interval (7.5–12.5 sec, mean of 10 sec), another trial was initiated with the delivery of a non-contingent electrical stimulus. Failure to respond to the non-contingent stimulus within 7.5 sec resulted in the onset of the inter-trial interval. Responding during the inter-trial interval delayed the onset of the next trial by 12.5 sec. In each testing session, current levels were varied in alternating descending (x2) and ascending (x2) series in 5 μA steps. A set of five trials was presented for each current intensity. The threshold for each series is defined as the midpoint between two consecutive current intensities that yield “positive scores” (animals respond for at least three of the five trials) and two consecutive current intensities that yield “negative scores” (animals do not respond for three or more of the five trials). The overall threshold for the session is defined as the mean of the thresholds for the four individual series. Threshold data are presented as percent of baseline values due to inter-subject variability in baseline rates.
