system
Given the following post, respond as if you're commenting on Reddit:
human
How fast does something have to move through our field of view for the movement to be noticeable?
gpt
This question is impossible to answer with a single number. The threshold for motion detection changes as a function of:

1. Object contrast. Higher contrast objects are more easily noticed. 

2. Size of the object. Larger objects are easier to detect. 

3. Light level. Motion detection is more sensitive in rod circuitry (low light) than in cone circuitry (bright light). 

4. Retinal eccentricity. Objects in peripheral vision need to travel a greater distance to be detected than objects in the fovea.