Individual differences in stimulus identification, rule induction, and generalization of learning.

In the field of stimulus generalization, an old yet unresolved discussion pertains to what extent stimulus misidentifications contribute to the pattern of conditioned responding. In this article, we perform cluster analysis on six datasets (four published datasets and two unpublished datasets, included N = 950) to examine the relationship between interindividual differences in (a) stimulus identification, (b) patterns of generalized responding, and (c) verbalized generalization rules. The datasets were obtained from online predictive learning tasks where participants learned associations between colored cues and the presence or absence of a hypothetical outcome. In these datasets, stimulus identification and expectancy ratings were assessed in separate phases to a range of colors varying between blue-green. Using cluster analyses on performance during stimulus identification, we identified different subgroups of participants (good vs. bad identifiers). In all six datasets, we found a close relationship between the pattern of stimulus identification and the shape of the expectancy gradient across the test dimension between the identified subgroups. Furthermore, participants classified as good identifiers were more likely to report a similarity generalization rule than a relational or linear rule, suggesting that individual differences in stimulus identification are related to individual differences in generalization rules. These findings suggest that greater consideration should be given to interindividual variability in stimulus identification, inductive rules, and their relationship in explaining patterns of generalized responses. (PsycInfo Database Record (c) 2023 APA, all rights reserved).

Learning-induced biases in the ongoing dynamics of sensory representations predict stimulus generalization.

Sensory stimuli have long been thought to be represented in the brain as activity patterns of specific neuronal assemblies. However, we still know relatively little about the long-term dynamics of sensory representations. Using chronic in vivo calcium imaging in the mouse auditory cortex, we find that sensory representations undergo continuous recombination, even under behaviorally stable conditions. Auditory cued fear conditioning introduces a bias into these ongoing dynamics, resulting in a long-lasting increase in the number of stimuli activating the same subset of neurons. This plasticity is specific for stimuli sharing representational similarity to the conditioned sound prior to conditioning and predicts behaviorally observed stimulus generalization. Our findings demonstrate that learning-induced plasticity leading to a representational linkage between the conditioned stimulus and non-conditioned stimuli weaves into ongoing dynamics of the brain rather than acting on an otherwise static substrate.

Neural substrates of human fear generalization: A 7T-fMRI investigation.

Fear generalization - the tendency to interpret ambiguous stimuli as threatening due to perceptual similarity to a learned threat - is an adaptive process. Overgeneralization, however, is maladaptive and has been implicated in a number of anxiety disorders. Neuroimaging research has indicated several regions sensitive to effects of generalization, including regions involved in fear excitation (e.g., amygdala, insula) and inhibition (e.g., ventromedial prefrontal cortex). Research has suggested several other small brain regions may play an important role in this process (e.g., hippocampal subfields, bed nucleus of the stria terminalis [BNST], habenula), but, to date, these regions have not been examined during fear generalization due to limited spatial resolution of standard human neuroimaging. To this end, we utilized the high spatial resolution of 7T fMRI to characterize the neural circuits involved in threat discrimination and generalization. Additionally, we examined potential modulating effects of trait anxiety and intolerance of uncertainty on neural activation during threat generalization. In a sample of 31 healthy undergraduate students, significant positive generalization effects (i.e., greater activation for stimuli with increasing perceptual similarity to a learned threat cue) were observed in the visual cortex, thalamus, habenula and BNST, while negative generalization effects were observed in the dentate gyrus, CA1, and CA3. Associations with individual differences were underpowered, though preliminary findings suggested greater generalization in the insula and primary somatosensory cortex may be correlated with self-reported anxiety. Overall, findings largely support previous neuroimaging work on fear generalization and provide additional insight into the contributions of several previously unexplored brain regions.

The influence of reward magnitude on stimulus memory and stimulus generalization in categorization decisions.

