Comparison of two methods for correcting ocular artifacts in the EEG

A major problem in the study of brain potentials is the occurrence of ocular artefacts in electro-encephalograms. OAs can be monitored by placing electrodes near the eyes and recording electro-oculograms. In the paper, two OA correction methods based on simulations are compared; the Jervis method and the vandenBerg method. In most simulations, the residual (the difference between the original EEG and the EEG after correction) is smaller in amplitude and variance for the vandenBerg method than for the Jervis method. When eye movements and blinks are given different factors, the blinks are not removed completely. For both methods, the residual of the blinks increases with the differences between the model parameters for the blinks and for eye movements. The occurrence of a slow negative wave greatly disturbs the estimated parameters and thus the residuals of the Jervis method. For the vandenBerg method, there is only a very small effect. The conclusion from correcting a recorded data set, which does not contain a slow negative wave, is that, for these data, there is no evidence that one method is better than the other

A spatiotemporal dipole model of the Stimulus Preceding Negativity (SPN) prior to feedback stimuli

Ten subjects performed a time production task, in which they were instructed to press a button four seconds after the presentation of an auditory stimulus. Two seconds after the button press they received either auditory or visual feedback on the temporal accuracy of their response. In such a paradigm negative slow brain potentials can be recorded preceding the response (Movement Preceding Negativity, MPN) as well as preceding the feedback stimulus (Stimulus Preceding Negativity, SPN). Spatiotemporal dipole modelling is used to gain insight in the possible generators of MPN and SPN. From the models it follows that the MPN can be described by one contralateral radial dipole and a bilateral pair of tangential dipoles. All three dipoles are located near central electrode positions, so the generators of the MPN probably reside within the motor cortex. The SPN is modelled by a bilateral frontotemporal pair of dipoles, hypothetically representing activation of the Insulae Reili. The insular cortex is involved in the processing of affective-motivational input, such as carried by the feedback in the present paradigm. However, processing of the information content of the feedback stimulus might by itself also activate the frontal cortex. Both the response and the feedback stimulus are followed by a positive peak, which can be described by the same deep posterior dipole. Both peaks probably represent a P3, which is related to context updating

The international 10-20 system revisited : cartesian and spherical co-ordinates

Methods like dipole source localization require an exact specification of the co-ordinates of electrode positions. Different values for the co-ordinates of F3, F4, P3 and P4 are encountered in literature. This is due to the,unexpected complexity of the calculations involved and aggravated by an inaccurate but widely used control procedure for the placement of these electrodes. We present a table of co-ordinates for the 10–20 system together with a method for determining the co-ordinates of mid-way positions within the 10–20 system. The consequence of using erroneous co-ordinates on the accuracy of dipole source localization is discussed