Acquisition of bar-pressing under interval and ratio-schedules in the presence and absence of stimuli correlated with water delivery

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1 EUROPEAN JOURNAL OF BEHAVIOR ANALYSIS 2009, 10, NUMBER 1 (SUMMER 2009) 19 Acquisition of bar-pressing under interval and ratio-schedules in the presence and absence of stimuli correlated with water delivery Emilio Ribes-Iñesta, Jazmín Carvajal, Ulises Valdez, Carlos Torres, and Alfredo Mayoral University of Guadalajara After one-session shaping, eight rats were exposed to interval or ratio schedules in the absence and attenuation of the usual dispenser-based stimuli correlated with the delivery of water, and eight rats to normal delivery water conditions. Half of the rats were exposed to a (fixed-interval) FI 1-min schedule; the other half was exposed to a (fixed-ratio) FR 10 schedule of water reinforcement. Experimental phases consisted of two exposures to an FI or FR schedule interpolated with one (continuous reinforcement) CRF schedule, and a final (fixed-time) FT period. Rats did not show typical scallop-like or break-andrun patterns of responding. Rats responded more in the absence of reinforcement delivery correlated stimuli during the CRF interpolated phase than rats in the presence of such stimuli. Responding and obtained reinforcers were higher during the CRF schedule than under the FI and FR schedules. Responding during the FT period was similar to that in the prior FI or FR phase. The results are discussed in terms of the central role of transitions from CRF reinforcement in order to achieve high rates and characteristic patterns of FI and FR schedules. Keywords: Interval schedules; Ratio schedules; Rats; Continuous reinforcement; Reinforcementdelivery-correlated-stimuli Simple schedules of intermittent reinforcement originated in the analysis of the extinction ratio in conditioning (Skinner, 1938). Fixedinterval (FI) and fixed ratio (FR) schedules consisted in the manipulation of the extinction ratio through periodic delivery of reinforcement for a given response and in the independent manipulation of the time between successive reinforcers, respectively. Later on (Ferster & Skinner, 1957), intermittent schedules were systematized in terms of variations from the continuous reinforcement procedure (CRF). Fixed- and variable-interval schedules consisted in prescribing a non-functional time for the reinforcement of a single response, so that they could be conceived as CRF schedules with an Address correspondence to the first author at Centro de Estudios e Investigaciones en Comportamiento, Apartado Postal 5-374, Zapopan, México. ribes@cencar.udg.mx 19 unsignalled time-out requirement from the previous reinforcement. Fixed- and variable-ratio schedules could be seen as a direct transformation of the one-response requirement of the CRF schedule (Fixed-ratio 1) into a requirement of more than one response per reinforcement. The study of reinforcement schedules developed as the systematic analysis and combination of response-reinforcer requirements based upon two prescription rules: the passage of time (interval schedules), or the counting of responses (ratio schedules) prior to the response to be reinforced (Schoenfeld & Cole, 1972). Ferster and Skinner (1957) showed that interval and ratio performances followed different modes of transition and patterning as documented through cumulative records. Although parametric restrictions applied, fixedinterval performance was usually characterized

2 20 Emilio Ribes-Iñesta, Jazmín Carvajal, Ulises Valdez, Carlos Torres, and Alfredo Mayoral as a positively accelerated pattern of responding between reinforcers (a scallop-like pattern, in the case of longer intervals or during initial training) or as a post-reinforcer zero-response state followed by a high-rate state (in the case of shorter intervals or after extended training), while fixed-ratio performance consisted of a constant, high-rate pattern of responding preceded by a post-reinforcement pause (steplike pattern). The consistency of these findings suggested two different processes regulating scheduled-controlled behavior: temporal discrimination and counting behavior. Although other factors participate in both types of schedule-controlled performance, it has been widely accepted that their determination heavily relies on the discrimination of the time elapsed since the preceding