Depression of the cat cortical visual evoked potential by soman

The effects of intravenous administration of the anticholinesterase agent soman (pinacolyl methylphosphonofluoridate, 3-15 micrograms/kg) on the visual evoked potential (VEP) were examined in cats using phase-reversed sine wave grating stimuli of different spatial frequencies and contrasts. Doses of 5-7 micrograms/kg caused a depression of the VEP across all spatial frequencies in an abrupt, non-graded fashion. Studies in which contrast was varied showed that VEP depression resulted primarily from a decrease in the system gain rather than a change in the contrast sensitivity, and that response depression increased with increasing contrast. The dominant changes in gain revealed by these studies are consistent with a modulation of potassium conductance in the cell membrane which previous studies have shown to be dependent on a cholinergic mechanism.