A phase plane representation of rat exploratory behavior

High mirror symmetry in mouse exploratory behavior

The physicality of the world in which the animal acts-its anatomical structure, physiology, perception, emotional states, and cognitive capabilities-determines the boundaries of the behavioral space within which the animal can operate. Behavior, therefore, can be considered as the subspace that remains after secluding all actions that are not available to the animal due to constraints. The very signature of being a certain creature is reflected in these limitations that shape its behavior. A major goal of ethology is to expose those constraints that carve the intricate structure of animal behavior and reveal both uniqueness and commonalities between animals within and across taxa. Exploratory behavior in an empty arena seems to be stochastic; nevertheless, it does not mean that the moving animal is a random walker. In this study, we present how, by adding constraints to the animal's locomotion, one can gradually retain the 'mousiness' that characterizes the behaving mouse. We then introduce a novel phenomenon of high mirror symmetry along the locomotion of mice, which highlights another constraint that further compresses the complex nature of exploratory behavior in these animals. We link these findings to a known neural mechanism that could explain this phenomenon. Finally, we suggest our novel finding and derived methods to be used in the search for commonalities in the motion trajectories of various organisms across taxa.

Spatial Disorientation Under Dark Conditions Across Development in an Alzheimer's Disease Mouse Model

Alzheimer's disease (AD) is associated with hippocampal neuropathology and cognitive impairments, including wandering behavior or becoming lost in a familiar environment. Wandering behavior is severe and manifests early in life for people with specific genetic mutations. Genetic mouse models of AD have been developed to characterize the onset and progression of behavioral deficits that represent human behaviors, such as wandering, to test the efficacy of therapeutics. It is not clear if current assessments of mouse models capture the onset of AD or a snapshot of its progression. Sequential analysis of open field behavior provides a robust, quick test to dissociate navigation cues that contribute to spatial disorientation, a feature of wandering. Despite potential utility in evaluating this feature of AD, little work has been reported using animal models of dementia in this task. Thus, we examined the use of different sources of information to maintain spatial orientation at two prodromal ages in female transgenic CRND8 AD (n = 17) and Control mice (n = 16). These mice exhibit amyloid plaques, a hallmark neuropathological feature of AD, that are associated with cognitive dysfunction at ∼three months of age. Spatial disorientation was observed at two months and more severely at four months under dark conditions, but performance was spared when visual environmental cues were available. This study provides documentation of impaired self-movement cue processing in AD mice, establishing the dark open field as a behavioral tool to characterize spatial disorientation associated with AD. These findings may accelerate future assessments of novel therapeutic interventions for neurological disorders.

Cue polarization and representation in mouse home base behaviors

The nature of the representation guiding spatial navigation has been investigated extensively; however, most of this work has used behavioral tasks that involved learning the location of food reward or an escape platform. In contrast, relatively few studies have focused on the spatial representation of a home base, a ubiquitous feature of open-field behavior, and its ability to be encoded relative to environmental cues. The current set of experiments investigated acquisition and retention of the location of home base establishment. In general, proximal cues anchored the position of the home base during acquisition sessions across all four experiments. Although mice established a home base during retention sessions, previous experience did not influence its position during retention sessions. These observations demonstrate that stimulus control of home base position depends on access to proximal cues. Further work is needed to determine the extent that home base establishment may provide a framework to encode goal-directed spatial behaviors.

Sexually dimorphic organization of open field behavior following moderate prenatal alcohol exposure

BACKGROUND

Prenatal alcohol exposure (PAE) can produce deficits in a wide range of cognitive functions but is especially detrimental to behaviors requiring accurate spatial information processing. In open field environments, spatial behavior is organized such that animals establish "home bases" marked by long stops focused around one location. Progressions away from the home base are circuitous and slow, while progressions directed toward the home base are non-circuitous and fast. The impact of PAE on the organization of open field behavior has not been experimentally investigated.

METHODS

In the present study, adult female and male rats with moderate PAE or saccharin exposure locomoted a circular high walled open field for 30 minutes under lighted conditions.

RESULTS

The findings indicate that PAE and sex influence the organization of open field behavior. Consistent with previous literature, PAE rats exhibited greater locomotion in the open field. Novel findings from the current study indicate that PAE and sex also impact open field measures specific to spatial orientation. While all rats established a home base on the periphery of the open field, PAE rats, particularly males, exhibited significantly less clustered home base stopping with smaller changes in heading between stops. PAE also impaired progression measures specific to distance estimation, while sex alone impacted progression measures specific to direction estimation.

CONCLUSIONS

These findings support the conclusion that adult male rats have an increased susceptibility to the effects of PAE on the organization of open field behavior.

Differential organization of open field behavior in mice following acute or chronic simulated GCR exposure

Astronauts undertaking deep space travel will receive chronic exposure to the mixed spectrum of particles that comprise Galactic Cosmic Radiation (GCR). Exposure to the different charged particles of varied fluence and energy that characterize GCR may impact neural systems that support performance on mission critical tasks. Indeed, growing evidence derived from years of terrestrial-based simulations of the space radiation environment using rodents has indicated that a variety of exposure scenarios can result in significant and long-lasting decrements to CNS functionality. Many of the behavioral tasks used to quantify radiation effects on the CNS depend on neural systems that support maintaining spatial orientation and organization of rodent open field behavior. The current study examined the effects of acute or chronic exposure to simulated GCR on the organization of open field behavior under conditions with varied access to environmental cues in male and female C57BL/6 J mice. In general, groups exhibited similar organization of open field behavior under dark and light conditions. Two exceptions were noted: the acute exposure group exhibited significantly slower and more circuitous homeward progressions relative to the chronic group under light conditions. These results demonstrate the potential of open field behavior organization to discriminate between the effects of select GCR exposure paradigms.

Organization of exploratory behavior under dark conditions in female and male rats

Sexually dimorphic performance has been observed across humans and rodents in many spatial tasks. In general, these spatial tasks do not dissociate the use of environmental and self-movement cues. Previous work has demonstrated a role for self-movement cue processing in organizing open field behavior; however, these studies have not directly compared female and male movement characteristics. The current study examined the organization of open field behavior under dark conditions in female and male rats. Significant differences between female and male rats were observed in the location of stopping behavior relative to a cue and the topography exhibited during lateral movements. In contrast, no sex differences were observed on measures used to detect self-movement cue processing deficits. These results provide evidence that female and male rats are similar in their use of self-movement cues to organize open field behavior; however, other factors may be contributing to differences in performance.

