Classes Of Stimuli Research Articles

Identification of Neurophysiological Markers of Verbal Information Processing Using Cognitive Evoked Potentials for Studying Schizophrenia Spectrum Disorders.

Cognitive evoked potentials were studied in 40 students of higher school with high and low (control) scores obtained on the Schizotypal Personality Questionnaire (SPQ) in response to the presentation of visual verbal information in the form of agreed word-combinations. In the first series of presentations, it was necessary to refer the attribute to one of the categories and to read the noun silently (non-target condition). In the second series, the attribute should be read silently, and then the following noun has to be categorized (target condition). There has been performed a cluster analysis of the evoked potential curves obtained in response to the noun presentation in the target and non-target conditions in the groups of participants with high and low scores gained on SPQ. Processing of the verbal stimulus under passive reading conditions and under the conditions of word categorization within the frameworks of a simple context has shown that in the group with low scores on the schizotypal questionnaire, lateralization of the N150 component to the left side was observed in contrast to the group with high scores. In this group, increase of the N400 component amplitude was found in response to the words presented for their passive reading in comparison with the categorization condition. On the contrary, in the group with high schizotypy scores, the N400 component appeared to be insensitive to the type of the task, i.e. neurophysiological differences were not expressed between reading and performing stimuli categorization task. These persons were found to have the decreased amplitude of the late positive component as compared to the control group under the condition of passive reading of the words. Increase of the late negative wave amplitude was registered in response to the target words subjected to categorization in comparison with reading in both groups of participants. Insufficiency of language lateralization and deficit connected with the language networks being activated automatically at the initial stage of word recognition are observed in the examined persons with schizotypy. Additionally, processing of the information in these persons at the stage of lexical-semantic processes is weakly modulated by the task imposing different requirements on the level of mental activity control. Some reduction of mnestic process activation is also possible, which is manifested during passive reading of the words, but not in the conditions of their categorization. The research conducted may serve as a starting point for a more detailed and long-term study of the fundamental mechanisms of impairment in information processing in the course of clinically evident pathology formation. In the applied aspect, the work may be considered as a contribution to the ongoing search for neurophysiological markers for early diagnosis of schizophrenia spectrum disorders.

Attentional adjustment in priming tasks: control strategies depend on context

Goal-directed behavior is assumed to require processes of attentional biasing to counter unwanted action tendencies elicited by distracting stimulus information. This is particularly so if stimulus categories that define the target and the distractor frequently reverse, requiring participants to respond to previously ignored stimulus categories and vice versa. In the current study, we investigated control strategies under such conditions. Specifically, we assessed trial-to-trial modulation of distractor-interference (i.e., congruency sequence effect, CSE) in a temporal flanker task associated with repetition versus alternation of the assignment of stimulus category (i.e., digits, letters) to targets and distractors (i.e., the character presented second or first, respectively) under conditions of a long SOA of 1000 ms (Experiment 1A) and 1200 ms (Experiment 1B). Whereas previous research, using a shorter SOA, suggested temporal-order control (i.e., the occurrence of a CSE in both repetition and—albeit less pronounced—alternation trials), lengthening the distractor-target SOA resulted in a CSE confined to repetition trials, suggesting strong or exclusive reliance on stimulus categories for attentional control (Experiment 1A and B). Adding a redundant stimulus feature (i.e., color), discriminating targets and distractors, eliminated the difference of CSE patterns in repetition and alternation trials (Experiment 2). Together, our results suggest that the strength of concurrently applied control strategies or the choice of a particular control strategy depend on contextual factors.

Investigating interactions between types of order in categorization

This study simultaneously manipulates within-category (rule-based vs. similarity-based), between-category (blocked vs. interleaved), and across-blocks (constant vs. variable) orders to investigate how different types of presentation order interact with one another. With regard to within-category orders, stimuli were presented either in a “rule plus exceptions” fashion (in the rule-based order) or by maximizing the similarity between contiguous examples (in the similarity-based order). As for the between-category manipulation, categories were either blocked (in the blocked order) or alternated (in the interleaved order). Finally, the sequence of stimuli was either repeated (in the constant order) or varied (in the variable order) across blocks. This research offers a novel approach through both an individual and concurrent analysis of the studied factors, with the investigation of across-blocks manipulations being unprecedented. We found a significant interaction between within-category and across-blocks orders, as well as between between-category and across-blocks orders. In particular, the combination similarity-based + variable orders was the most detrimental, whereas the combination blocked + constant was the most beneficial. We also found a main effect of across-blocks manipulation, with faster learning in the constant order as compared to the variable one. With regard to the classification of novel stimuli, learners in the rule-based and interleaved orders showed generalization patterns that were more consistent with a specific rule-based strategy, as compared to learners in the similarity-based and blocked orders, respectively. This study shows that different types of order can interact in a subtle fashion and thus should not be considered in isolation.

