Rituals, stereotypy and compulsive behavior in animals and humans

Environmentally enriched rearing environments reduce repetitive perseveration in caged mink, but increase spontaneous alternation

Studies spanning 15 species (including American mink, Neovison vison) demonstrate that within similarly-housed populations, individuals displaying high levels of stereotypic behaviour (SB) typically show perseverative responding (e.g. during set-shifting, or reversal/extinction learning). Similar correlations in autism and schizophrenia suggest this indicates captivity-induced cortico-striatal circuit dysfunction. However, this pattern does not prove developmental impairment: SB, perseveration and their inter-correlations also occur in normal humans. We therefore differentially-reared enriched versus non-enriched mink to investigate whether treatments that exacerbate SB correspondingly increase perseveration (Study 1). Enriched-rearing did reduce SB and perseverative response repetition (in two-choice guessing tasks), while increasing spontaneous alternation: a strategy yielding more rewards, and suggesting enhanced hippocampal development. This complements previous research demonstrating cortical/hippocampal impairments and reduced behavioural flexibility in non-enriched animals, with implications for research animals and wild animals captive-raised for reintroduction into nature. Consistent with previous data, highly stereotypic subjects repeated guessing task responses most rapidly, suggesting disinhibition during repetition. However, unexpectedly, SB and perseveration did not co-vary across individuals. We therefore suggest that behavioural changes manifest as increased perseveration are important but do not fully explain captive animals' SBs, possible reasons including the contributory role of differential motivations for underlying source behaviours. Re-analyses of old data (Study 2) confirmed that spontaneous alternation is profitable; and demonstrated that the precise methods used for quantifying perseveration and SB can modify the strength of apparent relationships between them, as can statistically controlling for feeding motivation: as predicted, partialling out motivational effects increased the variance in SB predicted by perseveration.

Religious thought and behavior

While earlier approaches to religious thought and practice searched for 'magic bullet' approaches to explain religious thought and behavior, seeing it as an example of irrationality, illusion, integrative force, symbolism, or false explanations of origins, cognitive scientific approaches have suggested that we see it rather as an aggregate of the products of various cognitive mechanisms. Studies in the cognitive science of religion, informed by experimental work, have converged on a standard model of explaining religious thought and behavior by focussing on the role of minimally counter-intuitive concepts, agent and animacy detection, ritual representations, notions of contagion and contamination avoidance, theory of mind, coalitions, and moral intuitions. WIREs Cogn Sci 2012 doi: 10.1002/wcs.1189 For further resources related to this article, please visit the WIREs website.

Video telemetry and behavioral analysis discriminate between compulsive cleaning and compulsive checking in obsessive-compulsive disorder

Obsessive-compulsive disorder (OCD) patients exhibit compulsive acts that share key characteristics that discriminate them from normal behaviors. In OCD, factor analysis of symptomatology has identified separate clusters (contamination/cleaning; harming/checking; symmetry/ordering; hoarding). Here we used video analysis of the motor characteristics of OCD compulsions derived from two separate clusters, checking and cleaning, in order to determine whether behavioral differences exist in the way these two compulsions are performed. We compared 22 behavioral components (acts) of 12 OC-cleaning rituals and 25 OC-checking rituals. A normal activity with the identical theme was matched for each OC ritual as a control. For each ritual and control, we measured 22 parameters (such as the duration and frequency of act performance), and the levels of functionality, by evaluating the degree to which the act appears to contribute toward achieving its goal. We found that both OC-cleaning and OC-checking rituals differed from their respective control activity, and that they also differ between themselves in seven out of the 22 parameters. OC-cleaning involved increased repetition of functional activity whereas OC-checking involved a relatively increased non-functional activity. These results suggest that OCD cleaning and checking rituals are sufficiently different to justify their division into different subtypes and presumably are sub-served by different mechanisms. A better understanding of the relationship between those behavioral parameters derived from video-telemetry, and the parameters assessed by means of clinical and neurobiological tools, would improve our understanding of the nosological significance of compulsive symptoms and contribute to advancing endophenotypic exploration of the heterogeneity of OCD.