Effects of acquired vestibular pathology on the organization of mouse exploratory behavior

Rodent open field behavior is highly organized and occurs spontaneously in novel environments. This organization is disrupted in mice with vestibular pathology, suggesting vestibular signals provide important contributions to this behavior. A caveat to this interpretation is that previous studies have investigated open field behavior in adult mice with congenital vestibular dysfunction, and the observed deficits may have resulted from developmental changes instead of the lack of vestibular signals. To determine which aspects of open field behavior depend specifically on vestibular signals, mouse movement organization was examined under dark and light conditions at two time points, 1 and 2 months, after bilateral chemical labyrinthectomy. Our results show that acquired vestibular damage selectively disrupted the organization of open field behavior. Access to visual environmental cues attenuated, but did not eliminate, these significant group differences. Improvement in movement organization from the first to the second testing session was limited to progression path circuity. These observations provide evidence for the role of the vestibular system in maintaining spatial orientation and establishes a foundation to investigate neuroplasticity in brain systems that process self-movement information.

A framework to identify structured behavioral patterns within rodent spatial trajectories

Animal behavior is highly structured. Yet, structured behavioral patterns-or "statistical ethograms"-are not immediately apparent from the full spatiotemporal data that behavioral scientists usually collect. Here, we introduce a framework to quantitatively characterize rodent behavior during spatial (e.g., maze) navigation, in terms of movement building blocks or motor primitives. The hypothesis that we pursue is that rodent behavior is characterized by a small number of motor primitives, which are combined over time to produce open-ended movements. We assume motor primitives to be organized in terms of two sparsity principles: each movement is controlled using a limited subset of motor primitives (sparse superposition) and each primitive is active only for time-limited, time-contiguous portions of movements (sparse activity). We formalize this hypothesis using a sparse dictionary learning method, which we use to extract motor primitives from rodent position and velocity data collected during spatial navigation, and successively to reconstruct past trajectories and predict novel ones. Three main results validate our approach. First, rodent behavioral trajectories are robustly reconstructed from incomplete data, performing better than approaches based on standard dimensionality reduction methods, such as principal component analysis, or single sparsity. Second, the motor primitives extracted during one experimental session generalize and afford the accurate reconstruction of rodent behavior across successive experimental sessions in the same or in modified mazes. Third, in our approach the number of motor primitives associated with each maze correlates with independent measures of maze complexity, hence showing that our formalism is sensitive to essential aspects of task structure. The framework introduced here can be used by behavioral scientists and neuroscientists as an aid for behavioral and neural data analysis. Indeed, the extracted motor primitives enable the quantitative characterization of the complexity and similarity between different mazes and behavioral patterns across multiple trials (i.e., habit formation). We provide example uses of this computational framework, showing how it can be used to identify behavioural effects of maze complexity, analyze stereotyped behavior, classify behavioral choices and predict place and grid cell displacement in novel environments.

Social spatial cognition: social distance dynamics as an identifier of social interactions

We suggest that socio-spatial behavior, which is an interaction between social and spatial cognition, can be viewed as a set of excursions that originate and end in close proximity to another individual(s). We present an extension of earlier studies that perceived spatial behavior in individual animals as a series of excursions originating from a particular location. We measured here the momentary distance between two individuals (social distance) to differentiate among eight possible types of social excursion originating in a state of proximity between excursion-participants. The defined excursion types are based on whether or not the excursion initiator also concludes the excursion, whether or not the excursion starts and ends at the same location, and the dynamics of the distance between excursion participants. We validated this approach to socio-spatial behavior as a set of excursions using it to analyze the behavior of the two sexes in rodents, of normal vs. stereotyped rats, as well as of different rodent species. Each of these groups displays a prevalent excursion type that reflects a distinct social dynamics. Our approach offers a useful and comprehensive tool for studying socio-spatial cognition, and can also be applied to distinguish among different social situations in rodents and other animals.

Effects of string length on the organization of rat string-pulling behavior

The string-pulling paradigm has been adapted to investigate many psychological phenomena across a range of animal species. Although varying string length has been shown to influence performance, the nature of the representation remains to be determined. Across three experiments, rats were shaped to pull string to receive food reinforcement. Either string length or reinforcement rate was manipulated to examine the influence on string-pulling behavior. Experiment 1 demonstrated that varied string length was sufficient to elicit an odor discrimination. Subsequent experiments provided evidence that varying string length (Experiment 2) and reinforcement rate (Experiment 3) produced qualitatively distinct patterns of string-pulling behavior. In Experiment 2 rats that received a long string were more likely to pull in the probe string to the end, yet no differences were observed in approach time between short and long groups. However, in Experiment 3 rats that received low reinforcement were less likely to pull in the probe string to the end and were slower to approach the string to begin pulling. These results are consistent with rats using temporal and motivational characteristics to guide responding during string-pulling behavior.

Environmental determinants of behavioural responses to short-term stress in rats: Evidence for inhibitory effect of ambient landmarks

Behavioural responses to stress occur in an environment-dependent manner. Complex environments require flexible behavioural coping strategies and chronic stress usually generates psychomotor inhibition. Here, we examine if short-term stress also exerts an inhibitory effect on novelty-seeking, exploratory behaviours. Rats underwent acute restraint stress or were left undisturbed, and their neuroendocrine and behavioural responses were assessed at short- and long-term time points. Animals were individually tested in the open field task (OFT) and the corridor field task (CFT) with and without a central object for free exploration and novelty seeking behaviour. Stress-related psychomotor alterations were measured by path speed, path length, number of stops and thigmotaxis in both tasks. Short-term stress activated the hypothalamic-pituitary-adrenal axis causing elevated plasma corticosterone levels. Stress also impacted psychomotor functions in terms of motivational changes (higher speed and longer path) only in the central-object variations of the OFT and CFT. Moreover, stress-induced emotional alterations were manifested by a higher number of stops and thigmotactic behaviour only in the central-object condition. These findings suggest that environmental landmarks determine the type and direction of exploratory behaviour under transient stress.

