Mapping Brain Activity with Electrocorticography: Resolution Properties and Robustness of Inverse Solutions.

Electrocorticography (ECoG) is an electrophysiological technique that records brain activity directly from the cortical surface with high temporal (ms) and spatial (mm) resolution. Its major limitations are in the high invasiveness and in the restricted field-of-view of the electrode grid, which partially covers the cortex. To infer brain activity at locations different from just below the electrodes, it is necessary to solve the electromagnetic inverse problem. Limitations in the performance of source reconstruction algorithms from ECoG have been, to date, only partially addressed in the literature, and a systematic evaluation is still lacking. The main goal of this study is to provide a quantitative evaluation of resolution properties of widely used inverse methods (eLORETA and MNE) for various ECoG grid sizes, in terms of localization error, spatial dispersion, and overall amplitude. Additionally, this study aims at evaluating how the use of simultaneous electroencephalography (EEG) affects the above properties. For these purposes, we take advantage of a unique dataset in which a monkey underwent a simultaneous recording with a 128 channel ECoG grid and an 18 channel EEG grid. Our results show that, in general conditions, the reconstruction of cortical activity located more than 1 cm away from the ECoG grid is not accurate, since the localization error increases linearly with the distance from the electrodes. This problem can be partially overcome by recording simultaneously ECoG and EEG. However, this analysis enlightens the necessity to design inverse algorithms specifically targeted at taking into account the limited field-of-view of the ECoG grid.

[Competitive pacing in a patient with DDD pacemaker and bigeminal ventricular extrasystoles]. / Stimolazione competitiva in un portatore di pacemaker DDD con extrasistoli ventricolari bigemine.

The ECG recorded from a patient with DDD pacemaker showed variable responses of the pacing system to bigeminal ventricular extrasystoles, dependent on the coupling interval of premature beats. For relatively short coupling intervals, the premature spontaneous event was detected by the pacemaker, inhibiting both atrial and ventricular output, and resulting in a relatively long pacing pause. In slightly less premature end-diastolic extrasystoles, in contrast, the pacing system delivered an atrial spike that was superimposed upon the spontaneous premature QRS complex (pseudo-pseudofusion); under these circumstances, the atrial spike was followed, at the end of the programmed atrioventricular interval, by a ventricular spike falling on the extrasystolic T wave apex (competitive ventricular pacing). This phenomenon, however, did not express a sensing malfunction, but was due to post-atrial ventricular blanking (PAVB), a short period initiated by the atrial spike during which ventricular sensing is temporarily disabled, so that no signal can be detected. Finally, whenever premature end-diastolic impulses occurred after PAVB, during the brief interval defined ventricular safety pacing, the spontaneous event was sensed, being followed by an earlier-than-expected ventricular spike, whose prematurity was aimed at avoiding the occurrence of an artificial impulse upon the T wave of extrasystole. In conclusion, despite several not sensed ventricular extrasystoles and competitive pacing, no sensing malfunction was present. This case demonstrates how complex can be the electrocardiographic analysis of a DDD pacemaker, owing to the many complicating phenomena related to this pacing mechanism.