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.

     

Stimulus generalization of the effects of punishment

Three pigeons were trained to respond to seven spectral stimulus values ranging from 490 to 610 mμ and displayed in random order on a response key. After response rates had equalized to these values, a brief electric shock was administered when the subject (S) responded to the central value (550 mμ) while positive reinforcement for all values was maintained. Initially, there was broad generalization of the resulting depression in response rate, but the gradients grew steeper in the course of testing. When punishment was discontinued, the rates to all values recovered, and equal responding to all stimuli was reattained by two of the Ss. Stimulus control over the effects of punishment was clearly demonstrated in the form of a generalization gradient; this probably resulted from the combined effects of generalization of the depression associated with punishment and discrimination between the punished value and neutral stimuli.     

Stimulus generalization along a light flicker rate continuum after discrimination training with several S−'s

Four pigeons were trained with VI reinforcement to peck a key which was briefly illuminated by a flickering light. Generalization gradients were then obtained with nine different rates of flicker, four faster than S+ and four slower. Two birds were then trained to discriminate between S+ and the fastest stimulus (S−). These birds were then trained to discriminate between S+ and the two fastest stimuli, alternated as S−'s. This procedure was continued, adding one new S− at a time, until all four stimuli faster than S+ were S−'s. The remaining two birds were trained on this latter discrimination without intervening training. In a final stage, using the first two birds, the slowest stimulus was added as a fifth S−. Generalization gradients in extinction were obtained from each bird after each stage of training. As more stimuli from one end of the continuum served as S−'s, responding increased in the presence of stimuli from the other end of the continuum, and the gradient tended to become flattened at this end.     

Discriminative effects of massed extincttion

Prior studies have reported that generalization gradients are not steepened if periods of non-reinforcement in S− follow and are not interspersed with periods of reinforcement in S+. Sharper gradients are produced by this massed-extinction procedure if it is preceded by prior discriminative training on a dimension orthogonal to the S+, S− dimension. The present study, using pigeons, found that generalization gradients along the wavelength dimension were steepened by massed-extinction sessions in 570 nm that had been preceded by: (1) discriminative training in which the S+ was a 550-nm light and the S− was a black vertical line superimposed on the 550-nm light; (2) non-differential reinforcement training with a 550-nm light and a black vertical line superimposed on the 550-nm light; (3) reinforcement training with only the 550-nm light. Massed-extinction sessions were administered until the response rate in the presence of the 570-nm stimulus was one-tenth of the mean response rate in the presence of the 550-nm stimulus during prior reinforcement training. Prior studies have used a time-dependent criterion, rather than a response-rate criterion of extinction, and this difference may be responsible for the differences in the effects of massed extinction on stimulus control.     

Discrimination learning with and without “errors”

Responses to S− (“errors”) are not a necessary condition for the formation of an operant discrimination of color. Errors do not occur if discrimination training begins early in conditioning and if S+ and S− initially differ with respect to brightness, duration and wavelength. After training starts, S−'s duration and brightness is progressively increased until S+ and S− differ only with respect to wavelength. Errors do occur if training starts after much conditioning in the presence of S+ has occurred or if S+ and S− differ only with respect to wavelength throughout training. Performance following discrimination learning without errors lacks three characteristics that are found following learning with errors. Only those birds that learned the discrimination with errors showed (1) “emotional” responses in the presence of S−, (2) an increase in the rate (or a decrease in the latency) of its response to S+, and (3) occasional bursts of responses to S−.     

Application of a graded choice procedure to obtain errorless learning in children

A simultaneous, two-choice color discrimination was carried out with three groups of four- to seven-year-old children. For Groups I and II, the opportunity to respond to the incorrect stimulus was controlled (graded) over three different conditions. First, only a red light (S+) and its retractable bar were presented (16 trials for Group I and 316 trials for Group II). Second, a green light (S−) was added with its correlated bar retracted for 14 trials. Third, 40 trials were given with both stimuli on and their correlated retractable bars extended. The opportunity to respond to S− was not graded for Group III children. They experienced only the third condition applied to Groups I and II. Responses to S+ were reinforced for all three groups, while responses to S− were not. Children in the first two groups made from zero to three responses to S−, while the control children emitted 11 to 46 errors. The results demonstrate that fading in S− or presenting S− early in the training procedure are sufficient, but not necessary conditions for errorless learning.     

