Open field (animal test) explained

Developed by Calvin S. Hall, the open field test is an experimental test used to assay general locomotor activity levels, anxiety, and willingness to explore in animals (usually rodents) in scientific research.^{[1] [2] [3] [4]} However, the extent to which behavior in the open field measures anxiety is controversial.^[5] The open field test can be used to assess memory by evaluating the ability of the animal to recognize a stimulus or object. Another animal test that is used to assess memory using that same concept is the novel object recognition test.^[6]

Concept

Animals such as rats and mice display a natural aversion to brightly lit open areas. However, they also have a drive to explore a perceived threatening stimulus. Decreased levels of anxiety lead to increased exploratory behavior. Increased anxiety will result in less locomotion and a preference to stay close to the walls of the field (thigmotaxis).^{[7] [4]}

Experimental design

The open field is an arena with walls to prevent escape. Commonly, the field is marked with a grid and square crossings. The center of the field is marked with a different color to differentiate from the other squares. In the modern open field apparatus, infrared beams or video cameras with associated software can be used to automate the assessment process.^[8] Behavioral patterns measured in the open field test include:^[9]

• Line crossings – Frequency with which the rodent crosses grid lines with all four paws (a measure of locomotor activity), sometimes divided into activity near the wall and activity in the center.
• Center square entries – Frequency with which the rodent enters the center square with all four paws.
• Center square duration – Duration of time spent in the central square.
• Rearing – Frequency with which the rodent stands on its hind legs in the field. Rearing-up behavior in which the forepaws of the animal are unsupported and the similar behavior in which the forepaws rest against the walls of the enclosure have different underlying genetic and neural mechanisms and unsupported rearing might be a more direct measure of anxiety.^{[10] [5] [4]}
• Stretch attend postures – Frequency with which rodent demonstrated forward elongation of the head and shoulders followed by retraction to the original position. High frequency indicates high levels of anxiety.
• Defecation and urination – The frequency of defecation and urination is controversial. Some scientists argue that increase in defecation shows increased anxiety. Other scientists disagree and state that defecation and urination show signs of emotionality but cannot be assumed to be anxiety.

Criticisms

The assumption that the test is based on conflict has been heavily criticized. Critics point out that when measuring anxiety each choice should have both positive and negative outcomes. This leads to more dependable observations which the OFT does not present.

When the test was first developed, it was pharmacologically validated through the use of benzodiazepines, a common anxiety medication. Newer drugs such as 5-HT-1A partial agonists and selective serotonin reuptake inhibitors, which have also been proven to treat anxiety, show inconsistent results in this test.

Due to the idiopathic nature of anxiety, animal models have flaws that cannot be controlled. Because of this it is better to do the open field test in conjunction with other tests such as the elevated plus maze and light-dark box test.^[11]

Different results can be obtained depending on the strain of the animal.^[4] Different equipment and grid lines may cause different results.^[12]

See also

• Animal models of depression
• Forced swim test
• Hole-board test
• Tail suspension test

Notes and References

1. Denenberg VH . Open-field behavior in the rat: what does it mean? . Annals of the New York Academy of Sciences . 159 . 3 . 852–9 . July 1969 . 5260302 . 10.1111/j.1749-6632.1969.tb12983.x . 1969NYASA.159..852D . 39079157 . Victor Denenberg .
2. Hall CS, Ballachey EL . A study of the rat's behavior in a field: a contribution to method in comparative psychology . University of California Publications in Psychology . 1932 . 6 . 1–12 .
3. Stanford SC . The Open Field Test: reinventing the wheel . Journal of Psychopharmacology . 21 . 2 . 134–5 . March 2007 . 17329288 . 10.1177/0269881107073199 . 37028127 .
4. Book: Crusio WE, Sluyter F, Gerlai RT, Pietropaolo S . Wim Crusio . Robert Gerlai . Behavioral Genetics of the Mouse . 2013 . . Cambridge, United Kingdom. 978-1-107-03481-5 . The genetics of exploratory behavior . 148–154 . WE . Crusio.
5. Sturman O, Germain PL, Bohacek J . Exploratory rearing: a context- and stress-sensitive behavior recorded in the open-field test . Stress . 21 . 5 . 443–452 . September 2018 . 29451062 . 10.1080/10253890.2018.1438405 . 19952338 .
6. Antunes M, Biala G . The novel object recognition memory: neurobiology, test procedure, and its modifications . Cognitive Processing . 13 . 2 . 93–110 . May 2012 . 22160349 . 3332351 . 10.1007/s10339-011-0430-z .
7. Ennaceur A . Tests of unconditioned anxiety - pitfalls and disappointments . Physiology & Behavior . 135 . 55–71 . August 2014 . 24910138 . 10.1016/j.physbeh.2014.05.032 . 19974405 .
8. Samson AL, Ju L, Ah Kim H, Zhang SR, Lee JA, Sturgeon SA, Sobey CG, Jackson SP, Schoenwaelder SM . 6 . MouseMove: an open source program for semi-automated analysis of movement and cognitive testing in rodents . Scientific Reports . 5 . 16171 . November 2015 . 26530459 . 4632026 . 10.1038/srep16171 . 2015NatSR...516171S .
9. Book: Crusio WE, Sluyter F, Gerlai RT, Pietropaolo S . Behavioral Genetics of the Mouse . 2013 . . Cambridge, United Kingdom. 978-1-107-03481-5 . Ethogram of the mouse . 17–22 .
10. Crusio WE . Genetic dissection of mouse exploratory behaviour . Behavioural Brain Research . 125 . 1–2 . 127–32 . November 2001 . 11682103 . 10.1016/S0166-4328(01)00280-7 . 28031277 .
11. Ramos A . Animal models of anxiety: do I need multiple tests? . Trends in Pharmacological Sciences . 29 . 10 . 493–8 . October 2008 . 18755516 . 10.1016/j.tips.2008.07.005 .
12. Kulesskaya N, Voikar V . Assessment of mouse anxiety-like behavior in the light-dark box and open-field arena: role of equipment and procedure . Physiology & Behavior . 133 . 30–8 . June 2014 . 24832050 . 10.1016/j.physbeh.2014.05.006 . 35795658 .