Knowledge Base

Definition

This is the number of turns, either clockwise or counterclockwise, of 360° (if not otherwise defined).

A rotation can be based on the movement of a body point or on the change in direction of a body axis.

Calculation

Body Point Rotation

The software scores a rotation event when the cumulative turn angle of the chosen body point exceeds the threshold value (default: a complete rotation, that is, 360°).

Consider the simplified example in the figure below. Left: the original track, starting from the sample at time 0. Here we consider the center point but the calculation is the same for the nose-point and the tail-base point. Middle: The turn angles (α) calculated for the body point of samples 1, 2, 3, etc. Right: cumulative turn angle. For details about turn angles, see the dependent variable Turn Angle.

At sample 7, the cumulative turn angle exceeds 360°. Therefore, a rotation event is scored. The cumulative turn angle is reset and consecutive turn angles are summed until the cumulative turn angle, again, exceeds the rotation threshold.

Body Axis Rotation

This method is based on the turn angle formed between the body axis at two consecutive sample times (k-1 and k). As a body axis, you can choose the segment joining the center-point and the nose-point, or the segment between the tail-base and the center-point.

The software scores a rotation event when the cumulative turn angle of the chosen body axis α+α+... exceeds the threshold value (default: a complete rotation, that is, 360°). Note that the angles depend only on the orientation of the body axis, not the position of the body center.

Note: This rotation method is only available if your experiment is set to Center-point, nose-point and tail-base detection.

Which Method Shall I Choose?

• Choose Body point rotation if the track clearly shows the rotation movement that you want to quantify. For example, a mouse walking along the walls of a cage, or a fish larva swimming in a well. In that case the trajectory of the center point is sufficient to characterize the behavior.
• Choose Body axis rotation if the subject is spinning around its own axis and you want to quantify exactly that behavior.
• Choose Body axis rotation if the subject is performing micro-rotations and you do not want to count them. Micro-rotations occur when the subject makes small repetitive movements of the body that cause the center point to rotate, while the subject itself does not walk along a circle. Such rotations are hardly noticeable in the track plots. That could occur for example while a rodent does body grooming. Note that the center point rotates while the body axis does not. If micro-rotations are a problem in your trials and you do not want them included in the results, choose Body axis rotation.
• Tip: To eliminate or reduce the detection of micro-rotations when using Body-point rotation, apply a track-smoothing filter based on minimum distance moved. See The Minimal Distance Moved Smoothing Method.
• More generally, you can use the Minimum Distance Moved filter to rectify the trajectory of the subject, so that the software ignores the moments when the subject is still or moves little. In this example a MDM of 25 cm was used to make the larger-scale rotations more visible.

How to Specify Rotation

1. Click the Add button next to Rotation.
2. Under Select, choose one of the two methods Body points rotation or Body axis rotation. If you choose Body axis rotation, select which axis you want to use to count rotation: Axis: Center-point to Nose-point or Axis: Tail-base to Center-point.
3. Under Settings, select whether EthoVision XT should count rotations in the Clockwise direction or in the Counter-clockwise direction.
4. In the Count every … rotation list, specify how rotations should be counted. The default value of 1 means that every 360° turn in the specified direction is counted as one rotation. If you select 0.5 in this list, every 180° turn is counted as one rotation.
5. By entering a threshold angle in the Threshold box, you can compensate for turns in the direction opposite to the one you selected in step 2. Tip: Visualize the rotation events in the Integrated Visualization to validate your choice of the threshold value.
6. Enter a value of Minimum Distance Moved if you want to remove the samples for which the distance from the last sample selected in the track is too short to represent actual movement. By default this is 2 cm / 0.78 inch. Turn angles and Rotation are re-calculated according to this filter. If you do not want to apply any filter, enter 0.
7. If you track three body points (center-point, nose-point and tail-base point), and you select Body point rotations, in the Body Points tab select the body points you want to use to calculate the rotations.
8. Complete the procedure to add the variable. See Calculate Statistics: Procedure.

The Role of the Threshold Angle

Suppose that in the figure shown at the top of this page the subject makes a counterclockwise turn between sample 2 and sample 3 (α), then it makes a new turn in the clockwise direction (α) before completing a rotation.

The threshold angle allows you to compensate for turns in the opposite direction. In this example we are interested in clockwise rotations. Depending on the value of the turn angle α in the counterclockwise direction, one of the following occurs:

• The turn angle α is smaller than or equal to the threshold angle. EthoVision XT continues to calculate the cumulative turn angle until it exceeds the corresponding rotation count. A turn in the opposite direction means that the cumulative angle is reduced at that sample.
• The turn angle α is larger than the threshold angle. EthoVision XT resets the cumulative turn angle (R in the figure below) and uses a new series of samples to calculate the cumulative turn angle until the latter exceeds the corresponding rotation count.

The rationale for applying a threshold angle is that if the animal makes large turns in the opposite direction, the trajectory can no longer be described as a circle-like pattern, and the subject is likely to have changed its behavior. The counting of cumulative turn angle should therefore be reset.

Notes

• When you apply a Minimum Distance Moved filter, samples are filtered with the Direct method for Minimal Distance Moved. This filter changes the values of turn angle at a specific sample. Consider the following example (movement is from A to B): The filter sets samples C and D equal to B since their distance from B is shorter than the threshold entered. The turn angle for sample E is the angle formed with A and B (=C, D). See Turn Angle.
• To count both clockwise and counter-clockwise rotations, repeat the steps from step 1 and specify the alternative option. As a result, you have two instances of the variable Rotation, one for clockwise rotations and the other for counter-clockwise rotations.

Source: EthoVision XT 19 Help, Noldus Information Technology