Behavioral and Neural Subsystems of Rodent Exploration

Animals occupy territories in which resources such as food and shelter are often distributed unevenly. While studies of exploratory behavior have typically involved the laboratory rodent as an experimental subject, questions regarding what constitutes exploration have dominated. A recent line of research has utilized a descriptive approach to the study of rodent exploration, which has revealed that this behavior is organized into movement subsystems that can be readily quantified. The movements include home base behavior, which serves as a central point of attraction from which rats and mice organize exploratory trips into the remaining environment. In this review, we describe some of the features of this organized behavior pattern as well as its modulation by sensory cues and previous experience. We conclude the review by summarizing research investigating the neurobiological bases of exploration, which we hope will stimulate renewed interest and research on the neural systems mediating rodent exploratory behavior.

Antisense oligonucleotide therapy rescues disruptions in organization of exploratory movements associated with Usher syndrome type 1C in mice

Usher syndrome, Type 1C (USH1C) is an autosomal recessive inherited disorder in which a mutation in the gene encoding harmonin is associated with multi-sensory deficits (i.e., auditory, vestibular, and visual). USH1C (Usher) mice, engineered with a human USH1C mutation, exhibit these multi-sensory deficits by circling behavior and lack of response to sound. Administration of an antisense oligonucleotide (ASO) therapeutic that corrects expression of the mutated USH1C gene, has been shown to increase harmonin levels, reduce circling behavior, and improve vestibular and auditory function. The current study evaluates the organization of exploratory movements to assess spatial organization in Usher mice and determine the efficacy of ASO therapy in attenuating any such deficits. Usher and heterozygous mice received the therapeutic ASO, ASO-29, or a control, non-specific ASO treatment at postnatal day five. Organization of exploratory movements was assessed under dark and light conditions at two and six-months of age. Disruptions in exploratory movement organization observed in control-treated Usher mice were consistent with impaired use of self-movement and environmental cues. In general, ASO-29 treatment rescued organization of exploratory movements at two and six-month testing points. These observations are consistent with ASO-29 rescuing processing of multiple sources of information and demonstrate the potential of ASO therapies to ameliorate topographical disorientation associated with other genetic disorders.

Spatial Memory: Mice Quickly Learn a Safe Haven

New work on innate escape behavior shows that mice spontaneously form a spatially precise memory of the location of shelter, which is laid down quickly and updated continuously.

Scale Invariance in Lateral Head Scans During Spatial Exploration

Universality connects various natural phenomena through physical principles governing their dynamics, and has provided broadly accepted answers to many complex questions, including information processing in neuronal systems. However, its significance in behavioral systems is still elusive. Lateral head scanning (LHS) behavior in rodents might contribute to spatial navigation by actively managing (optimizing) the available sensory information. Our findings of scale invariant distributions in LHS lifetimes, interevent intervals and event magnitudes, provide evidence for the first time that the optimization takes place at a critical point in LHS dynamics. We propose that the LHS behavior is responsible for preprocessing of the spatial information content, critical for subsequent foolproof encoding by the respective downstream neural networks.

Otolith dysfunction alters exploratory movement in mice

The organization of rodent exploratory behavior appears to depend on self-movement cue processing. As of yet, however, no studies have directly examined the vestibular system's contribution to the organization of exploratory movement. The current study sequentially segmented open field behavior into progressions and stops in order to characterize differences in movement organization between control and otoconia-deficient tilted mice under conditions with and without access to visual cues. Under completely dark conditions, tilted mice exhibited similar distance traveled and stop times overall, but had significantly more circuitous progressions, larger changes in heading between progressions, and less stable clustering of home bases, relative to control mice. In light conditions, control and tilted mice were similar on all measures except for the change in heading between progressions. This pattern of results is consistent with otoconia-deficient tilted mice using visual cues to compensate for impaired self-movement cue processing. This work provides the first empirical evidence that signals from the otolithic organs mediate the organization of exploratory behavior, based on a novel assessment of spatial orientation.

Machine vision methods for analyzing social interactions

Recent developments in machine vision methods for automatic, quantitative analysis of social behavior have immensely improved both the scale and level of resolution with which we can dissect interactions between members of the same species. In this paper, we review these methods, with a particular focus on how biologists can apply them to their own work. We discuss several components of machine vision-based analyses: methods to record high-quality video for automated analyses, video-based tracking algorithms for estimating the positions of interacting animals, and machine learning methods for recognizing patterns of interactions. These methods are extremely general in their applicability, and we review a subset of successful applications of them to biological questions in several model systems with very different types of social behaviors.

Coordination of sniffing and whisking depends on the mode of interaction with the environment

Smell and touch convey most of the information that nocturnal rodents collect in their natural environments, each via its own complex network of muscles, receptors and neurons. Being active senses, a critical factor determining the integration of their sensations relates to the degree of their coordination. While it has been known for nearly 50 years that sniffing and whisking can be coordinated, the dynamics of such coordination and its dependency on behavioral and environmental conditions are not yet understood. Here we introduce a novel non-invasive method to track sniffing along with whisking and locomotion using high-resolution video recordings of mice, during free exploration of an open arena. Active sensing parameters in each modality showed significant dependency on exploratory modes (“Outbound”, “Exploration” and “Inbound”) and locomotion speed. Surprisingly, the correlation between sniffing and whisking was often as high as the bilateral inter-whisker correlation. Both inter-whisker and inter-modal coordination switched between distinct high-correlation and low-correlation states. The fraction of time with high-correlation states was higher in the Outbound and Exploration modes compared with the Inbound mode. Overall, these data indicate that sniffing–whisking coordination is a complex dynamic process, likely to be controlled by multiple-level inter-modal coordinated loops of motor-sensory networks.

Three-dimensional space: locomotory style explains memory differences in rats and hummingbirds

While most animals live in a three-dimensional world, they move through it to different extents depending on their mode of locomotion: terrestrial animals move vertically less than do swimming and flying animals. As nearly everything we know about how animals learn and remember locations in space comes from two-dimensional experiments in the horizontal plane, here we determined whether the use of three-dimensional space by a terrestrial and a flying animal was correlated with memory for a rewarded location. In the cubic mazes in which we trained and tested rats and hummingbirds, rats moved more vertically than horizontally, whereas hummingbirds moved equally in the three dimensions. Consistent with their movement preferences, rats were more accurate in relocating the horizontal component of a rewarded location than they were in the vertical component. Hummingbirds, however, were more accurate in the vertical dimension than they were in the horizontal, a result that cannot be explained by their use of space. Either as a result of evolution or ontogeny, it appears that birds and rats prioritize horizontal versus vertical components differently when they remember three-dimensional space.

