Locating VEP equivalent dipoles in magnetic resonance images

Effects of flash mode and intensity on P2 component latency and amplitude

Alzheimer's disease (AD) groups manifest flash visual-evoked potential (VEP) P2 component delays compared to healthy control groups. However, using P2 latency to categorize individual patients and controls yields low accuracy. Additionally, several laboratories have failed to replicate the basic between group P2 latency findings. The sporadic failure to find the P2 delay and, when found, its failure to classify patients and controls accurately may reflect the use of non-optimal stimuli or recording sites.

OBJECTIVE

This was a parametric investigation of stimulation and recording methods in healthy college students.

METHOD

Using an extended recording montage of 64 electrodes, 10 stimulus conditions (5 flash intensities through open and closed eyes) were evaluated for their P2 effects.

RESULT

The optimal recording site (O2) yielded the most reliable latencies and amplitudes across a range of stimulus intensities. Flash intensity did not affect P2 latency or amplitude. Flashes delivered through closed eyelids produced a flash VEP but delivery through open eyes produced a pattern VEP lacking a flash P2 component.

CONCLUSION

This accounts for the failure of some laboratories using open eyes to replicate the P2 delay in AD groups.

SIGNIFICANCE

Optimal flash VEP conditions include closed eyes and recording from O2. Flash intensity is unimportant.

Noninvasive identification of human central sulcus: a comparison of gyral morphology, functional MRI, dipole localization, and direct cortical mapping

The locations of the human primary hand cortical somatosensory and motor areas were estimated using structural and functional MRI, scalp-recorded somatosensory-evoked potential dipole localization, expert judgments based on cortical anatomy, and direct cortical stimulation and recording studies. The within-subject reliability of localization (across 3 separate days) was studied for eight normal subjects. Intraoperative validation was obtained from five neurosurgical patients. The mean discrepancy between the different noninvasive functional imaging methods ranged from 6 to 26 mm. Quantitative comparison of the noninvasive methods with direct intraoperative stimulation and recording studies did not reveal a significant mean difference in accuracy. However, the expert judgments of the location of the sensory hand areas were significantly more variable (maximum error, 39 mm) than the dipole or functional MRI techniques. It is concluded that because expert judgments are less reliable for identifying the cortical hand area, consideration of the findings of noninvasive functional MRI and dipole localization studies is desirable for preoperative surgical planning.

High-resolution EEG: a new realistic geometry spline Laplacian estimation technique

BACKGROUND

A new realistic geometry (RG) spline Laplacian estimation technique has been developed for high-resolution EEG imaging.

METHODS

Estimation of the parameters associated with the spline Laplacian is formulated by seeking the general inverse of a transfer matrix. The number of spline parameters, which need to be determined through regularization, is reduced to one in the present approach, thus enabling easy implementation of the RG spline Laplacian estimator.

RESULTS

Computer simulation studies have been conducted to test the feasibility of the new approach in a 3-concentric-sphere head model. The new technique has also been applied to human visual evoked potential data with a RG head model.

CONCLUSIONS

The present numerical and experimental results demonstrate the feasibility of the new approach and indicate that the RG spline Laplacian can be estimated easily from the surface potentials and the scalp geometry.

Displaying electroencephalographic dipole sources on magnetic resonance images

A simple, inexpensive method of displaying electroencephalographic (EEG) dipole sources on magnetic resonance images (MRIs) is presented. It consists of measuring the head according to the 10-20 system but instead of placing electrodes, benzonatate capsules (Tessalon Perles) (100 mg) are affixed to the patient's scalp. MRI is obtained with the capsules in place. In addition to the routine images, thin-section (1.0-1.3-mm) scans in a three-dimensional volume are obtained and the coordinates for each electrode position ascertained. The capsules are then replaced by electrodes and a waking and sleep recording is performed with a digital EEG instrument. Phenomena of interest are then averaged and interfaced with a source analysis program. The three-dimensional electrode coordinates are placed in a file and used to establish the electrode cloud on the basis of which source analysis proceeds. The three-dimensional source locations are then superimposed on the MRIs. The method is useful in the workup of epilepsy patients, by relating focal epileptogenic activity to definable lesions, and it also allows more precise localization of normal EEG phenomena.