"Short-stops" in rats with fimbria-fornix lesions: evidence for change in the mobility gradient

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.

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.

Decreased investigatory head scanning during exploration in learning-impaired, aged rats

"Head scanning" is an investigatory behavior that has been linked to spatial exploration and the one-trial formation or strengthening of place cells in the hippocampus. Previous studies have demonstrated that a subset of aged rats with normal spatial learning performance show head scanning rates during a novel, local-global cue-mismatch manipulation that are similar to those of young rats. However, these aged rats demonstrated different patterns of expression of neural activity markers in brain regions associated with spatial learning, perhaps suggesting neural mechanisms that compensate for age-related brain changes. These prior studies did not investigate the head scanning properties of aged rats that had spatial learning impairments. The present study analyzed head scanning behavior in young, aged-unimpaired, and aged-impaired Long Evans rats. Aged-impaired rats performed the head scan behavior at a lower rate than the young rats. These results suggest that decreased attention to spatial landmarks may be a contributing factor to the spatial learning deficits shown by the aged-impaired rats.

The role of the hippocampal theta rhythm in non-spatial discrimination and associative learning task

The configural association theory and the conflict resolution model propose that hippocampal function is involved in learning negative patterning tasks (A+, B+, AB-). The first theory suggests a critical role of the hippocampus in the formation of configural representations of compound stimuli, in which stimuli A and B are presented simultaneously. The second theory hypothesizes that the hippocampus is important for inhibiting the response to a stimulus that is in conflict with response tendencies. Although these theories propose different interpretations of the link between hippocampal function and non-spatial discrimination tasks, they both predict that the hippocampus is involved in the information processing of compound stimuli in negative patterning tasks. Recently, our electrophysiological approach has shown that the hippocampal theta power correlate with response inhibition in a negative patterning task, positive patterning, simultaneous/serial feature negative task. These findings provide strong support for the assumption of the conflict resolution model that the role of the hippocampus in learning is to inhibit responses to conflicting stimuli during non-spatial stimulus discrimination tasks.

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.

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.

Otolithic information is required for homing in the mouse

Navigation and the underlying brain signals are influenced by various allothetic and idiothetic cues, depending on environmental conditions and task demands. Visual landmarks typically control navigation in familiar environments but, in the absence of landmarks, self-movement cues are able to guide navigation relatively accurately. These self-movement cues include signals from the vestibular system, and may originate in the semicircular canals or otolith organs. Here, we tested the otolithic contribution to navigation on a food-hoarding task in darkness and in light. The dark test prevented the use of visual cues and thus favored the use of self-movement information, whereas the light test allowed the use of both visual and non-visual cues. In darkness, tilted mice made shorter-duration stops during the outward journey, and made more circuitous homeward journeys than control mice; heading error, trip duration, and peak error were greater for tilted mice than for controls. In light, tilted mice also showed more circuitous homeward trips, but appeared to correct for errors during the journey; heading error, trip duration, and peak error were similar between groups. These results suggest that signals from the otolith organs are necessary for accurate homing performance in mice, with the greatest contribution in non-visual environments.

Attentive scanning behavior drives one-trial potentiation of hippocampal place fields

The hippocampus is thought to play a critical role in episodic memory by incorporating the sensory input of an experience onto a spatial framework embodied by place cells. Although the formation and stability of place fields requires exploration, the interaction between discrete exploratory behaviors and the specific, immediate, and persistent modifications of neural representations required by episodic memory has not been established. We recorded place cells in rats and found that increased neural activity during exploratory head-scanning behaviors predicted the formation and potentiation of place fields on the next pass through that location, regardless of environmental familiarity and across multiple testing days. These results strongly suggest that, during the attentive behaviors that punctuate exploration, place cell activity mediates the one-trial encoding of ongoing experiences necessary for episodic memory.

