Diazepam and Δ-9-THC: contrasting effects on the discrimination of speech sounds in nonhuman primates

Three adult male baboons were trained on a psychophysical procedure to discriminate five synthetic, steady-state vowel sounds (/a/, /æ/, /ε/, /U/, and /⊃/) from one another. A pulsed train of one vowel comprised the reference stimulus during a session. Animals were trained to press a lever and release the lever only when this reference vowel sound changed to one of the comparison vowels. All animals learned the vowel discriminations rapidly and, once learned, performed the discriminations at the 95-100% correct level. The IM administration of diazepam (0.32, 1.0, 3.2, and 10.0 mg/kg) produced dose-dependent decrements in vowel discriminability. The diazepam-induced decrements in vowel discriminability were correlated with the degree of spectral frequency differences found among the different vowels, with lower vowel discriminability scores found for those vowels with smaller spectral differences from the reference vowel. In contrast, oral administration of Δ-9-THC (0.32, 1.0, 3.2, and 5.6 mg/kg) produced no decrements in vowel discriminability.