A Behavioral Task Modeling 'Everyday Memory' in an Event Arena to Foster Allocentric Representations for Rodents

Summary

The goal of this optimized ‘everyday memory’ protocol in an event arena was to employ a stable home-base that encourages the use of allocentric spatial representations. This animal model provides an effective test-bed for future research into the formation and retention of event memories using behavioral and physiological techniques.

Abstract

The event arena provides an optimal platform to investigate learning and memory. The appetitive everyday memory task described in this paper provides a robust protocol for the investigation of episodic and spatial memory in rodents, which specifically fosters allocentric memory representation. Rats are trained to find and dig for food during the encoding phase and, after a time delay, rats are given a choice to find the reward food pellet in the correct location. There are two key elements that promote the use of an allocentric strategy in this protocol: 1) rats start from different start locations within and between sessions, 2) a stable home-base is deployed where rats have to carry their food to eat. By means of these modifications, we effectively encourage the rodents to use allocentric spatial representations to perform the task. In addition, the task provides a good paradigm for within-subject experimental design and allows experimenters to manipulate different conditions to reduce variability. Used in conjunction with behavioral and physiological techniques, the resulting rodent model provides an effective test-bed for future research into memory formation and retention.

Introduction

To investigate the neurobiology of learning and memory, invasive techniques are required, which are not generally feasible in humans. Thus, for over a century, behavioural protocols have been designed for laboratory animals to model various forms of human memory. The design and choice of both task and apparatus are central to the success of effective models of human memory. Numerous paradigms have been developed with diverse complexity, ranging from simple classical and instrumental conditioning protocols^1,2,3 to mazes such as the T-maze⁴, radial arm maze⁵, Barnes maze⁶, watermaze⁷, and the cheese-board maze⁸. Yet, while these tasks capture facets of associative learning and spatial navigation, they cannot be used unambiguously to study the memory representation of momentary events (i.e., episodic-like memory). And, although novel object recognition⁹ and permutations of this spontaneous memory task, such as object-place memory¹⁰, have provided valuable insights into recognition memory, they do not test explicit recall of events. To address this demand, the event arena was specifically developed, and its use has enabled research into long-term, paired-associate memory encoding and recall^11,12,13 as well as the encoding and recall of discrete events happening in a familiar space^{14,15,16,17,18}. The latter theme is the focus of this manuscript.

The event arena is a large, square, open-field area where events occur for rodents. The size of the arena can be scaled to accommodate either rats or mice, and rodents are encouraged to enter and explore. A typical example of an event that takes place within the arena is the finding and retrieval of food from a sandwell at a specific location. The event arena is designed for such appetitive tasks, in which rats or mice are trained to search for, find, and dig up food. It capitalizes on their natural tendency to carry food back to a dark environment, which in this case is located adjacent to the arena, where they then eat it. After minimal training to dig for food, rodents take to this task naturally and perform well in the encoding trial, and in the recall choice trial, which follows the encoding trials after a short, 30-min delay. In a choice trial, several sandwells (i.e., locations for digging) are available, but only one is rewarded.

Different tasks can be performed within the event arena (e.g., spatial memory, episodic-like memory, and paired-associate learning). Given the interest in developing effective models of episodic-like memory, the following protocol was developed, in which the location where food can be found is altered daily. In this task, rodents are required to remember where the event of digging for, and successfully retrieving, a food reward happened most recently within the event arena. The protocol outlined below entails an encoding trial in which rats search for a sandwell in a new place each day followed, after a delay, by a recall choice trial, where the recently encoded sandwell location is rewarded, while the other, alternative sandwells in different locations do not contain accessible food. Remembering where the food was on a previous day is not helpful: the correct location has to be encoded and remembered, at least for a while, each day. Accordingly, we have introduced the term 'everyday memory' to capture the form of memory modeled in this task, which we, as humans, use on a daily basis. A human example of everyday memory is remembering where one has parked one's car at the shopping mall (Figure 1A) or has put one's glasses down around the house. In this protocol, all intra- and extra-arena cues are all stable, just as they are in the settings of our everyday lives (i.e., homes, offices, car parks, etc.). Thus, rodents must remember where something happened most recently within a familiar environment (Figure 1B). The task is analogous to, but an improvement on, the delayed-matching-to-place (DMP) task in the watermaze¹⁹. Being an appetitive task, it exploits rodents' natural behavior to forage for food²⁰, instead of their desire to escape from the water. However, as in the watermaze⁷, there are no local cues differentiating correct from incorrect locations; animals must use recall rather than recognition to locate the correct sandwell location after varying memory delay durations.

Figure 1: Everyday memory. (A) Human everyday memory. Schematic showing a green car parked in a car park. After a delay, the driver attempts to remember exactly where she parked her car. (B) Animal everyday memory. Schematic showing a rat digging and retrieving a pellet from a sandwell at a location within the event arena. After a delay, the rat is given a choice trial with multiple incorrect sandwells (gray) and one correct sandwell (green). Please click here to view a larger version of this figure.