Sedative and anxiolytic effects of the extracts of the leaves of Stachytarpheta cayennensis in mice

The leaves are used ethnomedicinally in Nigeria and other parts of the world for insomnia and anxiety among other uses. The investigations sought scientific evidence for the ethnomedicinal use of the leaves for the management of insomnia and anxiety as well as the neural mechanisms for the activities. The sedative and anxiolytic effects of the extracts of the leaves of Stachytarpheta cayennensis were examined in this study. The methanolic extract (5-50 mg/kg, i.p.) as well as the ethylacetate (10-50 mg/kg, i.p.), butanol and aqueous fractions (5-50 mg/kg, i.p.) of the extract were examined. Sedation was assessed as reduced novelty-induced rearing (NIR), reduced spontaneous locomotor activity (SLA) and increased pentobarbitone-induced sleeping time (PIST) in mice. The anti-anxiety effect (methanol 2.5-5.0; butanol 5.0; aqueous 20.0; ethylacetate 25.0 mg/kg, i.p.) was assessed using an elevated plus maze. LD50 was calculated for the extract and the fractions after the intraperitoneal route of administration using the Locke method. The methanolic extract, the butanol and the aqueous fractions inhibited rearing and spontaneous locomotion but prolonged pentobarbitone induced sleep. The ethylacetate fraction however increased both rearing and locomotion and decreased pentobarbitone sleeping time. The butanol and aqueous fractions, but not the methanol extract showed indices of open arm avoidance consistent with anti-anxiety effect. Naltrexone (2.5 mg/kg, i.p.) reversed the inhibition of rearing, locomotion and prolongation of pentobarbitone sleep due to the aqueous fraction of the extract. Flumazenil (2mg/kg, i.p.) abolished the effects of both methanolic extract and the butanol fraction on rearing, locomotion, pentobarbitone sleep and anxiety model. The methanolic extract, the butanol and aqueous fractions possess sedative activity while the ethylacetate fraction possesses stimulant property. The anxiolytic effect was found in both the aqueous fraction and the butanol fraction but not in the main methanol extract and also not in the ethylacetate fraction. Flumazenil, blocked the effect of the leaves of Stachytarpheta cayennensis on rearing, locomotion and elevated plus maze suggesting that GABA receptors are involved in the observed sedative and anxiolytic activities. This study also found opioid receptors involved in the sedative activity of the leaves of Stachytarpheta cayennensis. The rationale for the ethnomedicinal use of the leaves for the management of insomnia and anxiety were confirmed scientifically in this study.

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.

Fornix transection impairs exploration but not locomotion in ambulatory macaque monkeys

Prompted by the theoretical prediction that damage to the hippocampal system should abolish exploratory behavior, the present study examined exploratory movements in control monkeys (CON) and monkeys with transection of the fornix (FNX), a major input/output pathway of the hippocampus. CON and FNX monkeys were introduced to a novel octagonal chamber for six daily sessions, each lasting 20 min. Both groups visited, punctuated by stops, the majority of the floor space of the environment in each of the sessions. The exploratory movements of CON and FNX groups were not significantly different on most of the measures taken over six consecutive days. These measures included cumulative distance traveled, number and duration of stops, traveling patterns, and proportion of time spent in each of 12 designated zones of floor space. The high degree of similarity in behavior between CON and FNX groups suggests that an intact hippocampal system is not necessary for the display of normal exploratory movement per se. On the other hand, the CON and FNX groups did behave differently on two measures. First, the CON group exhibited a decrement in distance traversed over consecutive epochs within the first test session, whereas FNX animals did not. Second, on those days in which the chamber was made visually asymmetrical, the CON animals tended to show a predilection for spending proportionally more time within one particular quadrant of the chamber. These observations are consistent with the idea that interrupting normal hippocampal system function by means of fornix transection is detrimental to learning about the spatial layout of environments. We therefore suggest that while monkeys with fornix transection still display intact locomotor and exploratory behavior patterns, their new learning of visuospatial context is impaired.

Home bases formed to visual cues but not to self-movement (dead reckoning) cues in exploring hippocampectomized rats