Stimulus generalization as a function of the delay between training and testing procedures: a reevaluation

Three groups of 12 pigeons each were trained to discriminate between lights of 550 mμ (S^D), correlated with 1-min variable-interval reinforcement and 570 mμ (S^Δ), correlated with extinction. Group A was tested for wavelength generalization in extinction 1 min after meeting the discrimination criterion; Group B was tested 24 hr later; Group C was tested 24 hr later after a 3-min (reinforced) warm-up with the S^D. The post-discrimination gradient of Group B was significantly flatter and showed significantly greater area shift than that of Groups A and C. The gradient of Group C was similar to that of Group A, indicating that the warm-up eliminated the effect of the delay period.     

Human peak shift: Analysis of the effects of three-stimulus discrimination training

In a series of studies, the effects of different types of intradimensional discrimination training on human auditory frequency generalization were examined. When subjects were trained with a single S− located on one or the other side of S+, postdiscrimination gradients were displaced away from S−. Subjects trained with two negative stimuli both on one side of S+ showed a greater extent of displacement with true peak shift. In a second experiment the procedures were repeated with two fixed amounts of training: either 12 or 42 training trials. Again the subjects trained with two negative stimuli showed more shift than those trained with one S−, and this effect was independent of amount of training. Experiment 3 showed increased peak shift when two positive stimuli surrounded a central S− as compared to groups with a single S+ and S−. The general conclusion is that training with more difficult, three-stimulus discrimination problems results in enhanced peak shift.     

Stimulus control of schedule-induced activity in pigeons during multiple schedules

Stimulus control of schedule-induced general activity was demonstrated with pigeons using multiple schedules of response-independent food delivery. In Experiment 1, the introduction of food during a multiple variable-time 30-second variable-time 30-second schedule produced a tenfold increase in activity above the no-food baseline. Each pigeon developed stable differential activity rates during the components (correlated with red and green lights) of a multiple variable-time 30-second extinction schedule. Lengthening the extinction component from 1 to 7 minutes increased the rate differences and produced a reliable pattern of responding during S− (the stimulus correlated with extinction): Activity rate was high immediately following the change from S+ (the stimulus correlated with variable-time 30-second) to S−, then decreased abruptly and remained low throughout the middle of the interval, and subsequently showed a positively accelerated increase until the stimulus changed to S+. In Experiment 2, three pigeons were exposed to a mixed variable-time extinction schedule prior to a multiple variable-time extinction schedule. Auditory rather than visual stimuli were used to determine the generality of Experiment 1 results. The multiple- versus mixed-schedule results indicated that stimulus control of activity occurred for two of the birds, but rate differences between S+ and S− were much less than those demonstrated with visual stimuli. A direct comparison of visual and auditory stimulus control in Experiment 3 supported this conclusion. These parallels between the stimulus control of reinforced responding and that of schedule-induced activity suggest that the stimulus control of induced activity may be a factor in operant stimulus control.     

The effect of negative stimulus presentations on observing-response rates

Theories of observing differ in predicting whether or not a signal for absence of reinforcement (S−) is capable of reinforcing observing responses. Experiments in which S− was first removed from and then restored to the procedure have yielded mixed results. The present experiments suggest that failure to control for the direct effect of presenting S− may have been responsible. Pigeons and operant procedures were used. Experiment 1 showed that presentations of S−, even when not contingent on observing, can raise the rate of an observing response that was reinforced only by presentations of a signal (S+) that accompanied a schedule of food delivery. Experiment 2 showed that this effect resulted from bursts of responding that followed offsets of S−. Experiment 3 showed that, when the presence of S− was held constant, lower rates occurred when S− was dependent on, rather than independent of, observing. These results support theories that characterize S− as incapable of reinforcing observing responses.     