The developmental dynamics of behavioral growth processes in rodent egocentric and allocentric space

In this review I focus on how three methodological principles advocated by Philip Teitelbaum influenced my work to this day: that similar principles of organization should be looked for in ontogeny and recovery of function; that the order of emergence of behavioral components provides a view on the organization of that behavior; and that the components of behavior should be exhibited by the animal itself in relatively pure form. I start by showing how these principles influenced our common work on the developmental dynamics of rodent egocentric space, and then proceed to describe how these principles affected my work with Yoav Benjamini and others on the developmental dynamics of rodent allocentric space. We analyze issues traditionally addressed by physiological psychologists with methods borrowed from ethology, EW (Eshkol-Wachman) movement notation, dynamical systems and exploratory data analysis. Then we show how the natural origins of axes embodied by the behavior of the organism itself, are used by us as the origins of axes for the measurement of the developmental moment-by-moment dynamics of behavior. Using this methodology we expose similar principles of organization across situations, species and preparations, provide a developmental view on the organization of behavior, expose the natural components of behavior in relatively pure form, and reveal how low level primitives generate higher level constructs. Advances in tracking technology should allow us to study how movements in egocentric and allocentric spaces interlace. Tracking of multi-limb coordination, progress in online recording of neural activity in freely moving animals, and the unprecedented accumulation of genetically engineered mouse preparations makes the behavioral ground plan exposed in this review essential for a systematic study of the brain/behavior interface.

Ten ways to improve the quality of descriptions of whole-animal movement

The demand for replicability of behavioral results across laboratories is viewed as a burden in behavior genetics. We demonstrate how it can become an asset offering a quantitative criterion that guides the design of better ways to describe behavior. Passing the high benchmark dictated by the replicability demand requires less stressful and less restraining experimental setups, less noisy data, individually customized cutoff points between the building blocks of movement, and less variable yet discriminative dynamic representations that would capture more faithfully the nature of the behavior, unmasking similarities and differences and revealing novel animal-centered measures. Here we review ten tools that enhance replicability without compromising discrimination. While we demonstrate the usefulness of these tools in the context of inbred mouse exploratory behavior they can readily be used in any study involving a high-resolution analysis of spatial behavior. Viewing replicability as a design concept and using the ten methodological improvements may prove useful in many fields not necessarily related to spatial behavior.

Knots: attractive places with high path tortuosity in mouse open field exploration

When introduced into a novel environment, mammals establish in it a preferred place marked by the highest number of visits and highest cumulative time spent in it. Examination of exploratory behavior in reference to this “home base” highlights important features of its organization. It might therefore be fruitful to search for other types of marked places in mouse exploratory behavior and examine their influence on overall behavior.

Examination of path curvatures of mice exploring a large empty arena revealed the presence of circumscribed locales marked by the performance of tortuous paths full of twists and turns. We term these places knots, and the behavior performed in them—knot-scribbling. There is typically no more than one knot per session; it has distinct boundaries and it is maintained both within and across sessions. Knots are mostly situated in the place of introduction into the arena, here away from walls. Knots are not characterized by the features of a home base, except for a high speed during inbound and a low speed during outbound paths. The establishment of knots is enhanced by injecting the mouse with saline and placing it in an exposed portion of the arena, suggesting that stress and the arousal associated with it consolidate a long-term contingency between a particular locale and knot-scribbling.

In an environment devoid of proximal cues mice mark a locale associated with arousal by twisting and turning in it. This creates a self-generated, often centrally located landmark. The tortuosity of the path traced during the behavior implies almost concurrent multiple views of the environment. Knot-scribbling could therefore function as a way to obtain an overview of the entire environment, allowing re-calibration of the mouse's locale map and compass directions. The rich vestibular input generated by scribbling could improve the interpretation of the visual scene.

Exploration is a central component of human and animal behavior that has been studied in rodents for almost a century. It is presently one of the main models for studying the interface between behavior, genetics, drugs, and the brain. Until recently the exploration of an open field by rodents has been considered to be largely stochastic. Lately, this behavior is being gradually deciphered, revealing reference places called home bases, from which the animals perform roundtrips into the environment, tracing well-trodden paths whose features contribute to our understanding of navigation, locational memory, cognition-, and emotion-related behavior. Using advanced computational tools we discover so-called knots, preferred places visited sporadically by mice. Mice perform in these places twists and turns. The measurement of speed on the way in and out of knots reveals that they are attractive for the mice. Knot formation is enhanced by stress, suggesting that stress-related arousal assigns these locales with a special significance that is reinstated by subsequent visits to them. The twists and turns could provide the mouse with multiple views that turn knots into navigational landmarks as well as with rich vestibular input that might improve the perception and subsequent interpretation of the visual input.

Fractionating dead reckoning: role of the compass, odometer, logbook, and home base establishment in spatial orientation

Rats use multiple sources of information to maintain spatial orientation. Although previous work has focused on rats' use of environmental cues, a growing number of studies have demonstrated that rats also use self-movement cues to organize navigation. This review examines the extent that kinematic analysis of naturally occurring behavior has provided insight into processes that mediate dead-reckoning-based navigation. This work supports a role for separate systems in processing self-movement cues that converge on the hippocampus. The compass system is involved in deriving directional information from self-movement cues; whereas, the odometer system is involved in deriving distance information from self-movement cues. The hippocampus functions similar to a logbook in that outward path unique information from the compass and odometer is used to derive the direction and distance of a path to the point at which movement was initiated. Finally, home base establishment may function to reset this system after each excursion and anchor environmental cues to self-movement cues. The combination of natural behaviors and kinematic analysis has proven to be a robust paradigm to investigate the neural basis of spatial orientation.

