Choice reaction and unequal stimulus frequencies in an absolute judgment situation

Previous investigators have reported discrepant results for Ss in a choice reaction time (CRT) situation when stimuli are not equi-probable. Sixty Ss participated in an absolute judgment CRT task under three conditions of equi-probable stimuli and three of unequally probable stimuli. The results indicate that previous findings may be determined by a threshold dependent upon the effect of unequal stimulus frequencies and the utilities of different response strategies.     

Choice reaction and unequal stimulus frequencies in an absolute judgment situation

Previous investigators have reported discrepant results for Ss in a choice reaction time (CRT) situation when stimuli are not equi-probable. Sixty Ss participated in an absolute judgment CRT task under three conditions of equi-probable stimuli and three of unequally probable stimuli. The results indicate that previous findings may be determined by a threshold dependent upon the effect of unequal stimulus frequencies and the utilities of different response strategies.     

Choice, time allocation, and response rate during stimulus generalization

Six pigeons were trained to discriminate between two noise intensities using a procedure that assessed choice, time allocation, and response rate simultaneously and independently. Responses on the left or right key (R1 or R2) were respectively correct in the presence of two different intensities, S1 and S2. After a correct response, reinforcement became available for pecks on the center key. Reinforcement density for R1¦S1 relative to R2¦S2 was varied across experimental conditions. Generalization tests followed extensive training at each condition. As a function of stimulus intensity, proportions of initial choices of R2, of time spent in R2-initiated components, and of center-key responses emitted in R2-initiated components all yielded sigmoidal gradients of similar slope, which shifted slightly in location when relative reinforcement density changed. Changeovers were maximal where initial choice proportions approximated 0.5. Gradients relating the absolute number of center-key responses to stimulus intensity were also roughly sigmoidal, but were more sensitive to changes in reinforcement density. Gradients of momentary response rate also depended on reinforcement density. During training, large but transitory shifts in choice responding occurred when reinforcement density changed, while differences in momentary response rate developed slowly, suggesting separate control of choice and response rate by the contingencies of reinforcement.     

Double discrimination avoidance with a single stimulus

Four white rats were trained to avoid electric shocks by depressing a bar after the onset of a stimulus and releasing it after termination of the stimulus. All subjects acquired the behavior of holding the bar in the presence of the stimulus and releasing it in the absence of the stimulus, but one animal almost always required a "priming" shock. The others exhibited a high level of shock avoidance.     

Choice and delay of reinforcement: Effects of terminal-link stimulus and response conditions

In two experiments, pigeons were exposed to concurrent-chains schedules in which a single initial-link variable-interval schedule led to access to terminal links composed of fixed-interval or fixed-delay schedules. In Experiment 1, an 8-s (or 16-s) delay to reinforcement was associated with the standard key, while reinforcer delay values associated with the experimental key were varied from 4 to 32 s. The results of Experiment 1 showed undermatching of response ratios to delay ratios with terminal-link fixed-delay schedules, whereas in some pigeons matching or overmatching was evident with the fixed-interval schedules. In Experiment 2, one pair of reinforcer delay values, either 8 versus 16 s or 16 versus 32 s, was used. In the first condition of Experiment 2, different delays were associated with different keylight stimuli (cued condition). In the second condition, different terminal-link delays were associated with the same stimulus, either a blackout (uncued-blackout condition) or a white key (uncued-white condition). To examine the role of responses emitted during delays, the keys were retracted during a delay (key-absent condition) in the third condition and responses were required by a fixed-interval schedule in the fourth condition. Experiment 2 demonstrated that the choice proportions for the shorter delay were more extreme in the cued condition than in the uncued-blackout condition, and that the response requirement imposed by the fixed-interval schedules did not affect choice of the shorter delay, nor did the key-absent and key-present conditions. These results indicate that the keylight-stimulus conditions affected preference for the shorter of two delays and that the findings obtained in Experiment 1 depended mainly on the keylight-stimulus conditions of the terminal links (i.e., the conditioned reinforcing value of the terminal-link stimuli).     

Effect of compatibility of S-R mapping on reactions toward the stimulus source

This research was concerned with separating the effects of three varieties of S-R compatibility: reactions toward the stimulus source, compatibility of S-R mapping, and display-control arrangement correspondence. In experiments 1 and 2, subjects pressed a green or red key located on the left and right in response to the onset of a green or red stimulus presented in a left or right window. Half of the subjects pressed the key which corresponded to the color of the stimulus (compatible S-R mapping) while the other half pressed the alternate colored key (incompatible S-R mapping). In the compatible mapping task, reactions were faster when location of stimulus and response corresponded than when they did not while, in the incompatible task, reactions were faster when location of stimulus and response did not correspond. This apparent reversal in the tendency to react toward the stimulus source was attributed to display- control arrangement correspondence rather than to logical recoding of the directional cue. Experiment 3 established that faster reactions toward the stimulus source occured only under compatible mapping instructions.     

Mechanisms of second-order conditioning with a backward conditioned stimulus

Five conditioned suppression experiments with rats examined the conditions under which backward pairings endow a first-order conditioned stimulus (CS1) with the ability to serve as a secondary reinforcer. Experiments 2-5B found evidence for excitatory second-order conditioning (SOC) if, during first-order pairings, the US-CS1 interval was 0 s rather than 3 s. Levels of SOC were comparable after forward and backward pairings (Experiments 1-3), and were unaffected by extinction of CS1 after SOC (Experiment 3). These results suggest that forward and backward CS1s support SOC for the same reason, and they call into question the need to invoke any special mechanism such as memory integration.     

