Influence of check and field size on the visual evoked magnetic response to a pattern shift stimulus

A. Slaven, C. Degg, Richard A. Armstrong

Research output: Unpublished contribution to conferencePosterpeer-review


A decrease in the check size of a pattern shift stimulus increases the latency and amplitude of the visual evoked potential (VEP) P100. In addition, for a given check size, decreasing the size of the stimulus field increases the latency and amplitude of the P100. These results imply that the central regions of the retina make a significant contribution to the generation of the electrical P100. However, the corresponding magnetic P100m may have a different origin. We have studied the effects of check and field size on the P100m in five normal subjects using a DC-Squid, second-order gradiometer. Magnetic responses were recorded at the positive maximum of the P100m over the occipital scalp to six check sizes (10-100') presented in a large (13 degrees 34') and small (5 degrees 14') field and to a large check (100') presented in seven field sizes (1 degree 45' - 15 degrees 10'). No responses were recorded to any check size with a small field. Decreasing the check size presented in a large field increased latency of the P100m by approx. 30 ms while the amplitude of the response decreased with the largest reduction occurring between 70' and 12' checks. Using a large check, latency increased and amplitude decreased as the field size was reduced. The latency changes in response to check and field size were similar to those described for the VEP although the magnitudes of the magnetic changes were greater. Unlike the VEP, amplitude responses were maximal when large checks were presented in a large stimulus field. This suggests that regions outside the central retina make a more significant contribution to the visual evoked magnetic response than they do to the VEP, and that the P100m may be useful clinically in the study of diseases that affect the more peripheral regions of the retina.
Original languageEnglish
Publication statusIn preparation - 1993
EventMeeting of the Society for Experimental Optometry - Birmingham (UK)
Duration: 26 Jul 199327 Jul 1993


ConferenceMeeting of the Society for Experimental Optometry
CityBirmingham (UK)

Bibliographical note

Papers presented at the meeting of The Society for Experimental Optometry, in Birmingham, UK, on 26 – 27 July 1993, Ophthal Physiol Opt 14(1), 0275-5408.


  • pattern shift stimulus
  • latency
  • amplitude
  • visual evoked potential
  • P100


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