Acquisition and Extinction of the Classically Conditioned Eyelid Response in the Albino Rabbit

Comparisons of the performance curve of a classical conditioning group with the curves of control groups provided unequivocal evidence that elicitation of eyelid responses to the conditioned stimulus was acquired by associations formed between the conditioned stimulus and the unconditioned stimulus. The eyelid reflex elicited by stimulation of the cornea is a prompt, stable response which does not appear to show qualitative variations in different mammals (1). Nevertheless, the literaSCIENCE, VOL. 136 o n M ay 1 9, 2 00 8 w w w .s ci en ce m ag .o rg D ow nl oa de d fr om ture on eyelid conditioning contains few references to infrahuman species (2) and none on the rabbit. Yet the albino rabbit, because of its docility, large eyelid, and low spontaneous blink rate appears to be particularly suited for classical studies of eyelid conditioning. The conditioning apparatus we employed has been described in detail (3), but it was modified for the present study to accommodate rabbits. In the experimental sessions the rabbit was restrained within a Plexiglas box 9 inches long, 4 inches wide, and 8 inches high. Gross head movements were reduced by placing the rabbit's head through an adjustable stock comprising the front of the box and by placing restraining straps above the snout and across the head. Positioned 9 inches in front of the rabbit was a 6-inch speaker and a rod supporting an air jet and spring-return potentiometer. A silk thread was attached to a rod mechanically coupled at 90 degrees to the shaft of the potentiometer; and a Velcro tab connected to the other end of the silk thread was attached to the rabbit's right upper eyelid by meshing with a permanently set Velcro countertab glued to the upper lid. The signal from the potentiometer, generated by movement of the eyelid, was amplified and then recorded by an inkwriting penmotor. Electronic timers controlled the duration of the conditioned stimulus (600 msec) and unconditioned stimulus (100 msec), and the interval between these stimuli (500 msec). The conditioned stimulus (CS) was an 800-cy/sec tone of 72 db SPL, and the unconditioned stimulus (UCS) was an 80-mm puff of compressed nitrogen delivered to the dorsal region of the right cornea. To insure continual exposure of the cornea, the rabbit's right lower eyelid was taped open. Forty albino rabbits, 85 to 90 days of age, were assigned to one of four groups for 8 days of acquisition training and 3 days of experimental extinction. In acquisition, one control group received the CS alone (group CS-A), another received the UCS alone (group UCS-A), and a third control group (group R) received random presentations of the CS alone (CS-R) and the UCS alone (UCS-R). The fourth group (group CS-UCS) was the classical conditioning group and received paired presentations of the CS and UCS at a CS-UCS interval of 500 msec. Groups CS-UCS, CS-A, and UCS-A consisted of 12, 10, and 10 rabbits, respectively, 18 MAY 1962 and received 82 acquisition trials per day at randomized intertrial intervals of 15, 20, and 25 seconds (mean 20 seconds). Group R consisted of eight rabbits and received 82 CS alone and 82 UCS alone trials. The random presentation of CS alone and UCS alone trials was restricted within two trial blocks, and the intertrial intervals were randomized at 5, 10, and 15 seconds (mean I'DOeconds). In extinction, all groups received 82 CS alone trials per day at randomized intertrial intervals of 15, 20, and 25 seconds (average 20 seconds). In acquisition all eyelid closures of at' least 1-mm deflection from the baseline were recorded from 0 to 525 msec after initiation of the trial. In extinction the interval was extended to 600 msec. Figure I presents the results of plotting percentage of responses for group CS-UCS in acquisition and extinction. Multiple t-test comparisons of mean percentage of responses among the control groups revealed no significant differences in acquisition or extinction; and their performance curves are presented collectively in Fig. I by hash lines indicating the upper boundary of percentage of responses. The percentage of acquisition responses among the control conditions did not increase over days, and never exceeded a 4.5 percent level of responding exhibited by the UCS-R condition on day 3. In contrast, group CS-UCS demonstrated a steady increase in per-