Reward magnitude is a central concept in most theories of preferential decision making and learning. However, it is unknown whether variable rewards also influence cognitive processes when learning how to make accurate decisions (e.g., sorting healthy and unhealthy food differing in appeal). To test this, we conducted 3 studies. Participants learned to classify objects with 3 feature dimensions into two categories before solving a transfer task with novel objects. During learning, we rewarded all correct decisions, but specific category exemplars yielded a 10 times higher reward (high vs. low). Counterintuitively, categorization performance did not increase for high-reward stimuli, compared with an equal-reward baseline condition. Instead, performance decreased reliably for low-reward stimuli. To analyze the influence of reward magnitude on category generalization, we implemented an exemplar-categorization model and a cue-weighting model using a Bayesian modeling approach. We tested whether reward magnitude affects (a) the availability of exemplars in memory, (b) their psychological similarity to the stimulus, or (c) attention to stimulus features. In all studies, the evidence favored the hypothesis that reward magnitude affects the similarity gradients of high-reward exemplars compared with the equal-reward baseline. The results from additional reward-judgment tasks (Studies 2 and 3) strongly suggest that the cognitive processes of reward-value generalization parallel those of category generalization. Overall, the studies provide insights highlighting the need for integrating reward- and category-learning theories. (PsycInfo Database Record (c) 2020 APA, all rights reserved).

Pigeons exhibit flexibility but not rule formation in dimensional learning, stimulus generalization, and task switching.

A prominent model of categorization (Ashby, Alfonso-Reese, Turken, & Waldron, 1998) posits that 2 separate mechanisms-one declarative, one associative-can be recruited in category learning. These 2 systems can effectively be distinguished by 2 task structures: rule-based (RB) tasks are unidimensional and encourage analytic processing, whereas information-integration (II) tasks are bidimensional and encourage nonanalytic associative learning. Humans and nonhuman primates have been reported to learn RB tasks more quickly than II tasks; however, pigeons and rats have shown no learning speed differences are thus believed to lack the declarative system. In the present trio of experiments, we further explored pigeons' dimensional category learning. We replicated the finding that pigeons learn RB and II tasks at equal speeds. Further, we found that stimulus generalization performance was equivalent on both tasks. We also explored the effect of switching from one task to another. Task switches between phases of training as well as within individual training sessions posed little difficulty for pigeons; they quickly and flexibly switched their categorization responses with no cost in choice speed or accuracy. Together, our data indicate that, although pigeons may lack the capacity to form explicit dimensional rules, their associative learning system is both powerful and flexible. Further exploration of this associative system would help us better appreciate possible contributions of the declarative system. (PsycInfo Database Record (c) 2020 APA, all rights reserved).

The effect of tDCS on recognition depends on stimulus generalization: Neuro-stimulation can predictably enhance or reduce the face inversion effect.

This article reports results from three experiments that investigate how a particular neuro-stimulation procedure is able, in certain circumstances, to selectively increase the face inversion effect by enhancing recognition for upright faces, and argues that these effects can be understood in terms of the McLaren-Kaye-Mackintosh (MKM) theory of stimulus representation. We demonstrate how a specific transcranial direct current stimulation (tDCS) methodology can improve performance in circumstances where error-based salience modulation is making face recognition harder. The 3 experiments used an old/new recognition task involving sets of normal versus Thatcherized faces. The main characteristic of Thatcherized faces is that the eyes and the mouth are upside down, thus emphasizing features that tend to be common to other Thatcherized faces and so leading to stronger generalization making recognition worse. Experiment 1 combined a behavioral and event-related potential study looking at the N170 peak component, which helped us to calibrate the set of face stimuli needed for subsequent experiments. In Experiment 2, we used our tDCS procedure (between-subjects and double-blind) in an attempt to reduce the negative effects induced by error-based modulation of salience on recognition of upright Thatcherized faces. Results largely confirmed our predictions. In addition, they showed a significant improvement on recognition performance for upright normal faces. Experiment 3 provides the first direct evidence in a single study that the same tDCS procedure is able to both enhance performance when normal faces are presented with Thatcherized faces, and to reduce performance when normal faces are presented with other normal faces (i.e., male vs. female faces). We interpret our results by analyzing how salience modulation influences generalization between similar categories of stimuli. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

Taxonomic relations evoke more fear than thematic relations after fear conditioning: An EEG study.