Glutamate abnormalities in obsessive compulsive disorder: neurobiology, pathophysiology, and treatment

Obsessive compulsive disorder is prevalent, disabling, incompletely understood, and often resistant to current therapies. Established treatments consist of specialized cognitive-behavioral psychotherapy and pharmacotherapy with medications targeting serotonergic and dopaminergic neurotransmission. However, remission is rare, and more than a quarter of OCD sufferers receive little or no benefit from these approaches, even when they are optimally delivered. New insights into the disorder, and new treatment strategies, are urgently needed. Recent evidence suggests that the ubiquitous excitatory neurotransmitter glutamate is dysregulated in OCD, and that this dysregulation may contribute to the pathophysiology of the disorder. Here we review the current state of this evidence, including neuroimaging studies, genetics, neurochemical investigations, and insights from animal models. Finally, we review recent findings from small clinical trials of glutamate-modulating medications in treatment-refractory OCD. The precise role of glutamate dysregulation in OCD remains unclear, and we lack blinded, well-controlled studies demonstrating therapeutic benefit from glutamate-modulating agents. Nevertheless, the evidence supporting some important perturbation of glutamate in the disorder is increasingly strong. This new perspective on the pathophysiology of OCD, which complements the older focus on monoaminergic neurotransmission, constitutes an important focus of current research and a promising area for the ongoing development of new therapeutics.

Repeated assessment of exploration and novelty seeking in the human behavioral pattern monitor in bipolar disorder patients and healthy individuals

Background

Exploration and novelty seeking are cross-species adaptive behaviors that are dysregulated in bipolar disorder (BD) and are critical features of the illness. While these behaviors have been extensively quantified in animals, multivariate human paradigms of exploration are lacking. The human Behavioral Pattern Monitor (hBPM), a human version of the animal open field, identified a signature pattern of hyper-exploration in manic BD patients, but whether exploratory behavior changes with treatment is unknown. The objective of this study was to assess the sensitivity of the hBPM to changes in manic symptoms, a necessary step towards elucidating the neurobiology underlying BD.

Methodology and Principal Findings

Twelve acutely hospitalized manic BD subjects and 21 healthy volunteers were tested in the hBPM over three sessions; all subjects were retested one week after their first session and two weeks after their second session. Motor activity, spatial and entropic (degree of unpredictability) patterns of exploration, and interactions with novel objects were quantified. Manic BD patients demonstrated greater motor activity, extensive and more unpredictable patterns of exploration, and more object interactions than healthy volunteers during all three sessions. Exploration and novelty-seeking slightly decreased in manic BD subjects over the three sessions as their symptoms responded to treatment, but never to the level of healthy volunteers. Among healthy volunteers, exploration did not significantly decrease over time, and hBPM measures were highly correlated between sessions.

Conclusions/Significance

Manic BD patients showed a modest reduction in symptoms yet still demonstrated hyper-exploration and novelty seeking in the hBPM, suggesting that these illness features may be enduring characteristics of BD. Furthermore, behavior in the hBPM is not subject to marked habituation effects. The hBPM can be reliably used in a repeated-measures design to characterize exploration and novelty seeking and, in parallel with animal studies, can contribute to developing treatments that target neuropsychiatric disease.

Cortico-striatal oxidative status, dopamine turnover and relation with stereotypy in the deer mouse

The deer mouse presents with spontaneous stereotypic movements that resemble the repetitive behaviours of obsessive-compulsive disorder (OCD), and demonstrates a selective response to serotonin reuptake inhibitors. OCD has been linked to altered redox status and since increased dopamine signalling can promote stereotypies as well as oxidative stress, we investigated whether the severity of deer mouse stereotypy may be associated with altered dopamine turnover and cortico-striatal redox status. Deer mice were separated into high (HSB), low (LSB) and non-stereotypy (NS) groups. Frontal cortical and striatal dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), as well as superoxide dismutase (SOD) activity, reduced (GSH) and oxidised (GSSG) glutathione and glutathione redox index, were analysed as markers for regional dopamine turnover and oxidative stress, respectively. Dopamine and its metabolites and SOD activity did not differ across the stereotypy groups. Significantly reduced GSH and GSSG and increased glutathione redox index were only observed in the frontal cortex of HSB animals. Frontal cortical GSH and GSSG were inversely correlated while glutathione redox index was positively correlated with stereotypy. Deer mouse stereotypy is thus characterised by a deficient glutathione system in the frontal cortex but not striatum, and provides a therapeutic rationale for using glutathione-active antioxidants in OCD. The evidence for a primary frontal lesion has importance for future OCD research.