Spatial theory proposes that the hippocampus contributes to exploratory behavior allowing animals to acquire information about their environment. In the present study, the exploratory movements of control rats, bulbectomized (anosmic) rats and hippocampectomized rats using the neurotoxin N-methyl-D-aspartate (NMDA) were monitored on a large circular table without walls and around which visual cues were manipulated. The rats displayed organized spatial behavior in that they developed home bases, one or more places operationally defined as those in which they spent a preponderance of time, in which they moved slowly, and to which they returned after excursions. Control rats and hippocampectomized rats were similar in that they established home bases: (i) adjacent to a proximal stable or moving visual landmark; (ii) in relation to more distant visual room cues; and (iii) in relation to contextually conditioned visual cues. Nevertheless, in exploratory tests given under infrared light, a wavelength to which rats are insensitive, control rats and bulbectomized rats established one or more home bases that were not dependent upon surface (e.g. olfactory) cues, whereas home base behavior was absent/fragmented in hippocampectomized rats. Thus, exploratory behavior, as exemplified by home base behavior, is organized in control and hippocampectomized rats in relation to visual cues, but is not organized in hippocampectomized rats when visual cues are absent. This result is discussed in relation to the idea that the hippocampus contributes to spatial behavior that is dependent upon guidance (dead reckoning) derived from self-movement cues.

Movements of exploration intact in rats with hippocampal lesions

Prompted by the theoretical prediction that damage to the hippocampus should abolish exploratory behavior, the present study examined exploratory movements in control rats and rats with hippocampal lesions produced with the neurotoxin N-methyl d-aspartate (NMDA). In four daily 30-min sessions, control and hippocampal rats were exposed to an open circular table under room lighting. Both control and hippocampal rats spent a majority of time near, and organized trips away from, a portion of the table (home base) near a large cue placed proximal to the table. On Day 1, control and HPC rats made equal numbers of head orientations and a comparable number of trips, featuring equal travel distance and numbers of stops. By Day 4, dwell times near the home base increased and other movements decreased in the control rats but the activity profile of Day 1 persisted in the hippocampal rats. The high degree of similarity in behavior between hippocampal and control rats on Day 1 and the persistence of this behavior in hippocampal rats on Day 4 suggests that the hippocampus is not necessary for the display of normal exploratory movements per se. The absence of habituation of exploration in hippocampal rats is discussed in relation to contemporary theories of hippocampal function.

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.

Activity density in the open field: a measure for differentiating the effect of psychostimulants

Traditional open-field activity measures do not provide a sharp behavioral differentiation across psychomotor stimulants such as d-amphetamine (AMPH) and cocaine (COC) in the mouse. We used Software for the Exploration of Exploration (SEE) to investigate and develop a novel behavioral endpoint to characterize the "structure" of AMPH- and COC-induced locomotor behavior in two inbred strains of mouse, C57BL/6 (B6) and DBA/2 (D2). We suggest a measure we term "activity density" as a means to differentiate the behavioral effects of COC and AMPH. Activity density is defined as the activity divided by the range over which it took place. It characterizes the restriction of behavioral repertoire that does not result merely from inactivity. In both the B6 and D2 mice, AMPH increased activity density in a dose-dependent fashion by restricting the range of activity compared with COC doses producing the same level of activity. While AMPH restricted the range in both genotypes, characterizing the geographical region in which the restriction took place further differentiated the genotypes. The newly developed activity density measure thus provides a more general measure than stereotypy of the path, and can differentiate the effects of AMPH and COC both within and across genotypes.

The development of spatial capacity in piloting and dead reckoning by infant rats: Use of the huddle as a home base for spatial navigation

Two forms of spatial navigation, piloting using external cues and dead reckoning using self-movement cues, are manifest in the outward and homeward trips of adult rats exploring from a home base. Here, the development of these two forms of spatial behavior are described for rats aged 14-65 days using a new paradigm in which a huddle of pups or an artificial huddle, a small heat pad, served as a home base on an open circular table that the rats could explore. When moving away from both home bases, the travel distance, path complexity, and number of stops of outward trips from the home base increased progressively with age from postnatal day 16 through 22. When returning to the home bases, the return trips to the home base were always more direct and had high travel velocities even though travel distance increased with age for the longest trips. The results are discussed in relation to the ideas that: (1) the pups pilot on the outward portion of their excursion and dead reckon on the homeward portion of their excursion, and (2) the two forms of navigation and associated spatial capacity are interdependent and develop in parallel and in close association with locomotor skill.

