ABSTRACT
BACKGROUND: Recreational ketamine use has been on the rise worldwide. Previous studies have demonstrated that it disrupts various memory systems, but few studies have examined how it affects learning and frontal functioning. The present study investigates the effects of repeated ketamine self-administration on frontal fluency, attention, learning, and memory along the verbal/nonverbal axis. METHODS: Twenty-five ketamine users and 30 healthy controls took a battery of neuropsychological tests. Frontal fluency was measured by the Verbal Fluency Test for semantic organization ability and the Figural Fluency Test for nonverbal executive functioning. Learning and memory were measured with the Chinese Auditory-Verbal Learning Test for acquisition and retention abilities of verbal information, as well as with the Continuous Visual Memory Test for nonverbal information. Participants also took several tests tapping subdomains of attention. To test for the potential effects of other drug use, 10 polydrug controls were included for comparison with the ketamine users and healthy controls. RESULTS: Ketamine users had impaired verbal fluency, cognitive processing speed, and verbal learning. Verbal learning impairment was strongly correlated with estimated lifetime ketamine use. Ketamine users showed no impairments in figural fluency, sustained attention, selective attention, visual learning, or verbal/nonverbal memory. However, heavier lifetime ketamine use was significantly correlated with deficits in verbal memory (both immediate recall and delayed recall) and visual recognition memory. Deficits in cognitive processing speed and verbal learning persisted even after polydrug controls were included in the control group, but their inclusion did make the impairment in verbal fluency barely reach statistical significance. CONCLUSIONS: This study suggests that repeated ketamine use causes differential impairment to multiple domains of frontal and medial temporal functioning, possibly specific to verbal information processing.
