Answer: The open field test is a behavioral test that measures overall locomotion. It can also be used as a measure of anxiety.
From DataBase Center for Life Science (DBCLS)
In the open field paradigm, an animal is placed in a large square box with walls. The animal (often a rodent, such as a rat or mouse) is free to move about the box for a set amount of time. The experimental design was developed by Calvin S. Hall.
The open field test is equipped with tools to track the position of the animal.. One method is to divide the area into a grid, with infrared beams projecting from one side to the other. If this beam is broken, then it indicates the animal had crossed from one square of the grid to the next.
A more sophisticated method to measure locomotion is to use a open field with an overhead camera. The camera is able to track more precisely the position of the animal, and can give a more accurate measure of distance traveled. Also, the overhead camera can quantify other behaviors such as rearing or defecation.
What does the open field test measure?
The most robust measurement collected by the open field is locomotor activity. This is basically how much the animal moves around. Interpreting these data is straightforward, as higher locomotor activity counts means more movement.
In addition to measuring locomotion, an open field test is also used to evaluate anxiety in rats or mice. The idea is that rodents prefer to be surrounded rather than exposed out in the open. In other words, a rodent will naturally prefer to be close to the walls rather than being exposed in the center of the open field. Usually, behavioral researchers will define the middle third to be the center of the open field. It is an evolutionary maintained behavior to avoid predators which may be hunting in the open, so they tend to hug the walls which provide them a little bit of cover.
What variables affect open field performance?
Mice and rats are especially sensitive to new environments. The normal reaction many rodents exhibit when first put into a new place is exploration. They will spend a lot of time examining their environment when they are first placed into the open field apparatus. Therefore, the locomotor counts are usually higher at the beginning, then decrease as the animal feels more comfortable in their surroundings.
Animals that have received a psychostimulant (such as caffeine, cocaine, or amphetamine) prior to testing will show a greater number of beam breaks or locomotion, as they move around the box more than their saline treated counterparts.
If a rodent is given an anxiolytic drug (one that decreases anxiety; commonly drugs such as benzodiazepines like Xanax) then they will spend more time in the center of the open field as opposed to the sides (The open field as a paradigm to measure the effects of drugs on anxiety-like behaviors: a review). Antidepressants may also increase time spent in the center.
The open field test has been criticized because of it’s low ethological validity. Critics suggest that a flat terrain, four cornered environment does not resemble anything that a mouse or rat in the wild would experience.
How to interpret results from open field test
The most straightforward measures include the following variables:
Distance traveled. This is a measure of how far the animal had moved over the course of the testing protocol. Although rats and mice are small, they can travel somewhere around 20-30 meters in five minutes. When analyzing these data, you can expect these numbers to increase after psychostimulant exposure. On the other hand, these numbers decrease if the rat is given a sedative drug.
Time spent in center region as a fraction of total time spent. This is a measure for anxiety like behavior. These values are often expressed as a percentage; increases in the amount of time spent in the center suggest an anxiolytic effect. So, giving the mouse a drug such as a benzodiazepine may lead to decreased center time.
Number of entries into center region. This variable is also a measure of anxiety like behaviors. Once the region defined as the “center” has been defined, each entry into this center region counts as an event. More events observed indicate anxiolytic effect of the drug.