Stimulus generalization following extra-dimensional training in educationally subnormal (severely) children.

The use of discrimination learning paradigms in the study of attentional transfer is discussed. The technique of go/no-go discrimination learning followed by stimulus generalization testing is contrasted with the more familiar simultaneous learning paradigm followed by a shift in the relevant cues. In the former paradigm the effect of training a discrimination on one dimension on the slope of the stimulus generalization gradient on an independent gradient dimension (extra-dimensional training) is assessed. A steepening of the gradient relative to appropriate control procedures is taken as evidence of positive attentional transfer. The relevance of the technique to the detailed study of attentional transfer in educationally subnormal (severely) (ESN(S)) children is considered. In Expt. I nine ESN(S) children were trained in a go/no-go discrimination involving stimuli differing in orientation, and were generalization tested on a dimension that was orthogonal, namely hue. Of the six subjects who learnt the discrimination five showed clear decremental gradients on the hue dimension. In contrast a Pseudo-Discrimination group (PD) of eight subjects matched to those in the TD group showed no gradients. These subjects were not trained in the orientation discrimination, but were reinforced for responding on 50 per cent of each of the S+ and S- stimulus presentations. They thus received equal exposure to, but no differential training on, the orientation dimension. An S+ only group of four subjects who received no exposure to the orientation stimuli showed no gradients when stimulus generalization testing on the hue continuum was carried out. The result is discussed in terms of transfer deriving from stimulus control by relational aspects of the stimuli; in terms of control by constant irrelevant stimuli; and in terms of the study of stimulus control in ESN(S) children. In Expt. II the influence of the codability of the colours on the location of the peak of the stimulus generalization gradients in the TD group is investigated.     

Interocular generalization: a study of mirror-image reversal following monocular discrimination training in the pigeon

Generalization gradients along a continuum of angular orientation were obtained from four pigeons, following monocular training on a discrimination between a 45° oblique line (S+) and a 135° oblique line (S−). All pigeons were trained on a chain DRO VI 1 schedule of reinforcement. Generalization gradients obtained with the trained and untrained eye were compared. All pigeons responded maximally to the 45° line (S+) when tested with only the trained eye open. During generalization tests of interocular transfer with only the untrained eye open, three pigeons responded maximally to S− (135°), the mirror-image of the stimulus associated with reinforcement during training (45°). The other pigeon failed to show interocular transfer of the discrimination. Interocular reversal of left-right mirror-image stimuli has not been reported for any other species.     

Differential autoshaping to common and distinctive elements of positive and negative discriminative stimuli

The learning by hungry pigeons of a discrimination between two successively presented compound visual stimuli was investigated using a two-key autoshaping procedure. Common and distinctive stimulus elements were simultaneously presented on separate keys and either followed by food delivery, S+, or not, S−. The subjects acquired both between-trial and within-trial discriminations. On S+ trials, pigeons pecked the distinctive stimulus more than the common stimulus; before responding ceased on S− trials, they pecked the common stimulus more than the distinctive one. Mastery of the within-display discrimination during S+ trials preceded mastery of the between-trials discrimination. These findings extend the Jenkins-Sainsbury analysis of discriminations based upon a single distinguishing feature to discriminations in which common and distinctive elements are associated with both the positive and negative discriminative stimuli. The similarity of these findings to other effects found in autoshaping—approach to signals that forecast reinforcement and withdrawal from signals that forecast nonreinforcement—is also discussed.     

Stimulus generalization following extradimensional and intradimensional training in educable retarded children

Five related experiments investigating stimulus generalization following go/no-go discrimination training of educable retarded children are reported. Experiment 1 employed an Extradimensional paradigm in which generalization testing was on the hue dimension following training on an independent (orientation) dimension. Following True discrimination training only 25% of children showed a decremental stimulus generalization gradient on the hue dimension, though all children exhibited flat gradients in Pseudodiscrimination and S+ only control groups. An increase in difficulty of the orientation discrimination in Experiment 2 did not increase the number of decremental gradients. In Experiment 3, children who exhibited decremental gradients in Experiments 1 and 2 underwent further generalization testing with modified stimuli to establish a symmetrical gradient peaked at a hue S+ to be employed in Experiments 4 and 5. In these experiments an Intradimensional paradigm was employed with S+ and S? stimuli drawn from the hue dimension. Excitatory control by S+ and inhibitory control by S? were demonstrated, as were inhibitory consequences of S? such as peak and area shift.     

