2. Experiment 1
Extinction and reversal learning data were collected under varying training conditions using two CSs. Conditioned responding was alternately acquired for one CS and extinguished for both CSs in blocks of multiple sessions. We propose an arousal-mediated learning model to account for conditioned response probabilities across all training conditions. Model parameters were estimated and possible variations of the model are considered. Finally, the model is extended to account for latencies—intervals between CS onset and conditioned response.
2.1. Methods
2.1.1. Subjects
Five male Sprague–Dawley rats originally were obtained from Harlan (Indianapolis, IN) and were maintained in a temperature- and humidity-controlled environment on a 12-h light/12-h dark cycle with lights on at 7:00 am. Rats weighed approximately 300 g and were maintained at approximately 80% of their adult weights (±20 g) by postsession feeding of rat chow. Prior to the present study, all rats participated in a study examining the effects of systemic injections of dopaminergic compounds on conditioned approach responses. Given the previous conditioning history, no additional preliminary training was necessary. All studies were carried out in accordance with the Guide for Care and Use of Laboratory Animals as adopted by the National Institutes of Health. University of Michigan's Committee on the Use and Care of Animals approved all experimental protocols.
2.1.2. Apparatus
Six Med Associates® (St. Albans, VT, USA) operant conditioning chambers were used. Each chamber was approximately 30 cm long, 24 cm wide, and 21 cm high, and housed in a sound-attenuating cubicle with a ventilation fan. A dipper (0.1-ml reservoir) that could deliver liquid food (Vanilla Ensure®) was centered on the front panel within an approximately 4.1 cm (h) × 3.5 cm (w) aperture with its bottom edge 2 cm above a grid floor. An LED recessed in the roof of the aperture could be turned on to illuminate the aperture and was used as the conditional stimuli. An infrared photobeam located immediately above the dipper receptacle recorded head entries into the aperture and were the primary dependent measure in the present study. Control of experimental events and data recording were conducted with Med Associates interfacing and programming. Sessions occurred 5 days per week at approximately the same time.
2.1.3. Procedures
All sessions consisted of 16 Pavlovian conditioned approach trials with two different visual conditional stimuli and were approximately 40 min in duration. During the first Acquisition condition for three rats, 8 trials per session consisted of illuminating the aperture with a steady light for 15 s prior to a 7-s presentation of the dipper (hereafter CS+ trials). Another 8 trials per session consisted of flashing the aperture light on and off every 0.1 s for 15 s prior to 7 s of no dipper presentation (hereafter CS− trials). The CS+ and CS− stimuli were reversed for the other two rats. Prior to all CS+ and CS− trials, a variable intertrial interval (ITI) was arranged with an average of 120 s (range 75 s to 165 s). Presentation of CS+ and CS− stimuli were chosen randomly throughout all sessions with the exception that each stimulus could only be presented at most twice in succession.