COMPONENT-SPECIFIC EFFECTS OF PHYSOSTIGMINE ON THE CAT VISUAL-EVOKED POTENTIAL

Pattern visual evoked potentials (VEPs) were recorded from the pial su rface of the cat primary visual cortex prior to and following the intr avenous administration of physostigmine, an agent which blocks the enz yme responsible for the breakdown of synaptically released acetylcholi ne. The control VEP was composed of a small initial positive deflectio n (P1), a subsequent large negative wave (Nl) and a second large posit ive wave (P2). Following physostigmine, the amplitude of P1-N1 was dim inished whereas that of N1-P2 increased. These effects were long lasti ng and were blocked by prior treatment with scopolamine, a result cons istent with mediation by a muscarinic cholinergic pathway. Waveform su btraction revealed that the physostigmine-sensitive component had a sl ow, negative polarity waveform while the physostigmine-insensitive com ponent 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 t o yield the conventional VEP.