Investigating the role of serotonin in visual orientation processing using an ecstasy (MDMA)-based research model.
A growing body of evidence suggests that regular ecstasy (3,4-methylenedioxymethamphetamine) use causes lasting changes to central serotonergic functioning in humans, including in the occipital lobe. Serotonin may play a role in visual orientation processing, mediated in the occipital lobe, via lateral inhibition. The tilt aftereffect is an illusion apparent following adaptation to stimuli angled 5-50 degrees from vertical and thought to be affected by lateral inhibition between occipital neurons. A recent study identified an enhanced tilt aftereffect among ecstasy users, but only in a subset that were recently abstinent from amphetamines. The current study examined the effects of ecstasy use, cannabis use and their interacting effect on the magnitude of the tilt aftereffect among participants with no recent history of amphetamine consumption. Eleven ecstasy users, 15 cannabis users, 15 ecstasy plus cannabis users and 15 drug-naïve controls were compared on the magnitude of the tilt aftereffect elicited following adaptation to stimuli angled 15, 30, 40 or 60 degrees from vertical. At a 40 degrees adaptation condition, ecstasy users had a greater magnitude of the tilt aftereffect compared to those that had not taken the drug. Additionally, the extent of ecstasy use was positively associated with the magnitude of the tilt aftereffect generated following 15, 30 and 40 degrees adaptation conditions, but not at 60 degrees. Given that lateral inhibition mediates the tilt aftereffect following adaptation to 5-50 degrees , the findings of a relationship between ecstasy use and tilt magnitude at the 15-40 degrees but not 60 degrees adaptation conditions support a role for serotonin in visual orientation processing via lateral inhibition.