Motor and cognitive stereotypies in the BTBR T+tf/J mouse model of autism

The BTBR T+tf/J inbred mouse strain displays a variety of persistent phenotypic alterations similar to those exhibited in autism spectrum disorders (ASDs). The unique genetic background of the BTBR strain is thought to underlie its lack of reciprocal social interactions, elevated repetitive self-directed grooming, and restricted exploratory behaviors. In order to clarify the existence, range, and mechanisms of abnormal repetitive behaviors within BTBR mice, we performed detailed analyses of the microstructure of self-grooming patterns and noted increased overall grooming, higher percentages of interruptions in grooming bouts and a concomitant decrease in the proportion of incorrect sequence transitions compared to C57BL/6J inbred mice. Analyses of active phase home-cage behavior also revealed an increase in stereotypic bar-biting behavior in the BTBR strain relative to B6 mice. Finally, in a novel object investigation task, the BTBR mice exhibited greater baseline preference for specific unfamiliar objects as well as more patterned sequences of sequential investigations of those items. These results suggest that the repetitive, stereotyped behavior patterns of BTBR mice are relatively pervasive and reflect both motor and cognitive mechanisms. Furthermore, other pre-clinical mouse models of ASDs may benefit from these more detailed analyses of stereotypic behavior.

A novel, multiple symptom model of obsessive-compulsive-like behaviors in animals

BACKGROUND

Current animal models of obsessive-compulsive disorder (OCD) typically involve acute, drug-induced symptom provocation or a genetic association with stereotypies or anxiety. None of these current models demonstrate multiple OCD-like behaviors.

METHODS

Neonatal rats were treated with the tricyclic antidepressant clomipramine or vehicle between days 9 and 16 twice daily and behaviorally tested in adulthood.

RESULTS

Clomipramine exposure in immature rats produced significant behavioral and biochemical changes that include enhanced anxiety (elevated plus maze and marble burying), behavioral inflexibility (perseveration in the spontaneous alternation task and impaired reversal learning), working memory impairment (e.g., win-shift paradigm), hoarding, and corticostriatal dysfunction. Dopamine D2 receptors were elevated in the striatum, whereas serotonin 2C, but not serotonin 1A, receptors were elevated in the orbital frontal cortex.

CONCLUSIONS

This is the first demonstration of multiple symptoms consistent with an OCD-like profile in animals. Moreover, these behaviors are accompanied by biochemical changes in brain regions previously identified as relevant to OCD. This novel model of OCD demonstrates that drug exposure during a sensitive period can program disease-like systems permanently, which could have implications for current and future therapeutic strategies for this and other psychiatric disorders.

Parent-child DRD4 genotype as a potential biomarker for oppositional, anxiety, and repetitive behaviors in children with autism spectrum disorder

The primary objective of the present study was to examine whether a combination of parent-child DRD4 genotypes results in more informative biomarkers of oppositional, separation anxiety, and repetitive behaviors in children with autism spectrum disorder (ASD). Based on prior research indicating the 7-repeat allele as a potential risk variant, participants were sorted into one of four combinations of parent-child genotypes. Owing to the possibility of parent-of-origin effects, analyses were conducted separately for mother-child (MC) and father-child (FC) dyads. Mothers completed a validated DSM-IV-referenced rating scale. Partial eta-squared (ηp(2)) was used to determine the magnitude of group differences: 0.01-0.06=small, 0.06-0.14=moderate, and >0.14=large. Analyses indicated that children in MC dyads with matched genotypes had the least (7-/7-) and most (7+/7+) severe mother-rated oppositional-defiant (ηp(2)=0.11) and separation anxiety (ηp(2)=0.19) symptoms. Conversely, youths in FC dyads with matched genotypes had the least (7-/7-) and most (7+/7+) severe obsessive-compulsive behaviors (ηp(2)=0.19) and tics (ηp(2)=0.18). Youths whose parents were both noncarriers had less severe tics than peers with at least one parental carrier, and the effect size was large (ηp(2)=0.16). There was little evidence that noncarrier children were rated more severely by mothers who were carriers versus noncarriers. Transmission Disequilibrium Test analyses provided preliminary evidence for undertransmission of the 2-repeat allele in youths with more severe tics (p=0.02). Parent genotype may be helpful in constructing prognostic biomarkers for behavioral disturbances in ASD; however, findings are tentative pending replication with larger, independent samples.

