Extrapolation of serial stimulus patterns by rats

This experiment determined if rats could extrapolate a familiar serial sequence of diminishing food quantities by accurately anticipating a novel quantity added to the end of the sequence. In 13 days of training, rats ran in a straight runway to obtain quantities of food pellets presented in sequential order. A strongly monotonic group received repetitions of a formally simple pattern of 14-7-3-1 pellets of food, while weakly monotonic and nonmonotonic groups received formally more complex 14-5-5-1 and 14-3-7-1 patterns, respectively. In subsequent transfer, a 0-pellet quantity was added to each pattern, thus extending pattern length to five elements. Results of the very first pattern repetition containing the added 0-pellet element indicated that rats in the strongly monotonic condition, but not in the others, anticipated the reduced quantity before actually experiencing it. This result supports a cognitive, rule-learning hypothesis for serial learning by rats.     

Overshadowing in matching-to-sample: Reduction in sample-stimulus control by differential sample behaviors

Separate groups of pigeons were trained to high levels of accuracy on 0-delay matching-to-sample with sample-response requirements that were either differential or nondifferential with respect to the sample stimuli. Differential subjects produced the comparisons by completing a differential-reinforcement-of-low-rates-of-responding 3-sec (DRL 3″) requirement during one sample and a fixed-ratio (FR 10) requirement during the other. Nondifferential subjects produced the comparisons by completing the same schedule requirement (either DRL 3″ or FR 10) for both samples. Following acquisition to criterion, the DRL and/or FR sample-response requirements were replaced by a nondifferential single-peck (CRF) requirement in order to assess the degree to which the samples had acquired control over choice in each group. This change disrupted performance in all subjects, but the disruption was greater for the differential birds, which generally performed at lower levels of accuracy and required more sessions of retraining to reach criterion levels of accuracy than the nondifferential birds. Follow-up experiments revealed that comparison choices by the differential birds were primarily controlled by their DRL vs. FR sample-specific behaviors. The relatively poor performance of the differential group during testing with CRF requirements suggests that the cue arising from the birds’ differential sample behaviorshad also overshadowed the sample stimuli for conditional control over choice. The unique, and rather unusual, aspect of this overshadowing effect is that it occurred in spite of the fact that the overshadowed cue (that provided by the samples) was necessary for producing the cue that resulted in overshadowing (the differential sample behaviors). This finding has potentially important implications for the differential outcomes effect in conditional discrimination learning and for attentional processes in compound-cue situations in general.     

Overshadowing and associability change: Examining the contribution of differential stimulus exposure

In two appetitive conditioning experiments with rats, we investigated the mechanisms responsible for demonstrations of the superior associability of overshadowed conditioned stimuli (CSs) relative to control CSs. In Experiment 1, we investigated whether previous demonstrations were a consequence of differences in the relationship between the CSs and the unconditioned stimulus (US) or of differences in the conditions of exposure to the CSs. Rats received trials with X, Y, and an AB compound, but no delivery of the US (X–, Y–, AB–). A subsequent AY–, AX+, BY + test discrimination revealed that the AY/BY component of the discrimination was solved more readily than the AY/AX component—suggesting the contribution of an exposure effect. In Experiment 2, we better equated the conditions of exposure between A and Y by using AB+, XY+, X– training in Stage 1. In Stage 2, instrumental responses were rewarded during an AY compound. A final test revealed that Y took better control of instrumental responding than did A. The results of these experiments are discussed in terms of classical and contemporary theories of learning and attention.     

Failure to block control by a relevant stimulus

Pigeons were trained to depress a treadle in the presence of a discriminative stimulus, either a tone or illumination of red houselights, in order to obtain access to grain or avoid electric shock. In avoidance training, the auditory discriminative stimulus yielded faster acquisition than did the visual one. In appetitive training, the visual discriminative stimulus yielded faster acquisition than the auditory one. Experiments 2 and 3 used these stimuli in Kamin’s (1969) blocking design. In Experiment 2, when the pigeons were trained to depress a treadle in the presence of tone to obtain grain and then red light was added as the redundant stimulus, the light acquired stimulus control over treadlepressing; blocking was not observed. In Experiment 3, when the pigeons were trained to depress a treadle in the presence of red light to avoid electric shock and then tone was added as the redundant stimulus, the tone acquired stimulus control over treadle-pressing. Again, blocking was not observed. The implications of these results for several models of stimulus control are discussed.     

