Knowledge Base

Advanced Detection Settings: Method

Aim

To choose:

• The detection method, that is, how EthoVision XT finds the subject over the background. Once you have selected the method, specify the background image (for the detection methods Static Subtraction, Dynamic Subtraction and Differencing only) and the level of contrast between subject and background.
• The method for estimating the body points. See Methods for Estimating the Body Points.

Gray Scaling

Choose Gray Scaling when:

• The animal's grayness differs from the background in all parts of the arena.
• The background does not change during a trial.
• Lighting is even (minimal shadows and reflections) during the trial.

Example: Tracking a white rat in a uniform black open field with no bright objects.

Static Subtraction

Choose Static Subtraction when:

• The Gray Scaling method does not work, for example because other objects in the arena have a similar color as the animal.
• The background does not change in time.
• The light is constant during the trial.

Example: Tracking a white rat in an open field with unavoidable reflections or bright objects.

Dynamic Subtraction

Choose this method when during trials light conditions gradually change or the background changes, for example because bedding material is kicked around, food pellets are dropped, droppings appear, etc.

Example: Tracking a mouse in a home cage provided with bedding material. The activity of the mouse causes the bedding to change appearance in the video image.

Differencing

Choose this method when there is a lot of variation in contrast between a subject and the background within an arena.

Example: There is a gradient in light intensity over the apparatus that cannot be removed, or a strong difference in the color of the fur of the animal, like in hooded rats.

Gray Scaling

Aim

To detect the subject, under the assumption that its color is always different from that of the background (no shadows, gradients etc.).

How Gray Scaling Works

The video image is converted to monochrome. Each pixel in the image has a gray scale value, ranging from 0 (black) to 255 (white). With Gray scaling, you define which range of gray scale values should be considered as the subject. The remaining gray scale values are considered as background. The Gray scaling method does not make use of a background image; the subject is determined by its gray scale values only.

Procedure

1. In the Detection Settings pane, click Advanced, then Method. Select Gray scaling.
2. Release the subject in the arena, or position the media file at a point where the subject is moving.
3. Move the two sliders next to Range or type the values in the corresponding fields to define the lower and higher limits of gray scale values for the subject. Make sure there are no areas in the background with gray scale values that fall in this range, otherwise they will be detected as the subject.
4. In the Video window, check the quality of detection.
   • If the detected area (by default, in yellow) is too large, or other objects are detected as the subject, reduce the range in either direction (brighter or darker).
   • If the detected area is too small relative to the real subject, increase the range at least in one direction (brighter or darker).
   • Areas marked as noise (by default, in orange) indicate that the gray scale range is too wide: you need to narrow it in at least one direction.
5. Move the sliders until the subject (or the part which is of interest) is detected fully, and the noise is minimized. Check that the subject is properly detected in all parts of the arena by moving the video slider, or by waiting for the live animal to move.

Important: The whole animal's body must be detected for optimal tracking. Proceed with the Contour adjustments to optimize body detection.