From random walks to informed movement

The analysis of animal movement is a large and continuously growing field of research. Detailed knowledge about movement strategies is of crucial importance for understanding eco‐evolutionary dynamics at all scales – from individuals to (meta‐)populations. This and the availability of detailed movement and dispersal data motivated Nathan and colleagues to published their much appreciated call to base movement ecology on a more thorough mechanistic basis. So far, most movement models are based on random walks. However, even if a random walk might describe real movement patterns acceptably well, there is no reason to assume that animals move randomly. Therefore, mechanistic models of foraging strategies should be based on information use and memory in order to increase our understanding of the processes that lead to animal movement decisions. We present a mechanistic movement model of an animal with a limited perceptual range and basic information storage capacities. This ‘spatially informed forager’ constructs an internal map of its environment by using perception, memory and learned or evolutionarily acquired assumptions about landscape attributes. We analyse resulting movement patterns and search efficiencies and compare them to area restricted search strategies (ARS) and biased correlated random walks (BCRW) of omniscient individuals. We show that, in spite of their limited perceptual range, spatially informed individuals boost their foraging success and may perform much better than the best ARS. The construction of an internal map and the use of spatial information results in the emergence of a highly correlated walk between patches and a rather systematic search within resource clusters. Furthermore, the resulting movement patterns may include foray search behaviour. Our work highlights the strength of mechanistic modelling approaches and sets the stage for the development of more sophisticated models of memory use for movement decisions and dispersal.     

Gender-Specific Differences in the Relationship between Autobiographical Memory and Intertemporal Choice in Older Adults

As the population of older adults grows, their economic choices will have increasing impact on society. Research on the effects of aging on intertemporal decisions shows inconsistent, often opposing results, indicating that yet unexplored factors might play an essential role in guiding one''s choices. Recent studies suggest that episodic future thinking, which is based on the same neural network involved in episodic memory functions, leads to reductions in discounting of future rewards. As episodic memory functioning declines with normal aging, but to greatly variable degrees, individual differences in delay discounting might be due to individual differences in the vitality of this memory system in older adults. We investigated this hypothesis, using a sample of healthy older adults who completed an intertemporal choice task as well as two episodic memory tasks. We found no clear evidence for a relationship between episodic memory performance and delay discounting in older adults. However, when additionally considering gender differences, we found an interaction effect of gender and autobiographical memory on delay discounting: while men with higher memory scores showed less delay discounting, women with higher memory scores tended to discount the future more. We speculate that this gender effect might stem from the gender-specific use of different modal representation formats (i.e. temporal or visual) during assessment of intertemporal choice options.     

Remembrance of odors past: human olfactory cortex in cross-modal recognition memory

Episodic memory is often imbued with multisensory richness, such that the recall of an event can be endowed with the sights, sounds, and smells of its prior occurrence. While hippocampus and related medial temporal structures are implicated in episodic memory retrieval, the participation of sensory-specific cortex in representing the qualities of an episode is less well established. We combined functional magnetic resonance imaging (fMRI) with a cross-modal paradigm, where objects were presented with odors during memory encoding. We then examined the effect of odor context on neural responses at retrieval when these same objects were presented alone. Primary olfactory (piriform) cortex, as well as anterior hippocampus, was activated during the successful retrieval of old (compared to new) objects. Our findings indicate that sensory features of the original engram are preserved in unimodal olfactory cortex. We suggest that reactivation of memory traces distributed across modality-specific brain areas underpins the sensory qualities of episodic memories.     

Modelling nematode movement using time-fractional dynamics

We use a correlated random walk model in two dimensions to simulate the movement of the slug parasitic nematode Phasmarhabditis hermaphrodita in homogeneous environments. The model incorporates the observed statistical distributions of turning angle and speed derived from time-lapse studies of individual nematode trails. We identify strong temporal correlations between the turning angles and speed that preclude the case of a simple random walk in which successive steps are independent. These correlated random walks are appropriately modelled using an anomalous diffusion model, more precisely using a fractional sub-diffusion model for which the associated stochastic process is characterised by strong memory effects in the probability density function.     

