Thanks for the great suggestion, Mahonka.
Posting sample images might help, but it could also be misleading because in a static image, a stall could look the same as a flowing vessel. In many cases, it is only when the movie is running that you can tell the difference. That said, we could post images representing a series of 4 or 5 frames that might capture some of the more difficult examples of stalls.
We intend to add a link to more detailed tutorial content for those seeking more advanced annotation guidance. Stay tuned...
I think part of being able to distinguish between true positives (actual stalls) and false positives (a flowing vessel that looks like a stall) is understanding the 3D nature of the movie. In some sense it is a movie that plays back over time. But in another sense, each subsequent frame of the movie represents a deeper layer of tissue.
Take the following difficult case as an example. If the vessel is oriented vertically through the stack of images, then you would be viewing a cross-section of the vessel. So instead of looking like a winding snake, it would simply appear as a dot (and the green outline would be a small circle).
In this case, if blood were flowing, the dot would appear relatively consistent throughout the movie. But if the dot disappeared completely for a few frames, and then became relatively solid thereafter, that could be a sign of a stall.
I may also be worth mentioning that although we provide participants with many opportunities to annotate stalled vessels, the natural occurrence rate is much lower. The natural frequency of stalls in wild type (normal) animals is about 0.5% and in animals with Alzheimer's disease it is about 2%. This means that it is very important to not miss a possible stall. So even if you aren't 100% sure, it is better to catch a stall than to miss one. Each and every stall discovered by the crowd will be individually examined by experts in the lab to verify that it is a true stall.
Thanks again for your thoughtful question and for joining Stall Catchers!
All the best,