# Head direction

Definition

The smallest angle formed by the Head direction line of the current sample relative to a line parallel to the x-axis in the coordinate system. See also Head directed to zone

The Head direction line is determined for each sample, based on the contour of the detected subject. It originates from the nose-point but it does not necessarily pass through the center-point. The angle a is assigned to the dependent variable Head direction for that sample.

note Head direction is not available if your experiment is set to:

▪Center-point detection.

▪Color marker tracking.

▪Live Mouse Tracker.

▪Center-point, nose-point and tail-base detection, with two subjects per arena tracked with Deep learning.

Examples

Below: An example of Head direction when exported in the raw data file (Analysis > Export > Raw Data; locate the Head direction column).

The angle is negative (-180°, 0] when the Head direction line departing from the nose-point lies left to the line parallel to the x-axis. The angle is positive [0, +180°] in the opposite case (in this example the x-axis is in the default position; it is horizontal and points to the right). The top-left picture shows the Head direction at the sample highlighted below.

important Note that the value of Head direction depends on the orientation of the x-axis set in the Arena Settings.

Range

Head direction ranges from -180° to +180°. Therefore, values like -179 and +179 represent very similar orientations (head pointing to the left). Take this into account when interpreting the raw data.

How to specify Head direction

1.In the Analysis profile, click the Add button next to Head direction.

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

Notes

▪Head direction is a very different thing than the Turn angle based on the nose point. The latter variable measures the direction of movement, not the orientation of the head. See Turn angle

▪You can check the head direction line during data acquisition. Click the Show/Hide button on the toolbar, select Track Features and make sure Head direction is selected. Next, let the animal move in the arena or play the video file. The real time Head direction values are shown in the Analysis Results and Scoring pane.

▪The mean, standard deviation and variance are calculated with circular statistics. The mean Head direction represents the average orientation of the animal relative to the x-axis. See Statistics available

▪Head direction is calculated relative to the orientation of the x-axis you have chosen in the Arena Settings used for that trial. By default, the x-axis is horizontal and pointing to the right. If the x-axis is not horizontal or is pointing to another direction, Head direction is calculated based on that axis direction.

▪note If you swap nose- and tail-base points, or interpolate those two points, the Head direction line is recalculated as the angle formed by the segment joining the nose-point and the center-point and the x-axis (see the figure below).

▪If you edit the position of the nose-point or center point, the value of Head direction does not change.

▪Be careful when interpreting values of Head direction relative to objects, for example two zones, or values extracted from different arenas.

example In a mirror test, we want to measure the head direction of the fish, and compare the data between two arenas. The mirror is placed at opposite sides in Arena 1 and Arena 2. You can see that the same orientation relative to the mirror is measured with different angles:

▪For a negative angle in Arena 1: Angle in Arena 2 = Angle in Arena 1 + 180° (see the figure below, top).

▪For a positive angle in Arena 1: Angle in Arena 2 = Angle in Arena 1 - 180° (bottom).

To compare the results between arenas, convert angles from Arena 2 to Arena 1 (or vice versa).

Applications

Head direction is useful for studies of spatial orientation and searching behavior. For this you can, for example, use the average and variation in Head direction.