The Open Field Test: reinventing the wheel

A molecular geneticist advised me recently that no study of a new strain of mutant mice would survive peer review unless the Open Field Test has been used to profile their locomotor (ambulatory) activity. This would certainly help to explain why the number of publications extracted by keywords ‘open field’ with ‘locomotor activity’ has increased fourfold in the last 10 years, but it’s worrying. Although it is not at all certain what the Open Field Test actually measures, it is absolutely clear that it does not give a simple index of the status of motor output. It is easy to see why the Open Field Test is so alluring. The modest approach is to score the number of times the rodent crosses a matrix of lines that has been painted on the floor of the arena. Higher-tech labs have the option of using activity meters, which record the number of times the animal intercepts parallel beams of light that span the arena. When two adjacent infrared beams are intercepted, the animal is deemed to have engaged in locomotor activity rather than vegetative motor activities, which are scored as single beam breaks. The procedure is apparently so straightforward that neither the rodents nor the humans require specialist training. However, a review, written 30 years ago by Walsh and Cummins (1976) should be compulsory reading for anyone contemplating using the Open Field Test. They discuss its shortcomings in detail. High on their list was the variability in the protocols, leading them to question: ‘When is an open field not an open field? ’ Judging from a random sample of recent studies that have used this test, nothing has changed. As in those days, the enclosure is still made of just about anything: Plexiglas, anodized aluminium, polypropylene or even painted (black or white) plywood. It can be any size or shape: round, square, rectangular or a circular corridor. Scoring can start either immediately, several minutes or several hours after the animals have been placed in the apparatus. The behaviour can be scored for as long as you choose, e.g. 2min or 4h. Husbandry, litter size, animals’ age, prior handling, ambient odours, whereabouts the animal is placed in the field and light intensity will all differ from lab to lab and influence experimental outcome, but details are rarely specified. Of particular relevance is the problem that differences in ambulation across different genotypes can change (even reverse) with repeated testing. As a consequence, a single snapshot could be misleading. The possibility of secondorder changes in behaviour, resulting fro genotype and environment, adds to the s 2005). At best, this wide range of experimenta ficult to compare one study with another. uninterpretable. ‘The dangers of unqualifi genetic, experimental and testing backgro ated’ (Walsh and Cummins, 1976). It is worth reflecting that the Open Fi designed to score defaecation as a measur ality’ (Hall, 1934). Later, it was used to sc induced by interventions such as elect administration of psychotropic drugs. Bro the Maudsley inbred (reactive and non-r the effects of some environmental varia cluded that movement in the open field wa exploratory drive (Broadhurst, 1957, 1958) Subsequent refinements of the test incl versus oblique movements around the fi Animals that spent the greatest time in th field are regarded as less fearful (or ‘an prefer the perimeter. It was generally agre and penetration of the central zone of the most informative measures from this tes was acknowledged that differences in anim could distort other behavioural measures and Stanford, 1989). Given that individual differences in l found measures of emotionality, it follows tionality will confound measures of locom 1997). Yet, these days, this complicatio ignored. There are numerous reports of di of line crosses or beam breaks in genetic types but, in the absence of other behavi no way of knowing whether these are e motor output, exploratory drive or the free The only reliable way to measure anim motor activity is to record their movement home cage. (Broadhurst (1957) and other 2000) have noted the lack of correlation the open field and cage activity, confirm