Morphological measurement of the SEP using a dynamic time warping algorithm

Warp-averaging event-related potentials

OBJECTIVE

To align the repeated single trials of the event-related potential (ERP) in order to get an improved estimate of the ERP.

METHODS

A new implementation of the dynamic time warping is applied to compute a warp-average of the single trials. The trilinear modeling method is applied to filter the single trials prior to alignment. Alignment is based on normalized signals and their estimated derivatives. These features reduce the misalignment due to aligning the random alpha waves, explaining amplitude differences in latency differences, or the seemingly small amplitudes of some components.

RESULTS

Simulations and applications to visually evoked potentials show significant improvement over some commonly used methods.

CONCLUSIONS

The new implementation of the dynamic time warping can be used to align the major components (P1, N1, P2, N2, P3) of the repeated single trials. The average of the aligned single trials is an improved estimate of the ERP. This could lead to more accurate results in subsequent analysis.

Evaluation of brain-stem auditory evoked potentials using dynamic time warping

Dynamic time warping is a procedure whereby portions of a temporal sequence of values are stretched or shrunk to make it similar to another sequence. This procedure can be used to align the brain-stem auditory evoked potentials recorded from different subjects prior to averaging. The resultant warp-average more closely resembles the wave form of a typical subject than the conventional average. Dynamic time warping can also be used to compare one brain-stem auditory evoked potential to another. This comparison can show the differences that result from changes in a stimulus parameter such as intensity or repetition rate. When a patient's wave form is compared to a normal template, warping can identify the peaks in the patient's wave form that correspond most closely to the peaks in the normal template. Compared to an experienced human interpreter, warping is very accurate in identifying the waves of normal brain-stem auditory evoked potentials (error rate between 0 and 4%) and reasonably accurate in identifying the peaks in abnormal wave forms (error rate between 3 and 18%).

Comparison of magnetic resonance imaging with somatosensory testing in MS suspects

Two hundred patients suspected of having multiple sclerosis (MS), including 42 with progressive myelopathy and 11 with optic neuritis, were investigated with somatosensory evoked potentials (SEPs) and magnetic resonance imaging (MRI). Most had minimal neurological deficit, the mean Kurtzke scale being 2.65. There were 117 patients had two or more MRI lesions suggestive of MS, with a total of 527 lesions identified; 290 (55%) involved the somatosensory pathways, most commonly lying in the mid-periventricular region (thalamo-cortical radiations). There was good correlation between positive and negative MRIs and SEPs. The MRI was abnormal more frequently than the SEP, except in progressive myelopathy when both were abnormal with equal frequency. It is proposed that some cases of myelopathy in MS may be due to periventricular rather than spinal pathology. The morphology of the MRI lesion would favor conduction block not slowing of the SEP as being the prime abnormality. This appeared to be true of leg SEPs.