  Although the open-field test has been widely used, its reliability and
compatibility are frequently questioned. Although many indicating parameters
were introduced for this test, they did not take data distributions into
consideration. This oversight may have caused the problems mentioned above.
Here, an exploratory approach for the analysis of video records of tests of
elderly mice was taken that described the distributions using the least number
of parameters. First, the locomotor activity of the animals was separated into
two clusters: dash and search. The accelerations found in each of the clusters
were distributed normally. The speed and the duration of the clusters exhibited
an exponential distribution. Although the exponential model includes a single
parameter, an additional parameter that indicated instability of the behaviour
was required in many cases for fitting to the data. As this instability
parameter exhibited an inverse correlation with speed, the function of the
brain that maintained stability would be required for a better performance.
According to the distributions, the travel distance, which has been regarded as
an important indicator, was not a robust estimator of the animals' condition.