When fear is generalized, knowledge based on concepts is also retrieved. Concepts have two very different relations: thematic relations based on the co-occurrence of events or scenarios, and taxonomic relations based on similarity or shared features. However, it remains unclear whether thematic and taxonomic relationships differentially affect fear generalization. To clarify the underlying cognitive mechanisms of these relations, the current study combined the classical fear conditioning procedure with electroencephalography (EEG). Forty participants were conditioned to a neutral word by pairing the presentation of the word with an unpleasant electrical pulse. A different stimulus was not paired with the electrical pulse. Next, during generalization testing, thematically related or taxonomic-related words were presented. Behavioral responses (shock expectancy and response time) and brain responses (event-related potentials [ERP] and oscillation activity) were recorded. Behavioral results showed that taxonomic relations initiated higher shock expectancy compared with thematic relations, and that conceptual relations did not affect response times. Taxonomic relations induced larger P2 components than thematic relations, and danger generalization stimuli initiated smaller P600 components than safe generalization stimuli. In addition, the magnitudes of alpha and beta oscillations were larger for danger generalization stimuli. These results suggested that taxonomic stimuli generalize broader responses compared with thematic relations after fear conditioning. Therefore, we report a possible new electrophysiological evidence for the presentation of fear generalization. These findings aid our understanding of fear generalization at the concept level and have clinical implications for the cognitive treatment of anxiety disorders.

Modeling Hierarchical Versus Random Exposure Schedules in Pavlovian Fear Extinction: No Evidence for Differential Fear Outcomes.

In exposure therapy, the client can either be confronted with the fear-eliciting situations in a hierarchical way or in a random way. In the current study we developed a procedure to investigate the effects of hierarchical versus random exposure on long-term fear responding in the laboratory. Using a fear conditioning procedure, one stimulus (CS+) was paired with an electric shock (US), whereas another stimulus was not paired with the shock (CS-). The next day, participants underwent extinction training including presentations of the CS-, CS+ and a series of morphed stimuli between the CS- and CS+. In the hierarchical extinction condition (HE; N = 32), participants were first presented with the CS-, subsequently with the morph most similar to the CS-, then with the morph most similar to that one, and so forth, until reaching the CS+. In the random extinction condition (RE; N = 32), the same stimuli were presented but in a random order. Fear responding to the CS+, CS- and a new generalization stimulus (GS) was measured on the third day. Higher expectancy violation, t(62) = -2.67, p = .01, physiological arousal, t(62) = -2.08, p = .04, and variability in US-expectancy ratings, t(62) = -2.25, p = .03, were observed in the RE condition compared to the HE condition, suggesting the validity of this novel procedure. However, no differences between the RE and HE condition were found in fear responding as tested one day later, F(1, 62) < 1. In conclusion, we did not find evidence for differential long-term fear responding in modeling hierarchical versus random exposure in Pavlovian fear extinction.

Behavioural Response to the Environmental Changes of Various Types in Lister-Hooded Male Rats.

The animal preference for complexity is most clearly demonstrated when the environmental change takes the form of an increase in complexity. Therefore, one of the potential difficulties in interpretation is that the preference for perceptual novelty may be confounded with the change in environmental complexity. In this study, the environmental complexity was controlled by manipulating with tunnels inside the experimental chamber. Adding new tunnels triggered a very profound change in behaviour, which was demonstrated by the animals' prolonged stay in the proximity of the novel objects, sniffing, touching, and climbing on top of the tunnels. The removal of the tunnels from the test arena turned out to have the least influence on behaviour compared to the other manipulations used in this study. The reduction of complexity of the tunnels had a moderate effect on rat behavior. Tunnels are important elements in the rats' environment, since they provide various possibilities for hiding, resting or moving inside the tunnel. They may be treated as a good example of affordances in rat-environment interactions. The results of this study may therefore serve as a basis for constructing a modified theory of animal curiosity which could incorporate the concept of ecological psychology.

The Effect of Outcome Probability on Generalization in Predictive Learning.

People apply what they learn from experience not only to the experienced stimuli, but also to novel stimuli. But what determines how widely people generalize what they have learned? Using a predictive learning paradigm, we examined the hypothesis that a low (vs. high) probability of an outcome following a predicting stimulus would widen generalization. In three experiments, participants learned which stimulus predicted an outcome (S+) and which stimulus did not (S-) and then indicated how much they expected the outcome after each of eight novel stimuli ranging in perceptual similarity to S+ and S-. The stimuli were rings of different sizes and the outcome was a picture of a lightning bolt. As hypothesized, a lower probability of the outcome widened generalization. That is, novel stimuli that were similar to S+ (but not to S-) produced expectations for the outcome that were as high as those associated with S+.

