Effects of locus of a brief stimulus in a second-order schedule with pigeons

JENNIFER J. RICHARDS 0 DEREK E. BLACKMAN 0 0 University College , Cardiff CFI IXL, Wales On the completion by pigeons of four equal fixed intervals on one key, a light on a second key signaled that one peck on that key would be followed by food. In condition A, a brief stimulus of a further color was produced on the first key by the pecks that ended the first three (but not the fourth) fixed intervals. In condition B, no brief stimuli occurred at the end of the first three fixed intervals (tandem schedule). In condition C, the unpaired brief stimulus was presented on the second key after the pecks on the first key that ended the first three fixed intervals. An ABACA reversal design was used. Postreinforcement pauses were longer in condition B (tandem) than in condition A, an effect similar to that reported in similar conventional one-key second-order schedules. Postreinforcement pauses in condition C, with the brief stimulus on the second key, were also longer than in condition A, with the brief stimulus on the first key, although similar pauses were observed after the brief stimuli in both conditions. The locus of the brief stimulus appears to affect the control it exerts over behavior in a second-order schedule. - stimulus seem therefore to be partly determined by its locus. The present study attempted to assess the importance of this variable in a within-subjects design. Rose and Fantino (1978) and Squires et al. (1975) reported the effects of two-key procedures similar to conventional second-order schedules of the form FR x (FI y) with brief stimuli either at the end of every component except the last (Snp) or at the end of all components (Sp), The completion of each fixed-interval component on one key led to a light's being presented on a second key. This stimulus occurred only for a brief period, and no pecks on the second key were required except when that key was illuminated at the end of the final fixed-interval component, when a peck was followed by food. Since the same stimulus was presented on the second key at the end of every component, it was inter mittently paired with food, so this schedule is reminiscent of a second-order schedule of the form FR x (FI y: Sp) (Stubbs, 1971). Squires et al. (1975) reported the tendency of pigeons to peck at the illuminated second key at the end of successive fixed interval components. This was taken as a measure of the pigeons' failure to discriminate their position in the sequence of stimulus presentations between successive food deliveries. Rose and Fantino (1978) compared this two-key procedure FR 4 (FI 60 sec) with a tandem schedule FR 4 (FI 60 sec) and with an unpaired brief stimulus schedule FR 4 (FI 60 sec: Snp). With all three schedules, a stimulus appeared on the second key at the completion of the final FI 60-sec component and a response was then required on this key for food. With the tandem schedule, the second key was not illuminated at the end of unreinforced fixed-interval components. In the unpaired condition, the second key was briefly illuminated at the end of unrein forced components, but in a color different from that with which the key was illuminated at the end of the final component. Again, subjects pecked the paired brief stimulus on the second key and postreinforcement pauses were shorter in this schedule than in the tandem schedule. However, the subjects did not peck the unpaired brief stimulus on the second key, and there appears, from Rose and Fantino's (1978) report, to be some evidence that postreinforcement pauses were longer in the unpaired stimulus condition than in either the tandem or the paired stimulus condition (see their Figure 4, p. 339, and Figure 5, p. 400). This possibility is difficult to evaluate because Rose and Fantino compared between schedules presented within experimental sessions, and postreinforcement pauses differed in one condition as a function of the schedule with which it was being compared (see their Figure 2, p. 397, and Figure 4, p. 399). The present experiment was designed to investigate further the effects of unpaired brief stimuli in a second-order schedule. A two-key procedure was employed similar to that reported by Rose and Fantino (1978) and Squires et al. (1975), in that the completion by a peck on one key of a final fixed interval component led to the illumination of a key on which a peck was then followed by food. A reversal design was used, in one condition of which the completion of unrein forced components by a peck on the first key was marked by the brief illumination (in a different color) of that key, as in conventional second-order schedules with un paired stimuli. The effects of this procedure were compared with a condition (tandem) in which no brief stimuli were presented at the end of unrein forced components and with a condition in which the brief stimulus was presented at the end of un reinforced components but on the second key, as in the Rose and Fantino study. Subjects Three adult male homing pigeons were maintained at 80% of their free-feeding weights, with water and grit constantly avail able in their home cages. All birds had previously been trained to eat from a food magazine, but were otherwise experimentally naive. Apparatus A standard two-key Lehigh Valley pigeon test chamber was used, with interior working dimensions of 34 x 34 x 34 em, To operate either key, a minimum force of .1 N was required. The noise level in the chamber was 64 dB. The chamber illu mination was provided by a houselight (2.8 W, 24 V dc). Eight seconds' access to mixed grain was used as the reinforcer. Electromechanical apparatus was used in conjunction with a Commodore PET microprocessor for programming the experiment and recording data. Procedure Each session began with the presentation of the houselight, schedule key light, and masking noise. The removal of these three stimuli marked the end of each session. The subjects were at least 21 h food deprived at the beginning of each session. Each session ended after the 25th reinforcer or after 50 min, whichever occurred sooner. The subjects were shaped to peck the key to the right of the food hopper, and they were then exposed to a continuous rein forcement schedule for two sessions. This key was illuminated with a red stimulus except during food delivery. The third training session started with the illumination of the left key with a white stimulus which was present throughout the session, including food deliveries. The first response on this white key produced the red stimulus on the right key. The pigeons had been trained to peck the right key for food in earlier sessions, and throughout the training and subsequent experiment it remained the only key on which a response might produce food. Although the left key remained illuminated during food presentation, the right key did not in this and all subsequent sessions in all phases. In this session, each subject pecked the right stimulus very soo (...truncated)