Darting behavior: a quantitative movement pattern designed for discrimination and replicability in mouse locomotor behavior

In the open-field behavior of rodents, Software for Exploring Exploration (SEE) can be used for an explicit design of behavioral endpoints with high genotype discrimination and replicability across laboratories. This ability is demonstrated here in the development of a measure for darting behavior. The behavior of two common mouse inbred strains, C57BL/6J (B6) and DBA/2J (D2), was analyzed across three different laboratories, and under the effect of cocaine or amphetamine. "Darting" was defined as having higher acceleration during progression segments while moving less during stops. D2 mice darted significantly more than B6 mice in each laboratory, despite being significantly less active. These differences were maintained following cocaine administration (up to 20mg/kg) and only slightly altered by amphetamine (up to 5mg/kg) despite a several fold increase in activity. The replicability of darting behavior was confirmed in additional experiments distinct from those used for its design. The strategy leading to the darting measure may be used to develop additional discriminative and replicable endpoints of open-field 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.

Fimbria-fornix lesions disrupt the dead reckoning (homing) component of exploratory behavior in mice

Exploration is the primary way in which rodents gather information about their spatial surroundings. Thus, spatial theories propose that damage to the hippocampus, a structure thought to play a fundamental role in spatial behavior, should disrupt exploration. Exploration in rats is organized. The animals create home bases that are central to exploratory excursions and returns, and hippocampal formation damage alters the organization of exploration by disrupting returns. Mice do not appear to readily establish home bases in novel environments, thus, for this species, it is more difficult to establish the contribution of the hippocampus to exploration. The purpose of the present study was threefold: develop a task in which mice center their exploration from a home base, determine whether the exploratory behavior is organized, and evaluate the role of fimbria-fornix lesions on exploration. Mice were given a novel exploratory task in which their nesting material was placed on a large circular table. Video records of control and fimbria-fornix mice were made in both light and dark (infrared light) conditions. Exploration patterns (outward trips, stops, and homeward trips) were reconstructed from the video records. Control mice centered their activity on their bedding, from which they made circuitous outward trips marked by many stops, and periodic direct returns. The bedding-centered behavior and outward trips of the fimbria-fornix mice were similar to those of the control mice, but significantly fewer direct return trips occurred. The direct homeward trips observed under light and dark conditions were consistent with a dead-reckoning strategy, in which an animal computes its present position and homeward trajectory from self-movement cues generated on the outward trip. Because the fimbria-fornix lesions disrupted the homeward component of exploratory trips, we conclude that the fimbria-fornix may contribute to dead reckoning in mice. The results also show that the home-bedding methodology facilitates the establishment of a home base by mice, thus providing a useful methodology for studies with mice.

A differential involvement of the shell and core subterritories of the nucleus accumbens of rats in attentional processes

The nucleus accumbens comprises of two anatomically distinct subterritories: an inner core and an outer shell region. The distinct pattern of the core and shell input and output targets suggests that these two regions may mediate different behavioral processes. Using N-methyl-D-aspartate excitotoxic lesions in either the core or shell region, we investigated whether we can dissociate functionally these two subterritories. N-Methyl-D-aspartate-lesioned, sham-lesioned and non-operated animals were tested for locomotor activity in an open field and in two behavioral paradigms known to evaluate attentional deficits, namely the pre-pulse inhibition of the acoustic startle reflex and latent inhibition, measured in a two-way active avoidance paradigm. The shell-lesioned animals showed a small but significant hyperactivity in the open field when compared to the core-lesioned and to control animals. In the pre-pulse inhibition paradigm, core-lesioned animals demonstrated reduced pre-pulse inhibition to the two high pre-pulse intensities (80 dB[A], 84 dB[A]). In the active avoidance paradigm, whereas no lesion effects were detected in the non-pre-exposed groups, clear attenuation of latent inhibition was found in the shell-lesioned rats, in comparison to both core-lesioned and control rats, due to improved avoidance performance of the shell-pre-exposed group. From these results we suggest that the two subterritories of the nucleus accumbens are differentially involved in attention-related processes: the core lesion leads to significant disruption of pre-pulse inhibition while the shell lesion leads to heightened activity and significant attenuation of latent inhibition.

The hippocampus as an olfacto-motor mechanism: were the classical anatomists right after all?

