Effects of dipole position, orientation and noise on the accuracy of EEG source localization

BACKGROUND: The electroencephalogram (EEG) reflects the electrical activity in the brain on the surface of scalp. A major challenge in this field is the localization of sources in the brain responsible for eliciting the EEG signal measured at the scalp. In order to estimate the location of these sources, one must correctly model the sources, i.e., dipoles, as well as the volume conductor in which the resulting currents flow. In this study, we investigate the effects of dipole depth and orientation on source localization with varying sets of simulated random noise in 4 realistic head models. METHODS: Dipole simulations were performed using realistic head models and using the boundary element method (BEM). In all, 92 dipole locations placed in temporal and parietal regions of the head with varying depth and orientation were investigated along with 6 different levels of simulated random noise. Localization errors due to dipole depth, orientation and noise were investigated. RESULTS: The results indicate that there are no significant differences in localization error due tangential and radial dipoles. With high levels of simulated Gaussian noise, localization errors are depth-dependant. For low levels of added noise, errors are similar for both deep and superficial sources. CONCLUSION: It was found that if the signal-to-noise ratio is above a certain threshold, localization errors in realistic head models are, on average the same for deep and superficial sources. As the noise increases, localization errors increase, particularly for deep sources

Auditory change detection in schizophrenia: sources of activity, related neuropsychological function and symptoms in patients with a first episode in adolescence, and patients 14 years after an adolescent illness-onset

<p>Abstract</p> <p>Background</p> <p>The event-related brain response mismatch negativity (MMN) registers changes in auditory stimulation with temporal lobe sources reflecting short-term echoic memory and frontal sources a deviance-induced switch in processing. Impairment, controversially present at the onset of schizophrenia, develops rapidly and can remain independent of clinical improvement. We examined the characteristics of the scalp-recorded MMN and related these to tests of short-term memory and set-shifting. We assessed whether the equivalent dipole sources are affected already at illness-onset in adolescence and how these features differ after a 14-year course following an adolescent onset. The strength, latency, orientation and location of frontal and temporal lobe sources of MMN activity early and late in the course of adolescent-onset schizophrenia are analysed and illustrated.</p> <p>Methods</p> <p>MMN, a measure of auditory change-detection, was elicited by short deviant tones in a 3-tone oddball-presentation and recorded from 32 scalp electrodes. Four dipole sources were placed following hypothesis-led calculations using brain electrical source analysis on brain atlas and MR-images. A short neuropsychological test battery was administered. We compared 28 adolescent patients with a first episode of schizophrenia and 18 patients 14 years after diagnosis in adolescence with two age-matched control groups from the community (n = 22 and 18, respectively).</p> <p>Results</p> <p>MMN peaked earlier in the younger than the older subjects. The amplitude was reduced in patients, especially the younger group, and was here associated with negative symptoms and slow set-shifting. In first-episode patients the temporal lobe sources were more ventral than in controls, while the left cingular and right inferior-mid frontal sources were more caudal. In the older patients the left temporal locus remained ventral (developmental stasis), the right temporal locus extended more antero-laterally (illness progression), and the right frontal source moved antero-laterally (normalised).</p> <p>Conclusion</p> <p>From the start of the illness there were differences in the dipole-model between healthy and patient groups. Separate characteristics of the sources of the activity differences showed an improvement, stasis or deterioration with illness-duration. The precise nature of the changes in the sources of MMN activity and their relationship to selective information processing and storage depend on the specific psychopathology and heterogeneous course of the illness.</p