Desynchronization of cortical rhythms following cutaneous stimulation: effects of stimulus repetition and intensity, and of the size of corpus callosum.

OBJECTIVE: Event-related desynchronization (ERD) and synchronization (ERS) of the Rolandic electroencephalographic (EEG) rhythms following brief, innocuous electrocutaneous stimulation were studied with respect to stimulus intensity and repetition and the size of corpus callosum (CC). METHODS: EEG was recorded using 82 closely spaced electrodes in 13 right-handed subjects. The subjects received 650 brief electrical stimuli to the right index finger at irregular intervals (6-12 s) in 5 blocks. The intensities of the stimuli varied randomly at 20, 30, 50, 65 and 80% of pain threshold. RESULTS: Mu- and beta-ERD of 0.3-0.6 s latency over the contra- and ipsilateral S1/M1 area was observed in all subjects. Post-stimulus beta-ERS over the contra- and ipsilateral frontal cortices with a peak latency of 0.6-0.8 s was found in 9 subjects. Stimuli presented in the second half of the experiment were followed by a smaller ipsilateral mu-ERD and smaller contra- and ipsilateral beta-ERD than stimuli applied in the first block. Mu- and beta-ERD and beta-ERS distinguished weak (20%) from intermediate and strong stimuli (>35%) but not the intermediate from strong stimuli. The amplitude of ipsilateral beta-ERS correlated positively with the size of intermediate truncus of CC (r(9)=0.71, P<0.05). In contrast, ipsilateral ERD showed no significant correlations with the size of CC. CONCLUSIONS: Habituation of ipsilateral mu-ERD and bilateral beta-ERD and beta-ERS suggests that these cortical responses are parts of the orienting response, and fail to disentangle fine intensity gradations. Ipsilateral beta-ERS appears to be mediated by the transcallosal fiber system.

Source activity in the human secondary somatosensory cortex depends on the size of corpus callosum.

If corpus callosum (CC) mediates the activation of the secondary somatosensory area (SII) ipsilateral to the side of stimulation, then the peak latencies of the contra- and ipsilateral SII activity as well as the amplitude of the ipsilateral SII activity should correlate with the size of CC. Innocuous electrical stimuli of five different intensities were applied to the ventral surface of the right index finger in 15 right-handed men. EEG was recorded using 82 closely spaced electrodes. The size of CC and of seven callosal regions was measured from the mid-sagittal slice of a high-resolution anatomical MRI. The activation in the contralateral and ipsilateral SII was evaluated using spatio-temporal source analysis. At the strongest stimulus intensity, the size of the intermediate part of the callosal truncus correlated negatively with the interpeak latency of the sources in ipsi- and contralateral SII (r = -0.83, P < 0.01). Stepwise regression analysis showed that the large size of the intermediate truncus of CC was paralleled by a latency reduction of peak activity of the ipsilateral SII, whereas both contra- and ipsilateral peak latencies were positively correlated. The peak amplitude of the ipsilateral SII source correlated positively with the size of the intermediate truncus of CC, and with the peak amplitudes of sources in the primary somatosensory cortex (SI) and in the mesial frontal cortex. The results suggest that in right-handed neurologically normal men, the size of the intermediate callosal truncus contributes to the timing and amplitude of ipsilateral SII source activity.