Answer: The open field test is a behavioral test that measures overall locomotion. It can also be used as a measure of anxiety.
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 device itself is able to track the position of the animal, and it can do so in a few different ways. One possible 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. Animals that have received a psychostimulant (such as caffeine or amphetamine) prior to testing will show a greater number of beam breaks, as they move around the box more than their saline treated counterparts.
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.
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. If a rodent is given an anxiolytic drug (one that decreases expression of anxiety) then they will spend more time in the center of the open field as opposed to the sides.