Stimulus generalization of gravity

In two experiments, squirrel monkeys were exposed to centrifugally generated, artificial gravity and trained to respond for food reinforcement at selected gravity (g) levels. Experiment I involved a single g value; in Exp. II, subjects were trained to discriminate among two or three g values. After training, generalization tests were administered over a 1.1-g to 2.1-g range. The major findings were as follows: (a) single-stimulus training yielded a linear relationship between percentage of responding and magnitude of gravity. (b) Two-valued discrimination training produced gradient peaks which were shifted from S(D) in a direction away from S(Delta). This effect was cancelled when S(D) was located equidistant between two S(Delta) stimuli. (c) Gradient form was independent of the S(D)-S(Delta) difference, but related to continuum location and/or intensity of discriminative stimuli.     

Effects of S+ and S- separation on gradient shifts in humans

Following single stimulus training, responding during a generalization test tends to be distributed around the positive stimulus (S+). However, if participants are trained instead to discriminate the S+ from a negative stimulus (S-), the response gradient often shifts away from the S- and toward the opposite end of the stimulus continuum. In this experiment, the author examined the basis of gradient shifts with 72 college undergraduates. The research especially examined how gradient shifts are affected by the physical similarity of the S+ and the S- and by the ease with which the two stimuli can be compared. For the former manipulation, the author randomly assigned participants to either a control condition in which only the S+ was shown or a discrimination condition in which the S- was either near to or far from the S+ on the continuum. For the latter manipulation, the author randomly assigned participants to a condition in which S+ and S- presentations were separated by intervals of 1, 15, or 30 s. The results showed that marked shifts occurred when the S+ and S- were relatively similar, but temporal separations did not affect responding.     

Generalization of conditioned fear along a dimension of increasing fear intensity