reinforcement and of the number of responses since reinforcement, respectively (Crossman, Heaps, Nunes, & Alferink, 1974; DeCasper & Zeiler, 1977; Harzem, Lowe, & Spencer, 1978; Priddle-Higson, Lowe, & Harzem, 1976). However, reinforcement involves a different contingency in the case of interval and ratio schedules. If contingency consists in how the distribution of responses determines the distribution of reinforcers (Schoenfeld, Cole, Lang, & Mankoff, 1973), in ratio schedules the frequency of reinforcers is directly dependent on the frequency of responses, whereas in interval schedules the temporal restriction imposed by the schedule breaks this dependence and reinforcement distribution tends to be regular or constant, irrespectively of variations in response frequency (Dews, 1970). Ferster and Skinner (1957) stressed the importance of gradual and consistent transitions from CRF schedules to interval and ratio reinforcement in order to obtain stable patterns of performance typical of both types of schedules. Although shaping is used as a default procedure to condition an operant response (as bar-pressing or key-pecking), given the topography of responses normally selected and the size of the experimental chamber, rats and pigeons, if allowed, show operant levels higher than zero prior to any shaping or reinforcement procedure (Skinner, 1938; Ribes & López- Valadéz, 1979). The effect of the shaping (or auto-shaping) procedure is to increase the likelihood of the contact between a given response and the presentation of a stimulus consequence. The limited experimental literature on CRF schedules shows that, compared to intermittent interval and ratio reinforcement, continuous reinforcement produces less variation in several properties or dimensions of responding: topography, effort, duration, and location (Eckerman & Lanson, 1969; Margulies, 1961; Notterman & Mintz, 1965; Stebbins & Lanson, 1962). If simple intermittent schedules are seen as cases of local extinction parameters, response patterning typical of interval and ratio schedules would result from the sequence of local extinction parameters manipulated after stable performance under continuous reinforcement. The immediate exposure to ratio and interval schedules after shaping, without the consistent presentation of continuous reinforcement, should prevent that behavior could make contact with programmed contingencies of reinforcement. Millenson and Hurwitz (1961) found that after training under a CRF, extinction increased the dispersion of two IRT classes with longer runs of only one of the IRT classes. Boren (1955) and Mechner (1958) suggested that these changes in extinction after continuous reinforcement may constitute the mechanism through which fixed ratio patterning is developed, since runs in extinction centered along the fixed-ratio requirement. A similar mechanism may take place during fixedinterval conditioning. Reinforcement delivery involves other stimulus changes in the experimental chamber, namely, the sound of the mechanism providing the reinforcer at the time that the general light of the chamber is switched off and the water or food dispenser s light is switched on. Skinner (1938) pointed out that conditioning the lever-pressing response in rats actually consisted of a chain of reflexes ending with seizing the food. The sound of the food magazine and the propioceptive stimulation, correlated with lever pressing at the moment of the reinforcer delivery, were thought to be important intermediate variables affecting the acquisition and rate of the conditioned response. Skinner showed that secondary conditioning in acquisition

3 Acquisition Under Interval and Ratio-Schedules 21 could be obtained by following lever pressing with the sound of the food magazine previously correlated with food delivery. He also showed that similar extinction effects could be observed by eliminating the sound of the magazine and maintaining food deliveries, or by leaving the magazine empty but maintaining the correlated sound. Skinner explained these effects in terms of the discriminative and secondary reinforcing properties of the various stimuli in the chain of reflexes involved in pressing the lever for food. Present accounts of interval and ratio schedule-controlled performance are based on discriminatory or inhibitory processes that depend on the occurrence of the reinforcing stimulus after shaping and exposure to