Cholinergic agents and the McCollough effect

The strength of the McCollough effect (ME), a pattern-contingent colour aft ereffect, has been shown to be inversely related to acetylcholine, being si gnificantly strengthened by (anticholinergic) scopolamine and weakened by ( cholinergic) physostigmine delivered before adapting to the ME stimuli. The purpose of the present study was (i) to establish whether the effect of pr eadaptation scopolamine is linearly dose-dependent and (ii) to investigate the effects of scopolamine and physostigmine delivered between adaptation a nd testing. In experiment i, ten healthy male volunteers who received place bo, or 0.6 mg, 1.2 mg, or 1.8 mg scopolamine before adapting to ME stimuli showed a significant linear dose-dependence over tests repeated from 10 to 70 min after adaptation. In experiment 2 twelve male volunteers adapted to ME stimuli and then received placebo, 1.2 mg oral scopolamine, or 0.75 mg s ubcutaneous physostigmine. On subsequent repeated testing, strength of the ME was increased by scopolamine and decreased by physostigmine relative to placebo. Both experiments were double-blind double-dummy repeated measures. These data support the view that the ME is a product of inhibitory mechani sms in the visual system rather than processes involved in associative lear ning.