Shifts In Magnitude Of Reward Research Articles

Dissociating the effects of alternative early-life feeding schedules on the development of adult depression-like phenotypes

Early-life adversity is associated with increased vulnerability to depression in humans, and depression-like phenotypes in animals. However, different types of adverse experience may leave different signatures in adulthood. We experimentally manipulated the Amount of food delivered to European starling nestlings and the begging Effort required to obtain food during early development. Here, we report behavioural data in adulthood from a task that assessed sensitivity to shifts in reward magnitude characteristic of depression-like low mood. Birds that had experienced Hard Effort were more food motivated than birds that had experienced Easy Effort. Both Effort and Amount affected sensitivity to shifts in reward magnitude: Hard Effort birds showed an enhanced negative contrast effect following loss of reward (‘disappointment’), and Lean Amount birds failed to show a normal positive contrast effect following gain in reward (a lack of ‘elation’). Therefore, the feeding schedule experienced for just 10 days in early life caused enduring effects on feeding motivation and sensitivity to reward loss/gain consistent with human depression. Furthermore, the contrast effects were specific to different types of adversity. These results highlight the importance of early-life feeding schedules in the development of depression-like phenotypes.

Mesencephalic substrate of reward: lesion effects

Psychophysical studies suggest that reward-relevant neurons in the posterior mesencephalon (PM) form a caudal extension of the axonal pathway that mediates the rewarding effectiveness of electrical stimulation of the medial forebrain bundle. The present study sought to further characterize the reward-relevant functional link between these two regions by assessing changes to the rewarding effectiveness of caudal medial forebrain bundle stimulation (ventral tegmental area, VTA) subsequent to electrolytic lesions of different PM sites. A total of 13 rats were tested, 11 of these at bilateral VTA stimulation sites. Overall, rewarding effectiveness was reduced in five rats and pontentiated in four. The presence and magnitude of the effects were site-, current- and time-dependent, and ranged from 0.1 to 0.4 log 10 unit shifts in reward magnitude, with most effects falling below 0.3 log 10 units. Generally, these effects became apparent approximately two weeks after the lesion. In addition to these effects, PM lesions placed on or close to the midline also produced small transient reductions in rewarding effectiveness immediately after the lesion, an effect that disappeared within three days. Conversely, lateral PM lesions were associated either with no immediate effects or with small transient potentiations of reward. The finding that lesions of the PM placed on the midline, just off the midline or laterally all altered the rewarding effectiveness of VTA stimulation suggests that the reward-relevant circuitry is distributed diffusely throughout the PM.

Some effects of pimozide and of shifts in sucrose concentration on lick rate, duration, and interlick interval

Multiple dependent measures were employed to characterize the licking behavior of rats exposed to shifts in reward magnitude or injected with pimozide (PIM). Nondeprived rats licked either a 32% (n = 14) or 4% (n = 15) sucrose solution in daily 10 min sessions. Rats in the 32% condition were then down-shifted to either a 16% (n = 7) or 4% (n = 7) sucrose solution. Rats in the 4% condition were up-shifted to either 16% (n = 7) or 32% (n = 8) sucrose solution. The response profiles generated by those rats shifted to a lower reward magnitude were contrasted with either rats shifted from a 32% sucrose solution to a no-reward (plain tap water) condition, or with rats maintained on a 32% sucrose solution and administered the neuroleptic PIM (0.5 mg/kg or 1.0 mg/kg). Rats down-shifted from a 32% to 4% sucrose solution generated response profiles more similar to rats shifted to plain tap water than rats maintained on a 32% sucrose solution and administered PIM. These results suggested that PIM treatment is not functionally equivalent to either a shift to no-reward or to a shift to reduced reward conditions.

Age of first appearance of simultaneous and successive negative contrast in infant rats.

Simultaneous (SimNC) and successive negative contrast (SNC), two paradoxical effects that are related to shifts in reward magnitude, were studied in rats from 11 to 17 days of age. Experiments 1 and 2 looked at the emergence of SimNC at 11, 14, and 17 days. At all of these ages, rats were able to discriminate between the large (milk suckling) and small (dry suckling) reward odor cues as measured by attachment latencies, but only the 14- and 17-day-olds showed SimNC. At none of the three ages was a discrimination formed to the differential odor cues in the alley in terms of runway speeds. Experiment 3 was designed as a further test of the absence of SimNC at 11 days. In this experiment, the pups were placed directly on the dam's ventrum to facilitate attachment. As in the earlier experiment, the 11-day-old pups discriminated between the odors signaling the two reward conditions but did not show the SimNC effect. In Experiment 4, SNC was measured in pups 14 and 17 days of age. SNC was shown at 17 days but not at 14 days in the attachment latency measure and at neither age in the run measure. These four experiments extend our earlier findings that the paradoxical effects that emerge out of intermittent schedules of reward magnitude occur earlier than those associated with single abrupt shifts in reward magnitude.

Age of first appearance of simultaneous and successive negative contrast in infant rats.