The present study investigated the extent to which fear generalization in humans is determined by the amount of fear intensity in nonconditioned stimuli relative to a perceptually similar conditioned stimulus. Stimuli consisted of graded emotionally expressive faces of the same identity morphed between neutral and fearful endpoints. Two experimental groups underwent discriminative fear conditioning between a face stimulus of 55% fear intensity (conditioned stimulus, CS+), reinforced with an electric shock, and a second stimulus that was unreinforced (CS−). In Experiment 1 the CS− was a relatively neutral face stimulus, while in Experiment 2 the CS− was the most fear-intense stimulus. Before and following fear conditioning, skin conductance responses (SCR) were recorded to different morph values along the neutral-to-fear dimension. Both experimental groups showed gradients of generalization following fear conditioning that increased with the fear intensity of the stimulus. In Experiment 1 a peak shift in SCRs extended to the most fear-intense stimulus. In contrast, generalization to the most fear-intense stimulus was reduced in Experiment 2, suggesting that discriminative fear learning procedures can attenuate fear generalization. Together, the findings indicate that fear generalization is broadly tuned and sensitive to the amount of fear intensity in nonconditioned stimuli, but that fear generalization can come under stimulus control. These results reveal a novel form of fear generalization in humans that is not merely based on physical similarity to a conditioned exemplar, and may have implications for understanding generalization processes in anxiety disorders characterized by heightened sensitivity to nonthreatening stimuli.Fear generalization occurs when a fear response acquired to a particular stimulus transfers to another stimulus. Generalization is often an adaptive function that allows an organism to rapidly respond to novel stimuli that are related in some way to a previously learned stimulus. Fear generalization, however, can be maladaptive when nonthreatening stimuli are inappropriately treated as harmful, based on similarity to a known threat. For example, an individual may acquire fear of all dogs after an aversive experience with a single vicious dog. In this case, recognizing that a novel animal is related to a feared (or fear-conditioned) animal is made possible in part by shared physical features to the fear exemplar, such as four legs and a tail. On the other hand, fear generalization may be selective for those features that are associated with natural categories of threat; a harmless dog may not pose a threat, but possesses naturally threatening features common to other threatening animals, such as sharp teeth and claws. Moreover, the degree to which an individual fearful of dogs responds with fear may be related to either the physical similarity to the originally feared animal (e.g., from a threatening black dog to another black dog), or the intensity of those threatening features relative to the originally feared animal (e.g., sharp teeth from one animal to sharp teeth of another animal). Therefore, fear generalization based on perceptual information may occur via two routes—similarity to a learned fear exemplar along nonthreatening physical dimensions or along dimensions of fear relevance. Given that fear generalization often emerges as a consequence of conditioning or observational learning, it is important to determine which characteristics of novel stimuli facilitate fear generalization and the extent to which generalization processes can be controlled.Early explanations of stimulus generalization emphasized that an organism''s ability to generalize to nonconditioned stimuli is related to both the similarity and discriminability to a previously conditioned stimulus (CS) (Hull 1943; Lashley and Wade 1946). While Lashley and Wade (1946) argued that generalization was simply a failure of discriminating between a nonconditioned stimulus (CS−) and the reinforced CS (CS+), contemporary views contend that generalization enables learning to extend to stimuli that are readily perceptually distinguished from the CS (Pearce 1987; Shepard 1987; McLaren and Mackintosh 2002). This latter view has been supported by empirical studies of stimulus generalization in laboratory animals (Guttman and Kalish 1956; Honig and Urcuioli 1981). In these studies, animals were reinforced for responding to a CS of a specific physical quality such as color, and then tested with several different values along the same stimulus dimension as the CS (e.g., at various wavelengths along the color spectrum). Orderly gradients of responses are often reported that peak at or near the reinforced value and decrease as a function of physical similarity to the CS along the stimulus dimension (Honig and Urcuioli 1981). Further generalization was shown to extend from the CS+ to discriminable nonconditioned stimuli, suggesting that generalization is not bound to the organism''s ability to discriminate stimuli (Guttman and Kalish 1956, 1958; Shepard 1987).Interestingly, when animals learn to distinguish between a CS+ and a CS−, the peak of behavioral responses often shift to a new value along the dimension that is further away from the CS− (Hanson 1959). For instance, when being trained to discriminate a green CS+ and an orange CS−, pigeons will key peck more to a greenish-blue color than the actual CS+ hue. Intradimensional generalization of this sort is reduced when animals are trained to discriminate between two or more stimulus values that are relatively close during conditioning (e.g., discriminating a green-yellow CS+ from a green-blue CS−), suggesting that the extent of generalization can come under stimulus control through reinforcement learning (Jenkins and Harrison 1962). Spence (1937) described the transposition of response magnitude as an effect of interacting gradients of excitation and inhibition formed around the CS+ and CS−, respectively, which summate to shift responses to values further from the inhibitory CS− gradient. In all, early theoretical and empirical treatments of stimulus generalization in nonhuman animals revealed that behavior transfers to stimuli that are physically similar, but can be discriminated from a CS, and that differential reinforcement training can both sharpen the stimulus gradient and shift the peak of responses to a nonreinforced value.Although this rich literature has revealed principles of generalization in nonhuman animals, few studies of fear generalization have been conducted in humans (for review, see Honig and Urcuioli 1981; Ghirlanda and Enquist 2003). Moreover, the existing human studies have yet to consider the second route through