Chronotopographical analysis of the pattern onset visual evoked magnetic response (VEMR): implications for waveform peak identification.
The topography of the visual evoked magnetic response (VEMR) to a pattern onset stimulus was studied in five normal subjects using a single channel BTi magnetometer. Topographic distributions were analysed at regular intervals following stimulus onset (chronotopography). Two distinct field distributions were observed with half field stimulation: (1) activity corresponding to the C11 m which remains stable for an average of 34 msec and (2) activity corresponding to the C111 m which remains stable for about 50 msec. However, the full field topography of the largest peak within the first 130 msec does not have a predictable latency or topography in different subjects. The data suggest that the appearance of this peak is dependent on the amplitude, latency and duration of the half field C11 m peaks and the efficiency of half field summation. Hence, topographic mapping is essential to correctly identify the C11 m peak in a full field response as waveform morphology, peak latency and polarity are not reliable indicators.
Degg
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Department of Vision Sciences, Aston University, Birmingham, England.
Slaven
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A
Armstrong
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RA
D000328:Adult; D001931:Brain Mapping; D005074:Evoked Potentials, Visual / Q000502:physiology*; D005260:Female; D006801:Humans; D008280:Magnetics; D008297:Male; D010364:Pattern Recognition, Visual / Q000502:physiology*; D013997:Time Factors