The effect of background illumination on pattern onset visual evoked potentials.

The early part (first 200 msec) of pattern onset VEPs elicited by a dartboard pattern was studied in conditions of varying level of background illumination. The effect of pattern adaptation and pattern blurr was also studied. The observed complex behaviour of the main negativity within this part of the VEP can be best described in terms of a composite of two independent negative peaks labelled N100 and N130. In high luminance conditions peak N100 was dominant and the presence of N130 was indicated only by a 'notch' on the rising slope of the negativity. As luminance decreased the situation was reversed and N130 became a dominant feature of the negative wave. This finding did not depend on the particular choice of reference site. For checkerboard stimulation the same features were present, but variability of the VEP wave form was greater than in the case of dartboard stimulation. Present results relate the well-known pattern specific properties of the negativity in onset VEPs to N100 only, whereas N130 is not pattern specific. Lower and upper half-field stimulation produced peaks of opposite polarity at 100 msec but no change was observed in polarity of N130. These findings support the suggestion that these two parts of the negativity in pattern onset VEPs may have different cortical sources.