continuous reinforcement, and gradual steps in the transition from continuous to intermittent reinforcement (Ferster & Skinner, 1957; Carr & Reynolds, 1974; Harzem, Lowe, & Priddle-Higson, 1978; Staddon, 1970, 1974; Zeiler & Solano, 1982). However, there is no research about two important procedural variables related to acquisition of schedule-controlled behavior: 1) the effects of the immediate exposure to intermittent reinforcement after shaping, and 2) the effects of the absence or presence of the reinforcement delivery correlated stimuli (RDCS), such as the sound and light of the dispenser or turning the house light off. If prior continuous reinforcement and the stimuli correlated with the delivery of reinforcers are important for the acquisition and maintenance of schedulecontrolled behavior, then the performance of interval and ratio schedules should change in their absence. Hypotheses regarding time and count discriminations should be redirected toward the discriminative properties of stimuli correlated with the presentation of the reinforcer, and the role of continuous reinforcement as a necessary antecedent of response patterning under intermittent schedules. Typical ratio schedule performance should be disrupted in a larger degree than interval schedule performance because of the dependence relation that the former schedule prescribes between response and reinforcement frequencies. An experiment was done on bar pressing in rats, to evaluate the effects of omitting sustained continuous reinforcement before exposing the rats to non-signaled fixed interval or ratio schedules. To evaluate the effects of the sound and light correlated with the delivery of water on the acquisition and patterning of performance under these schedules, half of the rats were not exposed to them. CRF and Fixed-time (FT) reinforcement probes were included in order to test the sensitivity of behavior to reinforcement frequency and contingency, after direct exposure to intermittent interval and ratio schedules. Method Subjects Four groups of four male Wistar old albino rats each, six-months old (Groups 1 and 2), and twelve-months old (Groups 3 and 4) at the start of the experiment, were used. All of the rats were experimentally naive and maintained under a daily schedule of 22½ hrs water deprivation. After each experimental session, the rats had free access to water for 30 min. They also had free access to solid food (Purina Chow) in their home cages; the rats weights ranged between 380 and 400 gr. The rats were run once a day in four separate experimental chambers from 9:00 to 10:00, from 10:00 to 11:00 AM, and from 5 to 7:30 PM, six days per week for 70 sessions. Apparatus Four standard operant conditioning chambers for rats, two Coulbourn Instruments (E10-10TC) and two custom-made (30 x 25.5 x 32 cm), were housed in sound-attenuating cubicles. Each chamber had a water dispenser (E14-05) at the bottom center of the front panel, as well as one lever on the right side of the water dispenser, 26 mm from the right wall, and 25 mm from the grid floor. The lever required 0.24 N to operate. One 28-V bulb provided general illumination and was located in the upper section of the front panel above the water dispenser; a second 28-V bulb was located in the water dispenser itself. For Groups 1 and 2, the sound produced by the water dispenser was attenuated by slowing down the delivery mechanism. The attenuation of the sound by the dispenser did not delay the delivery of water.

4 22 Emilio Ribes-Iñesta, Jazmín Carvajal, Ulises Valdez, Carlos Torres, and Alfredo Mayoral Table 1. Description of the experimental design. Phase 1 30 Sessions Phase 2 5 sessions Phase 3 30 sessions Phase 4 5 sessions Groupn 1 W/O CS FI 1 CRF FI 1 FT 1 Group 2 W CS FI 1 CRF FI 1 FT 1 Group 3 W/O CS FR 10 CRF FR 10 FT Yoked Group 4 W CS FR 10 CRF FR 10 FT Yoked W/O CS: Without correlated stimuli W CS: With correlated stimuli The house light was on all the time and the light of the water dispenser was never lit. For Groups 3 and 4 the water dispenser mechanism operated normally, and the house light was out and the light of the water dispenser on when water was delivered. A reinforcer consisted of a 0.01 cm 3 drop of water available for 3 s in a small cup. All responses were recorded with their time of occurrence. An AT386-TURBO PC computing system with a DIO Paraport interface scheduled events and recorded responses. The program was