Generalization of response patterns in a multiple peak procedure.

Stimulus generalization is typically assessed by analyzing overall response rates. Studies of generalization of response-rate patterns across time are less common, despite the ubiquitous nature of time and the strong temporal control over behavior in the natural world. Thus, we investigated generalization of response-rate patterns across time using a multiple peak procedure in pigeons. The frequency (fast or slow) at which the color of a keylight changed signaled a fixed-interval (FI) 5-s or 20-s schedule, counterbalanced across subjects. In peak trials, the frequency of keylight-color changes was varied. For the fast and slow training stimuli, response rates in peak trials were controlled by the arranged FI schedule value; they increased as the arranged reinforcer time approached, and decreased thereafter. Response-rate patterns to all test stimuli were similar to response-rate patterns to the slow training stimulus for all subjects. Thus, overall, strong generalization from the slow training stimulus to all test stimuli was evident, whereas there was little to no generalization from the fast training stimulus. These findings extend past research examining generalization of temporally controlled response-rate patterns, and provide a useful starting point for future investigations of generalization of fixed-interval responding. A thorough understanding of generalization processes requires analysis of dependent variables other than overall response rates, especially when responding is likely to be temporally controlled.

Cortical responses to dynamic emotional facial expressions generalize across stimuli, and are sensitive to task-relevance, in adults with and without Autism.

Individuals with Autism Spectrum Disorders (ASD) report difficulties extracting meaningful information from dynamic and complex social cues, like facial expressions. The nature and mechanisms of these difficulties remain unclear. Here we tested whether that difficulty can be traced to the pattern of activity in "social brain" regions, when viewing dynamic facial expressions. In two studies, adult participants (male and female) watched brief videos of a range of positive and negative facial expressions, while undergoing functional magnetic resonance imaging (Study 1: ASD n = 16, control n = 21; Study 2: ASD n = 22, control n = 30). Patterns of hemodynamic activity differentiated among facial emotional expressions in left and right superior temporal sulcus, fusiform gyrus, and parts of medial prefrontal cortex. In both control participants and high-functioning individuals with ASD, we observed (i) similar responses to emotional valence that generalized across facial expressions and animated social events; (ii) similar flexibility of responses to emotional valence, when manipulating the task-relevance of perceived emotions; and (iii) similar responses to a range of emotions within valence. Altogether, the data indicate that there was little or no group difference in cortical responses to isolated dynamic emotional facial expressions, as measured with fMRI. Difficulties with real-world social communication and social interaction in ASD may instead reflect differences in initiating and maintaining contingent interactions, or in integrating social information over time or context.

Tactile recognition of visual stimuli: Specificity versus generalization of perceptual learning.

Sensory substitution devices aim at assisting a deficient sensory modality by means of another sensory modality. For instance, to perceive with visual-to-tactile devices, users learn to recognize visual stimuli through their tactile conversion. A crucial characteristic of learning lies in the ability to generalize, that is, the ability to extend the acquired perceptual abilities to both new stimuli and new perceptual conditions. The study reported here investigated the perceptual learning of tactile alphanumerical stimuli. The learning protocol consisted in alternating a repeated list of symbols with lists of new symbols. A first experiment revealed that, when each list consisted of 4 stimuli, recognition performance improved over time only for the repeated list. This result suggests that learning a small set of stimuli involves stimulus-specific learning strategies, preventing generalization. A second experiment revealed that increasing to six the set of learned stimuli results in higher generalization abilities. This result can be explained by greater difficulties in using stimulus-specific strategies in this case, thereby favouring the use of generalization strategies. Feature variability also appeared to be important to achieve generalization. Thus, as in visual perceptual learning, the involvement of stimulus-specific versus general strategies depends on task difficulty and feature variability. A third experiment highlighted that tactile perceptual learning generalizes to changes in orientation. These results are discussed in terms of brain plasticity as they influence the design of learning methods for using sensory substitution devices, with the aim to compensate visual impairments.