Mouse cognition-related behavior in the open-field: emergence of places of attraction

Spatial memory is often studied in the Morris Water Maze, where the animal's spatial orientation has been shown to be mainly shaped by distal visual cues. Cognition-related behavior has also been described along “well-trodden paths”—spatial habits established by animals in the wild and in captivity reflecting a form of spatial memory. In the present study we combine the study of Open Field behavior with the study of behavior on well-trodden paths, revealing a form of locational memory that appears to correlate with spatial memory. The tracked path of the mouse is used to examine the dynamics of visiting behavior to locations. A visit is defined as either progressing through a location or stopping there, where progressing and stopping are computationally defined. We then estimate the probability of stopping at a location as a function of the number of previous visits to that location, i.e., we measure the effect of visiting history to a location on stopping in it. This can be regarded as an estimate of the familiarity of the mouse with locations. The recently wild-derived inbred strain CZECHII shows the highest effect of visiting history on stopping, C57 inbred mice show a lower effect, and DBA mice show no effect. We employ a rarely used, bottom-to-top computational approach, starting from simple kinematics of movement and gradually building our way up until we end with (emergent) locational memory. The effect of visiting history to a location on stopping in it can be regarded as an estimate of the familiarity of the mouse with locations, implying memory of these locations. We show that the magnitude of this estimate is strain-specific, implying a genetic influence. The dynamics of this process reveal that locations along the mouse's trodden path gradually become places of attraction, where the mouse stops habitually.

Spatially guided behavior and spatial memory are central subjects in behavioral neuroscience. Many tasks have been developed for laboratory investigations of these subjects since no single task can reveal their full richness. Here we turn to the simplest and oldest “task”, which involves no task at all: introducing a mouse into a large arena and tracking its free behavior. Traditionally, the test is used for studying emotionality and locomotor behavior, using simple summaries of the mouse's path such as its length and the percent of time spent away from walls. More sophisticated computational analysis of the dynamics of the path enables us to separate visiting behavior at locations into stops and passings. Using this distinction, the mouse's path reveals quantifiable locational memory: the mouse's decision to stop in a location is based on its visiting history there. In some strains of mice, the visited locations gradually become places of attraction where the mouse stops habitually. In other strains, the phenomenon is not evident at all. Such quantifiable characterization of locational memory now enables further exploration of the senses that mediate this type of memory and allows measurement and comparisons across mouse strains and across genetic and pharmacological preparations.

Temporal stability of novelty exploration in mice exposed to different open field tests

We investigated behavioural activity and temporal distribution (patterning) of mouse exploration in different open field (OF) arenas. Mice of 129S1 (S1) strain were subjected in parallel to three different OF arenas (Experiment 1), two different OF arenas in two trials (Experiment 2) or two trials of the same OF test (Experiment 3). Overall, mice demonstrated a high degree of similarity in the temporal profile of novelty-induced horizontal and vertical exploration (regardless of the size, colour and shape of the OF), which remained stable in subsequent OF exposures. In Experiments 4 and 5, we tested F1 hybrid mice (BALB/c-S1; NMRI-S1), and Vitamin D receptor knockout mice (generated on S1 genetic background), again showing strikingly similar temporal patterns of their OF exploration, despite marked behavioural strain differences in anxiety and activity. These results suggest that mice are characterised by stability of temporal organization of their exploration in different OF novelty situations.

Sharing of the home base: a social test in rats

The open field provides abundant opportunities for a pair of rats to express social interactions. Rats demonstrate social proximity while exploring the open field and also during simultaneous occupancy of a home base (HB). The HB is defined as a place in the field for which rats show a long-term preference, both in terms of occupancy and as a starting and ending point of exploratory excursions. In the present study, the social proximity of pairs of rats treated with phencyclidine (PCP; 2 mg/kg) or saline (Sal), and rats treated with clozapine (CLZ; 1.3 mg/kg) alone or in combination with PCP (CLZ-PCP), was videotaped and analysed algorithmically. PCP was tested for its disruptive effects on social interactions, and CLZ was tested for its ability to reverse some forms of the disruptive effects of PCP. The results showed that PCP reduced the rate of pairs establishing a common HB and shortened social HB occupancy, but had no effect on episodes of social exploration in the field. These findings demonstrate that the antisocial effect of PCP cannot be generalized across the entire spectrum of behavioural states in the open field. CLZ further decreased rather than increased social HB occupancy. This effect was derived from the reduction in rate of pairs establishing a common HB.

Texture of locomotor path: a replicable characterization of a complex behavioral phenotype

A database of mouse locomotor path in spatial tests can be used to search in silico for behavioral measures that better discriminate between genotypes and are more replicable across laboratories. In this study, software for the exploration of exploration (SEE) was used to search a large database for a novel behavioral measure that would characterize complex movement paths. The database included mouse open-field behavior assessed in 3 laboratories, 7 inbred strains, several pharmacological treatments and hundreds of animals. The new behavioral measure, "path texture", was characterized using the local curvature of the path (the change of direction per unit distance, in degrees/cm) across several spatial scales, starting from scales smaller than the animal's body length and up to the scale of the arena size. Path texture analysis differs from fractal dimension analysis in that it does not assume self-similarity across scales. Path texture was found to discriminate inbred strains with relatively high broad-sense heritability (43%-71%) and high replicability across laboratories. Even genotypes that had similar path curvatures in some scales usually differed in other scales, and self-similarity across scales was not displayed by all genotypes. Amphetamine decreased the path curvature of C57BL/6 mice in small and medium scales, while having no effect on DBA/2J mice. Diazepam dose-dependently decreased the curvature of C57BL/6 mice across all scales, while 2 anxiogenic drugs, FG-7142 and pentylenetetrazole, increased it. Path texture thus has high potential for behavioral phenotyping and the study of drug effects in the mouse.

Early social and physical deprivation leads to reduced social motivation in adulthood in Wistar rats

Behavioural abnormalities in adulthood may have their origin in a disturbed interaction with the environment during postnatal development. We tested the consequences for adult social motivation of early deprivation (ED) of rat pups from mothers and littermates relative to nonhandled (NH) pups. Early deprivation was performed at room or warm ambient temperatures, cold-ED and warm-ED, respectively, and during either the dark or light phase of the daily cycle. In adulthood, rats that were unrelated and unfamiliar but of the same treatment group were introduced in pairs to an open field for a 30-min test. Social behaviour in home base and exploration modes was assessed using algorithmic analysis of the XY locations of the two rats. Findings revealed that Cold-ED induced a preference for a separate home base, which limited significantly the episodes of social interactions, in comparison to NH. Warm-ED had no comparable effect on the rats' social behaviour. These findings indicate that ED under ambient conditions that constitute severe thermal stress for rat pups leads to development of reduced social motivation in adulthood.