Spatial and configural factors in compound stimulus processing by pigeons

Three matching-to-sample experiments examined whether spatial or configural factors determined how the element arrangement of compound sample stimuli influenced matching accuracy in pigeons. Seven types of compound stimuli were tested. The arrangement of color and line-orientation elements in these compounds varied in terms of the spatial separation between the elements, the degree of consistency in element spatial location, and the number of bounded areas containing the elements. Matching accuracy was examined upon initial exposure to the compounds, during asymptotic conditions of shared attention, and with variation of sample durations ranging from .04 to 5.935 sec. In all three experiments, when spatial proximity, locational certainty, and the number of lines were precisely controlled or equated, no evidence for the proposed configural processing of “unified” compounds was found (Lamb & Riley, 1981). Element spatial separation, and to a lesser degree perceptual limitations, determined compound performance. These results question our lab’s previous evidence for configural compound processing by pigeons (Lamb, 1988; Lamb & Riley, 1981). They suggest instead that pigeons independently and separately process the individual elements of color/line-orientation compounds, with element separation determining the distribution of processing between the elements.     

Overshadowing of geometric cues by a beacon in a spatial navigation task

In three experiments, we examined whether overshadowing of geometric cues by a discrete landmark (beacon) is due to the relative saliences of the cues. Using a virtual water maze task, human participants were required to locate a platform marked by a beacon in a distinctively shaped pool. In Experiment 1, the beacon overshadowed geometric cues in a trapezium, but not in an isosceles triangle. The longer escape latencies during acquisition in the trapezium control group with no beacon suggest that the geometric cues in the trapezium were less salient than those in the triangle. In Experiment 2, we evaluated whether generalization decrement, caused by the removal of the beacon at test, could account for overshadowing. An additional beacon was placed in an alternative corner. For the control groups, the beacons were identical; for the overshadow groups, they were visually unique. Overshadowing was again found in the trapezium. In Experiment 3, we tested whether the absence of overshadowing in the triangle was due to the geometric cues being more salient than the beacon. Following training, the beacon was relocated to a different corner. Participants approached the beacon rather than the trained platform corner, suggesting that the beacon was more salient. These results suggest that associative processes do not fully explain cue competition in the spatial domain.     

Overshadowing not potentiation of illness-based contextual conditioning by a novel taste

In four experiments using rat subjects, we investigated the effects of presenting a novel flavor cue at the time of pairing an environmental context with illness. In each experiment, the subjects were allowed to spend time in a distinctive cage before receiving an injection of LiCl. For some, plain water was made available on these conditioning trials; for others, a novel taste (HCl) was presented. We measured the strength of the context aversion by assessing the ability of the contextual cues to block the acquisition of an aversion to sucrose in a further phase of training. We found that initial training with HCl present made the context less effective as a blocking cue and concluded that the HCl had overshadowed learning about the context. We suggest that this blocking procedure provides a more accurate assessment of contextual aversion than does the consumption test that has more usually been used and that taste-context overshadowing may be a more robust phenomenon than has thus far been thought.     

Recovery from conditioned suppression to a partially overlapping compound stimulus

In a conditioned suppression paradigm, a partially overlapping compound stimulus signaled occurrences of electric shock. That compound CS consisted of 3 min of continuous illumination of the houselight with three discrete 5-see presentations of a tone superimposed. A .5-sec electric shock was coterminous with each tone presentation. Rats that received this treatment in early conditioning sessions showed considerable recovery from conditioned suppression to the houselight as the experiment progressed. However, the effect was not reversible, and it could not be demonstrated in rats that experienced extensive prior conditioning to the houselight alone. These results are discussed in relation to a hypothesis concerning the modulation of behavioral control exerted by elements of compound stimuli and as they relate to a recent theoretical model for Pavlovian fear conditioning.     

Unraveling sources of stimulus control in a temporal discrimination task

In temporal discriminations tasks, more than one stimulus may function as a time marker. We studied two of them in a matching-to-sample task, the sample keylight and the houselight that signaled the intertrial interval (ITI). One group of pigeons learned a symmetrical matching-to-sample task with two samples (2 s or 18 s of a center keylight) and two comparisons (red and green side keys), whereas another group of pigeons learned an asymmetrical matching-to-sample task with three samples (2 s, 6 s, and 18 s) and two comparisons (red and green). In the asymmetrical task, 6-s and 18-s samples shared the same comparison. In a subsequent retention test, both groups showed a preference for the comparison associated with the longer samples, a result consistent with the hypothesis that pigeons based their choices on the duration elapsed since the offset of the houselight (i.e., sample duration + retention interval). Results from two no-sample tests further corroborated the importance of the ITI illumination as a time marker: When the ITI was illuminated, the proportion of choices correlated positively with the retention interval; when the ITI was darkened, choices fell to random levels. However, the absolute value of choice proportions suggested that the sample stimulus was also a time marker. How multiple stimuli acquire control over behavior and how they combine remains to be worked out.     

