A Comparison of Two Methods Designed to Rapidly Estimate Thresholds of Rewarding Brain Stimulation
Four experiments evaluated the relative merits of two procedures for rapidly estimating thresholds of rewarding brain stimulation. In the first method termed autotitration, rats earned brain stimulation by pressing a lever, and the stimulation became progressively weaker in pulse frequency with continued responding. The depression of a second Lever restored the stimulation to its original strength. In the second protocol or timed method, rats worked through the same descending sequence of frequencies and eventually quit responding but, here, they had no control over resets of stimulation; resets were automatically triggered at timed intervals by the equipment. Experiment 1 showed that autotitration consistently produced higher reset thresholds than the threshold estimates of the timed method. The second experiment showed that autotitration’s reset thresholds climbed when the stimulation sequence began at twice the customary frequency but that thresholds derived from the timed method remained at control levels with this double-frequency test. Reset thresholds from autotitration were found in Experiment 3 to be strongly influenced by changes in only the first frequency of the sequence, but this manipulation had no effect on timed-method estimates. Finally, the last experiment illustrated how autotitration might miss or seriously underestimate the effect of a pharmacological treatment. Together, these data suggest that resets made in autotitration are influenced by the value of the reinforcer made available immediately after the reset but that the alternative, timed method, is free of this problem.
KeywordsBrain Stimulation Pulse Frequency Threshold Estimate Initial Frequency Msec Pulse
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