We investigated a phase-related-external-suction (PRES) method of bidirectional carotid stimulation which, unlike other methods, is not readily discriminable for direction (excitation vs. inhibition). Thirty-two subjects were first given 128 6-s PRES trials (64 each excitatory and inhibitory) which were signaled by two tones of differing frequency. There followed a 20-trial discrimination phase where subjects' task was to identify excitatory and inhibitory PRES trials (randomly presented) in terms of the two tone signals. Physiological (HR) discrimination was bidirectional (deceleration and acceleration for excitatory and inhibitory PRES trials, respectively), reflex-like (no habituation), but asymmetrical in magnitude (larger decelerations than accelerations), and topography (e.g., presence of a short latency deceleration). Group psychological discrimination was absent, although two subjects had a 100% hit rate on the discrimination test. There were, however, no systematic HR changes associated with these two subjects. Finally, the small-magnitude (2-3 bpm) physiological HR reflex was markedly augmented by what appeared to be a psychological, attentional factor. Accordingly, while the results indicated a dissociation between physiological and psychological differentiation, there was also evidence of a psychological factor (attention) influencing a physiologically induced reflex.