Are there between-country differences in motor behavior of obsessive-compulsive disorder patients?

BACKGROUND

Cross-cultural factors attributed to obsessive-compulsive disorder (OCD) that are widely investigated around the world are mostly epidemiological, with no respect to the impact of culture on the structure of OCD behavior itself.

METHODS

Nine Israeli and nine British OCD patients with respective non-OCD individuals were compared. To determine whether OCD symptoms are consistent across cultures, similarities in behavior were analyzed, as well as differences due to a country effect. In each country, nine OCD patients and nine non-OCD individuals were videotaped while performing the task that the patients attributed to their behavior.

RESULTS

Except for a significantly higher rate of repetition and higher performance of idiosyncratic acts, patients from both Israel and the United Kingdom showed high levels of similarities in 22 out of 24 parameters. Compared with Israeli subjects, British OCD patients had significantly longer chains of idiosyncratic acts, and a twice-higher prevalence of brief (1-2 second) idiosyncratic acts. Between-country differences were mild, possibly overridden by the conspicuous impact of OCD pathology, resulting in a similar OCD phenotype.

CONCLUSION

These results qualitatively and quantitatively emphasize the universal appearance of the compulsions in OCD symptoms.

Superstitiousness in obsessive-compulsive disorder

It has been speculated that superstitiousness and obsessivecompulsive disorder (OCD) exist along a continuum. The distinction between superstitious behavior italic>and superstitious belief, however, is crucial for any theoretical account of claimed associations between superstitiousness and OCD. By demonstrating that there is a dichotomy between behavior and belief, which is experimentally testable, we can differentiate superstitious behavior from superstitious belief, or magical ideation. Different brain circuits are responsible for these two forms of superstitiousness; thus, determining which type of superstition is prominent in the symptomatology of an individual patient may inform us about the primarily affected neurocognitive systems.

Pragmatic and idiosyncratic acts in human everyday routines: the counterpart of compulsive rituals

Our daily activities are comprised of motor routines, which are behavioral templates with specific goals, typically performed in an automatic fixed manner and without much conscious attention. Such routines can seem to resemble pathologic rituals that dominate the motor behavior of obsessive-compulsive disorder (OCD) and autistic patients. This resemblance raises the question of what differentiates and what is common in normal and pathologic motor behavior. Indeed, pathologic motor performance is often construed as an extended stereotyped version of normal everyday routines. In this study we applied ethological tools to analyze six motor routines performed by 60 adult human volunteers. We found that longer normal everyday routines included more repetitions, but not more types of acts, and that in each routine, most acts were performed either by all individuals (pragmatic acts) or by only one individual (idiosyncratic components). Thus, normal routines consist in a relatively rigid part that is shared by all individuals that perform the routine, and a flexible part that varies among individuals. The present results, however, do not answer the question of whether the flexible individual part changes or remains constant over routine repetition by the same person. Comparing normal routines with OCD rituals revealed that the latter comprise an exaggeration of the idiosyncratic component. Altogether, the present study supports the view that everyday normal routines and pathologic rituals are opposite processes, although they both comprise rigid motor behavioral sequences.

Turning order into chaos through repetition and addition of elementary acts in obsessive-compulsive disorder (OCD)

A concept and methodology derived from an animal model provided the framework for a study of rituals in obsessive compulsive disorder (OCD) patients and yielded objective and observable criteria applicable for compulsive rituals across patients. The employed ethological approach should be able to reveal and identify a common structure underlying OCD rituals, pointing to shared psychopathology. Eleven OCD rituals performed by patients in their own home were videotaped and compared with the behaviour of healthy individuals instructed to perform the same rituals. The videotaped rituals were deconstructed into visits to specific locations or objects (ritual space), and to the acts performed at each location/object (ritual basic components). Quantitative analyses revealed that compulsiveness emanates from the expansion of repeats for some acts and visits, and from the addition of superfluous act types. Best discrimination between OCD and control rituals (90.9% success) was provided by the parameter "maximum of act repeats in a ritual" (R(2)=0.77). It is suggested that the identified properties of compulsive behaviour are consistent with a recent hypothesis that ritualized behaviour shifts the individual's attention from a normal focus on structured actions to a pathological attraction onto the processing of basic acts, a shift that invariably overtaxes memory. Characteristics and mechanisms of compulsive rituals may prove useful in objective assessment of psychiatric disorders, behavioural therapy, and OCD nosology.

