Alpha-band desynchronization in human parietal area during reach planning
Introduction
Reaching for an object is a type of complex hand movement performed by humans and primates, and is often called visually guided reaching or praxis. We reach for objects with an incredibly high degree of precision in daily life. Such behavior appears to be effortless, even with unexpected perturbations such as an object relocation (Prablanc and Martin, 1992, Pisella et al., 2000). However, reaching movements require integrated information regarding the object’s position and orientation to guide the hand to the object with accuracy, whereas information regarding the object’s shape and size determines how the fingers move opposite to the thumb to grasp the object. Recent functional imaging studies that used functional MRI or PET in humans have revealed that the posterior parietal cortex (PPC) plays an important role in controlling praxis movements by continuously integrating sensory information regarding the body state and environment (Culham et al., 2006). However, these modern techniques provide temporal resolution that is insufficient for reliable quantitative analysis of activation times.
In humans and primates, motor-related activity has been successfully investigated through electroencephalographic (EEG) oscillatory activity analysis. It is well known that event-related desynchronization (ERD) in the alpha-band (8–13 Hz) starts about 1.5 s before the onset of movement (Pfurtscheller and Berghold, 1989). This activity is presumed to reflect cortical activity related to movement planning (Pfurtscheller and Lopes da Silva, 1999). Experimental data suggest that alpha ERD represents an electrophysiological correlate of activated cortical areas that is related to information processing, selective attention, and motor preparation (Van Winsum et al., 1984, Pfurtscheller, 1992, Dujardin et al., 1993, Dujardin et al., 1995). Furthermore, Pfurtscheller et al. (2000) reported that in a motor task, the upper frequency mu rhythms (10–12 Hz) reflects a more somatotopic spatial ERD pattern than the lower frequency mu rhythms (8–10 Hz). This different behavior between the lower and upper alpha-band components indicates that the lower alpha ERD reflects general task demands and attentional processes that are not task-specific, whereas the upper alpha ERD develops when movement-related information is processed; therefore, it is task-specific (Pfurtscheller et al., 2000). In addition, the posterior dominant rhythm (PDR) is an idling rhythm, indicative of a relative decrease in conscious attention or visual processing (Pfurtscheller and Aranibar, 1977). Approximately 80% of healthy adults had a PDR between 9 and 11 Hz (Kellaway, 1990).
Movement-specific ERD has been recorded not only from scalp electrodes but also subdural electrodes (Toro et al., 1994). However, only a few investigations have used this approach to study more practical and coordinated movements such as reaching, catching, or grasping (Tombini et al., 2009, Van Der Werf et al., 2010, Virji-Babul et al., 2010).
The present study aimed to clarify the involvement of PPC in movement planning and execution by revealing upper alpha-band ERD in parietal area, when reaching for an object (i.e., target- and body-related movements). Therefore, we compared reaching and simple movements to determine whether the underlying neural sources of EEG activity for these movements can be distinguished as independent.
Section snippets
Participants
Subjects were 16 healthy right-handed university students (6 men, 10 women; age, 22–25 years) with normal or corrected-to-normal vision and with no reported history of neurological or psychiatric illnesses. All subjects provided an informed consent for participating in the study. The Ethical Committee of Kyoto University approved the experimental protocol (No. E-929).
Recording conditions
EEG signals from 23 Ag/AgCl surface electrodes placed on the scalp were recorded according to the 10–10 International System. For
Results
Data from a subject was excluded from the statistical analysis because an insufficient number of trials were recorded. In addition, data from another subject including significant artifacts during left wrist extensions was excluded.
Fig. 2A presents grand-average mapped spatially enhanced alpha ERD for reaching and wrist extension. In the grand-averaged ERD, the 12-Hz frequency band revealed the highest power decreases relative to baseline (Fig. 2A). A three-way repeated-measures ANOVA revealed
Discussions
The aim of this study was to determine the temporal characteristics and spatial distribution of brain regions activated during the planning of reaching and wrist extension movements. We identified 3 main differences between these 2 movement conditions in the ERD analysis. The first difference was in the distribution of 12-Hz ERD that revealed a task-specific pattern (Figs. 2A and 3). We estimated the upper alpha-band activity involved in reach planning was located in the parietal area
Conclusions
In conclusion, the present EEG investigation supports the working hypothesis that alpha ERD in the parietal area provides complementary information on brain activity in humans during the preparation and execution of volitional reaching movements. Our main finding was that alpha ERD originating from the parietal area encodes the reaching movement, whereas alpha ERD originating from fronto-central (e.g., suppression of the mu rhythm) and the anterior region of the parietal area encodes wrist
Acknowledgements
This work was supported by Grants-in-Aid for Scientific Research (B) 26282218, 26293209, (C) 26330175 from the Ministry of Education, Culture, Sports, Science, and Technology of Japan (MEXT).
Conflict of interest: The authors declare no competing financial interests.
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