Topographic mapping and source localization of the pattern onset visual evoked magnetic response.

The topography of the visual evoked magnetic response (VEMR) to a pattern onset stimulus was investigated using 4 check sizes and 3 contrast levels. The pattern onset response consists of three early components within the first 200ms, CIm, CIIm and CIIIm. The CIIm is usually of high amplitude and is very consistent in latency within a subject. Half field (HF) stimuli produce their strongest response over the contralateral hemisphere; the RHF stimulus exhibiting a lower positivity (outgoing field) and an upper negativity (ingoing field), rotated towards the midline. LHF stimulation produced the opposite response, a lower negative and an upper positive. Larger check sizes produce a single area of ingoing and outgoing field while smaller checks produce on area of ingoing and outgoing field over each hemisphere. Latency did not appear to vary with change in contrast but amplitudes increased with increasing contrast. A more detailed topographic study incorporating source localisation procedures suggested a source for CIIm-4cm below the scalp, close to the midline with current flowing towards the lateral surface. Similar depth and position estimates but with opposite polarity were obtained for the pattern shift P100m previously. Hence, the P100m and the CIIm may originate in similar areas of visual cortex but reveal different aspects of visual processing.