Instructions and stimulus categorizing in a measure of stimulus generalization

In Experiment I, three groups of 20 Ss each were exposed to a light of 550 mμ (yellowish-green) for 60 sec and then viewed a random sequence of wavelengths with instructions to respond only to the original color. The instructions given the three groups were worded differently in an attempt to vary the strength of a set-to-discriminate assumed to be created by this procedure. The three groups produced similar gradients, each with a peak of responding at 540 mμ, in agreement with Kalish's (1958) published gradient for the 550 mμ standard stimulus value. It was suggested that the nature of the task is such that a strong discriminatory set is produced regardless of the wording of the instructions.

A temporal analysis of the gradient as it develops during the testing revealed that initially the peak of responding occurs at 550 mμ; but as testing progresses, it shifts gradually in the direction of the shorter wavelengths (purer greens). Experiment II was performed to test the generality of the phenomenon of regression to the primary color. Two groups of 20 Ss each were tested for generalization following exposure to 510 mμ (bluish-green) and 525 mμ (pure green), respectively. We predicted that the 510 mμ gradient would reveal a progressive shift toward the longer wavelengths (purer greens), whereas the 525 mμ gradient would show no tendency to shift. The results were strikingly in accord with these predictions.

We concluded that although a physiological process could not be ruled out, the verbal labeling of the standard stimulus value may well be responsible for the regression of the gradient toward the primary color.

     

Potentiation of the acoustic startle response by a conditioned stimulus paired with acoustic startle stimulus in rats

The hypothesis that the standard acoustic startle habituation paradigm contains the elements of Pavlovian fear conditioning was tested. In a potentiated startle response paradigm, a startle stimulus and a light conditioned stimulus (CS) were paired. A startle stimulus then was tested alone or following the CS. Freezing behavior was measured to index conditioned fear. The startle response was potentiated on CS trials, and rats froze more in CS than in non-CS periods. In Experiment 1, response to a previously habituated, weak startle stimulus was potentiated. In Experiment 2, response to the same stimulus used as the unconditioned stimulus (US) in training was potentiated. This CS-potentiated response retarded the course of response decrements over training sessions as compared with an explictly unpaired control group. Conditioned fear is a standard feature of this habituation paradigm, serves to potentiate the startle response, and provides an associative dimension lacking in the habituation process per se.     

Choice reaction time with a random foreperiod

Choice reaction latencies were measured at three different a priori probabilities for two stimulus alternatives. Unlike the results of some other studies, the mean latency of a given response was nearly the same whether the response was correct or incorrect. The discriminable stimuli were a 1000- or a 1700-Hz tone presented at 70 dB SPL. Latencies and standard deviations, based on about 17,000 observations, are reported for three observers. The data are compared with predictions of the optimum sequential model of Wald and Stone and two modifications of that random-walk model, one proposed by Link and Heath and the other proposed by Laming. Fast-guess analyses were also carried out. The three-parameter version of either the sequential or the modified random-walk models provided reasonably accurate predictions of the mean data for each observer. The parameters estimated by the fast-guess analysis were unrealistic. There are three obvious differences between this experiment and most previous choice reaction-time experiments. First is stimulus modality#x2014;we used an auditory signal, whereas most of the previous studies used a visual signal. Second, the observers practiced more in this experiment than in most previous experiments. Finally, there was a random foreperiod with a heavy penalty for anticipations. One or more of these factors is the probable reason for the discrepancy between our results and those of previous studies.     

The effects of stimulus sequence on event related potentials: A comparison of visual and auditory sequences

The effect of stimulus sequence on the waveform of the scalp-recorded event-related potential (ERP) was studied in the auditory and visual modalities. The results indicate that the sequential effects are qualitatively similar in the two modalities; however, an interaction appeared between the modalities and the stimulus effects. The degree of sequential effect was a function of whether or not the stimulus eliciting the ERP was the counted (target) event. This finding is discussed in terms of an expectancy model, and the interaction is attributed to variations in short-term memory for past events as a function of the level of processing required for each.     

Statistical analysis for finger tapping with a periodic external stimulus

In this paper we outline an experiment in which seven subjects attempted to tap in synchrony with a pacing stimulus with the interstimulus interval that varied in a cycle (T,T+dT,T, T-dT) repeating every fourth interval, where T represents an unexpected step change of an interstimulus interval. Conditions included three levels of T= (400, 500, and 600 msec.) with four levels of dT= 1, 3, 5, and 7% of T). The statistical analysis indicated that small changes in T lead to rapid matching of the interresponse intervals to the new interstimulus intervals even when the variation in the input is not apparent to subjects.     

Punishment as a discriminative stimulus and conditioned reinforcer with humans

Mental hospital patients were reinforced for responding in a two-response operant situation. When a noise was used to punish one of the responses, all subjects shifted to the unpunished one. When the noise was then paired with positive reinforcement, the subjects responded to produce the noise. Also, a novel response was reinforced by noise in the absence of other reinforcers. This study with humans extends the findings of previous studies with animals in revealing how a punishing stimulus can acquire discriminative or conditioned reinforcing properties.