The event arena has already been successfully utilized in investigations of 'everyday memory'. These are memories that are automatically encoded each day, retained in long-term memory, but often forgotten after relatively short time periods. Bast et al.¹⁴ showed monotonic delay-dependent event memory, which varies from excellent memory after short intervals to chance level after 24 h. The retention of memory can, however, be successfully enhanced by post-encoding novelty or, with multiple encoding trials, with extended trial spacing^15,17.

The event arena is versatile and relatively non-stressful; no aversive stimuli are used. The size of the arena, and the tasks it accommodates, can be adapted for both rats^14,15 and mice¹⁶. Also, as a land-based task, it is amenable to physiological recording and calcium imaging studies, unlike the watermaze²¹. Moreover, in accordance with the principles of the 3Rs (reduction, refinement, replacement), studies employing the event arena require fewer animals to obtain statistical power, as within-subject experimental designs are feasible (in which each animal serves as its own control for pharmacological interventions, optogenetic stimulations, etc.) and no aversive stimulation is required for motivation. Although initial training demands more time and occurs over more sessions than in, for example, novelty recognition tasks, once animals achieve a stable, asymptotic level of task performance, manipulations such as drug, vehicle-control, or optogenetic stimulation may be interspersed with a relatively small number of additional training sessions¹⁷. In addition, distinct facets of representation come under direct experimental control in the event arena, such as the nature of the spatial representation employed when solving the task.

The issue of representation concerns the mental framework employed by rats when remembering where recent events happen¹⁸. Do they remember where the food is located, or do they only remember how to get to the food? Rats can use allocentric (map-like) or egocentric (body-centered) spatial representations to solve an appetitive task within the arena¹⁸. However, to control and identify the spatial strategy employed by each experimental subject when performing the task, there are distinct training protocols that are able to selectively promote the use of only one spatial representation. Usually, an egocentric-based representation is employed when rats take their food reward back to the same location from which they started the day's trial, which allows several opportunities to remember the reward location during runs back and forth. This spatial strategy can be employed regardless of whether the start location is changed from day to day or kept constant. In contrast, an allocentric representation is favored when rats are required to carry food reward to a fixed home-base location at the side of the arena, which is different from the changing starting locations. There are numerous advantages of allocentric representations with respect to the brain's storage capacity.

In this paper, we have outlined the home-base protocol, which encourages the employment of only an allocentric representation. We have provided representative results for this task, which clearly illustrate the advantages of using this rodent model of 'everyday memory' in the investigation of learning and memory and highlights how allocentric representations of episodic-like spatial memory can be promoted.

Protocol

The methods described in this paper have been approved by the University of Edinburgh Ethical Review Committee; they are compliant with the UK Animals (Scientific Procedures) Act 1986 and the European Communities Council Directive of 24 November 1986 (86/609/EEC) legislation governing the maintenance of laboratory animals and their use in scientific experiments. NOTE: The experimental subject of the protocol outlined below is Lister-hooded rats, but it can be adapted for other rodent strains.<…

Representative Results

This stable home-base protocol has been used to successfully train rats to learn this everyday memory task using allocentric representations. There are two important elements in this protocol. First, animals start from different black boxes (e.g., East, South, and West) within and between sessions (Figure 7A). There are two encoding trials and one recall choice trial per session (or probe trial instead of the choice trial in some cases), all starting from an alternate startbox. This encourag…

Discussion

Humans automatically encode single events in everyday life. We readily recall some events and forget others. The episodic-like everyday memory protocol described above provides a robust method for researchers wishing to investigate this type of memory (episodic memory) in rodents. Because the task involves the daily act of finding and retrieving food pellets from a defined location, the natural instinct of rodents to forage for food is exploited. The task rests on the reasonable assumption that the act of finding and dig…

Materials

Camera	CCTVFirst	N/A
Event Arena	University of Edinburgh (designed and built in house)	University of Edinburgh (designed and built in house)	Event arena for everyday memory task
Lister-hooded rats	Charles River UK	603
Multitimer Labview	University of Edinburgh (designed and built in house)	University of Edinburgh (designed and built in house)
Pneumatics, frames, screws of event arena	RS Components Ltd.	University of Edinburgh (P. Spooner)	Tools for building event arena
Sandwells	Adam Plastics (http://www.adamplastics.co.uk)	University of Edinburgh (P. Spooner)	Sandwells for arena
Startboxes	Adam Plastics (http://www.adamplastics.co.uk)	University of Edinburgh (P. Spooner)
Video recording	Windows 10 computers with OBS software, Blackmagic Decklink Mini Recorder cards	N/A