A unified model of spatial and episodic memory

Medial temporal lobe structures including the hippocampus are implicated by separate investigations in both episodic memory and spatial function. We show that a single recurrent attractor network can store both the discrete memories that characterize episodic memory and the continuous representations that characterize physical space. Combining both types of representation in a single network is actually necessary if objects and where they are located in space must be stored. We thus show that episodic memory and spatial theories of medial temporal lobe function can be combined in a unified model.     

Aging, eye movements, and object-location memory

This study investigated whether "intentional" instructions could improve older adults' object memory and object-location memory about a scene by promoting object-oriented viewing. Eye movements of younger and older adults were recorded while they viewed a photograph depicting 12 household objects in a cubicle with or without the knowledge that memory about these objects and their locations would be tested (intentional vs. incidental encoding). After viewing, participants completed recognition and relocation tasks. Both instructions and age affected viewing behaviors and memory. Relative to incidental instructions, intentional instructions resulted in more accurate memory about object identity and object-location binding, but did not affect memory accuracy about overall positional configuration. More importantly, older adults exhibited more object-oriented viewing in the intentional than incidental condition, supporting the environmental support hypothesis.     

Neocortical connectivity during episodic memory formation

During the formation of new episodic memories, a rich array of perceptual information is bound together for long-term storage. However, the brain mechanisms by which sensory representations (such as colors, objects, or individuals) are selected for episodic encoding are currently unknown. We describe a functional magnetic resonance imaging experiment in which participants encoded the association between two classes of visual stimuli that elicit selective responses in the extrastriate visual cortex (faces and houses). Using connectivity analyses, we show that correlation in the hemodynamic signal between face- and place-sensitive voxels and the left dorsolateral prefrontal cortex is a reliable predictor of successful face-house binding. These data support the view that during episodic encoding, "top-down" control signals originating in the prefrontal cortex help determine which perceptual information is fated to be bound into the new episodic memory trace.     

Diffusion models for population dynamics incorporating individual behavior at boundaries: applications to refuge design.

We construct models for dispersal of a population which incorporate the response of individuals to interfaces between habitat types. The models are based on random walks where there may be a bias in the direction an individual moves when it encounters an interface. This sort of dispersal process is called skew Brownian motion. Our models take the form of diffusion equations with matching conditions across the interface between regions for population densities and fluxes. We combine the dispersal models with linear population growth models which assume that the population growth rate differs between regions of different habitat types. We use those models to study issues of refuge design. We specifically consider how the effectiveness of buffer zones depends on their size, quality, and the population's response to the interface between the buffer zone and the refuge.     

Territory surveillance and prey management: Wolves keep track of space and time

Identifying behavioral mechanisms that underlie observed movement patterns is difficult when animals employ sophisticated cognitive‐based strategies. Such strategies may arise when timing of return visits is important, for instance to allow for resource renewal or territorial patrolling. We fitted spatially explicit random‐walk models to GPS movement data of six wolves (Canis lupus; Linnaeus, 1758) from Alberta, Canada to investigate the importance of the following: (1) territorial surveillance likely related to renewal of scent marks along territorial edges, to reduce intraspecific risk among packs, and (2) delay in return to recently hunted areas, which may be related to anti‐predator responses of prey under varying prey densities. The movement models incorporated the spatiotemporal variable “time since last visit,” which acts as a wolf's memory index of its travel history and is integrated into the movement decision along with its position in relation to territory boundaries and information on local prey densities. We used a model selection framework to test hypotheses about the combined importance of these variables in wolf movement strategies. Time‐dependent movement for territory surveillance was supported by all wolf movement tracks. Wolves generally avoided territory edges, but this avoidance was reduced as time since last visit increased. Time‐dependent prey management was weak except in one wolf. This wolf selected locations with longer time since last visit and lower prey density, which led to a longer delay in revisiting high prey density sites. Our study shows that we can use spatially explicit random walks to identify behavioral strategies that merge environmental information and explicit spatiotemporal information on past movements (i.e., “when” and “where”) to make movement decisions. The approach allows us to better understand cognition‐based movement in relation to dynamic environments and resources.     