Food-Induced Brain Activity in Children with Overweight or Obesity versus Normal Weight: An Electroencephalographic Pilot Study.

Although increased food cue reactivity is evidenced to be crucial to the development and maintenance of pediatric obesity, virtually nothing is known about the underlying neurophysiological aspects of food cue reactivity in children with obesity. Therefore, this study aimed at investigating neural characteristics in children with overweight or obesity using electroencephalography (EEG). Electrophysiological brain activity was measured using EEG frequency band analysis in n = 9 children with overweight or obesity versus n = 16 children with normal weight (8-13 years) during the presentation of high- and low-calorie food pictures and images of appealing non-food stimuli. Children with overweight or obesity showed significantly increased relative central beta band activity induced by high-calorie foods and appealing non-food stimuli compared to children with normal weight. Beyond significant effects of the scalp region on EEG activity, non-significant effects of stimulus category or weight status were seen for theta and alpha frequency bands. This study demonstrated elevated beta band activity in children with overweight or obesity when viewing high-calorie food stimuli. Beta band activity may, thus, be a valuable target for neuromodulatory interventions in children with overweight or obesity.

Alzheimer’s pathology disrupts cortical processing of repeated stimuli in cognitively intact older adults

AbstractBackgroundWe used functional imaging (fMRI) and generative modelling to investigate effects of Alzheimer’s disease (AD) pathology (β‐amyloid/Aβ, pathological tau) on cortical mechanisms that support differentiation of novel and repeated stimuli.Method66 subjects (45 Older Adults (OA), 21 Younger Adults) underwent an fMRI task involving stimuli of novel and repeated scenes and objects (stimulus conditions; scenes and objects, context condition; repetition); 42 OA also had PIB‐PET measurement of Aβ and flortaucipir (FTP)‐PET of pathological tau. fMRI data were decomposed into functional networks using group spatial Independent Component Analysis (ICA). We used Dynamic Causal Modelling (DCM) to predict local cortical activity from the time‐course associated with stimulus‐related activity and stimulus repetition. AD pathology was defined as Entorhinal cortex FTP‐SUVR (EC‐tau) and continuous global Aβ. A hierarchical approach uncovered the optimal model that best described our data while considering subject variability due to Aβ and EC‐tau.ResultFive ICA networks were selected for DCM analysis (Figure 1), two input; related to stimulus category (LOC Objects>Scenes: t(65) = 21.64 p<0.001; PPA Scenes>Objects: t(65) = 22.4 p<0.001) and three higher order showing significant effects of repetition (DMN (t(65) = 14.04 p<0.001, MTL t(65) = 11 p<0.001, SAL ‐13.14, p<0.001). Time‐courses modelled using DCM included all possible connections (except between LOC and PPA), allowing repetition to modulate any of these connections with stimuli modelled as inputs to LOC and PPA. From this analysis we observed very strong evidence that increased Aβ removes the inhibition of the DMN by MTL leading to over stimulation when stimuli are repeated (Commonality(repeated) ‐0.38 Pp = 1; Ab(repeated) 1.99, Pp = 1). Conversely, we observed very strong evidence that increased EC‐tau removes the inhibition of the MTL by DMN when stimuli are repeated leading to over stimulation when stimuli are repeated (Commonality(repeated) ‐0.29, Pp = 1, EC‐tau(repeated) 1.7, Pp = 1).ConclusionAD pathology disrupts the normal cortical processes that subserve the ability to recognise repeated stimuli in cognitively intact OA. Critically, global Aβ disinhibits the DMN and EC‐tau disinhibits the MTL, two previously described functional phenomena linked to AD pathology. These models provide a systems‐level account of how pathology affects interactions between functional brain networks that may underlie cognitive decline.

Bilingualism can cause enhanced monitoring and occasional delayed responses in a flanker task.

