The effects of fluoxetine and lorazepam on visual perceptual learning in healthy adults

Abstract

Purpose: The selective serotonin reuptake inhibitor (SSRI) fluoxetine has been shown to significantly enhance visual cortex plasticity in adult animals. This effect was blocked by administration of a benzodiazepine. The aim of this study was to assess whether fluoxetine would enhance visual perceptual learning in healthy adults and whether the benzodiazepine Lorazepam would reverse this effect.

Methods: 20 male participants were randomized to a 3 week course of fluoxetine (20mg per day) or placebo. Psychometric functions of performance on a motion direction discrimination task were measured for two different motion directions at baseline and at weekly intervals for 4 weeks after the first drug dose. Transcranial magnetic stimulation was also used to measure phosphene thresholds, an index of visual cortex excitability, at weekly intervals. During the final 5 days of drug administration, participants were trained extensively on the motion discrimination task at a fixed motion direction. Psychometric functions were re-measured the day after training and the following day 2 hours after a single dose of Lorazepam (0.0625 mg). A final set of measurements were made one week after training.

Results: There were no significant differences between the placebo and fluoxetine groups in the amount of perceptual learning, the rate of learning or the transfer of learning to the untrained motion direction. However fluoxetine did tend to reduce phosphene thresholds. Lorazepam significantly impaired task performance for the untrained motion direction but not the trained motion direction (F = 3.3, p = 0.04).

Conclusions: Fluoxetine did not enhance perceptual learning; however the fluoxetine group did show a tendency for increased visual cortex excitability. Learning due to transfer was vulnerable to Lorazepam whereas learning due to training was not. This suggests the existence of separate neural mechanisms for learning and transfer within perceptual learning paradigms.

Keywords: 503 drug toxicity/drug effects • 650 plasticity • 755 visual cortex