Furthermore, we observed a direct relationship between spectral indexes and levels of cognitive functioning in all subjects participating in the study. In control subjects, as expected, the source of delta BROs was located in the precuneus, whereas in DOC patients, delta BROs were not recognizable and no precuneal localization was possible. Cortical sources of the maximum blink-related delta power, corresponding to the positive peak of the delta BROs, were estimated by standardized Low Resolution Electromagnetic Tomography. Three-second-lasting EEG epochs centred on each blink instance were analyzed in both time- (BROs) and frequency domains (event-related spectral perturbation or ERSP and intertrial coherence or ITC).
To test these hypotheses, electroencephalographic (EEG) activity at rest was recorded in 12 healthy controls and nine patients with DOC (four vegetative states, and five minimally conscious states). We therefore hypothesized that the source of delta BROs in healthy subjects could be located in the precuneus and that delta BROs could be absent or reduced in patients with disorders of consciousness (DOC). Furthermore, fMRI studies have shown that precuneal activity is reduced or missing when consciousness is low or absent. Delta BROs have been linked with continuous gathering of information from the surrounding environment, which is classically attributed to the precuneus. Recently, blink-related delta oscillations (delta BROs) have been observed in healthy subjects during spontaneous blinking at rest.
In otherwords, such a procedure preserves the graphic representation ofthe data without considering the true polarity.This would not be a problem when on-line averaged evokedpotentials with known waveform and polarity are exported (pro-vided that one reverses the polarity off-line so they match with In this case, in fact, the procedureof exporting data as ASCII files, by assigning negative values to thesamedeflections(becausetheyarereallydownwardfromaspatialpoint of view), gives rise to an error of inverted polarity.
This poses a serious problem when the reference coordinatesof the recording device, concerning vice versa the polarity of thedeflections with respect to the isoelectric line, are such that posi-tive deflections are downward. On the contrary, data exported in the form of filetext (with an ASCII code) only allow reproducing signals visual-izedontherecordingdevicescreen.But,indoingso,thewaveformis reconstructed referring to an internal code, according to whichnegative values are assigned to the points situated in the inferiorpart of the picture (with respect to an equatorial line of symme-try). We believe that the polarity originating fromthe EDF files (i.e., positive-negative) should be considered correct,because EEG data exported in the form of EDF files, according tothe specifications of the format itself, faithfully represent originalraw data. In particular, the blink-relateddelta oscillation, that was a biphasic negative-positive waveformin the above-mentioned paper, has turned out to be reversed inpolarity (i.e., positive-negative) in the recent recordings.The only difference in the EEG data processing with respect tothe previous recordings was in the export file format: EDF (recent)vs ASCII (previous). Subsequent recordings collected during a follow-up of our studieson blink-related EEG have not produced results overlapping thosereported in the previous paper.
0 kommentar(er)
