The effects of cocaine on speech sound discriminations was examined to determine whether cocaine's previously demonstrated effect in reducing speech sound discriminability was dependent upon either the type of stimuli employed (simple tones versus complex speech) or the procedure (stimulus detection versus stimulus discrimination). Because of demonstrated similarities in the way that baboons and humans discriminate speech, and in the way the CNS is thought to encode and process speech sounds in these two species, baboons were trained to perform a choice procedure to identify the occurrence of different synthetic vowel sounds (/a/, /æ/, /ɛ/, /U/, and /љ/). Animals held down a lever and released the lever only when one of four target vowels sounded, and not when a fifth, standard vowel sounded. Acute IM administration of cocaine (0.0032–1.0 mg/kg) produced dose-dependent decreases in vowel discriminability that were mostly due to elevations in false alarms (i.e., releases to the standard vowel) following cocaine. Cocaine also shortened reaction times to the stimuli in two of three baboons, but to a much lesser extent than observed previously. These results suggest that cocaine may interfere with the ability of the CNS to process the acoustic cues in speech sounds, and that the effects of cocaine on reaction times may depend upon the complexity of the reaction time procedure employed.

Three adult baboons were trained using a psychophysical procedure to discriminate between different synthetic vowel sounds (/a/, /æ/, /ɛ/, /U/, and /⊃/). Baboons pressed and held a lever down to produce a pulsed train of a single reference vowel that served as the standard stimulus. Animals were trained to release the lever only when this standard vowel sound changed to one of the four remaining comparison vowels. A lever release within 1.5 s of this change in vowel sounds was defined as a correct detection of the change from the standard vowel to one of the comparison vowels, and was reinforced. All baboons readily learned the vowel discriminations and detected vowel changes at the 90–100% correct performance level. Acute IM administration of cocaine prior to test sessions (0.00032–3.2 mg/kg) produced dose-dependent decrements in vowel discriminability. At the same time, cocaine shortened lever release latencies (reaction times) to the vowel stimuli in two of three baboons. The cocaine-induced decrements in vowel discriminability were correlated with the degree to which frequency differences occurred among the different vowels in that lower vowel discriminability scores were found for those vowels with smaller spectral differences from the standard vowel. Further, false alarm rates were not systematically affected by cocaine, indicating that the cocaine-induced decrements in vowel discrimination accuracy occurred in the absence of systematic changes in the reliability of the baboons' discrimination performances.