A reverse-translational study of dysfunctional exploration in psychiatric disorders: from mice to men

CONTEXT

Bipolar mania and schizophrenia are recognized as separate disorders but share many commonalities, which raises the question of whether they are the same disorder on different ends of a continuum. The lack of distinct endophenotypes of bipolar mania and schizophrenia has complicated the development of animal models that are specific to these disorders. Exploration is fundamental to survival and is dysregulated in these 2 disorders. Although exploratory behavior in rodents has been widely studied, surprisingly little work has examined this critical function in humans.

OBJECTIVES

To quantify the exploratory behavior of individuals with bipolar mania and schizophrenia and to identify distinctive phenotypes of these illnesses.

DESIGN

Static group comparison by the use of a novel human open field paradigm, the human Behavioral Pattern Monitor (BPM).

SETTING

Psychiatric hospital.

PARTICIPANTS

Fifteen patients with bipolar mania and 16 patients with schizophrenia were compared with 26 healthy volunteers in the human BPM. The effects of amphetamine sulfate, the selective dopamine transporter inhibitor GBR12909, and the genetic knockdown of the dopamine transporter were compared with controls in the mouse BPM.

MAIN OUTCOME MEASURES

The amount of motor activity, spatial patterns of activity, and exploration of novel stimuli were quantified in both the human and mouse BPMs.

RESULTS

Patients with bipolar mania demonstrated a unique exploratory pattern, characterized by high motor activity and increased object exploration. Patients with schizophrenia did not show the expected habituation of motor activity. Selective genetic or pharmacologic inhibition of the dopamine transporter matched the mania phenotype better than the effects of amphetamine, which has been the criterion standard for animal models of mania.

CONCLUSIONS

These findings validate the human open field paradigm and identify defining characteristics of bipolar mania that are distinct from those of schizophrenia. This cross-species study of exploration calls into question an accepted animal model of mania and should help to develop more accurate human and animal models, which are essential to the identification of the neurobiological underpinnings of neuropsychiatric disorders.

Obsessive-compulsive disorder: a disorder of pessimal (non-functional) motor behavior

OBJECTIVE

To determine whether in addition to repetitiveness, the motor rituals of patients with obsessive-compulsive disorder (OCD) involve reduced functionality due to numerous and measurable acts that are irrelevant and unnecessary for task completion.

METHOD

Comparing motor rituals of OCD patients with behavior of non-patient control individuals who were instructed to perform the same motor task.

RESULTS

Obsessive-compulsive disorder behavior comprises abundant acts that were not performed by the controls. These acts seem unnecessary or even irrelevant for the task that the patients were performing, and therefore are termed 'non-functional'. Non-functional acts comprise some 60% of OCD motor behavior. Moreover, OCD behavior consists of short chains of functional acts bounded by long chains of non-functional acts.

CONCLUSION

The abundance of irrelevant or unnecessary acts in OCD motor rituals represents reduced functionality in terms of task completion, typifying OCD rituals as pessimal behavior (antonym of optimal behavior).

Developing translational animal models for symptoms of schizophrenia or bipolar mania

Animal models have long been used to explore hypotheses regarding the neurobiological substrates of and treatments for psychiatric disorders. Early attempts to develop models that mimic the entirety of the diagnostic syndromes in psychiatry have evolved into more appropriate efforts to model specific symptoms. Such an approach reflects the facts that even in patients, clinical symptoms transcend diagnostic categories, and the specific etiologies of psychiatric disorders are unknown. An animal model can only be identified adequately by specifying both the manipulation (drug, lesion, strain) used to induce abnormalities and the measure(s) used to characterize them. A wide range of pharmacological, lesion, and developmental manipulations have been combined with various measures of information processing to develop useful animal models that parallel human tests. Prepulse inhibition of startle, event-related potential (ERP) measures of auditory gating, and Cambridge neuropsychological test automated battery (CANTAB) measures of cognition are examples of measures that can be used in both rodents and humans and that are robustly altered in both psychiatric disorders and animals manipulated with appropriate drugs or lesions. The further development of cross-species models is critically important, given the new opportunities for the development and registration of specific treatments for the cognitive disorders of schizophrenia that are not ameliorated by available drugs. In moving beyond the focus on psychotic symptoms to the cognitive symptoms of schizophrenia, animal models that do not involve manipulations of dopamine D(2) receptors but that do utilize information processing measures that are correlated with cognitive disturbances are receiving increased attention. Here, selected examples of how cross-species measures of psychiatric disorders are developed and validated are discussed. Specific animal paradigms that parallel the specific domains of cognition that are altered in schizophrenia provide one focus of the review. Another focus includes efforts to develop new human models of psychiatric symptoms that are designed to parallel existing tests used in rodents.