Mice lacking leukocyte common antigen-related (LAR) protein tyrosine phosphatase domains demonstrate spatial learning impairment in the two-trial water maze and hyperactivity in multiple behavioural tests

Leukocyte common antigen-related (LAR) protein is a cell adhesion molecule-like receptor-type protein tyrosine phosphatase. We previously reported that in LAR tyrosine phosphatase-deficient (LAR-Delta P) mice the number and size of basal forebrain cholinergic neurons as well as their innervation of the hippocampal area was reduced. With the hippocampus being implicated in behavioural activity aspects, including learning and memory processes, we assessed possible phenotypic consequences of LAR phosphatase deficiency using a battery of rodent behaviour tests. Motor function and co-ordination tests as well as spatial learning ability assays did not reveal any performance differences between wildtype and LAR-Delta P mice. A spatial learning impairment was found in the difficult variant of the Morris water maze. Exploration, nestbuilding and activity tests indicated that LAR-Delta P mice were more active than wildtype littermates. The observed hyperactivity in LAR-Delta P mice could not be explained by altered anxiety or curiosity levels, and was found to be persistent throughout the nocturnal period. In conclusion, behavioural testing of the LAR-Delta P mice revealed a spatial learning impairment and a significant increase in activity.

Mice lacking the UbCKmit isoform of creatine kinase reveal slower spatial learning acquisition, diminished exploration and habituation, and reduced acoustic startle reflex responses

Brain-type creatine kinases B-CK (cytosolic) and UbCKmit (mitochondrial) are considered important for the maintenance and distribution of cellular energy in the central nervous system. Previously, we have demonstrated an abnormal behavioral phenotype in mice lacking the B-CK creatine kinase isoform, regarding exploration, habituation, seizure susceptibility and spatial learning. The phenotype in these mice was associated with histological adaptations in the hippocampal mossy fiber field size. Here, mice lacking the ubiquitous mitochondrial creatine kinase isoform (UbCKmit-/- mice) showed, when subjected to a similar battery of behavioral tasks, diminished open field habituation and slower spatial learning acquisition in the Morris water maze task, but normal sensory or motor functions. A reduced acoustic startle response, higher threshold, and lack of prepulse inhibition were observed in UbCKmit-/- mice, suggesting that the unconditioned reflexive responsiveness is not optimal. Our findings suggest a role for mitochondrial CK-mediated high-energy phosphoryl transfer in synaptic signalling in the acoustic signal response network and hippocampal-dependent learning circuitry of brain. Finally, we demonstrate that UbCKmit has a widespread occurrence in the cell soma of neuronal nuclei along the rostro-caudal axis of the brain, i.e. cortex, midbrain, hindbrain, cerebellum and brainstem, similar to the occurrence of B-CK. This may explain the similarity of phenotypes in mice lacking B-CK or UbCKmit. We predict that the remaining functional intactness of the cytosolic B-CK reaction and perhaps the compensatory role of other phosphoryl transfer systems are sufficient to sustain the energy requirements for basic sensory, motor and physiological activities in UbCKmit-/- mice.

The dynamics of spatial behavior: how can robust smoothing techniques help?

A variety of setups and paradigms are used in the neurosciences for automatically tracking the location of an animal in an experiment and for extracting features of interest out of it. Many of these features, however, are critically sensitive to the unavoidable noise and artifacts of tracking. Here, we examine the relevant properties of several smoothing methods and suggest a combination of methods for retrieving locations and velocities and recognizing arrests from time series of coordinates of an animal's center of gravity. We accomplish these by using robust nonparametric methods, such as Running Median (RM) and locally weighted regression methods. The smoothed data may, subsequently, be segmented to obtain discrete behavioral units with proven ethological relevance. New parameters such as the length, duration, maximal speed, and acceleration of these units provide a wealth of measures for, e.g., mouse behavioral phenotyping, studies on spatial orientation in vertebrates and invertebrates, and studies on rodent hippocampal function. This methodology may have implications for many tests of spatial behavior.

Behavior of the gray short-tailed opossum (Monodelphis domestica) in the open field and in response to a new object, in comparison with the rat

We compared the behavior of the gray short-tailed opossums (Monodelphis domestica) and Long-Evans rats during repeated exposures to the open-field (OF) test. Animals were videotaped for 10 min on four consecutive days. A new object was placed in the center of the field on the third day and it was present there again on the fourth day. The rate of locomotor activity in the opossum was always higher than that in the rat. On the first exposure to the open field, both species showed strong thigmotaxy. On the second day, opossums shifted a significant part of their activity to the internal and central parts of the field, while thigmotaxy dominated in the rats' behavior till the end of the experiment. The frequency and time of exploration of a new object placed on the central square was higher in the opossums than in rats. They also showed higher frequency of rearings and lower defecation scores, while the time of grooming was similar to the rats'. These results, that are consistent with those of our earlier experiments in the elevated plus maze (EPM), show that in response to novelty Monodelphis opossums change their behavior from defensive to exploratory faster than rats and then explore it more intensely. These differences may be either a result of different ecologies or evolution of the two species.

Open-field behavior withstands drastic changes in arena size

Individual voles were tested on successive days under increasing, decreasing, or randomly changing arena size. Locomotor behavior was adjusted to arena size by (i) preserving the same level of activity, (ii) taking longer but less frequent trips in smaller arenas in contrast to taking more frequent yet shorter trips in the larger arenas, and (iii) moving in the entire space available for exploration in the smaller arenas in contrast to remaining along the walls of the open field in the larger arenas. The effect of testing order was minimal, probably being related to increased novelty under increasing arena size, as opposed to habituation under decreasing arena size, when parts of the same area were re-explored. These behavioral changes averaged up to a two-fold difference compared with the larger six-fold change in the perimeter and 44-fold change in the area of the open field. The modest change in open-field behavior indicates that it has a solid spatio-temporal structure that withstands extensive environmental changes. This behavioral stability and consistency further validates studies of pharmacological, neurological, and genetic preparations that use behavior in a small open field as representation of the general pattern of behavior.

SEE locomotor behavior test discriminates C57BL/6J and DBA/2J mouse inbred strains across laboratories and protocol conditions

Conventional tests of behavioral phenotyping frequently have difficulties differentiating certain genotypes and replicating these differences across laboratories and protocol conditions. This study explores the hypothesis that automated tests can be designed to quantify ethologically relevant behavior patterns that more readily characterize heritable and replicable phenotypes. It used SEE (Strategy for the Exploration of Exploration) to phenotype the locomotor behavior of the C57BL/6 and DBA/2 mouse inbred strains across 3 laboratories. The 2 genotypes differed in 15 different measures of behavior, none of which had a significant genotype-laboratory interaction. Within the same laboratory, most of these differences were replicated in additional experiments despite the test photoperiod phase being changed and saline being injected. Results suggest that well-designed tests may considerably enhance replicability across laboratories.