Complex cognitive tasks require different stages of processing (i.e. conflict monitoring, attentional resource allocation and stimulus categorisation). Performance differences between bilinguals and monolinguals on conflict tasks can be affected by the balance of these sub-processes. The current study investigated the effect of bilingualism on these sub-processes during a conflict task with medium monitoring demand. Behavioural responses and evoked potentials from bilinguals and monolinguals were examined during a flanker task with 25% incongruent trials. Behavioural differences were analysed by means of averaged response times and exponentially modified Gaussian analyses of response time distributions. For evoked potentials, the study focussed on N2 (reflecting conflict monitoring) and P3 responses (reflecting allocation of attentional resources for cognitive control). Bilinguals had significantly longer response distribution tails compared to monolinguals. Bilinguals were shown to have a more pronounced N2 and smaller P3 compared to monolinguals, independent of condition, suggesting a different balance of sub-processes for the two groups. This suggests that bilinguals were engaged more strongly in monitoring processes, leading to the allocation of fewer attentional resources during stimulus categorisation. Additionally, the P3 amplitudes were negatively related with the length of response distribution tails for bilinguals. These results are consistent with enhanced conflict monitoring in bilinguals that led to reduced engagement of attentional resources for stimulus categorisation. This enhanced conflict monitoring could lead to occasional extremely slow responses. Thus, the bilingual experience appears to impact the balance of cognitive control processes during conflict tasks, which might only be reflected in a minority of responses.

Touchscreen-based approach-avoidance responses to appetitive and threatening stimuli

Background and objectivesIndividuals are thought to be biased towards approaching positive stimuli and avoiding negative stimuli. Yet, it is unclear whether this general pattern applies to all stimulus classes or whether biases are more specific. We expected significant approach biases towards two types of positive stimuli, appetitive foods and butterflies; and avoidance biases away from two types of negative stimuli, spoiled foods and spiders. MethodsA touchscreen-based Approach-Avoidance Task (AAT), using hand gestures toward or away from stimuli assessed biases. Questionnaires and image ratings assessed individual differences in stimulus evaluations. ResultsApproach biases for butterflies and appetitive foods were found, the latter being strongest towards individually liked foods. There was no avoidance bias for spoiled foods. An avoidance bias for spiders was found in individuals with elevated spider fear. LimitationsIncomplete counterbalancing precluded direct comparison between both positive and negative stimuli. ConclusionsBehavioural biases in the touchscreen AAT generally co-vary with individuals’ affective evaluation of the stimuli. Approach biases were elicited by positive stimuli independently of whether they were regularly (foods) or rarely (butterflies) approached in everyday life. This may hint towards a tendency to approach positive stimuli regardless of the specific category, whereas avoidance biases may be more stimulus specific.

The mechanisms supporting holistic perception of words and faces are not independent.

The question of whether word and face recognition rely on overlapping or dissociable neural and cognitive mechanisms received considerable attention in the literature. In the present work, we presented words (aligned or misaligned) superimposed on faces (aligned or misaligned) and tested the interference from the unattended stimulus category on holistic processing of the attended category. In Experiment 1, we found that holistic face processing is reduced when a face was overlaid with an unattended, aligned word (processed holistically). In Experiment 2, we found a similar reduction of holistic processing for words when a word was superimposed on an unattended, aligned face (processed holistically). This reciprocal interference effect indicates a trade-off in holistic processing of the two stimuli, consistent with the idea that word and face recognition may rely on non-independent, overlapping mechanisms.

Cetacean responses to violation of expectation paradigm in a free-swim context.

The investigation of individual responses to unexpected stimuli or outcomes provides insights into basic cognitive processes, such as mental representations, emotional states of surprise, and detections of anomalies. Three experiments using a violation of expectation paradigm were conducted with 12 belugas and 17 bottlenose dolphins in managed care to test two classes of stimuli (humans and objects) in manipulated sequences of familiar and unfamiliar humans (Study 1, trainers and strangers), familiar and unfamiliar objects (Study 2, typical enrichment devices and new objects), and finally objects and humans (Study 3). Gaze durations were assessed for each condition in a given study during free-swim contexts. The results supported previous findings that visual stimuli, regardless of class, were stimulating and intriguing for both belugas and bottlenose dolphins. Belugas were more likely to gaze longer at human and object stimuli and tended to gaze longer at unexpected experiences than control or expected experiences. Bottlenose dolphins showed similar trends except when objects were involved. Individual variability was present for both species with some individuals showing stronger patterns of responses for expected experiences than others. After 2years of intermittent experiments, belugas and bottlenose dolphins in managed care maintained their curiosity about visual stimuli, for which they received no primary reinforcement. Investigating responses to unexpected stimuli with animals in managed care may provide insight into how these animals respond to biologically relevant conditions, such as boat presence, predators, and unfamiliar conspecifics.