Ritual Behavior in Obsessive and Normal Individuals

Ritualized behavior is characteristic of obsessive-compulsive disorder (OCD), but it is also observed in other, nonclinical contexts such as children's routines and cultural ceremonies. Such behaviors are best understood with reference to a set of human vigilance–precaution systems in charge of monitoring potential danger and motivating the organism toward appropriate precautions. Ritualized behavior focuses attention on low-level representations of actions, probably leading to some measure of intrusion suppression. Cultural rituals too may be understood in this framework.

Behavioral and in vitro correlates of compulsive-like food seeking induced by operant conditioning in Aplysia

Motivated behaviors comprise appetitive actions whose occurrence results partly from an internally driven incentive to act. Such impulsive behavior can also be regulated by external rewarding stimuli that, through learning processes, can lead to accelerated and seemingly automatic, compulsive-like recurrences of the rewarded act. Here, we explored such behavioral plasticity in Aplysia by analyzing how appetitive reward stimulation in a form of operant conditioning can modify a goal-directed component of the animal's food-seeking behavior. In naive animals, protraction/retraction cycles of the tongue-like radula are expressed sporadically with highly variable interbite intervals. In contrast, animals that were previously given a food-reward stimulus in association with each spontaneous radula bite now expressed movement cycles with an elevated frequency and a stereotyped rhythmic organization. This rate increase and regularization, which was retained for several hours after training, depended on both the reward quality and its contingency because accelerated, stereotyped biting was not induced in animals that had previously received a less-palatable food stimulus or had been subjected to nonassociative reward stimulation. Neuronal correlates of these learning-induced changes were also expressed in the radula motor pattern-generating circuitry of isolated buccal ganglia. In such in vitro preparations, moreover, manipulation of the burst frequency of the bilateral motor pattern-initiating B63 interneurons indicated that the regularization of radula motor pattern generation in contingently trained animals occurred separately from an increase in cycle rate, thereby suggesting independent processes of network plasticity. These data therefore suggest that operant conditioning can induce compulsive-like actions in Aplysia feeding behavior and provide a substrate for a cellular analysis of the underlying mechanisms.

Modulation of quinpirole-induced compulsive-like behavior in rats by environmental changes: implications for OCD rituals and for exploration and navigation

Background

Rats treated chronically with the D_2–3 dopamine agonist quinpirole were previously proposed as an animal model of obsessive-compulsive disorder (OCD) since their behavior is based on repeated, compulsive-like persistent traveling between a few places in the open field. The aim of the present study was to determine properties of the physical environment that shape such behavior. For this, quinpirole-treated rats were first exposed to an arena with an array of objects (landmarks) and after the development of compulsive-like behavior, the arena was manipulated by multiplying the number of objects, changing their spacing, relocating object array, or removing the objects.

Results

When the number of objects was retained but they were spaced further apart, rat routes converged at 1–2 of the objects and at the corner at which the rats had been introduced into the arena (start corner). When object spacing was retained but their number was increased, the rats traveled between the objects with the routes converging only at the start corner. Finally, when object array was relocated to different places within the arena, the rats extended their routes from the start corner to the object array, regardless of array location.

Conclusion

Quinpirole-treated rats organized and updated their progression primarily according to the proximal layout of landmarks, but did so with excessive repetitions compared with saline-treated rats. The behavior of quinpirole-treated rats paralleled human OCD rituals that are linked to the immediate physical environment, featuring an excessive rate of performance. Finally, when only a few objects were present, they were perceived by the rats as positional cues (beacons) that routes converged at them. In contrast, in the presence of many objects, the routes passed between the objects as if using them as directional cues.