written in PASCAL 7.0. Procedure The rats were trained to press the lever through a hand-shaping procedure. Training ended when each rat obtained 100 reinforcers in a single session. Rats in Groups 1 and 2 were shaped under the attenuated reinforcement delivery procedure, whereas rats in Groups 3 and 4 were shaped under the normally operating dispenser mechanism. Table 1 describes the experimental design in this study. Half of the rats (R1 to R4, and R9 to R12) were exposed to two experimental phases with fixed-interval schedules and two probe periods. The other half of the rats (R5 to R8, and R13 to R16) were exposed to fixed-ratio schedules and two probe periods. Experimental phases and probe periods consisted of a fixed number of sessions, irrespectively of any performance criterion. Experimental phases consisted of 30 sessions in order to evaluate possible effects of an extended exposure to the initial reinforcement schedule. Probe periods consisted of 5 sessions. Each session lasted 60 min. Rats R1 to R4 and rats R9 to R12 were exposed to the following sequence: a) fixed-interval (FI) 1-min schedule, b) CRF probe, c) FI 1-min schedule, and d) fixed-time (FT) 1-min probe. Rats R5 to R8 and rats 13 to 16 were exposed to the following sequence: a) fixed-ratio (FR) 10 schedule, b) CRF probe, c) FR 10 schedule, and d) yoked FT probe. For rats R5 to R8 and rats R13 to 16, the value of the yoked FT schedule used in the probe period was calculated individually so as to match the mean interval between reinforcers in the previous FR experimental phase. Probes were designed to evaluate the effects of reinforcing every response or a constant frequency of reinforcement delivery after the exposure to interval or ratio schedules. Rats R1 to R8 were exposed to the absence or attenuation of stimuli correlated with the delivery of reinforcement (RDCS), whereas R9 to R16 were exposed to the normal procedure including these stimuli. Results Figure 1 shows the median number and standard deviation of responses and water deliveries per session for the four groups of rats. During the first phase, rats in the four groups responded under the direct exposure to FI or FR schedules of water. Responding was higher in Groups 1 and 3 without RDCS, although median number of bar-pressing responses never exceeded 30 responses per session. It was also observed that responding was higher in the FI groups than in the FR groups under equivalent conditions (presence or absence of RCS). Me-

5 Acquisition Under Interval and Ratio-Schedules 23 MEDIAN NUMBER OF RESPONSES AND WATER DELIVERIES PER SESSION GROUP 1 (W/O CORRELATED STIMULI) FI CRF FI FT GROUP 3 (W/O CORRELATED STIMULI) FR CRF FR FT FI CRF FI FT Figure 1. Shows the median number of responses and standard deviations for each group of rats during experimental and probe phases. dian number of water deliveries per session was about 20 for the FI groups, whereas it was lower than 10 in the FR groups. During the CRF probe, responding increased in the four groups of rats, with larger number of responses for rats in the FI group without RDCS. Median number of responses per session was slightly higher EXPERIMENTAL CONDITION 0 GROUP 2 (W CORRELATED STIMULI) GROUP 4 (W CORRELATED STIMULI) FR CRF FR FT Responses Water Deliveries than 300 in Group 1, and about 100 in Group 4 that showed the extreme values in this period. Median number of water deliveries was always lower than the median number of responses, since rats used to continue pressing the bar during the water delivery period. In the second experimental phase with repeated exposure to the FI and FR schedules, median number of responses and water deliveries were similar to those in the first phase, excepting Group 1 which showed more than 100 responses and around 40 water deliveries per session. During the last FT probe, median number of responses and water deliveries tended to be slightly lower than in the previous FI or FR period. Standard deviations show that variation in the number of responses was larger during CRF and FR than during FI sessions. Figure 2 shows the same data for each individual rat. Figure 3 shows the cumulative records for one rat of each group during the experimental and probe periods. Visual inspection of cumulative records show that response patterning under Figure 2. Shows the median number of responses and standard deviations for each rat during experimental and probe phases.

