Open field (animal test)

Developed by Calvin S. Hall, the open field test (OFT) is an experiment used to assay general locomotor activity levels and anxiety in rodents in scientific research and willingness to explore in rodents. However, the extent to which behavior in the open field measures anxiety is controversial.

Animals such as rats and mice display a natural aversion to brightly lit areas. They also have a drive to explore a perceived threatening stimulus. The result of these two conflicting drives is anxiety. Decreased anxiety leads to increased exploratory behaviour. Increased anxiety will result in less locomotor motion and preference for the edges of the field.

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.

Behavioral patterns measured in the open field test include:

The relation between the OFT and other tests of exploratory activity (elevated plus maze and emergence) have been analyzed in two mouse strains. Changes in these measures are often used to assess the sedative or stimulant effects of pharmacological agents.

Newer attempts have been made to analyse the OFT by quantifying the animal's moment-by-moment developmental dynamics. A recent study was able to show that mouse exploratory behavior consists of sequences of repeated motion: iterative processes that increase in extent and complexity, whose presumed function is a systematic active management of input acquired during the exploration of a novel environment.

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

In a modified OFT, animals would go to the center of the field if there were objects that the animal could explore. This shows that the standard OFT might not provide enough drive for exploration.

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 agonist and SSRIs which have also been proven to treat anxiety show inconsistent results with the OFT.

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