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.     

Stimulus delay and the reduction of errors in the transfer of stimulus control

Pigeons initially trained on a simultaneous discrimination of line orientation (S1) were subsequently transferred to a wavelength discrimination (S2). Three transfer procedures were employed. The abrupt-transfer Ss were “abruptly” switched from S1 to the S2 dimension. The stimulus-compounding Ss were trained on a compound stimulus consisting of S1 and S2 displayed in superimposition prior to the presentation of S2 alone. The stimulus-delay Ss were trained on a compound stimulus in which the presentation of the S1 component was delayed for successively longer intervals as a result of a correct response to the preceding trial. Stimulus-delay Ss transferred by responding to S2 prior to the presentation of S1 and the resulting formation of the compound. Ss transferred by the stimulus-compounding and the abrupt-transfer procedures displayed 5 and 10 times as many errors to the S2 dimension, respectively, as Ss receiving the stimulus-delay procedure.

     

Differential inhibition and stimulus generalization cannot account for value transfer in simultaneous discrimination learning by pigeons: Reply to Aitken

Aitken (1999) argues that, in a simultaneous discrimination, reports of the transfer of value from the positive to the negative stimulus can be more readily explained in terms of an artifact produced by the procedure in which differential inhibition accrues to the negative test stimuli during training, together with stimulus generalization (similarity between the positive and negative stimuli). We argue that (1) there is little evidence for differential inhibition, and it often occurs in the wrong direction; (2) value transfer can be demonstrated when differential value is established to the positive stimuli afterdiscrimination training, when differential inhibition is not likely to be a factor; and (3) on both logical and empirical grounds, stimulus similarity does not provide an adequate account of the transfer of value from the positive to the negative stimulus (i.e., the strongest evidence for value transfer occurs when there is least stimulus similarity). We propose that value transfer occurs whenever there is relatively little experience with the negative stimuli. However, when there is extended experience with the negative stimuli, contrast will be found.     

Symmetrical generalization decrements: Configural stimulus processing in human contingency learning

Models of associative learning differ in their predictions concerning the symmetry of generalization decrements. Whereas Pearce’s (1994) configural model predicts the same response decrement after adding elements to and after removing elements from a previously trained stimulus, elemental models, such as the replaced elements model and Harris’s (2006) model, anticipate more of a decrement for removing than for adding elements. In three contingency learning experiments, we manipulated the motion and the spatial arrangement of colored dots in order to induce configural or elemental processing by perceptual grouping. The results reliably showed symmetrical decrements for the added and removed groups. The manipulations of the stimuli had no effect on stimulus processing. This is in line with Pearce’s configural model, but it is at variance with the elemental models and previous studies.     

Poison-induced neophobia in rats: Role of stimulus generalization of conditioned taste aversions

Rats repeatedly injected with lithium chloride were subsequently tested drinking novel and familiar solutions of both casein hydrolysate and vinegar. Injections in the absence of edibles result in only a small, and sometimes not reliable, increased avoidance of the novel casein and vinegar solutions. In contrast, if subjects acquired an aversion to saccharin as a result of the lithium injections, this learned aversion generalized to casein hydrolysate, with the generalization greatly enhanced by novelty of the casein flavor. However, the saccharin aversions did not generalize to the novel vinegar solution nearly as much as to the novel casein flavor. These results suggest that previous observations of poison-induced neophobia were probably in part a result of the stimulus generalization of conditioned taste aversions and that in addition to test stimulus novelty some other factor, such as stimulus salience or similarity to the conditioned aversive flavor, is also involved in the generalization of learned taste aversions.     