The relations between behavior, olfactory input (monitored by recording the activity of the olfactory mucosa), and the spontaneous field potentials of the dentate gyrus were studied in freely moving rats. Bursts of 30-80 Hz (gamma) waves were elicited in the dentate gyrus when a rat sniffed at a variety of objects but were not elicited by auditory, somesthetic, or visual inputs and were not related to the occurrence of locomotion. The presence of gamma wave activity was associated with an enhancement of the population spike elicited in the dentate gyrus by stimulation of the perforant path. Odorized air blown into a nostril via a cannula, inserted under light urethane anesthesia, elicited a gamma wave response bilaterally in the dentate gyrus. These and other data were reviewed to support the general hypothesis that the hippocampus is primarily an olfacto-motor mechanism and does not play any unique role in learning and memory, cognitive mapping, or emotion. The role of the hippocampus in the control of some forms of motor activity is supported by numerous anatomical and electrophysiological studies, studies of the effect of hippocampal lesions on behavior, and studies of the effects of electrical or chemical stimulation of the hippocampus on behavior.

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.

Impact of cognitive impairment on wandering behavior

The purpose of this study was to explore cognitive impairment as a predictor of wandering rhythm and pattern in a sample of 25 demented residents from two long-term care settings. Parameters of rhythm indicating cycle frequency and structure were examined for wandering patterns (random, lapping, and pacing) and for nonwandering (direct) ambulation. All measures of cognitive impairment (Mini-Mental State Exam, Mattis Dementia Rating Scale, and a neuropsychologist's clinical rating) were significant predictors of parameters signifying frequency of wandering for random and lapping patterns but not for the pacing pattern. In addition, for nonwandering ambulation, cognitive impairment predicted some parameters of cycle structure (mean locomoting and nonlocomoting phase durations) but not those denoting frequency of ambulation. Results indicate that cognitive impairment plays an important role in determining the frequency of wandering cycles, but other factors may better explain parameters that characterize its cycle structure.

Contextual fear, gestalt memories, and the hippocampus

This review examines the relationship between exploration and contextual fear conditioning. The fear acquired to places or contexts associated with aversive events is a form of Pavlovian conditioning. However, an initial period of exploration is necessary to allow the animal to form an integrated memory of the features of the context before conditioning can take place. The hippocampal formation plays a critical role in this process. Cells within the dorsal hippocampus are involved in the formation, storage and consolidation of this integrated representation of context. Projections from the subiculum to the nucleus accumbens regulate the exploration necessary for the acquisition of information about the features of the context. This model explains why electrolytic but not excitotoxic lesions of the dorsal hippocampus cause enhanced exploratory activity but both cause deficits in contextual fear. It also explains why retrograde amnesia of contextual fear is greater than anterograde amnesia.

Anticholinergic effects in frogs in a Morris water maze analog

We determined the effect of two doses of the centrally acting anticholinergic drug, atropine sulfate (AS), on the performance of female Northern Leopard frogs (Rana pipiens) in a visual cue analog of the Morris water task. Untreated frogs learned the visually cued task, while frogs treated with 150 mg/kg AS were significantly slower than controls in learning to escape warm water by finding a visible platform, and there was a dose-dependent response, with frogs treated with 50 mg/kg AS performing midway between the higher dose and control frogs. These results suggest that the general role of the cholinergic system in learning is important in amphibians, and that this role is evolutionarily conserved across vertebrate species.

Calibrating space: exploration is important for allothetic and idiothetic navigation

Allothetic and idiothetic navigation strategies rely on very different cues and computational procedures. Allothetic navigation uses the relationships between external cues (visual, auditory, and olfactory) and mapping or geometrical calculations to locate places. Idiothetic navigation relies on cues generated by self-movement (proprioceptive cues or cues from optic, auditory, and olfactory flow, or efference copy of motor commands) and path integration to locate a present location and/or a starting point. Whereas it is theorized that exploratory behavior is used by animals to create a central representation of allothetic cues, it is unclear whether exploration plays a role in idiothetic navigation. Computational models suggest that either a reference frame, calibrated by exploration, or vector addition, without reference to exploration, could support path integration. The present study evaluated the contribution of exploration in these navigation strategies by comparing its contribution to the solution of both allothetic and idiothetic navigation problems. In two experiments, rats were trained to forage on an open table for large food pellets, which they then carried to a refuge to eat. Once trained, they were given probe trials from novel locations in either normal light, which permits the use of allothetic cues, or in infrared light, which requires the use of idiothetic cues. When faced with a new problem in either lighting condition, the rats first explored the foraging table before navigating directly home with the food. That exploration is equally important for allothetic and idiothetic navigation, suggests that both navigation strategies require a calibrated representation of the environment.

