range of 15-45 Hz was always the highest in the occipital areas, which was associated with the visual modality of stimuli. The EEG spectral power in the gamma range (35-45 Hz) was substantially higher at the first second of operation as compared with the second one testifying that EEG changes in this range are more related with the processes of perception and recognition than with mental operations with images.

[The cortical interactions in short time intervals during the search for verbal associations]. / Issledovanie korkovykh vzaimodeistvii v korotkikh intervalakh vremeni pri poiske verbal'nykh assotsiatsii.

Cortical connectivity was studied in tasks of generating the use of words in comparison with reading aloud the same words. These tasks were used earlier in PET and high-density ERP recording studies, which described both the functional anatomy and time course of involvement of cortical areas in word processing. We developed a new method for studying the synchrony of EEG spectral components within the short time intervals compatible with the duration of particular cognitive operations. The wavelet transform of the ERP records and calculation of correlations between the wavelet curves were used to reveal connections between cortical areas. Three stages of intracortical communications developing over the course of task performance were discovered: between the right and left frontal areas (0-200 ms after the stimulus presentation), between the left frontal and left posterior temporo-parietal areas (250-500 ms), and, finally, between the left temporal and right fronto-centro-temporal areas. These findings are in good agreement with the results of the previous PET and ERP studies and supplement them with the circuitry of cortical information transfer. Also, they suggest some differences in information processing during automated reading and performance of more complicated use-generation task.

[The spectral rearrangements of the EEG and the organization of the cortical connections during spatial and verbal thinking]. / Spektral'nye perestroiki EEG i organizatsiia korkovykh sviazei pri prostranstvennom i verbal'nom myshlenii.

Solution of spatial and verbal tasks was studied in 43 subjects using the advanced version of the method of intracortical interaction mapping (IIM) and analysis of EEG spectra in short (250 ms) intervals. Comparison between the IIM and coherence methods during solving simple motor tasks revealed some advantages of the IIM for studying of labile cortical connections. Several stages were separated in the process of solution of the mental task in accordance with patterns of cortical connections. The duration of the stages was similar in both kinds of tasks and corresponded to that revealed in psychological experiments. Cortical connections in the theta range were generalized and reflected the level of general tension. This unspecific tension was high for the whole period of the spatial task solution while in the verbal task the tension increased to the stage of decision making. Task-specific patterns were revealed in the beta range. The right parietal and temporal regions were the centers of EEG connections during solution of the spatial task while the left central area was essential for the verbal task solution. The left parietal area was involved in solution of both kinds of tasks. The frequency parameters of connections in the beta range were stable during the spatial task solving, and in the process of verbal task solving the dominant frequency changed after the initial (perception) stage probably due to switching to another thinking mechanism.

[Use of artificial neural networks in recognition of types of thinking by EEG]. / Ispol'zovanie iskusstvennykh neirosetei dlia raspoznavaniia tipa myslitel'nykh operatsii po EEG.i

Reflection in EEG parameters of the process of solving two types of mental tasks, i.e. spatial-imaginary and verbal-logical, was investigated in nine test-subjects. Mental efforts were found to specify EEG spatiofrequency patterns depending on the task type and subject's identity. These patterns were unique and reproducible enough to enable identification of this or the other type of mental task within several seconds of EEG record. Recognition was executed with the use of an artificial neural network algorithm in order to simultaneously weigh a wealth of signs of a specific EEG pattern. Recognition precision was 85% on the average.