Simultaneous (SimNC) and successive negative contrast (SNC), two paradoxical effects that are related to shifts in reward magnitude, were studied in rats from 11 to 17 days of age. Experiments 1 and 2 looked at the emergence of SimNC at 11, 14, and 17 days. At all of these ages, rats were able to discriminate between the large (milk suckling) and small (dry suckling) reward odor cues as measured by attachment latencies, but only the 14- and 17-day-olds showed SimNC. At none of the three ages was a discrimination formed to the differential odor cues in the alley in terms of runway speeds. Experiment 3 was designed as a further test of the absence of SimNC at 11 days. In this experiment, the pups were placed directly on the dam's ventrum to facilitate attachment. As in the earlier experiment, the 11-day-old pups discriminated between the odors signaling the two reward conditions but did not show the SimNC effect. In Experiment 4, SNC was measured in pups 14 and 17 days of age. SNC was shown at 17 days but not at 14 days in the attachment latency measure and at neither age in the run measure. These four experiments extend our earlier findings that the paradoxical effects that emerge out of intermittent schedules of reward magnitude occur earlier than those associated with single abrupt shifts in reward magnitude.

Multiple Shifts in Magnitude of Reward

Three groups of 5 rats were administered either large reward (10 pellets), small reward (2 pellets), or multiple shifts (Iarge-small-large-etc.) in an alleyway. The multiple-shift group received a total of 7 large and 6 small phases of reinforcement. Early in training the shifted group exhibited positive contrast effect to a shift to large reward and negative contrast effect to a shift to small reward. Later in training, the same group showed neither effect perhaps because experience with the shift provided a smaller discrepancy between the upshifts and downshifts in magnitude of reward.

Positive and negative successive contrast effects following multiple shifts in reward magnitude under high drive and immediate reinforcement

Multiple shifts in reward amount were given to rats in a straight runway. Both positive and negative contrast effects were observed. A positive contrast effect was observed, however, only if the shift from small to large reward occurred while the subject was evidencing a negative contrast effect. Implications for current conceptions of reinforcement contrast are discussed.

Positive Contrast following a Shift from Partial Punishment to Continuous Reinforcement

The primary emphasis in the area of incentive contrast has centered around the effects of shifts in reward magnitude. However, with increasing evidence of the broad nature of contrast phenomena, investigators have explored the effect under other experimental contingencies. One procedure of potential theoretical interest deals with shifts in percentage or frequency of reward. Lehr (1974) and Shanab, Birnbaum, and Cavallaro (1974) have recently found positive contrast following shifts from partial reinforcement to continuous reinforcement schedules. The purpose of the present study was to expand the scope of research on percentage shift to include an examination of preshift incentive level by introducing punishment as a partial determinant of incentive. Twenty-four albino rats, 100 days old, were initially given 40 trials of continuous reinforcement (10 .045-gm. pellets on each trial) in a straight, 1.2-m., wooden alley (4 trials/day). All subjects received a constant 20-sec. delay of reward throughout the experiment to hold down running speed and allow for the emergence of positive contrast. The alley was 10 cm wide, 13.75 cm high, and covered by a hinged Plexiglas top. During a subsequent 40-trial preshift-phase 3 conditions (8 subjects/group) experienced different reward schedules. As a control, continuous reinforced subjects received 10 .045-gm. Noyes pellets per trial throughout the experiment (4 trials/day). Subjects partially reinforced received 2 reinforced and 2 nonreinforced trials each day and subjects partially punished received 2 rewarded and 2 punished trials each day (punishment was superimposed on nonreinforcement). During this phase, the alley was modified such that bIack and white vertical stripes were placed over the grey interior and a grid floor was installed in the delay and goal sections of the alley. A Lehigh Valley, Model 1531 shock unit was employed to administer a brief (.5-sec. duration) electric shock at 2.5 mA. Following preshift training, in order to avoid the possible contaminating influence of conditioned fear, the alley was reconverted to grey interior and wooden floor and all subjects shifted to delayed continuous reinforcement (10 pellets on each trial). This postshift phase lasted 32 trials (4 trials/day). A 3 groups X 8 blocks of 4 trials repeated-measures analysis of variance was performed on total speeds during postshift. The finding of greatest interest was a significant interaction of groups X blocks (K,,I,, = 1.82, p < .05). Newman-Keuls post hoc procedure indicated that a significant positive contrast effect emerged on Blodc 4; the condition of partial punishment was superior to those of partial and continuous reinforcement, which did not differ (p < .05). Since parcial punishment would have been expected to have had a lower preshift-incentive level than partial reinforcement, the present experiment supporcs the position that in studies of percentage shift, preshift-incentive level determines the magnitude of contrast in postshift.

Sensitivity of sunfish (Lepomis gibbosus) to qualitative changes in reinforcement

Shifts in reward magnitude have often failed to produce rapid performance changes in the instrumental behavior (swimming speed) of fish. The present within-subjects experiment was conducted to determine if sunfish (Lepomis gibbosus) are sensitive to qualitative shifts in reinforcement. Shifts from pellet to mealworm (Tenebrio molitor) reinforcement produced large increases in straight-alley swimming speed, and a shift from mealworm to pellet reinforcement produced an equally large performance decrement. These results show that sunfish were sensitive to this qualitative shift in reinforcement. A shift-saliency hypothesis is advanced to account for the seemingly disparate reports in the literature.