Movement-related glutamate levels in rat hippocampus, striatum, and sensorimotor cortex

Changes in brain extracellular glutamate during movement stress were studied using in vivo microdialysis. Male Long-Evans rats were placed in a clear cylinder designed to elicit behavioral activation while undergoing microdialysis sampling from either the hippocampus, striatum or sensorimotor cortex. Glutamate levels were determined by high performance liquid chromatography with fluorescence detection in the dialysates taken before, during, and after exposure to the cylinder. Animals were in a behaviorally quiescent state before exposure to the cylinder, but they demonstrated increases in rearing, locomotion, and turning while in the cylinder. Dialysate glutamate levels were significantly enhanced in the samples taken while the rat was in the cylinder compared with samples taken before or after exposure to the cylinder. In a second study, rats were implanted with bilateral probes in the forelimb sensorimotor cortex, and one forelimb was immobilized by means of a plaster of paris cast. Glutamate, aspartate, serine, and taurine levels were quantified in casted animals. In casted animals, dialysate glutamate levels were lower on the side contralateral to the immobilized limb during both quiescence and movement stress. Aspartate and taurine, but not serine levels increased during movement stress in both the side contralateral and the side ipsilateral to the immobilized limb. These results suggest that there is extracellular overflow of glutamate and other neuroactive amino acids during spontaneous movement, and chronic disuse can suppress extracellular glutamate levels.

Wandering. A dementia-compromised behavior

The wandering behavior of individuals with dementia is a puzzling behavior, and strategies for responding to it often are poorly grounded. However, advances in the understanding of neuro-cognitive factors contributing to wandering now may provide important clues for designing nursing approaches. In this article, such advances are summarized, and implications and strategies for practice are delineated.

Neurotoxic lesions of the dorsal hippocampus and Pavlovian fear conditioning in rats

Electrolytic lesions of the dorsal hippocampus (DH) produce deficits in both the acquisition and expression of conditional fear to contextual stimuli in rats. To assess whether damage to DH neurons is responsible for these deficits, we performed three experiments to examine the effects of neurotoxic N-methyl-D-aspartate (NMDA) lesions of the DH on the acquisition and expression of fear conditioning. Fear conditioning consisted of the delivery of signaled or unsignaled footshocks in a novel conditioning chamber and freezing served as the measure of conditional fear. In Experiment 1, posttraining DH lesions produced severe retrograde deficits in context fear when made either 1 or 28, but not 100, days following training. Pretraining DH lesions made 1 week before training did not affect contextual fear conditioning. Tone fear was impaired by DH lesions at all training-to-lesion intervals. In Experiment 2, posttraining (1 day), but not pretraining (1 week), DH lesions produced substantial deficits in context fear using an unsignaled shock procedure. In Experiment 3, pretraining electrolytic DH lesions produced modest deficits in context fear using the same signaled and unsignaled shock procedures used in Experiments 1 and 2, respectively. Electrolytic, but not neurotoxic, lesions also increased pre-shock locomotor activity. Collectively, this pattern of results reveals that neurons in the DH are not required for the acquisition of context fear, but have a critical and time-limited role in the expression of context fear. The normal acquisition and expression of context fear in rats with neurotoxic DH lesions made before training may be mediated by conditioning to unimodal cues in the context, a process that may rely less on the hippocampal memory system.

Hippocampal lesions and path integration

Research on spatial problem-solving over the past two years has linked the hippocampus to path integration, that is, the use of movement-related cues to guide spatial behavior. Path integration may underlie the forms of place learning that are impaired by hippocampal damage. It remains a challenge to determine whether path integration is the central function of the hippocampus or but one of many.

