Three male baboons were trained to respond in an auditory psychophysical procedure to obtain reliable estimates of their auditory thresholds. Subsequently, animals were implanted with femoral catheters and retested at near-threshold levels of a 1.0-kHz tone. Beat-by-beat information was obtained on changes in interbeat interval (IBI) and both systolic and diastolic blood pressure (SBP and DBP) during tone presentations. IBIs decreased as stimulus intensity increased when animals correctly detected the stimuli, whereas no changes in IBI occurred when the animals did not detect the stimuli or when the response lever was removed in a probe session. No changes in SBP were observed, and one animal showed significant increases in DBP as stimulus intensity increased. Thresholds estimated from the overall functions relating changes in IBI to changes in stimulus intensity were approximately 7-9 dB higher than thresholds estimated from behavioral psychometric functions.
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