The effect of short-term training on repetition probability effects for non-face objects

The magnitude of repetition suppression (RS) is modulated by the probability of stimulus repetitions when measured by fMRI. This repetition probability effect (P(rep)) is usually expressed in a stronger RS when the probability of repetition trials is higher when compared to blocks with less frequent repetitions. Previous studies have suggested that the P(rep) effect depends on the extensive long-term experience of participants with the stimuli. At the same time, the how short-term learning affects RS and its modulation by P(rep) remains largely unknown. To address this issue, we used fMRI and measured the RS and its modulation by P(rep) for non-face objects (cars) before and after a 10-day long perceptual learning (PL) period and for unfamiliar faces as control. The results showed a significant P(rep) effect for faces within the Fusiform Face Area (FFA) and for cars within the Lateral Occipital Complex (LO) in the pre-training fMRI measurement session. Following the PL period, participants exhibited strong improvements in the subordinate categorization of the trained stimuli. Surprisingly, the magnitude of RS did not change as a function of training, but the P(rep) effect was absent in the post-training fMRI sessions for both stimulus categories. These results suggest that the predictive processes, measured by P(rep) modulation of RS, may be modulated by the short-term perceptual learning experience.

The contribution of the left precuneus to emotion memory in migraine without aura patients.

The impact of migraine without aura (MWoA) on cognitive function remains controversial, especially given the sparse literature on emotional memory. Twenty seven MWoA patients and 25 healthy controls (HCs) were enrolled in this cross-sectional study. Emotional memory behavior was evaluated by combining incidental encoding with intentional encoding of five emotional categories of visual stimulus [positive valence + high arousal (PH), negative valence + high arousal (NH), positive valence + low arousal (PL), negative valence + low arousal (NL), and neutral (N)]. The recollection performance (Pr) was measured and compared. Then, the neural relevance was explored by correlating the Pr with gray matter volume (GMV) and resting-state functional connectivity (rs-FC) based on structural and functional magnetic resonance imaging. No significant differences in recollection performance or emotional enhancement of memory effect were observed. However, MWoA patients were more sensitive to the valence and arousal of emotional stimuli under incidental encoding. Significantly, the Pr-PH under incidental encoding and Pr-PL under intentional encoding were negatively correlated with the GMV of the left precuneus, and the rs-FC between the left precuneus and putamen was positively correlated with Pr-PL under intentional encoding in MWoA patients. Our study demonstrated the tendency for the influence of migraine on emotional memory and revealed the left precuneus as a critical contributor to recollection performance, providing novel insights for understanding emotional memory and its neural mechanisms in MWoA patients.

From Bayes-optimal to heuristic decision-making in a two-alternative forced choice task with an information-theoretic bounded rationality model.

Bayes optimal and heuristic decision-making schemes are often considered fundamentally opposed to each other as a framework for studying human choice behavior, although recently it has been proposed that bounded rationality may provide a natural bridge between the two when varying information-processing resources. Here, we investigate a two-alternative forced choice task with varying time constraints, where subjects have to assign multi-component symbolic patterns to one of two stimulus classes. As expected, we find that subjects' response behavior becomes more imprecise with more time pressure. However, we also see that their response behavior changes qualitatively. By regressing subjects' decision weights, we find that decisions allowing for plenty of decision time rely on weighing multiple stimulus features, whereas decisions under high time pressure are made mostly based on a single feature. While the first response pattern is in line with a Bayes-optimal decision strategy, the latter could be considered as an instantiation of heuristic decision-making with cue discounting. When fitting a bounded rational decision model with multiple feature channels and varying information-processing capacity to subjects' responses, we find that the model is able to capture subjects' behavioral change. The model successfully reflects the simplicity of heuristics as well as the efficiency of optimal decision making, thus acting as a bridge between the two approaches.

Restricting movements of lower face leaves recognition of emotional vocalizations intact but introduces a valence positivity bias

Blocking facial mimicry can disrupt recognition of emotion stimuli. Many previous studies have focused on facial expressions, and it remains unclear whether this generalises to other types of emotional expressions. Furthermore, by emphasizing categorical recognition judgments, previous studies neglected the role of mimicry in other processing stages, including dimensional (valence and arousal) evaluations. In the study presented herein, we addressed both issues by asking participants to listen to brief non-verbal vocalizations of four emotion categories (anger, disgust, fear, happiness) and neutral sounds under two conditions. One of the conditions included blocking facial mimicry by creating constant tension on the lower face muscles, in the other condition facial muscles remained relaxed. After each stimulus presentation, participants evaluated sounds’ category, valence, and arousal. Although the blocking manipulation did not influence emotion recognition, it led to higher valence ratings in a non-category-specific manner, including neutral sounds. Our findings suggest that somatosensory and motor feedback play a role in the evaluation of affect vocalizations, perhaps introducing a directional bias. This distinction between stimulus recognition, stimulus categorization, and stimulus evaluation is important for understanding what cognitive and emotional processing stages involve somatosensory and motor processes.