6 24 Emilio Ribes-Iñesta, Jazmín Carvajal, Ulises Valdez, Carlos Torres, and Alfredo Mayoral FR and FI schedules in this experiment do not correspond to performances usually obtained after a sequential training involving shaping, continuous reinforcement and gradual transitions. FI performance did not show positive acceleration between reinforcers or two-state, short-ratio-like patterns, and FR performance did not show break-and-run patterns or changes Figure 3. Shows cumulative recordings for one rat of each group in each of the experimental and probe phases.

7 Acquisition Under Interval and Ratio-Schedules 25 in the PRP size correlated with reinforcement frequency. Discussion The results of this study show that, after exposure to a single shaping session, rats acquired lever pressing under FI and FR schedules, both in the presence and in the absence and attenuation of the stimuli usually correlated with reinforcement delivery (RDCS). However, the performances developed under these conditions, as well as those under subsequent exposure to a CRF schedule probe, were not similar to those obtained under standard procedures. The latter procedures, not only involve the sound of the reinforcement delivery mechanism and the illumination of the dispenser at the time the house light is turned off, but also include gradual transitions in the intermittent delivery of reinforcers after several sessions of shaping and continuous reinforcement. As a general result, the performances observed in this study differed from the idealized scalloping and pause-and-run patterns described in textbooks (Reynolds, 1968), as well as from the two-state patterns observed after extended training (Cumming & Schoenfeld, 1958; Ferster & Skinner, 1957; Schneider, 1969; Shull & Brownstein, 1970; Branch & Gollub, 1974). All rats without RDCS (except R7 under a FR schedule) responded in the first session after shaping. However, in spite of some sessions without responses during the initial phase, responding was different under the FI and the FR schedules. Rats under the FI schedule performed with an overall range of responses in the first session, and then showed an increasing and relatively stable frequency of responses and obtained reinforcers during the remainder of the experimental phases. In contrast, rats under the FR schedule either showed a large number of responses in the first session (100 to 500) or emitted less than 10 responses. In the following experimental phases, responding under the ratio schedules was generally variable. Rats in the presence of RDCS showed lower frequencies of responding than rats in the absence of RDCS. Different performances were also observed under the FI and the FR schedule during the initial phase. Rats under the FI schedule showed a high number of responses in the first session (70 to 500), obtaining from 20 to 35 reinforcers. In the following experimental phase, responding never exceeded 200 responses per session and obtained reinforcers varied from 8 to 35. Rats under the FR schedule emitted from 55 to 200 responses in the first session, but showed less variability in the following sessions than the rats in the absence of RDCS, obtaining less than 10 reinforcers in most sessions. During probe sessions, CRF performance was similar to that previously reported by Hurwitz (1958). Responding tended to accelerate after the first reinforcements, with periods of no responding alternated with further high frequency of responses. In the presence of the CRF, rats showed a large number of responses (a small number of them non-reinforced because they occurred during reinforcer presentation). Responding under FT schedules was slightly reduced relative to the preceding phase in all rats (Lachter, 1971; Lattal & Maxey, 1971; Lattal, 1972). The responding during the CRF probe shows that low responding under the FI and FR schedules was due to the peculiar contingencies that arose as result of the initial contact with intermittent reinforcement. In fact, in spite of the large number of responses during the CRF probe, responding during the second exposure to the FI and FR schedules did not increase, suggesting the importance of transitions in the development of local contingencies in intermittent schedules. If intermittent reinforcement is seen as a local parameter of extinction, the responding observed during FI and FR schedules fit with Schoenfeld (1968) remarks, that in extinction is obtained what has been reinforced in conditioning. Local extinction and conditioning are confounded when acquisition is developed under intermittent reinforcement, without a previous stable period of continuous reinforcement. The large number of reinforcers obtained during CRF probe in the absence of RDCS suggests that the terminal within-session decrease in response frequency reported under interval schedules, cannot be attributed to a satiation

