Parametric effects in the discrimination of intracranial stimulation: some methodological and analytical issues.

Rats were implanted with electrodes in the medial forebrain bundle at the level of the lateral hypothalamus. Electrical stimulation was established as a conditioned stimulus for responding in a modified two-way active shock avoidance procedure. Stimulus generalization experiments were conducted to determine the effects of pulse frequency, current intensity, pulse duration, and of train duration. It was found that the discriminative stimulus properties of the electrical stimulation co-varied in an orderly manner with variations in each of the parameters. The generalization data sets of frequency, intensity, and train duration could be fitted by linear functions in log-linear coordinates; the effects of pulse duration generated a biphasic gradient. An analysis in terms of charge revealed that train duration had the lowest slope, whereas frequency and intensity generated equally steep functions. The most striking differences occurred among conditions which were similar in terms of current intensity, pulse duration and number of pulses, but which differed in the temporal patterning of pulses. Some methodological and analytical aspects of discrimination and generalization with intracranial stimulation are discussed.