Extending SEE for large-scale phenotyping of mouse open-field behavior

SEE (Software for the Exploration of Exploration) is a visualization and analysis tool designed for the study of open-field behavior in rodents. In this paper, I present new extensions of SEE that were designed to facilitate its use for mouse behavioral phenotyping and, especially, for the problems of discrimination of genotypes and the replication of results across laboratories and experimental conditions. These extensions were specifically designed to promote a new approach in behavioral phenotyping, reminiscent of the approach that has been successfully employed in bioinformatics during recent years. The path coordinates of all animals from many experiments are stored in a database. SEE can be used to query, visualize, and analyze any desirable subsection of this database and to design new measures (endpoints) with increasingly better discriminative power and replicability. The use of the new extensions is demonstrated here in the analysis of results from several experiments and laboratories, with an emphasis on this approach.

Voles scale locomotion to the size of the open-field by adjusting the distance between stops: a possible link to path integration

In this study we show that social voles (Microtus socialis guentheri) preserve the same level of activity and spatio-temporal organization of behavior whether exploring a small (1 x 1 m) or a large (2 x 2 m) open field. In each open field, a vole established a home base from which it set on to round-trips of exploration; taking fewer but longer trips in the large open field, compared with more frequent but shorter trips in the small open field. Each trip comprised bouts of progression (locomotion) interrupted by stops. The number of stops per trip was the same for both large open field (longer trips) and small open field (shorter trips), and achieved by scaling the distance between stops according to the size of the open field. Voles traveled more along the walls in the large compared with the small open field. These adjustments in locomotor behavior to open field size were observed immediately after the voles were introduced into the arena, indicating that the perceived distances available for locomotion were identified by the voles immediately at the beginning of exploration. It is suggested that these properties of spontaneous exploration are an expression of navigation using visual landmarks and path integration.

Creatine kinase B-driven energy transfer in the brain is important for habituation and spatial learning behaviour, mossy fibre field size and determination of seizure susceptibility

Creatine kinases are important in maintaining cellular-energy homeostasis, and neuroprotective effects have been attributed to the administration of creatine and creatine-like compounds. Herein we examine whether ablation of the cytosolic brain-type creatine kinase (B-CK) in mice has detrimental effects on brain development, physiological integrity or task performance. Mice deficient in B-CK (B-CK-/-) showed no gross abnormalities in brain anatomy or mitochondrial ultrastructure, but had a larger intra- and infrapyramidal mossy fibre area. Nuclear magnetic resonance spectroscopy revealed that adenosine triphosphate (ATP) and phosphocreatine (PCr) levels were unaffected, but demonstrated an apparent reduction of the PCr left arrow over right arrow ATP phosphorus exchange capacity in these mice. When assessing behavioural characteristics B-CK-/- animals showed diminished open-field habituation. In the water maze, adult B-CK-/- mice were slower to learn, but acquired the spatial task. This task performance deficit persisted in 24-month-old, aged B-CK-/- mice, on top of the age-related memory decline normally seen in old animals. Finally, a delayed development of pentylenetetrazole-induced seizures (creating a high-energy demand) was observed in B-CK-/- mice. It is suggested that the persistent expression of the mitochondrial isoform ubiquitous mitochondrial CK (UbCKmit) in the creatine/phospho-creatine shuttle provides compensation for the loss of B-CK in the brain. Our studies indicate a role for the creatine-phosphocreatine/CK circuit in the formation or maintenance of hippocampal mossy fibre connections, and processes that involve habituation, spatial learning and seizure susceptibility. However, for fuelling of basic physiological activities the role of B-CK can be compensated for by other systems in the versatile and robust metabolic-energy network of the brain.

Quantification of a single exploratory trip reveals hippocampal formation mediated dead reckoning

A rat's proclivity to explore a novel environment presents a behaviorally rich paradigm to investigate the role of the hippocampus in spatial navigation. Here we describe a novel technique of behavioral analysis that is derived from a single exploratory trip. An exploratory trip was defined as a rat's departure from the home base that ended when the rat returned to the home base. The behavior observed on a single exploratory trip by a control animal is highly organized into outward and homeward segments. An outward segment is characterized by a slow circuitous progression from the home base marked by several stops. A homeward segment is characterized by a rapid direct return to the home base. The velocity attribute of the exploratory trip was quantified by estimating the point of inflection associated with the trip's cumulative moment-to-moment velocity distribution. The heading direction and variance of the homeward trip segment was analyzed with circular statistics. A comparison of the exploratory behavior of control animals and animals with damage to the fimbria-fornix indicated that the velocity and heading direction of the homeward portion of the trip depends upon the hippocampal formation. While control and fimbria-fornix rats had similar outward segments, the return paths of the fimbria-fornix rats were significantly slower, more circuitous, and more variable compared with that of the control rats. This result was also independent of testing in light or dark conditions. The lack of dependence on allothetic cues suggests that rats employ dead reckoning navigational strategies to initiate the homeward portion of exploratory movements. Methods to quantify exploratory behavior in terms of velocity and angular components provide an assessment of control behavior and the assessment of the behavior of rats with hippocampal formation damage that is easy to implement.

Dead reckoning (path integration) requires the hippocampal formation: evidence from spontaneous exploration and spatial learning tasks in light (allothetic) and dark (idiothetic) tests

Animals navigate using cues generated by their own movements (self-movement cues or idiothetic cues), as well as the cues they encounter in their environment (distal cues or allothetic cues). Animals use these cues to navigate in two different ways. When dead reckoning (deduced reckoning or path integration), they integrate self-movement cues over time to locate a present position or to return to a starting location. When piloting, they use allothetic cues as beacons, or they use the relational properties of allothetic cues to locate places in space. The neural structures involved in cue use and navigational strategies are still poorly understood, although considerable attention is directed toward the contributions of the hippocampal formation (hippocampus and associated pathways and structures, including the fimbria-fornix and the retrosplenial cortex). In the present study, using tests in allothetic and idiothetic paradigms, we present four lines of evidence to support the hypothesis that the hippocampal formation plays a central role in dead reckoning. (1) Control but not fimbria-fornix lesion rats can return to a novel refuge location in both light and dark (infrared) food carrying tasks. (2). Control but not fimbria-fornix lesion rats make periodic direct high velocity returns to a starting location in both light and dark exploratory tests. Control but not fimbria-fornix rats trained in the light to carry food from a fixed location to a refuge are able to maintain accurate outward and homebound trajectories when tested in the dark. (3). Control but not fimbria-fornix rats are able to correct an outward trajectory to a food source when the food source is moved when allothetic cues are present. These, tests of spontaneous exploration and foraging suggest a role for the hippocampal formation in dead reckoning.

