Component-specific effects of physostigmine on the cat visual evoked potential.

Pattern visual evoked potentials (VEPs) were recorded from the pial surface of the cat primary visual cortex prior to and following the intravenous administration of physostigmine, an agent which blocks the enzyme responsible for the breakdown of synaptically released acetylcholine. The control VEP was composed of a small initial positive deflection (P1), a subsequent large negative wave (N1) and a second large positive wave (P2). Following physostigmine, the amplitude of P1-N1 was diminished whereas that of N1-P2 increased. These effects were long lasting and were blocked by prior treatment with scopolamine, a result consistent with mediation by a muscarinic cholinergic pathway. Waveform subtraction revealed that the physostigmine-sensitive component had a slow, negative polarity waveform while the physostigmine-insensitive component was also slow, but positive in polarity. The fundamental nature of these components remains to be assessed. Nevertheless, the results indicate that waveforms of different polarity combine algebraically to yield the conventional VEP.