8 26 Emilio Ribes-Iñesta, Jazmín Carvajal, Ulises Valdez, Carlos Torres, and Alfredo Mayoral or habituation threshold, and it may be due to factors not yet identified (Carlton, 1961; Killeen, 1995; McSweeney & Roll, 1998; Reese & Hogenson, 1962). Most of the water available in each session was consumed. At the end of every session, water had to be refilled in each dispenser, showing that the rats consumed from 2 to 20 ml of water per session. The non-occurrence of the patterns usually associated with fixed-ratio and fixed-interval schedules may be an effect of the absence of exposure to continuous reinforcement and transitions prior to the exposure to interval or ratio schedules. Nevertheless, RDCS seemed to influence the total output of responses per session. Scallop-like, two-state, and break-and-run patterns have been considered as evidence of temporal discriminations and counting processes taking place in fixedinterval and fixed-ratio behavior, respectively. In the case of fixed-interval behavior, it has been assumed that the reinforcement occurrence plays the role of a discriminative or inhibitory stimulus regarding the next reinforcement occurrence. In the case of fixed-ratio behavior, the presentation of reinforcement produces a pause whose length is inversely related to the ratio requirement, and serves to discriminate the size of the required number of responses, like a response- counting marker. However, it seems that in our experiment, the presentation of the reinforcer with or without correlated stimuli was not sufficient to allow for the discrimination of the inter-reinforcement interval or of the number of responses required by the schedule. The importance of signaling the moment of reinforcement delivery has been shown by Reed (2003), who found that the presentation of a brief tone or a diffuse light correlated with the delivery of food reinforcement, enhanced the frequency of responding on FI schedules. However, the effect of signaling reinforcement delivery depended on the local rates correlated with the reinforcer presentation. Reed (1989) found that when three responses were required at the end of a variable interval, the signal increased response rate, whereas the same signal produced a decrease in rate when only one response was required at the end of the interval. These results suggest that reinforcement delivery correlated stimuli play different roles in FI and FR schedules. Since FI schedules impose restrictions to reinforcement and response distribution in time, reinforcement presentation may signal both eating behavior (reinforcement onset) and not responding (post-reinforcement pause) controlled by the absence of reinforcement in the immediate subsequent period (Schildkraut, 1982). On the contrary, CRF and FR schedules do not impose restrictions to reinforcement and response distribution. Thus, in CRF and FR schedules reinforcement presentation seems to interrupt ongoing operant behavior, at the time that induces additional responding after the consummatory response. This interpretation is partially supported by our finding that CRF reinforcement produced high response rates without reinforcement-correlated stimuli. Discriminating the time between reinforcers and the number of responses might be the outcome of a behaviorinterrupting effect of the stimuli accompanying reinforcement presentation (Guthrie, 1935, 1939; Mueller & Schoenfeld, 1954). Although rats in this experiment did not show the consistent or usual patterns found under fixed-interval and fixed-ratio schedules, responding was sensitive to the response-reinforcer contingency. Contrary to what could be expected from a single shaping session and the immediate exposition to the intermittent schedules, all rats except one (R7) responded on their first experimental session, and some of them with a large number of responses. Reinforcement presentation was effective without any stimulus correlated with its delivery. This finding is consistent with the effectiveness of the one-session shaping procedure and with the high rates developed under the CRF probes. The sensitivity to the reinforcement contingency is additionally confirmed by the fact that, in contrast to food reinforcement that consists in a pellet dropping down to a tray, water reinforcement is not an intruding event in the chamber, and the rat has to look for it inside the dispenser. The exposure to the CRF probe resulted in increases of response frequency, suggesting that responding was sensitive to the one-to-one dependency between responses and reinforcers and, therefore, to increments in the local density of reinforcers. On the other hand, the higher frequencies of responding obtained in the

