Cardiac responsiveness and differential conditioning.

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Abstract
Discrimination difficulty was systematically varied in 2 experiments involving a visual discrimination problem. During 60 trials of differential conditioning with a shock US [unconditional stimulus], beat-to-beat pulse-rate changes, measured for selected trials, showed a characteristic biphasic pattern on nonshock trials-initial deceleration followed by acceleration. Shock-produced changes were similar although acceleration was more prominent. Magnitude of change was related to discrimination difficulty and did not depend on training procedure (converging discriminable stimuli vs. maintaining fixed level of difficulty throughout). Results replicate a previous study and generally conform to Liddell''s analysis of differential conditioning. Vigilance is proposed as the intermediary process linking discrimination difficulty to magnitude of pulse-rate change.

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