Assessment of Mouse Judgment Bias through an Olfactory Digging Task

Summary

This article provides a detailed description of a novel mouse judgment bias protocol. Evidence of this olfactory digging task’s sensitivity to affective state is also demonstrated and its utility across diverse research fields is discussed.

Abstract

Judgment biases (JB) are differences in the way that individuals in positive and negative affective/emotional states interpret ambiguous information. This phenomenon has long been observed in humans, with individuals in positive states responding to ambiguity ‘optimistically’ and those in negative states instead showing ‘pessimism’. Researchers aiming to assess animal affect have taken advantage of these differential responses, developing tasks to assess judgment bias as an indicator of affective state. These tasks are becoming increasingly popular across diverse species and fields of research. However, for laboratory mice, the most widely used vertebrates in research and a species heavily relied upon to model affective disorders, only one JB task has been successfully validated as sensitive to changes in affective state. Here, we provide a detailed description of this novel murine JB task, and evidence of its sensitivity to mouse affect. Though refinements are still necessary, assessment of mouse JB opens the door for answering both practical questions regarding mouse welfare, and fundamental questions about the impact of affective state in translational research.

Introduction

Measuring affectively modulated judgment bias (henceforth JB) has proven to be a useful tool for studying the emotional states of animals. This innovative approach borrows from human psychology since humans experiencing positive or negative affective states (emotions and longer-term moods) reliably demonstrate differences in the way they process information^1,2,3. For example, humans experiencing anxiety or depression might interpret neutral facial expressions as negative, or neutral sentences as threatening^4,5. It is likely that these biases have an adaptive value and are therefore conserved across species^6,7. Researchers aiming to assess animal affect have cleverly taken advantage of this phenomenon, operationalizing optimism as the increased expectation of reward in response to neutral or ambiguous cues, and pessimism as the increased expectation of punishment or reward absence^8,9. Thus, in an experimental setting, optimistic and pessimistic responses to ambiguous stimuli can be interpreted as indicators of positive and negative affect, respectively^10,11.

Compared to other indicators of animal affect, JB tasks have the potential to be particularly valuable tools since they are capable of detecting both the valence and intensity of affective states^10,11. The ability of JB tasks to detect positive states (e.g., Rygula et al.¹²) is especially useful since most indicators of animal affect are limited to the detection of negative states¹³. During JB tasks, animals are typically trained to respond to a positive discriminative cue predicting reward (e.g., high-frequency tone) and a negative discriminative cue predicting punishment (e.g., low-frequency tone), before being presented with an ambiguous cue (e.g., intermediate tone)⁸. If in response to ambiguous cues an animal 'optimistically' performs the trained response for the positive cue (as if expecting reward), this indicates a positive judgment bias. Alternatively, if animals demonstrate the negative trained response to avoid punishment, this is indicative of 'pessimism' or negative judgment bias.

Since the development of the first successful JB task for animals by Harding and colleagues⁸, several JB tasks have been developed for a wide range of species across diverse research fields⁷. But despite their increasing popularity, animal JB tasks are often labor-intensive. Moreover, perhaps because they are methodologically different from the human tasks that inspired them, they sometimes produce null or counterintuitive results¹⁴ and commonly yield only small treatment effect sizes¹⁵. As a result, JB tasks can be challenging to develop and implement. In fact, for laboratory mice, the most widely used vertebrates in research^16,17 and a species heavily relied upon to model affective disorders¹⁸, only one JB task has been successfully validated as sensitive to changes in affective state¹⁹ despite many attempts over the past decade (see supplementary material of Resasco et al.¹⁹ for a summary). This article describes the recently validated murine JB task, detailing its biologically relevant design, and highlighting the ways that this humane task can be applied to test important hypotheses relevant to mouse affect. Overall, the protocol can be implemented to investigate the affective effects of any variable of interest on JB in mice. This would include categorical treatment variables as described here (drug or disease effects, environmental conditions, genetic background, etc.), or relationships with continuous variables (physiological changes, home cage behaviors, etc.).

Protocol

Experiments were approved by the University of Guelph's Animal Care Committee (AUP #3700), conducted in compliance with Canadian Council on Animal Care guidelines, and reported in accordance with ARRIVE (Animal Research: Reporting of In Vivo Experiments)20 requirements. 1. Experiment preparation Experimental design (see Table 1). NOTE: This behavioral test is a scent-based Go/Go digging task, in which mic…

Representative Results

Results presented here reflect relevant findings from Experiment 1 of Resasco et al.19. Subjects in this experiment were 18 female C57BL/6NCrl ('C57') and 18 Balb/cAnNCrl ('Balb') mice. Animals arrived at the facility at 3-4 weeks of age and were randomly assigned to environmentally enriched or conventional housing treatments (EH or CH, respectively) in mixed strain quartets25. Each cage contained one C57 and one Balb, in addition to two DBA/2NCrl mice being…

Discussion

The scent-based digging protocol and results outlined here demonstrate the ability of this novel JB task to detect changes in mouse affective state. The task thus presents a valuable tool for diverse fields of research. Similar to any JB task, to assess animal affect it is critical that animals first learn to discriminate between cues (step 4.7.3) and that the ambiguous stimulus is interpreted as intermediate (step 5.3). Though simple, meeting these requirements can be challenging, particularly in laboratory mice for whi…

Materials

Absolute ethanol	Commercial alcohol	P016EAAN	Dilute to 70% with distilled water, for cleaning
Centrifuge tubes	Fischer	55395	15 mL tubes used to dilute essences here. However, size may be selected based on total volume required for sample size
Cheerios (original)	Cheerios	N/A	Commercially available. Used as reward to train animals to enter annex cage for handling
Corncob bedding	Envigo	7092	Teklad 1/8 corncob bedding used in digging pots and animal cages
Cotton pads	Equate	N/A	Commercially available. Modified in lab to fit within tissue cassettes for scent dispensing
Digging pots	Rubbermaid	N/A	Commercially available. Containers were modified to fit the apparatus in the lab
Dried, sweetened banana chips	Stock and Barrel	N/A	Commercially available. High value reward in JB task
JB apparatus		N/A	The apparatus was made in the lab
JWatcher event recording software	Animal Behavior Laboratory, Macquarie University	Version 1.0	Freely available for download online. Used to score digging behavior during JB task
Mint extract	Fleibor	N/A	Commercially "Menta (Solución)". Discriminative stimulus odor
Rodent Diet	Envigo	2914	Teklad global 14% protein rodent maintenance diets. Low value reward in JB task and regular diet in home cage
SAS statistical software	SAS	Version 9.4	Other comparable software programs (e.g. R) are also appropriate
Vanilla extract	Fleibor		Commercially available "Vainilla (Solución)". Discriminative stimulus odor
Video camera	Sony	DCR-SX22	Sony handycam.