Leisure Activity,Brain β‐amyloid,and Episodic Memory in Adults with Down Syndrome

The present study provided an investigation of associations between leisure activity and early Alzheimer’s disease neuropathology (i.e., brain β‐amyloid) and episodic memory in a sample of 65 adults with Down syndrome (aged 30–53 years), at baseline and follow‐up, approximately three years apart. Findings indicated that leisure activity at baseline was not associated with brain β‐amyloid at baseline or change in brain β‐amyloid from baseline to follow‐up. Greater cognitively stimulating leisure activity at baseline was associated with better episodic memory at baseline, and greater social leisure activity at baseline was associated with less decline in episodic memory from baseline to follow‐up. High (as opposed to low) levels of social and overall leisure activity at baseline moderated the association between increase in brain β‐amyloid and decline in episodic memory, from baseline to follow‐up. Findings suggest that cognitively stimulating and social leisure activity could protect against the effect of Alzheimer’s disease neuropathology on episodic memory in adults with Down syndrome.     

Landmark recognition in Alzheimer's dementia: spared implicit memory for objects relevant for navigation

Background

In spatial navigation, landmark recognition is crucial. Specifically, memory for objects placed at decision points on a route is relevant. Previous fMRI research in healthy adults showed higher medial-temporal lobe (MTL) activation for objects placed at decision points compared to non-decision points, even at an implicit level. Since there is evidence that implicit learning is intact in amnesic patients, the current study examined memory for objects relevant for navigation in patients with Alzheimer’s dementia (AD).

Methodology/Principal Findings

21 AD patients participated with MTL atrophy assessed on MRI (mean MMSE = 21.2, SD = 4.0), as well as 20 age- and education-matched non-demented controls. All participants watched a 5-min video showing a route through a virtual museum with 20 objects placed at intersections (decision points) and 20 at simple turns (non-decision points). The instruction was to pay attention to the toys (half of the objects) for which they were supposedly tested later. Subsequently, a recognition test followed with the 40 previously presented objects among 40 distracter items (both toys and non-toys). Results showed a better performance for the non-toy objects placed at decision points than non-decision points, both for AD patients and controls.

Conclusion/Significance

Our findings indicate that AD patients with MTL damage have implicit memory for object information relevant for navigation. No decision point effect was found for the attended items. Possibly, focusing attention on the items occurred at the cost of the context information in AD, whereas the controls performed at an optimal level due to intact memory function.     

Long-term memory in Alzheimer's disease.

Recent findings have further characterized the neural and psychological bases of long-term memory failure in Alzheimer's disease. Convergent volumetric neuroimaging studies indicate that loss of episodic memory is specifically related to early-stage limbic-diencephalic pathology, and that non-mnemonic impairment is specifically related to later-stage temporal-neocortical pathology. Recent studies of Alzheimer's disease have also reported informative cognitive dissociations in semantic memory and implicit memory.     

Episodic memory: insights from semantic dementia

Semantic dementia, also known as the temporal lobe variant of fronto-temporal dementia, results in a progressive yet relatively pure loss of semantic knowledge about words, objects and people, and is associated with asymmetric, focal atrophy of the antero-lateral temporal lobes. Semantic dementia provides a unique opportunity to study the organization of long-term memory particularly since initial observations suggested sparing of episodic memory. Recent studies reveal, however, a more complex but theoretically revealing pattern. On tests of autobiographical memory, patients with semantic dementia show a 'reverse step function' with sparing of recall of events from the most recent 2 to 5 years but impairment on more distant life periods. Anterograde recognition memory for visual materials is extremely well preserved, except in the most deteriorated cases, although performance is heavily reliant upon perceptual information about the studied stimuli, particularly for items that are no longer known by the subjects. On tests of verbal anterograde memory such as word learning, performance is typically poor even for words which are 'known' to the patients. A source discrimination experiment, designed to evaluate familiarity and recollection-based anterograde memory processes, found that patients with semantic dementia showed good item detection, although recollection of source was sometimes impaired. Semantic knowledge about studied items and measures of item detection and source discrimination were largely independent. The implications of these findings for models of long-term memory are discussed. The results support the concept that episodic memory, or at least the recall of temporally specific autobiographical experiences, draws upon a number of separable memory processes, some of which can function independently of semantic knowledge.     