which fear responses may generalize—via gradients of fear relevance. While a wide range of neutral stimuli, such as tones or geometric figures, can acquire fear relevance through conditioning processes, other stimuli, such as threatening faces or spiders, are biologically prepared to be fear relevant (Lanzetta and Orr 1980; Dimberg and Öhman 1996; Whalen et al. 1998; Öhman and Mineka 2001). Compared with fear-irrelevant CSs, biologically prepared stimuli capture attention (Öhman et al. 2001), are conditioned without awareness (Öhman et al. 1995; Öhman and Soares 1998), increase brain activity in visual and emotional processing regions (Sabatinelli et al. 2005), and become more resistant to extinction when paired with an aversive unconditioned stimulus (US) (Öhman et al. 1975). Although the qualitative nature of the CS influences acquisition and expression of conditioned fear, it is unknown how generalization proceeds along a gradient of natural threat. For instance, human studies to date have all tested variations of a CS along physically neutral stimulus dimensions, such as tone frequency (Hovland 1937), geometric shape (Vervliet et al. 2006), and physical size (Lissek et al. 2008). These investigations implicitly assume that the generalization gradient is independent of the conditioned value (equipotentiality principle). In other words, since the stimuli are all equally neutral prior to fear learning, fear generalization operates solely as a function of similarity along the reinforced physical dimension. However, since fear learning is predisposed toward fear-relevant stimuli, generalization may be selective to those shared features between a CS+ and CS− that are associated with natural categories of threat. Examining generalization using fear-relevant stimuli is thus important to gain better ecological validity and to develop a model system for studying maladaptive fear generalization in individuals who may express exaggerated fear responses to nonthreatening stimuli following a highly charged aversive experience (i.e., post-traumatic stress disorder or specific phobias).To address this issue, the present study examined generalization to fearful faces along an intradimensional gradient of fear intensity. A fearful face is considered a biologically prepared stimulus that recruits sensory systems automatically for rapid motor responses (Öhman and Mineka 2001), and detecting fearful faces may be evolutionarily selected as an adaptive response to social signals of impending danger (Lanzetta and Orr 1980; Dimberg and Öhman 1996). During conditioning, an ambiguous face containing 55% fear intensity (CS+) was paired with an electric shock US, while a relatively neutral face (11% fear intensity) was explicitly unreinforced (CS−) (Experiment 1). Skin conductance responses (SCR) were recorded as a dependent measure of fear conditioning. Before and following fear conditioning, SCRs were recorded in response to face morphs of the same actor expressing several values of increasing fear intensity (from 11% to 100%; see Fig. 1). A total of five values along the continuum were used: 11% fear/88% neutral, 33% fear/66% neutral, 55% fear/44% neutral, 77% fear/22% neutral, and 100% fear. For clarity, these stimuli are herein after labeled as S1, S2, S3, S4, and S5, respectively.Open in a separate windowFigure 1.Experimental design. (A) Pre-conditioning included six presentations of all five stimulus values without the US. (B) Fear conditioning involved discriminative fear learning between the S3, paired with the US (CS+), and either the unreinforced S1 (Experiment 1) or the unreinforced S5 (Experiment 2) (CS−). (C) The generalization test included nine presentations of all five stimuli (45 total), with three out of nine S3 trials reinforced with the US. Stimuli are not drawn to scale.Testing generalization along an intradimensional gradient of emotional expression intensity allows for an examination of the relative contributions of fear intensity and physical similarity on the magnitude of generalized fear responses. If fear generalization is determined purely by the perceptual overlap between the CS+ and other morph values, without regard to fear intensity, then we would expect a bell-shaped generalization function with the maximum SCR centered on the reinforced (intermediate) CS+ value (S3), less responding to the directly adjacent, but most perceptually similar values (S2 and S4), and the least amount of responding to the most distal and least perceptually similar morph values (S1 and S5). This finding would be in line with stimulus generalization reported along fear-irrelevant dimensions (Lissek et al. 2008) and in stimulus generalization studies using appetitive instrumental learning procedures (Guttman and Kalish 1956). If, however, fear generalization is biased toward nonconditioned stimuli of high fear intensity, then an asymmetric generalization function should result with maximal responding to the most fear-intense nonconditioned stimuli. This finding would suggest that fear generalization is selective to the degree of fear intensity in stimuli, similar to studies of physical intensity generalization gradients in nonhuman animals (Ghirlanda and Enquist 2003). We predicted that the latter effect would be observed, such that the magnitude of SCRs will disproportionately generalize to stimuli possessing a greater degree of fear intensity than the CS+ (Experiment 1). A secondary goal was to determine whether fear generalization to nonconditioned stimuli can be reduced through discriminative fear learning processes. Therefore, a second group of participants was run for whom the CS− was the 100% fearful face (Experiment 2). In this case, we predicted that discriminative fear conditioning between the CS+ (55% intensity) and the most fear-intense nonconditioned stimulus would sharpen the generalization gradient around the reinforced CS+ value, and that responses to the most fear-intense stimulus would decrease relative to Experiment 1. Moreover, this discriminative fear-learning process may provide evidence that fear generalization is influenced by associative learning processes and is not exclusively driven by selective sensitization to stimuli of high fear relevance (Lovibond et al. 1993). Finally, we were interested to discover whether generalization processes would yield subsequent false memory for the intensity of the CS+ in a post-experimental retrospective report. In sum, the present study has implications for understanding how fear generalization is related to the degree of fear intensity of a nonconditioned stimulus, the extent to which discrimination training efforts can thwart the generalization process, and how fear generalization affects stimulus recognition.     