Rule-based generalisation in single-cue and differential fear conditioning in humans.

Fear generalisation refers to the spread of conditioned fear to stimuli similar but distinct from the original conditioned stimulus. In this study, participants were presented with repeated pairings of a conditioned stimulus with a shock, in either a single-cue or differential conditioning paradigm. Generalisation of fear was then tested by presenting stimuli that were novel, but similar to the conditioned stimulus along a spatial stimulus dimension. Dependent measures were online shock expectancy ratings and skin conductance level. A diverse range of generalisation gradients was observed, and the shape of the gradients for both expectancy ratings and skin conductance responses corresponded with participants' verbally reported rules. The findings point to an important role for cognitively controlled processes in human fear generalisation, and provide support for a single-system learning model. They also highlight the potential importance of cognitive reappraisal in clinical treatments for over-generalised fear.

Relationship of Reaction Time to Perception of a Stimulus and Volitionally Delayed Response.

BACKGROUND AND OBJECTIVE: Initiation of response in a simple reaction time (RT) task may precede conscious perception of the stimulus. Since volitionally delayed responses may require conscious perception of the stimulus before response initiation, it has been hypothesized that volitionally delayed responses will markedly delay RT. METHODS: We conducted two experiments with separate groups of healthy volunteers (n=16; n=13) who performed computerized simple and choice RT tasks. In the standard condition, we instructed the participants to respond to a visual stimulus by pushing a button as quickly as possible. In the second condition, we instructed the participants to respond after a slight volitional delay. The second experiment had an additional volitional delay condition in which we asked participants to delay their responses by an estimated 50% above their usual standard response. RESULTS: We found marked delays and increased variability when participants volitionally delayed their responses, averaging 322 ms for standard and 861 ms for delayed simple RTs (267% increase), and 650 ms for standard and 1018 ms for delayed choice RTs (157% increase). Effects did not differ across age, sex, or handedness. However, a minority of participants did not meaningfully delay their RT during the volitional delay conditions. CONCLUSIONS: On average, participants had marked delays when they tried to delay their responses slightly, but a subset of participants exhibited essentially no delay despite trying to delay. We suggest some potential mechanisms that future investigations might delineate.

Neural Substrates of Overgeneralized Conditioned Fear in PTSD.

OBJECTIVE: Heightened generalization of fear from an aversively reinforced conditioned stimulus (CS+, a conditioned danger cue) to resembling stimuli is widely accepted as a pathogenic marker of posttraumatic stress disorder (PTSD). Indeed, a distress response to benign stimuli that "resemble" aspects of the trauma is a central feature of the disorder. To date, the link between overgeneralization of conditioned fear and PTSD derives largely from clinical observations, with limited empirical work on the subject. This represents the first effort to examine behavioral and brain indices of generalized conditioned fear in PTSD using systematic methods developed in animals known as generalization gradients: the gradual decline in conditioned responding as the presented stimulus gradually differentiates from CS+. METHOD: Gradients of conditioned fear generalization were assessed using functional MRI and behavioral measures in U.S. combat veterans who served in Iraq or Afghanistan and had PTSD (N=26), subthreshold PTSD (N=19), or no PTSD (referred to as trauma control subjects) (N=17). Presented stimuli included rings of graded size, with extreme sizes serving as CS+ (paired with shock) and as a nonreinforced conditioned stimulus (CS-, a conditioned safety cue), and with intermediate sizes forming a continuum of similarity between CS+ and CS-. Generalization gradients were assessed as response slopes from CS+, through intermediate ring sizes, to CS-, with less steep slopes indicative of stronger generalization. RESULTS: Relative to trauma control subjects, PTSD patients showed stronger conditioned generalization, as evidenced by less steep generalization gradients in both behavioral risk ratings and brain responses in the left and right anterior insula, left ventral hippocampus, dorsolateral and dorsomedial prefrontal cortex, and caudate nucleus. Severity of PTSD symptoms across the three study groups was positively correlated with levels of generalization at two such loci: the right anterior insula and left ventral hippocampus. CONCLUSIONS: The results point to evidence of brain-based markers of overgeneralized fear conditioning related to PTSD. These findings provide further understanding of a central yet understudied symptom of trauma-related psychopathology.