On the acquisition and measurement of stimulus control in pigeons

Two experiments were performed to investigate the relationship between excitatory stimulus control (number of responses to a training stimulus) and dimensional stimulus control (generalization gradient slope). In experiment 1, after being trained to peck a green key, pigeons received either 20, 40, or 80 brief (.5, 2, 4, or 8 sec) presentations of a 45-deg line followed by reinforcement (12 groups) or 20, 40, or 80 reinforcements for pecking a continuously presented 45-deg line (3 groups). Number of reinforcements determined the slope (percent of total responses to 45 deg) of a subsequent line-angle generalization gradient, but number of responses to the 45-deg line in the test was controlled by total experience with 45 deg as measured by either total exposure time or total responses to 45 deg in training. In a second experiment, it was shown that increasing the number of days of pretraining to green decreased the slope of the gradient (in subjects given 2-sec presentations), but had no effect on number of responses to 45 deg in the test. Furthermore, continuous presentation yielded flatter gradients but more responding to the 45-deg line in the test than did 2-sec presentations. It was concluded that the measures of dimensional stimulus control and excitatory stimulus control reflect different processes because they vary differentially (sometimes in different directions) in response to the same independent variable manipulations.     

Differential generalization of habituation across contexts as a function of stimulus significance

Orienting to one of two levels of stimulus significance (a distress squeal or a simulated mimic squeal) in female hooded rats was measured by suppression of ongoing drinking. Subsequent generalization tests with the same stimulus presented in different contexts showed that generalization of habituation across contexts was a function of stimulus significance: Habituation to the distress squeal was restricted to the context in which it first had been presented, whereas habituation to the mimic squeal generalized across the different contexts.     

Behavioral contrast and inhibitory stimulus control

Two experiments examined the presumed relationship between behavioral contrast and inhibitory stimulus control. In Experiment I, pigeons were exposed to mult VI 1-min VI 1-min or mult VI 5-min VI 5-min during baseline training prior to mult VI 1-min VI 5-min discrimination training. Half of the subjects received a timeout (TO) component during baseline in order to reduce the degree of contrast during discrimination training. Only 3 of 8 subjects receiving the TO showed contrast while all other subjects showed various degrees of contrast. Postdiscrimination generalization gradients indicated excitatory rather than inhibitory control by the stimulus associated with the VI 5-min schedule. During baseline training in Experiment II, responding to all the generalization stimuli was reinforced. In addition, some subjects received the TO stimulus. The subjects were next exposed to mult VI 1-min EXT, mult VI 1-min VI 5-min, or just the VI 5-min component. Generalization gradients indicated inhibitory control by the stimulus associated with EXT or VI 5-min for 19 of 20 subjects even though some subjects did not show contrast. These results question the presumed relationship between behavioral contrast and inhibitory stimulus control.     

The effect of localizing the stimulus at the response key on generalization along the hue dimension in the pigeon

After training with a variable-interval schedule of positive reinforcement, pigeons were tested for stimulus generalization along the hue dimension. For one group, the stimulus was located on the response key. For a second group, the stimulus was located on a surface adjacent to the response key. The stimulus-on-key group produced the typical steep gradients normally found with hue stimuli; the stimulus-off-key group produced flat gradients. After discrimination training between the presence and absence of the hue stimulus, both groups produced decremental gradients. In a second experiment, naive pigeons were trained to peck a transparent key with the stimulus surface located approximately 3.8 cm behind the key. When tested for generalization, the hue gradients were decremental. The results suggest that location of the stimulus in the line of sight with pecking is a necessary condition for stimulus control by hue after nondifferential training.     

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.     

Effects of discrimination training on stimulus generalization in pigeons: Role of resistance to extinction and response-produced stimuli