Behavioral control by stimulus components of an imprinting object

In the first of three experiments, ducklings that had received prolonged exposure to the visibly moving imprinting object subsequently suppressed ongoing distress vocalization both during brief presentations of the moving object and during brief presentations of its initially neutral stimulus components (i.e., its auditory and static visual features). Only presentations of the moving object were followed by priming aftereffects (namely, enhancement of distress vocalization over a baseline rate). In Experiment 2, weak, but reliable, priming effects were detected after very long presentations of the auditory and static visual features. Experiment 3 found that these features strongly suppressed low, but not high, rates of distress vocalization, while the visibly moving object strongly suppressed both high and low rates. These studies suggest that initially neutral features of an imprinting object acquire the same sort of behavioral control as is exerted by the object when it is in motion, but that this control is somewhat weaker.     

Inhibitory control by one stimulus after an increase in the frequency of reinforcement associated with another stimulus

Pigeons were trained on a multiple variable-interval 5-min variable-interval 5-min schedule and then shifted to either a multiple variable-interval 1-min variable-interval 5-min or a multiple variable-interval 30-sec variable-interval 5-min schedule. A generalization test was subsequently administered along the dimension containing the stimulus associated with the variable-interval 5-min component. The generalization gradients for subjects that received multiple variable-interval 1-min variable-interval 5-min training were not consistent in shape. However, an incremental gradient was obtained from each subject that received multiple variable-interval 30-sec variable-interval 5-min training. Thus, a sufficiently large reduction in merely the relative frequency of reinforcement during a stimulus resulted in that stimulus’ acquiring inhibitory control over responding.     

Expression of a taste aversion conditioned with an odor-taste compound: Overshadowing is relatively weak in weanlings and decreases over a retention interval in adults

Adult rats were injected with lithium chloride (LiCl) after consumption of a novel flavor (chocolate milk) that either was or was not presented together with a novel ambient odor (banana) as a compound conditioned stimulus (CS). In Experiment 1, the adults’ consumption of the flavor 24 h after conditioning was compared with that of weanling rats given the same conditioning treatment on Postnatal Day 21. The results confirmed previous indications that the reduction in aversion observed for adults conditioned with the compound CS (overshadowing) was weak or nonexistent in weanlings. After a longer retention interval (21 days), there was no evidence of overshadowing in adults despite maintained retention of the basic conditioned aversion. In Experiment 2 this decrease in overshadowing after a long retention interval was replicated with adult animals and extended to a different method of testing. The form of the effect was the same as in Experiment 1: The decrease in overshadowing occurred over the retention interval without loss in retention of the basic taste aversion; the decrease in overshadowing was a consequence of anincrease in the flavor aversion displayed by animals conditioned with the compound CS. The impaired flavor aversion (i.e., the overshadowing) observed shortly after conditioning apparently was due to factors associated with memory retrieval, rather than to reduced attentional or associative strength.     

Overshadowing of environmental cues by an odor in toxicosis-based conditioning in rats

It was demonstrated that an odor presented to rats in a distinctive environment can interfere with toxicosis-based conditioning to the environmental cues. Rats poisoned when they drank unflavored water in a black compartment odorized with oil of eucalyptus exhibited no suppression of water intake when they were subsequently tested in the same compartment minus the odor. In contrast, rats poisoned in the same compartment minus the odor later exhibited reduced water consumption in that environment. Oil of eucalyptus proved to be as effective an overshadowing stimulus as a taste cue (a 1.5% NaCl solution).     

Tracking the serial advantage in the naming rate of multiple over isolated stimulus displays

Reading and Writing - The serial advantage, defined as the gain in naming rate in the serial over the discrete task of the same content, was examined between grades and types of content in English...     