Fear conditioning and stimulus generalization in patients with social anxiety disorder.

Although overgeneralization seems to be a hallmark of several anxiety disorders, this until now has not been investigated in social anxiety disorder (SAD). Therefore, we examined fear generalization in 26 SAD patients and 29 healthy controls (HC) using two faces as conditioned stimuli (CS+, CS-), and a loud scream and a fearful face as unconditioned stimulus (US). Generalization was tested by presenting both CS and four morphs of the two faces (generalization stimuli [GSs]), while ratings, heart rate (HR) and skin conductance responses (SCR) were recorded. Results revealed that SAD patients rated all stimuli as less pleasant and more arousing compared to HC. Moreover, ratings and SCR indicated that both groups generalized their acquired fear from the CS+ to GSs. Remarkably, only SAD patients showed generalization in HR responses (fear bradycardia). Overall, SAD seems not to be characterized by strong overgeneralization but discrepancies in fear responses to both conditioned and generalized threat stimuli.

Amnesic patients show superior generalization in category learning.

OBJECTIVE: Generalization is the application of existing knowledge to novel situations. Questions remain about the precise role of the hippocampus in this facet of learning, but a connectionist model by Gluck and Myers (1993) predicts that generalization should be enhanced following hippocampal damage. METHOD: In a two-category learning task, a group of amnesic patients (n = 9) learned the training items to a similar level of accuracy as matched controls (n = 9). Both groups then classified new items at various levels of distortion. RESULTS: The amnesic group showed significantly more accurate generalization to high-distortion novel items, a difference also present compared to a larger group of unmatched controls (n = 33). CONCLUSIONS: The model prediction of a broadening of generalization gradients in amnesia, at least for items near category boundaries, was supported by the results. Our study shows for the first time that amnesia can sometimes improve generalization. (PsycINFO Database Record

Linking perceptual learning with identical stimuli to imagery perceptual learning.

Perceptual learning is usually thought to be exclusively driven by the stimuli presented during training (and the underlying synaptic learning rules). In some way, we are slaves of our visual experiences. However, learning can occur even when no stimuli are presented at all. For example, Gabor contrast detection improves when only a blank screen is presented and observers are asked to imagine Gabor patches. Likewise, performance improves when observers are asked to imagine the nonexisting central line of a bisection stimulus to be offset either to the right or left. Hence, performance can improve without stimulus presentation. As shown in the auditory domain, performance can also improve when the very same stimulus is presented in all learning trials and observers were asked to discriminate differences which do not exist (observers were not told about the set up). Classic models of perceptual learning cannot handle these situations since they need proper stimulus presentation, i.e., variance in the stimuli, such as a left versus right offset in the bisection stimulus. Here, we show that perceptual learning with identical stimuli occurs in the visual domain, too. Second, we linked the two paradigms by telling observers that only the very same bisection stimulus was presented in all trials and asked them to imagine the central line to be offset either to the left or right. As in imagery learning, performance improved.

Stimulus generalization and operant context renewal.

Context renewal is the relapse of an extinguished response due to changing the stimulus context following extinction. Reinforcing operant responding in Context A and extinguishing in Context B results in relapse when either returning to Context A (ABA renewal) or introducing a novel Context C (ABC renewal). ABA renewal typically is greater than ABC renewal. The present study assessed whether renewal might be conceptualized through excitatory and inhibitory generalization gradients inferred from studies of stimulus generalization. We arranged one keylight-color alternation frequency for pigeons to signal reinforcement in Phase 1 and a different alternation frequency to signal extinction in Phase 2. During a subsequent test in extinction, we presented a range of keylight-alternation frequencies and found renewal to be a function of keylight-alternation frequency. Specifically, Phase-3 responding increased as keylight-alternation frequency differed from that arranged during extinction in Phase 2. Moreover, we observed a shift in the function beyond the originally reinforced keylight-alternation frequency arranged in training (i.e., peak shift). We discuss the relevance of these findings for conceptualizing stimulus-control processes governing generalization gradients for understanding the processes underlying context renewal.