Social attraction between rats in open field: long-term consequences of kindled seizures

Kindling of the amygdaloid complex in rats results in an enhanced emotionality frequently expressed by an elevated anxiety and defensive attitude toward other animals. Defensive attitude may have important consequences in social context and if tested in a large space it may eventually lead to social withdrawal. To test this hypothesis, rats were subjected to daily kindling sessions and their behavior was compared to implanted-sham and intact rats. Blood was collected after selected kindling trials for assessment of corticosterone response. Behavioral tests started 1 month after the last kindling trial and consisted of two open field sessions. A solitary rat was tested in the 1st session and pair of rats was tested simultaneously in the second session. Results showed that kindling changed the balance between exploration and occupation of a home base (HB) in the open field, in favor of higher preference of the home base occupancy. These results were apparent only during the social session leading to the conclusion that rats preferred to stay in the home base to maximize the proximity to a partner rat. This was supported by the observation that by increasing the occupancy of the HB, the kindled rats accomplished the longest concurrent presence with the partner rat in the common HB. We discuss the level of inter-rats aggression as a factor defining whether the anxious kindled rats will respond with increased or decreased social attraction in the open field test.

Rats and mice share common ethologically relevant parameters of exploratory behavior

Detailed studies of rat exploratory behavior reveal that it consists of typical behavior patterns having a distinct structure. Recently we have developed interactive software that uses as input the automatically digitized time-series of the animal's location for the visualization, analysis, capturing and quantification of these patterns. We use this software here for the study of BALB/cJtau mouse behavior. The results suggest that a considerable number of rat patterns are also present in the mouse. These ethologically-relevant patterns have a significant potential as a phenotyping tool.

Natural segmentation of the locomotor behavior of drug-induced rats in a photobeam cage

Recently, Drai et al. (J Neurosci Methods 96 (2000) 119) have introduced an algorithm that segments rodent locomotor behavior into natural units of 'staying in place' (lingering) behavior versus going between places (progression segments). This categorization, based on the maximum speed attained within the segment, was shown to be intrinsic to the data, using the statistical method of Gaussian Mixture Model. These results were obtained in normal rats and mice using very large (650 or 320 cm) circular arenas and a video tracking system. In the present study, we reproduce these results with amphetamine, phencyclidine and saline injected rats, using data measured by a standard photobeam tracking system in square 45 cm cages. An intrinsic distinction between two or three 'gears' could be shown in all animals. The spatial distribution of these gears indicates that, as in the large arena behavior, they correspond to the difference between 'staying in place' behavior and 'going between places'. The robustness of this segmentation over arena size, different measurement system and dose of two psychostimulant drugs indicates that this is an intrinsic, natural segmentation of rodent locomotor behavior. Analysis of photobeam data that is based on this segmentation has thus a potential use in psychopharmacology research.

SEE: a tool for the visualization and analysis of rodent exploratory behavior

The complexity of exploratory behavior creates a need for a visualization and analysis tool that will highlight regularities and help generating new hypotheses about the structure of this behavior. The hypotheses can then be formulated as algorithms that capture the patterns and quantify them. SEE is a Mathematica based software developed by us for the exploration of exploratory behavior. The raw data for SEE are a time series of the animal 's coordinates in space sampled at a rate that allows a meaningful computation of speeds. SEE permits: (i) a visualization of the path of the animal and a computation of the dynamics of activity; (ii) a decomposition of the path into several modes of motion (1st gear, 2nd gear, etc.) and a computation of the typical maximal speeds, the spatial spread, and the proportion of each of these modes; and(iii) a visualization of the location in the environment of stopping episodes, along with their dwell time. These visualizations highlight the presence of preferred places, including the animal's so-called home base, and permits a computation of the spatio-temporal diversity in the location of stopping episodes. The software also: (i) decomposes the animal's path into round trips from the home base, called 'excursions', and computes the number of stops per excursion; (ii) generates a visualization of the phase space (path+speed, traced in a three-dimensional graph) of any progression segment or list of such segments; and (iii) produces a visualization of the way places in the animal's operational world are connected to each other. SEE also permits the definition and computation of behavioral endpoints across any section of any database of raw data. The range of applicability of SEE to various experimental set ups, tracking procedures, species, and preparations is addressed in the discussion.

Methods to identify and characterize developmental neurotoxicity for human health risk assessment. I: behavioral effects

Alterations in nervous system function after exposure to a developmental neurotoxicant may be identified and characterized using neurobehavioral methods. A number of methods can evaluate alterations in sensory, motor, and cognitive functions in laboratory animals exposed to toxicants during nervous system development. Fundamental issues underlying proper use and interpretation of these methods include a) consideration of the scientific goal in experimental design, b) selection of an appropriate animal model, c) expertise of the investigator, d) adequate statistical analysis, and e) proper data interpretation. Strengths and weaknesses of the assessment methods include sensitivity, selectivity, practicality, and variability. Research could improve current behavioral methods by providing a better understanding of the relationship between alterations in motor function and changes in the underlying structure of these systems. Research is also needed to develop simple and sensitive assays for use in screening assessments of sensory and cognitive function. Assessment methods are being developed to examine other nervous system functions, including social behavior, autonomic processes, and biologic rhythms. Social behaviors are modified by many classes of developmental neurotoxicants and hormonally active compounds that may act either through neuroendocrine mechanisms or by directly influencing brain morphology or neurochemistry. Autonomic and thermoregulatory functions have been the province of physiologists and neurobiologists rather than toxicologists, but this may change as developmental neurotoxicology progresses and toxicologists apply techniques developed by other disciplines to examine changes in function after toxicant exposure.