Blood glucose kinetics during concentric and eccentric muscle contractions

Studies comparing the two classes of stimuli (concentric and eccentric) have shown differences in the improvement of cardiovascular, metabolic, and muscle strength gain. This is an experimental, quantitative, and prospective study that aimed to verify the effect of eccentric exercise on glycolytic consumption and kinetics. The blood glucose kinetics of 17 male subjects was evaluated during a treadmill exercise with a 10% declined floor and velocity that required a 60% metabolic activity of VO2max, for 30 minutes. Seventy-two hours later, the same subjects exercised on the treadmill with a 10% inclined floor and 60% VO2max, for 30 minutes. To quantify glucose, blood samples were collected before the exercise, every three minutes along the 30 minutes of physical activity, and five and 10 minutes after finishing the exercise. For the downward slope, there was a homogeneous group behavior for blood glucose dynamics during the exercise, which was characterized by a monotonic decrease of glucose levels until reaching a minimum value at experimental times between 20 and 30 min, followed by a progressive recovery toward initial values. For the acclivity condition, blood glucose dynamics did not follow such a homogeneous behavior. A set of different types of dynamics could be identified. Experimental data showed that the type of dynamics could be predicted, to some extent, by the basal blood glucose level of subjects. The type of floor slope (upward or downward) directly affected glycolytic consumption and kinetics for the individuals analyzed.

Age differences in visual statistical learning: Investigating the effects of selective attention and stimulus category.

A cognitive function that is of interest when investigating age-related changes is statistical learning-the ability to detect and extract regularities in sensory information from our rich, dynamic, and complex environment. A previous study has suggested that there were age differences in visual statistical learning, with older adults demonstrating visual statistical learning of attended and unattended information (due to the "hyper-binding effect"). In the present study, we were interested in investigating whether there are age differences in visual statistical learning and whether stimulus category influenced visual statistical learning of unattended information in older adults. We tested two stimulus categories: highly familiar line drawings and abstract shapes. Participants completed a selective-attention task, in which regularities were embedded into both the attended and unattended visual streams. Then, participants completed a triplet-discrimination task, which assessed their ability to extract regularities from the attended and unattended visual streams. We also implemented a 4-point confidence-rating scale in the triplet-discrimination task as an assessment of participants' awareness of these regularities. There were four key findings. First, selective attention modulates visual statistical learning, with greater visual statistical learning for attended information than for unattended information. Second, there were age differences in visual statistical learning, but these differences were only observed for visual statistical learning of attended information. Third, stimulus category did not affect visual statistical learning of unattended information in older adults. Fourth, visual statistical learning occurs with awareness of statistical regularities. Further research is warranted to investigate the age-related mechanisms underlying visual statistical learning. (PsycInfo Database Record (c) 2022 APA, all rights reserved).

Functional imaging analyses reveal prototype and exemplar representations in a perceptual single-category task

Similarity-based categorization can be performed by memorizing category members as exemplars or by abstracting the central tendency of the category – the prototype. In similarity-based categorization of stimuli with clearly identifiable dimensions from two categories, prototype representations were previously located in the hippocampus and the ventromedial prefrontal cortex (vmPFC) and exemplar representations in areas supporting visual memory. However, the neural implementation of exemplar and prototype representations in perceptual similarity-based categorization of single categories is unclear. To investigate these representations, we applied model-based univariate and multivariate analyses of functional imaging data from a dot-pattern paradigm-based task. Univariate prototype and exemplar representations occurred bilaterally in visual areas. Multivariate analyses additionally identified prototype representations in parietal areas and exemplar representations in the hippocampus. Bayesian analyses supported the non-presence of prototype representations in the hippocampus and the vmPFC. We additionally demonstrate that some individuals form both representation types simultaneously, probably granting flexibility in categorization strategies.

The deviation-from-familiarity effect: Expertise increases uncanniness of deviating exemplars.