基于统一过程方法的膜片钳系统软件设计

新一代膜片钳系统控制软件使用敏捷UP(统一过程)、可复用面向对象方法,多线程内存共享、事件同步和内存映射文件技术,具有响应速度快、内存消耗小、易移植、维护性能好等特点。实验结果表明,新系统显著提高了电压门控模式的系统响应速度,增加了配体门控模式的记录时间等。     

A reduction in hippocampal GABAA receptor α5 subunits disrupts the memory for location of objects in mice

The memory for location of objects, which binds information about objects to discrete positions or spatial contexts of occurrence, is a form of episodic memory particularly sensitive to hippocampal damage. Its early decline is symptomatic for elderly dementia. Substances that selectively reduce α5‐GABA_A receptor function are currently developed as potential cognition enhancers for Alzheimer's syndrome and other dementia, consistent with genetic studies implicating these receptors that are highly expressed in hippocampus in learning performance. Here we explored the consequences of reduced GABA_Aα5‐subunit contents, as occurring in α5(H105R) knock‐in mice, on the memory for location of objects. This required the behavioral characterization of α5(H105R) and wild‐type animals in various tasks examining learning and memory retrieval strategies for objects, locations, contexts and their combinations. In mutants, decreased amounts of α5‐subunits and retained long‐term potentiation in hippocampus were confirmed. They exhibited hyperactivity with conserved circadian rhythm in familiar actimeters, and normal exploration and emotional reactivity in novel places, allocentric spatial guidance, and motor pattern learning acquisition, inhibition and flexibility in T‐ and eight‐arm mazes. Processing of object, position and context memories and object‐guided response learning were spared. Genotype difference in object‐in‐place memory retrieval and in encoding and response learning strategies for object–location combinations manifested as a bias favoring object‐based recognition and guidance strategies over spatial processing of objects in the mutants. These findings identify in α5(H105R) mice a behavioral–cognitive phenotype affecting basal locomotion and the memory for location of objects indicative of hippocampal dysfunction resulting from moderately decreased α5‐subunit contents.     

The home range fractal: From random walk to memory-dependent space use

We present theoretical developments of the multi-scaled random walk (MRW) model for cognitive map-influenced space use by animals. The extensions include a unified space–time scaling function, and further details with respect to statistical properties of the spatial distribution of a set of locations. Supported by numeric simulations we show how memory effects may open for a complex, multi-scaled and self-organized – i.e., intrinsically driven – habitat utilization pattern with fractal dimensional properties. These properties allow for testing for MRW compliance by using parameters from classic movement models like Brownian motion, correlated random walk and Levy walks as null models. In terms of applied ecology, empirical confirmation of memory-influenced space use by individuals will have consequences for interpretation and statistical analyses of habitat utilization. For example, under memory map influence, re-visits to intra-home range locations do not represent independent events. Further, the MRW formulation specifically implies home range movements over a continuum of process rates, spanning a range of spatio-temporal scales in parallel, in violation of the traditional low order Markovian (scale-specific) model architecture. The MRW approach requires an extension of classic Boltzmann–Gibbs statistical mechanics, which rests on the premise that spatio-temporal memory effects are averaged out beyond micro-scales. We suggest that the emergent coherence between spatial and temporal scaling from the MRW approach may open for a more realistic statistical mechanics theory for population processes under terms of memory-influenced space use by individuals.     

