Why Zone Durations May Not Add Up to Total Trial Time
In EthoVision XT, if the sum of time spent in zones does not equal the total trial duration, it may be due to:
- The subject was not tracked for the entire trial.
- The zones do not fully cover the arena or zones overlap.
- The Zone exit threshold allows an animal to be counted in multiple zones at once.
1. The Subject Was Not Tracked for the Entire Trial
If the animal disappears from view or if detection is poor, some frames may be missing. When an animal is not detected for three or more consecutive samples, it won’t be counted in any zone. You can check this using the Subject Not Found indicator during the trial or in the Trial List (enable it via Show/Hide > Variables).
If there are frequent missing samples, improve detection or retrack the video. For short gaps, enabling Track Smoothing or manually interpolating in the Track Editor may help.
2. Zones Do Not Cover the Entire Arena, or Zones Overlap
Zone coverage problems often arise from:
- Gaps between zones due to imperfect alignment
- Overlapping zones in separate zone groups
Correct method (all areas within a single zone group):
Incorrect method (multiple zone groups causing gaps or overlaps):
Tip: Use the View checkboxes to hide other zone groups for clarity during editing.
3. Zone Exit Threshold Allows Dual-Zone Occupancy
The Zone Exit Threshold setting lets you define a buffer around a zone’s edge to avoid spurious exits due to slight movements:
Example:
- With a threshold of 0.00 cm, quick exits/re-entries are counted as separate visits.
- With a threshold of 0.5 cm, minor exits won’t be counted, reducing false positives.
However, if the threshold is too large, it may cause the animal to be counted in two adjacent zones simultaneously (e.g., between quadrants in a Morris Water Maze), artificially inflating zone durations. Set the threshold to 0.00 cm if mutual exclusivity is required.