Index for natural scenes starts from -1 in Neuropixels data

I went through the tutorial ecephys_quickstart.ipynb , and found that the indices for natural scenes in the downloaded data ( scene_presentations["frame"] in the notebook) range from -1 to 117, but I think the indices should actually range from 0 to 117, since there are only 118 natural scenes and the argument for cache.get_natural_scene_template ranges from 0 to 117. (All ranges above are inclusive.) What does -1 mean?

OS & version: Manjaro 21.0
Python version: 3.7.10
AllenSDK version: 2.8.0

Hi Bichidian,

Scene_presentations[“frame”] = -1 is the blank sweep. Values 0-117 correspond to the 118 natural scenes in the template.