Video analysis

The video records (resolution 704 × 576 pixels, 25 frames/s) were analyzed using tracking software EthoVision XT 9.0 (Noldus Information Technology, Netherlands). An “arena-image” (the region in the video image in which a moving subject-animal is tracked) was set up with calibration scale, enabling to convert pixel coordinates to real-space coordinates, with each pixel approximately equal to 0.26 cm (EthoVision XT Reference manual 2012; Noldus et al. 2001). The arena-image comprised the entire floor of the arena and bottom part of the wall (up to 15 cm high). The arena-image was then divided into virtual zones: the “central zone” (circular area with a diameter of 56 cm), the “middle zone” (a 21 cm ring outside the central zone), the “edge zone” (all the arena-image outside middle zone), and the “wall zone” (an outer part of the edge zone, covering only the wall of the physical arena).

Tracking started automatically 1 s after detection of “center of the body” point of the animal and stopped automatically after 300 s. EthoVision determined the body central point every 0.04 s and calculated distance moved between 2 consecutive frames (time points). To avoid overestimation due to noise in tracked points and changes of body shape, the movement duration and distance were counted after filtering out small movements of body center point, with threshold start velocity 0.08 cm/0.04 s and end velocity 0.07 cm/0.04 s. The start velocity is the velocity above which the subject was considered to start moving, whereas the end velocity is the velocity below which displacements of the subject’s body points is no longer attributed to locomotion but to system noise, body wobble or pivoting on the spot (Noldus Information Technology 2012; Noldus et al. 2001).

We measured the following parameters: the total distance moved by the vole (cm), duration of mobility (s), the mean and maximum velocity (cm/s), meandering coefficient (rad/cm), latency to reach the edge (s), frequencies of visits to the edge zone, wall-seeking (standing or jumping by the wall), and returns from the edge to the central zone. Duration of mobility is the total time (summation of all 0.04 s time intervals) when the animal was detected as moving based on the filter mentioned above. The mean velocity is the total distance moved by an individual divided by the duration of mobility. The maximum velocity is computed by the tracking software as the maximum frame-to-frame (i.e., across 0.04 s) change of the body center position observed in the entire trial (and multiplied by 25 to obtain cm/s units). The meandering coefficient is the mean frame-to-frame change in direction of movement (unsigned radians) of a subject relative to the distance moved by that subject. Frequency of visits to a particular zone was counted as the sum of instances the body central point was detected immediately following the entry to that zone.

We also calculated proportions of the total time spent moving (duration of mobility divided by total trial time, i.e., 300 s) and of the time spent in the edge and the central zones (duration of time spent in the respective zone divided by total trial time), and proportion of distance covered in the edge and in the central zone (relative to total distance). The proportion of time spent moving provides the same information as the duration of mobility, but we decided that presenting the results in this form will be more effective. The proportions of distance and time in the central zone were computed only for episodes following returns to the central zone (i.e., excluding the time immediately following releasing the animal in the arena).