Effects of stimulus spacing on steady state gradients of inhibitory stimulus control

When the response of pigeons is maintained to a number of stimulus wavelengths, but extinguished to one (S?), the birds peck more rapidly at stimuli near the S? than at more distant stimuli. The present study explores this “dimensional contrast” effect as a function of the number and spacing of test wavelengths. A fixed portion of the wavelength continuum was spanned by 5, 9, 13, or 49 stimuli, which appeared in random sequence behind a standard pecking key. At the end of each 20-sec trial, pecks to test stimuli produced a conditioned reinforcer (sometimes followed by food), while pecks to the S? stimulus produced only darkness. Dimensional contrast “shoulders” developed to test stimuli on either side of the S?; these shoulders were of approximately the same height and wavelength position for all but the 5-stimulus (widely spaced) condition, and were comparable to the original contrast results with 25 stimuli. The results strongly suggest that the extent and locus of contrast shoulders are largely independent of the number and spacing of test stimuli.     

External stimulus control of target-directed attack and biting elicited by tailshock

Two experiments evaluating the effects of external stimuli on attack and biting in rats elicited by electric tailshock are reported. In the first experiment, stimuli presented to four groups of test subjects consisted of a stimulus animal, a stimulus animal plus taped vocalizations of a rat experiencing shock, an inanimate object, and an inanimate object plus taped vocalizations. Subjects in a fifth (control) group were tested in the absence of these stimuli. The presence of another animal in the test situation significantly increased the amount of target-directed responding. A decrease in responding, rather than increase, was shown by the subjects tested under the stimulus-object plus taped-vocalization conditions. Experiment 2 investigated the salient features of the stimulus animal and found a combination of both olfactory and visual cues to be most effective in eliciting target-directed responding. These studies indicate that the amount of target-directed attack and biting shown in this situation may be related causally to the type of sensory input received by the test animal.     

Cross-modal transfer of stimulus control in the albino rat: A stimulus delay procedure

The present experiment demonstrated in a simultaneous discrete trial discrimination that the stimulus control of a rat’s leverpress response can be errorlessly transferred across stimulus modalities, i.e., from light to click location and from click to light location. Subsequent to acquisition of the original discrimination, the original and new discriminative stimuli were simultaneously presented for several sessions. Then the new discriminative stimulus was presented 3 sec prior to the onset of the original discriminative stimulus. Within the direction of transfer, e.g., from light to click location, the delay group emitted fewer trial and intertriai errors than the control group. As the new discriminative stimuli acquired control over responding, the response latency distributions were differentially affected. The results suggest that the transfer of control from the original to the new discriminative stimuli is mediated by the temporal aspects of the delay interval.     

Effects of group rearing on the control exerted by an imprinting stimulus

Prior research suggests that group rearing may attenuate a young bird’s tendency to approach and follow an imprinting stimulus. The present work examined the effect of group rearing on a different measure of attachment, suppression by the imprinting stimulus of distress calling induced by abrupt reductions in group size. In Experiment 1, ducklings were reared in groups of 12 or 3 over Days 1–6 posthatch, and each group received a total of 3.5 h of exposure to an imprinting stimulus. Subsequent tests revealed that, when the groups of ducklings were separated into smaller subgroups: (1) the fewer the ducklings in a subgroup, the more distress calls emitted; (2) a given number of birds separated from a larger group emitted more distress calls than an equal number from a smaller group; and (3) regardless of the subgroup size, fewer distress calls occurred when the imprinting stimulus was present than when it was absent. A second experiment revealed that when a group of 12 birds was first confronted with an imprinting stimulus on Day 6 posthatch, they reacted with an increase in distress calling and corner huddling (an aversive reaction). After several hours of exposure to the stimulus, however, its presence exerted a powerful suppressive effect on distress calling. It is concluded that the social bonds between the members of a brood do not preclude the formation of a social attachment to an imprinting stimulus subsequently encountered.     

Motivational cues as determiners of stimulus control in rats

Four rats were each trained to perform a light-intensity discrimination and a sound-intensity discrimination. For half of the subjects, light training sessions were preceded by food deprivation, and correct choices were reinforced with food. Sound training sessions, on the other hand, were preceded by water deprivation, and correct choices were reinforced with water. For the remaining subjects, light training sessions were associated with water deprivation, whereas sound sessions were associated with food deprivation. When the rats were tested in the presence of compounds of sound and light, choices tended to be controlled by light when testing was preceded by the deprivation condition associated with light discrimination task. Reliably fewer light-consistent choices were made under the other deprivation condition, though some preference for responding on the basis of light remained. Following extended training in the presence of all four combinations of light and sound stimuli, this preference was reduced somewhat. When additional testing sessions were preceded by combined food and water deprivation, the tendency to respond on the basis of either light or sound was shown to be related to both deprivation and reinforcement factors.