Visual evoked electrical and magnetic response to half-field stimulation using pattern reversal stimulation

Abstract

The visual evoked magnetic response lo half-field stimulation using pattern reversal was studied using a d.c. SQUID coupled to a second ordergrudiometer. The main component of the magnetic response consisted of a positive wave at around 100 ms (PIOOM). At the time this component was present the response to hiilf-field stimulation consisted of an outgoing magnetic field contralateral and extending to the midline. When the left half field was stimulated the outgoing field was over the posterior right visual cortex and when the right half field was stimulated it was over the left anterior visual cortex. These findings would correctly identify a source located in the contralateral visual cortex. The orientation of the dipoles was not that previously assumed to explain the paradoxical lateralization of the visual evoked potential. The results are discussed in terms of both electrical and magnetic models of the calcarine fissure.Topographic distribution of the visual evoked electrical potential (VEP) to pattern reversal stimulation has been the subject of many studies. Halliday's group, restricting themselves to a simple stimulus consisting of 50 min black and white checks in a field of 0 16 degrees radius, carried out studies of both full-field and half-field responses. On full-field stimulation, they found that the N75-P100-N145 complex was always maximal at a position ^ 5 cm above the occiput, but that the amplitude gradient rapidly reduced in more temporal locations'. With half-field stimulation, however, the PIOO component of the VEP appeared to be maximal both at the midline and ipsilateral to the half field stimulated. The response showed a more lateral distribution than under full-field stimulation and was clearly present at the most lateral occipital electrodes. However, on the contralateral side of the head the response was of lower amplitude and a separate response of opposite polarity consisting of a positive at 75, a negative at 105 and a positive at 135 ms was obtained. This component was clearly maximal at the most lateral occipital electrode position and reduced in amplitude around the midline^. Blumhardt and Halliday showed that the full-field response is in fact the algebraic summation of the responses to each half iield\ The PIOO complex is of highest amplitude at the mid-occipital electrode simply because contralateral negativity is of lowest amplitude at this point whichever half field is stimulated. When both the half fields are stimulated the positivity is clearly maximal at the occiput since it is not cancelled by both contralateral negativities, whereas at the most occipital electrodes, the fall-off of Ihe PIOO component is marked because it is in these