9 Acquisition Under Interval and Ratio-Schedules 27 absence of RDCS suggest that stimuli correlated with reinforcement do not necessarily develop conditioned reinforcing functions (Wike, 1966; Hendry, 1969; Davison & Baum, 2006). The results of this study reminds of the need to conceive schedule performance as the outcome of complex sequences of transitions of response-reinforcer contingencies. The quantitative effects of reinforcers seem to be contrived by the differential patterning of behavior resulting of response-reinforcer episodes. Responses occur as part of molar segments of behavior including other non-measured responses (Schoenfeld & Farmer, 1970; Staddon & Simmelhag, 1971). Further parametric research is needed to understand the patterning and quantitative relations between identified operant responses and other responses as an outcome of transitions from continuous to intermittent reinforcement. References Boren, J.J. (1955). Response rate and resistance to extinction as functions of fixed ratio. Unpublished doctoral dissertation, Columbia University. Branch, M.N., & Gollub, L.R. (1974). A detailed analysis of the effects of d-amphetamine on behavior under fixed-interval schedules. Journal of the Experimental Analysis of Behavior, 21, Carlton, P.L. (1961). The interacting effects of deprivation and reinforcement schedule. Journal of Experimental Analysis of Behavior, 4, Carr, E.G., & Reynolds, G.S. (1974) Temporal inhibition: Effects of changes of reinforcement and rate of responding. Journal of the Experimental Analysis of Behavior, 22, Crossman, E.K., Heaps, R.S., Nunes, D.L., & Alferink, L.A. (1974). The effects of number of responses on pause length with temporal variables controlled. Journal of the Experimental Analysis of Behavior, 22, Cumming, W.W., & Schoenfeld, W.N. (1958) Behavior under extended exposure to a highvalued fixed-interval schedule. Journal of the Experimental Analysis of Behavior, 1, Davison, M., & Baum, W.M. (2006). Do conditional reinforcers count? Journal of the Experimental Analysis of Behavior, 86, DeCasper, A.J., & Zeiler, M.D. (1977). Time limits for completing fixed ratios. IV. Components of the ratio. Journal of the Experimental Analysis of Behavior, 27, Dews, P.B. (1970) The theory of fixed-interval responding. In W.N. Schoenfeld (Ed.), The theory of reinforcement schedules (pp ). New York: Appleton Century Crofts. Eckerman, D.A., & Larson, R.N. (1969). Variability of response location for pigeons responding under continuous reinforcement, intermittent reinforcement, and extinction. Journal of the Experimental Analysis of Behavior, 27, Ferster, C.B., & Skinner, B.F. (1957) Schedules of Reinforcement. New York: Appleton Century Crofts. Guthrie, E.R. (1935). The psychology of learning. New York: Harper & Row. Guthrie, E.R. (1939). The effect of outcome on learning. Psychological Review, 46, Harzem, P., Lowe, C.F., & Priddle-Higson, P.J. (1978) Inhibiting function of reinforcement: Magnitude effects on variable-interval schedules. Journal of the Experimental Analysis of Behavior, 30, Harzem, P., Lowe, C.F., & Spencer, P.T. (1978) Temporal control of behavior: Schedule interactions. Journal of the Experimental Analysis of Behavior, 30, Hendry, D.P.(1969). Conditioned reinforcement. Homewood, Ill: Dorsey Press. Hurwitz, H. M. B. (1958). A source of error in estimating the number of reinforcements in a lever pressing apparatus. Journal of the Experimental Analysis of Behavior, 1, Killeen, P.R. (1995). Economics, ecologics, and mechanics: The dynamics of responding under conditions of varying motivation. Journal of the Experimental Analysis of Behavior, 64, Lachter, G.D. (1971). Some temporal parameters of non-contingent reinforcement. Journal of the Experimental Analysis of Behavior, 16, Lattal, K.A., & Maxey, G.C. (1971). Some effects of response independent reinforcers in

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