Net weighted movement

Definition

The signed, distance-weighted change in distance between two subjects from one sample to the next.

Net weighted movement is weighted by the distance between two subjects. Changes in positions of subjects which are at a large distance from each other have a lower weight, so they can be distinguished from movements at close distance, which have a different biological meaning.

Unlike Relative movement, this is a continuous variable (in distance units). The Net weighted movement is positive if the subject (Actor) is getting closer to another subject (Receiver), negative in the other case.

Calculation

Formula:

NWM_n = (D_k-1 - D_k) * abs (DS_k - DS_k-1) / (max (DS_k, DS_k-1))

Where:

D_k-1, D_k is the distance between the Actor's body point and the interpolated point of the Receiver calculated for two consecutive samples.

DS_k-1, DS_k is the distance between subjects for two samples.

In the example of the first figure in the topic Relative movement, the value of the variable depends on which subject is considered as Actor and which as Receiver. Similarly, Net weighted movement of subject A is positive relative to R. If the other subject was the Actor (see the second figure in the topic), the point Q would be defined for the other subject in the middle of the R_k-1-R_k segment. In that case D_k would be longer than D_k-1, thus Net weighted movement would be negative relative to R.

How to specify Net weighted movement

1.Click the Add button next to Net weighted movement.

2.In the Net Weighted Movement tab, under Maximum interaction distance, enter the distance above which you do not want to consider the subjects as interacting. (Default: 50 cm/16.69 inches)

3.Click the Body points tab. Select the body points of the focal subject (Actor) you want to use to calculate net weighted movement.

If you select two or three points, results are calculated for each point separately.

4.Click the Receivers tab.

▪Under Select, choose the subjects you want to consider as Receivers.

▪Under Body points, select the body points of the subjects selected above. If you select two or more subjects and points, results are calculated for each combination separately.

5.Complete the procedure to add the variable. See Calculate statistics: procedure.

Notes

▪Net weighted movement is calculated for all the subjects selected in the Data profile. Each subject displayed on the rows of your result table is considered as Actor. The subjects displayed on the columns are the Receivers.

▪If your experiment is set to Only center-point detection or Color marker tracking, the body point options are absent. Calculations are based on the center point.

Application

Net weighted movement can be used as an objective measure for the intensity of approach and avoidance behavior (Spruijt et al. 1992. Physiology & Behavior 51, 747-752). The advantage of this variable relative to Weighted movement to/from is that it integrates both. This means that you can analyze, for instance, the movement of subjects regardless of the direction towards or away from each other.