Shifts in magnitude of reward with humans in the “straightaway”

Two experiments, one with adults and one with children, were conducted to explore further the phenomenon of incentive contrast effects in a situation more analogous to the instrumental conditioning studies with rats than had previously been employed with humans. Subjects (44 adults and 44 children) rolled a marble along an inclined “ straightaway ” and into a goal cup at the end. The adult college students earned points toward their course grade for the completion of each trial, while the children were rewarded with tokens which were exchanged for candy. Both adults and children showed improvement in performance as training progressed, but no effect for reward magnitude was obtained, nor did changes in performance occur with shifts in magnitude of reward.

Simultaneous shifts in reward magnitude and level of food deprivation

Rats were employed in a 2 by 2 factorial design varying deprivation level (11 vs 20 g of food per day) and reward magnitude (1 vs 10 pellets) in Phase 1. In Phase 2, all groups were run to 10 pellets under low deprivation. Both Phase 1 deprivation level and Phase 1 reward magnitude exerted a significant effect on running speed in Phase 2. More importantly, the interaction of these two variables was not significant. The effect of Phase 1 reward magnitude decreased over the rewarded trials of Phase 2, while the effect of Phase 1 deprivation level did not. However, in a subsequent extinction phase, the effects of Phase 1 deprivation level disappeared.

Changes in running speed after incentive shifts

Rats experienced shifts in reward magnitude after 24, 54, or 108 trials to the first reward magnitude. All Ss were run in a two-way runway situation with six trials every day. As days of preshift training increased, the number of trials before postshift performance shifts occurred also increased. Since Ss with extended training and only one directional response do not take a long time to shift response rate (Crespi, 1942), the present results were related to the two responses that were required.

Runway performance of rats following amygdaloid lesions

Amygdaloid lesions in rats produced an initial increase in response latencies during the acquisition of a runway response which disappeared with further training. This disruption may have resulted from a lesion-produced interference with normal attentional processes. The lesions also decreased responsivity to shifts in magnitude of reward and decreased response latencies during extinction trials. This finding appears to reflect response perseveration by the amygdaloid rats.

Shifts in magnitude of reward and contrast effects in instrumental and selective learning: a reinterpretation.

Shifts in magnitude of reward and contrast effects in instrumental and selective learning: a reinterpretation.

Resistance to extinction following shifts in reward magnitude

Three groups of 12 rats each were trained to run a straight alley for four trials per day for 12 days with reward being three food pellets, followed by 10 additional days of training (4 trials/day). During the latter period, Ss in Group 3 — 3 continued to receive three pellets as reward on each trial. For the remaining Ss three of the four daily training trials were also followed by three pellets reward, but for Group 3 — 9 one daily trial was reinforced with nine pellets while for Group 3 — 1 one daily trial was reinforced with a single pellet. During a final single day of 20 extinction trials, Ss in Group 3 — 1 performed at a higher level than Ss in either Group 3 — 9 or 3 — 3, while no significant difference in resistance to extinction was observed for the latter groups. These results were interpreted as consistent with the frustration theory of extinction and partial reinforcement.

Repeated shifts in reward magnitude: evidence in favor of an associational and absolute (noncontextual) interpretation.

Repeated shifts in reward magnitude: evidence in favor of an associational and absolute (noncontextual) interpretation.

The effects of shifts in magnitude of reward on runway performance in immature and adult rats

Three groups of immature rats and three groups of adult rats were each trained for 20 preshift trials with a different quantity of reward: .1, .5, or 2.5 gm of wet mash. On 25 post-shift trials, each group was rewarded with .5 gm. In preshift training, amount of reward significantly differentiated immature groups but not adult groups. On postshift trials, how ever, reward values significantly differentiated adult groups but not immature groups.

Effect of Noncontingent Exposure to Shifts in Reward Magnitude on Subsequent Instrumental Runway Performance

This experiment extends the paradigm of nonresponse acquisition and extinction studies to the case of shifts in reward magnitude. In each of two replications, rats were first trained to traverse a runway for a reward of either 1 or 8 food pellets. Subgroups then received a series of direct placements (DP) into the goalbox with either the same or the opposite reward magnitude to that received in acquisition. This was followed by another series of running trials on which all Ss now received the reward magnitude opposite to that received in acquisition. The results indicated that on the first post-DP trial, speeds were a function only of the reward magnitude received on DP trials. Over all test trials, however, a complex pattern emerged For Ss shifted from 1 to 8 pellets, DP with 8 pellets produced faster performance changes and permanently faster running than DP with 1 pellet. For Ss shifted from 8 to 1 pellet, one replication showed no difference between groups which had received 1- or 8-pellet DPs in the rate at which performance dropped, while in the other replication the group receiving 1-pellet DPs showed no performance decrease at all. The implications of these results for two-process theories of instrumental reward learning are discussed briefly.