The first neuropsychological study of the S-enantiomer of ketamine, a noncompetitive NMDA antagonist, found that low and high subanaesthetic doses produced only nonsignificant impairment on most cognitive tasks in 12 healthy male volunteers. However, tasks involving divided and sustained attention, along with objective and subjective psychopathology scores, showed significant impairment that increased with dose. These results suggest that ketamine's effects on attention may model aspects of schizophrenia.
The NMDA neurotransmitter system's role in psychoses is not fully understood, but NMDA antagonists like phencyclidine and ketamine induce psychotic states similar to schizophrenia. Binocular depth inversion, a visual illusion impaired in psychotic states, was tested in 12 healthy male volunteers using stereoscopic pictures of natural and artificial objects. Two subanaesthetic doses of the psychotomimetic S-enantiomer of ketamine were compared with placebo. Despite dose-dependent subjective and objective psychopathology, no significant impairment of binocular depth perception occurred with (S)-ketamine. The findings suggest implications for memory, perception formation, and bottom-up processing in ketamine model psychosis and schizophrenia.