Generalization and discrimination of shape orientation in the pigeon

Pigeons learned to peck a green key on which parallelogram-shapes were projected; they then received generalization tests in which the orientation of the parallelogram was varied. Nondifferential training produced very little eventual stimulus control along the orientation dimension, but when training included S- trials (absence of the parallelogram) subjects responded consistently more to certain orientations than to others. Unlike typical results for visual generalization (e.g., line-tilt), the tilt gradients obtained for this complex stimulus were bimodal, supporting predictions on the basis of human perceptual data. However, unimodal gradients could be produced by specific discrimination training along the orientation dimension. Other forms of intradimensional training also produced relatively steep gradients, often characterized by unexpected but consistent secondary peaks. An attempt to obtain inhibitory gradients (S+: green key; S-: parallelogram on a green background) resulted in virtually zero responding all along the shape-orientation dimension; therefore, specific inhibitory control could not be evaluated. All these experiments suggest that definition of this complex stimulus dimension in terms of mere "angular orientation" is inappropriate, and alternative interpretations are discussed.     

Stimulus generalization and discrimination along the click-frequency (flutter) continuum in pigeons

Before tests for click-frequency generalization, pigeons had been reinforced for keypecks during one click frequency (S+). Some Ss received S+ training only, whereas other Ss also received unreinforced (S?) trials, during which the clicks were either absent (Experiments 1-3) or presented at some other frequency (faster or slower than S+: Experiment 4). When training included S+ trials only, birds responded approximately equally to all generalization test frequencies (0.0 to 53.5 pulses/sec, pps). Most Ss that had received both S+ and S? training trials responded fastest not during S+ but during click frequencies even further away from S? along the click-frequency dimension (peak shift). Complex bimodal gradients were obtained after training with S+ (1.6 pps) vs S? (0.0 pps); maximal responding generally occurred near S+ and at approximately 14.2 pps. Among other factors, the “nonorthogonality” of click absence (0.0 pps) to the click dimension seems crucially involved in producing these complex effects.     

Two determinants of the peak shift in human voluntary stimulus generalization

Two hundred and forty college students were divided into two groups, with training stimuli (from the brightness dimension) selected to produce small and large adaptation level shifts between discrimination training (to respond “same” to S+ and “different” to S—) and gen-eralization testing. These were further divided into three groups with discriminations expected to yield positive (toward brighter values), negative (toward dimmer values), or zero post-discrimination peak shifts. Half the subjects received brief discrimination training while half received extended training. A further group of 60 subjects were given exposure to the stimuli comparable to that of the extended training subjects, but were asked to rate the stimuli instead of being given discrimination training. The results indicated that two independent, additive sources of shift were active. One source, occurring in all groups, was interpreted as being due to the change in adaptation level from training to test. The other source of shift, occurring only in the groups with extended discrimination training, was interpreted as due to the establishment of asymmetrical decision criteria; the more traditional interpretation in terms of the interaction between excitatory and inhibitory gradients of response strength was also considered.