Perseveration on place reversals in spatial swimming pool tasks: further evidence for place learning in hippocampal rats

Animals with damage to the fimbria-fornix (FF) or cells of the hippocampus (HIP) can learn a place problem but cannot learn matching-to-place problems, which feature a series of place "reversals." The two experiments described in the present report were designed to examine the causes of impairment on reversal learning. In experiment 1, control, HIP, and FF groups were trained to asymptote on a place problem, and then the location of the platform was moved. Control rats learned the reversal response more quickly than the initial response; the HIP rats learned both problems at the same rate. Swim analysis showed that the impairment in the lesion group on the reversal response was aggravated by perseverative returns to the first learned place. In experiment 2, control and FF groups were trained on a task in which the platform was visible on three daily trials and hidden on one daily trial. After 10 days, the platforms were moved. In the reversal response, the FF group showed enhanced performance on the cue trials and severely impaired performance on the place trials relative to initial learning and control performance. Swim analysis showed that FF rats perseverated on the initial place response in place trials. These experiments provide further evidence for place learning in hippocampal rats and show that perseverative responses contribute to impairments in new learning. The results are discussed in relation to the idea that the hippocampus mediates spatial mapping and/or uses self-movement cues to solve spatial problems.

The neuroscience of spatial navigation: focus on behavior yields advances

The development of the water maze as a laboratory approach to the study of spatial navigation has led to a large amount of research on the brain mechanisms of this ecologically important behavior. The procedural simplicity of this task belies its underlying complexity, which can complicate the interpretation of data obtained with the standard water maze procedure. In this review, recent experiments that used novel training procedures and detailed analyses of behavior are evaluated, together with related experiments, to clarify the brain mechanisms involved in this behavior. Pharmacological, lesion, and unit recording experiments demonstrate the existence of forebrain circuits for spatial navigation that are considerably more varied and extensive than was previously proposed, and involve various extrahippocampal structures. The use of novel and specialized procedures, together with a continued detailed focus on the behavior of animals in the maze, appears to be the most promising approach to understanding the mechanisms of spatial navigation.

Evidence for extrahippocampal involvement in place learning and hippocampal involvement in path integration

Although there is a good deal of evidence that animals require the hippocampus for learning place responses, animals with damage to the afferent and efferent fibers coursing through the fimbria-fornix have been shown to acquire a place response. This finding suggests either that the cells of the hippocampus proper (CA1-4 and dentate gyrus), via their connections to the temporal lobe, can mediate place learning or that some extrahippocampal structure is sufficient. We examined this question using rats with ibotenic acid lesions of the cells of the hippocampus. Rats were pretrained to swim to a visible platform and then given probe trials on which the visible platform was removed. Video and kinematic analyses showed that the hippocampal rats expected to find the platform at its previous location because they swam directly to that location and paused and turned at that location after the platform was removed. Additional tests confirmed that they had learned a place response. There were, however, abnormalities in their swimming patterns, and despite having acquired one place response, they did not then acquire new place responses when only the hidden platform training procedure was used. These results demonstrate that place learning can be acquired by rats in which the hippocampus proper is removed. Contrasts between conditions in which hippocampal rats acquire a place response and conditions in which they fail suggests that the hippocampus may serve as an on line system for monitoring movement and integrating movement paths.

Similarities vs. differences in place learning and circadian activity in rats after fimbria-fornix section or ibotenate removal of hippocampal cells

Damage to either the fimbria-fornix or to the hippocampus can produce a deficit in spatial behavior and change in locomotor activity but the extent to which the two kinds of damage are comparable is not known. Here we contrasted the effects of cathodal sections of the fimbria-fornix with ibotenic acid lesions of the cells of the hippocampus (Ammon's horn and the dentate gyrus) on place learning in a swimming pool and on circadian activity. Rats in both ablation groups were impaired relative to control rats in learning a single place response but they did acquire the response as measured by swim latencies, errors, and by enhanced searching on probe trials. They were also more active than the control group on the test of activity. Nevertheless, the fimbria-fornix group was initially more impaired on learning and was more active than the hippocampal group. Analysis of the strategies used in learning indicated that the lesion groups were very similar to each other but different from the control group especially in that at asymptotic performance, rats in both lesion groups made rather tight loops as they swam toward the platform. This strategy likely contributed to the greater proportion of time they spent swimming in the correct quadrant on the subsequent probe trial. These findings confirm that rats with fimbria-fornix or hippocampal damage display impairments in place learning and are hyperactive but also show that there are lesion differences. The results are discussed with respect to the relative effectiveness of the lesions and the possibility that fibers in the fimbria-fornix may mediate some functions that are not attributable to the hippocampus.