The effects of the anticonvulsant drugs clonazepam and ethosuximide were examined in pigeons performing under a fixed-consecutive-number schedule with and without an added external discriminative stimulus. Under these schedules, food was delivered whenever subjects responded between and 8 and 12 times on one response key (work key), and then responded once on a second response key (reinforcement key). For one group, an external discriminative stimulus signalled completion of the response requirement on the work key, while no stimulus change was programmed for the other group. Clonazepam (0.06-0.75 mg/kg) produced dose-dependent decreases in percentage of reinforced runs and rate of responding for both groups. The magnitude of the accuracy-decreasing effect was generally greater in the group without the external discriminative stimulus. For this group, the higher doses of clonazepam produced pronounced increases in switching to the reinforcement key before completing the minimum requirement of eight consecutive responses on the work key. No consistent patterns of errors were evident for the subjects with the added external discriminative stimulus. Although ethosuximide (20-160 mg/kg) produced dose-dependent decreases in rate of responding, it had little effect on the percentage of reinforced runs or the run length distributions. These findings are consistent with previous reports indicating that clonazepam, but not ethosuximide, substantially disrupts performance under operant tasks requiring conditional discriminations. These data also suggest that the addition of an external discrimination stimulus attenuates the disruptive behavioral effects of clonazepam.

The effects of clonazepam (0.25-4.0 mg/kg) and phenobarbital (10-80 mg/kg) were examined in pigeons responding under a multiple fixed-ratio 50 fixed-interval 90-sec schedule offood delivery. When given acutely 30 min prior to testing, the lowest dose of each drug generally increased responding under both components of the multiple schedule, whereas high doses decreased responding. Time-course determinations revealed maximum rate-reducing effects when phenobarbital was given 30 min prior to testing; 15-and 30-min presession intervals were similarly effective with clonazepam. When a 120-min presession interval was used, responding generally exceeded control rates for clonazepam, but not for phenobarbital.If one accepts a prevalence of 0 .3 % to 0.6 % (Hauser, 1978), roughly 750,000 to 1,500,00 individuals suffer from epilepsy in the United States alone. Their treatment, in all likelihood, will involve prolonged exposure to one or more of the 17 antiepilepsy medications currently marketed (Swinyard, 1982). Although anticonvulsants are widely and effectively used in the treatment of epilepsy in its various forms, the behavioral side effects of such medications are poorly understood (Gibbs et al., 1982). Clinical investigations have not revealed the precise behavioral effects of such drugs, but they have provided evidence that anticonvulsants can deleteriously affect behavior. Recent research with nonhumans (summarized by Poling & Picker, in press) has compared the effects of antiepilepsy medications under a number of operant conditioning procedures. Findings provide a crude profile of the behavioral actions of several anticonvulsants, but detailed understanding of the behavioral actions of these agents necessitates further investigation. In an attempt to explore further the effects of anticonvulsants in nonhumans, the present study examined the effects of cIonazepam and phenobarbital on the responding of pigeons main- tained under a multiple fixed-ratio 50 fIxed-interval 90 sec (mult FR 50 FI 90-sec) schedule of food delivery. Although these drugs similarly increase errors in pigeons performing under repeated acquisition and delayedmatching-to-sample procedures (Picker & Poling, 1984; Picker, White, & Poling, 1985), their effects on schedulecontrolled responding have not been compared directly. Because cIonazepam is a benzodiazepine and phenobarbital a barbiturate, and because the clinical applications of the two drugs differ, determining the range of conditions under which they produce similar behavioral effects may be of some interest. METHOD Subjects and ApparatusSix experimentally-naive barren hen White Cameaux pigeons, maintained at 80% of free-feeding weights, served as subjects. The birds were individually housed with unlimited access to water and grit in a constantly illuminated room.Subjects were tested in three computer-controlled operant conditioning chambers, described in detail by Picker and Poling (1984). Each chamber contained a food hopper and three response keys; only the center key was operative i...