In Experiment 1, two groups (n = 10) of pigeons received 17 sessions of TD (true discrimination) or ND (nondifferential) training with line angles. Seventeen sessions of SS (single stimulus) training with a wavelength preceded this training and two followed it. Subsequent wavelength generalization testing in extinction revealed a sharper TD than ND gradient. This slope difference was evident from the very first test stimulus presentation and remained stable throughout testing. As a consequence of substantial overtraining, there was no reduction of response strength and no sharpening of generalization during testing for either group. In Experiment 2, two groups (n = 16) of pigeons received 10 sessions of TD or PD (pseudodiscrimination) training with line angles, followed by four sessions of SS training with a single wavelength. During this training and in subsequent wavelength generalization testing in extinction, brief blackouts separated stimulus presentations. Again, the TD group yielded the sharper gradient. Although responding weakened and the gradients sharpened during the test, these effects were comparable in the two groups. Furthermore, gradients based on the percentage of trials with at least one response showed the same TD-PD slope difference. This finding indicates that differential control over responding by response-produced feedback is inadequate to account for the TD-PD difference in generalization slope. Both experiments indicate that a purported difference in resistance to extinction is also an inadequate explanation.     

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.     

The acquisition of trace supraordinate stimulus control in pigeons

Match-to-sample and oddity-from-sample problems with four colors were acquired by two pigeons under the supraordinate control of a line tilt superimposed on samples, Since the supraordinate stimulus terminated before the comparison stimuli were presented, accurate matching and oddity performance indicated trace stimulus control as well, The temporal extent of trace control was assessed in one subject by presenting probes—trials without a line tilt on the sample—in which the basis of correct responding was the supraordinate stimulus presented on the previous trial, Trace supraordinate control did not extend between trials, Subsequently, the delay between the termination of the supraordinate stimulus and the presentation of the comparison stimuli was gradually increased within a trial, Both subjects were able to perform matching and oddity over longer delays, and eventually on probe trials, although accuracy decreased, Results were discussed in terms of instructional stimulus control and memory.     

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.     

Independent effects of stimulus and cycle duration in conditioning: The role of timing processes

Rats received delay conditioning procedures with a white-noise conditioned stimulus (CS), a food unconditioned stimulus (US), and head entries into the food cup as the conditioned response. The stimulus duration (S) and the interval between food deliveries (C) were varied between groups:S=15, 30, 60, and 120 sec;C=90, 180, and 360 sec. The stimulus/cycle duration ratio was negatively related to the asymptotic level of conditioning but had no effect on the rate of acquisition. Conditioning and timing of responses emerged together in training. Timing occurred during the CS-US interval (ISI) and the US-US interval (ITI), as evidenced by increasing response rate gradients that were steeper for shorter intervals. The effects of the stimulus/cycle ratio on conditioning were attributed to independent timing of theS andC durations. Serial-, parallel-, and single-process accounts of conditioning and timing are compared.     

The transfer of social exclusion and inclusion functions through derived stimulus relations

Previous studies have found that social exclusion can cause distress to those excluded. One method used to study social exclusion is through a virtual ball-toss game known as Cyberball. In this game, participants may be excluded from or included in the ball-toss game and typically report lower feelings of self-esteem, control, belonging, and meaningful existence following exclusion. Experiments 1 and 2 sought to explore the transfer of feelings of exclusion and inclusion through stimulus equivalence classes. In both experiments, participants were trained to form two three-member equivalence classes (e.g., A1–B1, B1–C1; A2–B2, B2–C2) and were tested with novel stimulus combinations (A1–C1, C1–A1, A2–C2, C2–A2). Thereafter, participants were exposed to the Cyberball exclusion and inclusion games. In these games, one stimulus (C1) from one equivalence class was assigned as the Cyberball inclusion game name, whereas one stimulus (C2) from the other equivalence class was assigned as the Cyberball exclusion game name. In Experiment 2, participants were only exposed to the Cyberball exclusion game. During a subsequent transfer test, participants were asked to rate how included in or excluded from they thought they would be in other online games, corresponding to members of both equivalence classes. Participant reported that they felt they would be excluded from online games if the games were members of the same equivalence class as C2. In contrast, participants reported that they felt they would be included in online games if the games were members of the same equivalence class as C1. Results indicated the transfer of feelings of inclusion (Experiment 1) and feelings of exclusion (Experiments 1 and 2) through equivalence classes.     

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.