Studies of the effects of drugs on equine performance require access to sensitive methods of measuring subtle behavioral changes. Variable interval (VI) reinforcement scheduling is a specific type of operant conditioning that is sensitive to drug effects even when overt clinical signs of the drug have diminished. In our VI studies, horses were conditioned to break a light beam with a head-bobbing movement and this behavior was reinforced with a reward of clean oats (approximately 30 mg/reinforcement). Initial training procedures included acclimatization to the behavioral equipment and fixed-ratio reinforcement scheduling. To establish baseline rates of behavior the horses were converted to a variable interval (60 seconds) reinforcement schedule and were kept on this schedule for the remainder of the experiments. Daily sessions lasted 30 minutes and responding rates remained remarkably stable even after long periods of inactivity. Responses and reinforcement were recorded and dispensed by use of an electromechanical relay system wired to an electric eye, an automatic feeder and a programming and recording system. Recently, responding rates from horses administered detomidine, hordenine, xylazine and saline were monitored and compared to responding rates in horses from previous studies using acepromazine, cocaine, methylphenidate, phenylbutazone and reserpine. Rates of behavior in these operant-conditioned horses were remarkably stable over time and readily detected subclinical pharmacological effects of drug administrations.
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