Humanlike entities deviating from the norm of human appearance are perceived as strange or uncanny. Explanations for the eeriness of deviating humanlike entities include ideas specific to human or animal stimuli like mate selection, avoidance of threat or disease, or dehumanization; however, deviation from highly familiar categories may provide a better explanation. Here it is tested whether experts and novices in a novel (greeble) category show different patterns of abnormality, attractiveness, and uncanniness responses to distorted and averaged greebles. Greeble-trained participants assessed the abnormality, attractiveness, uncanniness of normal, averaged, and distorted greebles and their responses were compared to participants who had not previously seen greebles. The data show that distorted greebles were more uncanny than normal greebles only in the training condition, and distorted greebles were more uncanny in the training compared to the control condition. In addition, averaged greebles were not more attractive than normal greebles regardless of condition. The results suggest uncanniness is elicited by deviations from stimulus categories of expertise rather than being a purely biological human- or animal-specific response.

A highly selective response to food in human visual cortex revealed by hypothesis-free voxel decomposition

Prior work has identified cortical regions selectively responsive to specific categories of visual stimuli. However, this hypothesis-driven work cannot reveal how prominent these category selectivities are in the overall functional organization of the visual cortex, or what others might exist that scientists have not thought to look for. Furthermore, standard voxel-wise tests cannot detect distinct neural selectivities that coexist within voxels. To overcome these limitations, we used data-driven voxel decomposition methods to identify the main components underlying fMRI responses to thousands of complex photographic images. Our hypothesis-neutral analysis rediscovered components selective for faces, places, bodies, and words, validating our method and showing that these selectivities are dominant features of the ventral visual pathway. The analysis also revealed an unexpected component with a distinct anatomical distribution that responded highly selectively to images of food. Alternative accounts based on low- to mid-level visual features, such as color, shape, or texture, failed to account for the food selectivity of this component. High-throughput testing and control experiments with matched stimuli on a highly accurate computational model of this component confirm its selectivity for food. We registered our methods and hypotheses before replicating them on held-out participants and in a novel dataset. These findings demonstrate the power of data-driven methods and show that the dominant neural responses of the ventral visual pathway include not only selectivities for faces, scenes, bodies, and words but also the visually heterogeneous category of food, thus constraining accounts of when and why functional specialization arises in the cortex.

Peripheral vision in matching-to-sample procedures.

Eye-tracking has been used to investigate observing responses in matching-to-sample procedures. However, in visual search, peripheral vision plays an important role. Therefore, three experiments were conducted to investigate the extent to which adult participants can discriminate stimuli that vary in size and position in the periphery. Experiment 1 used arbitrary matching with abstract stimuli, Experiment 2 used identity matching with abstract stimuli, and Experiment 3 used identity matching with simple (familiar) shapes. In all three experiments, participants were taught eight conditional discriminations establishing four 3-member classes of stimuli. Four different stimulus sizes and three different stimulus positions were manipulated in the 12 peripheral test phases. In these test trials, participants had to fixate their gaze on the sample stimulus in the middle of the screen while selecting a comparison stimulus. Eye movements were measured with a head-mounted eye-tracker during both training and testing. Experiment 1 shows that participants can discriminate small abstract stimuli that are arbitrarily related in the periphery. Experiment 2 shows that matching identical stimuli does not affect discrimination in the periphery compared to arbitrarily related stimuli. However, Experiment 3 shows that discrimination increases when stimuli are well-known simple shapes.

Reexamining nodality in equivalence classes.

Equivalence classes are defined according to the substitutability, or functional similarity, of the stimuli within a stimulus class. Several studies have demonstrated that the degree of functional similarity between stimuli in a class is dependent, in part, upon the number of nodes (intervening stimuli) between the stimuli. Higher nodal number is related to lower functional similarity. This effect is referred to as "nodality." There are three key factors that have not been simultaneously controlled for in the relevant studies: priming effects, reinforcement during training, and multiple stimulus functions of stimuli (sample, comparison, or both). In the present experiment, controlling for these factors, two 6-member, 4-node equivalence classes were established, and a within-class preference assessment was used to evaluate nodality. Of 12 participants, five achieved criterion accuracy (90%) during testing. These participants demonstrated nodality, showing preference for stimuli that were nodally proximal to a sample in the preference test. When distal comparisons were chosen, participants took longer, on average, to make the selection compared to selections of proximal stimuli. These findings are consistent with earlier studies demonstrating nodality, which suggests that nodality is a robust phenomenon and not an artifact of the factors that were controlled for in the present study.