Episodic-like memory in animals: psychological criteria, neural mechanisms and the value of episodic-like tasks to investigate animal models of neurodegenerative disease

The question of whether any non-human species displays episodic memory is controversial. Associative accounts of animal learning recognize that behaviour can change in response to single events but this does not imply that animals need or are later able to recall representations of unique events at a different time and place. The lack of language is also relevant, being the usual medium for communicating about the world, but whether it is critical for the capacity to represent and recall events is a separate matter. One reason for suspecting that certain animals possess an episodic-like memory system is that a variety of learning and memory tasks have been developed that, even though they do not meet the strict criteria required for episodic memory, have an 'episodic-like' character. These include certain one-trial learning tasks, scene-specific discrimination learning, multiple reversal learning, delayed matching and non-matching tasks and, most recently, tasks demanding recollection of 'what, where and when' an event happened. Another reason is that the neuronal architecture of brain areas thought to be involved in episodic memory (including the hippocampal formation) are substantially similar in mammals and, arguably, all vertebrates. Third, our developing understanding of activity-dependent synaptic plasticity (which is a candidate neuronal mechanism for encoding memory traces) suggests that its expression reflects certain physiological characteristics that are ideal components of a neuronal episodic memory system. These include the apparently digital character of synaptic change at individual terminals and the variable persistence of potentiation accounted for by the synaptic tag hypothesis. A further value of studying episodic-like memory in animals is the opportunity it affords to model certain kinds of neurodegenerative disease that, in humans, affect episodic memory. An example is recent work on a transgenic mouse that over-expresses a mutation of human amyloid precursor protein (APP) that occurs in familial Alzheimer's disease, under the control of platelet derived (PD) growth factor promoter (the PDAPP mouse). A striking age- and amyloid plaque-related deficit is seen using a task in which the mice have to keep changing their memory representation of the world rather than learn a single fact.     

Overview: The future of ecological risk assessment

Risk assessment has become a popular tool to help solve ecological problems. The basic concept is not new and has been applied to diverse decision problems. The application to ecological problems, especially complex ecological problems, is fairly recent and controversial. The fundamental and most important elements of the controversy revolve around two key points: (1) a person's implicit “world view;”; and (2) the assumption of who (or what) receives the benefits and who (or what) pays the costs for ecological “decisions.”; A person's attitude toward risk assessment is, at least implicitly, defined by a world view. It is this world view that defines how each of us reacts to risk assessment applied to ecological problems. How the question of benefits and costs is defined also defines the appropriate use, if any, of ecological risk assessment. The future of ecological risk assessment will almost certainly follow the course of other analytical tools: enthusiastic support, rapid, widespread adoption and use; then disillusionment and rapid replacement with newer approaches, but with continued use for a greatly constrained set of ecological issues.     

Differential consolidation and pattern reverberations within episodic cell assemblies in the mouse hippocampus

One hallmark feature of consolidation of episodic memory is that only a fraction of original information, which is usually in a more abstract form, is selected for long-term memory storage. How does the brain perform these differential memory consolidations? To investigate the neural network mechanism that governs this selective consolidation process, we use a set of distinct fearful events to study if and how hippocampal CA1 cells engage in selective memory encoding and consolidation. We show that these distinct episodes activate a unique assembly of CA1 episodic cells, or neural cliques, whose response-selectivity ranges from general-to-specific features. A series of parametric analyses further reveal that post-learning CA1 episodic pattern replays or reverberations are mostly mediated by cells exhibiting event intensity-invariant responses, not by the intensity-sensitive cells. More importantly, reactivation cross-correlations displayed by intensity-invariant cells encoding general episodic features during immediate post-learning period tend to be stronger than those displayed by invariant cells encoding specific features. These differential reactivations within the CA1 episodic cell populations can thus provide the hippocampus with a selection mechanism to consolidate preferentially more generalized knowledge for long-term memory storage.