Does anybody know why J-Cells track only upwards motion?
Here’s my theory:
1. Stuff that comes up, tend to eventually come down.
2. This coming down part can be quite traumatic if you happen to be underneath it. Especially if you’re a rat and you’ve been jumped upon by a cat.
3. I posit that J-Cells are the rat’searly warning system to spot a cat jumping on it. So it’s basically like NORAD for rats.
I was going to say the same thing except for birds of prey. They cast shadows. If the shadow of that bird is travelling up your mouse head, it’s probably coming toward you. If it’s travelling down it’s probably flying away from you and you are in the clear. Don’t know that for sure though…
Nope, in general the shadow would travel downards as the bird gets closer.
Hmmm. I think it probably matters on the angles involved with respect to the sun. I’d imagine that a mouse would want to be able to detect all the angles of approach though. But what do I know! I’m not a mouse!
Well, assuming the ground is level and the hawk approaches on straight line, the shadow will start off someway off and then move towards the mouse. This will appear as black blob moving downward. The only case in which the image would move upwards would be if the rat is on the side of a lake, and catches the reflection of diving raptor. I’m for the pouncing cat, but I guess this can be tested by deactivating the gene that activates J-cell and measuring reaction time when presented with video of a cat jumping towards the camera.
This is what I was imagining:
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As the bird flies toward the rat at the same altitude, it’s shadow would pass from in front of the rat to behind it (i.e. up it’s eyes). This assumes that the rat is facing toward the sun. By the time the bird is diving at the rat, it’s pretty much toast anyway 
Seems to me that it could just be a part of the normal visual reflex system. It’s advantagous to be able to respond reflexively to movement in any direction across the visual field. Otherwise you wouldn’t be able to actually track things visually. Imagine not being able to follow a moving object with your gaze. That’d kinda be a problem.
When I posted above I hadn’t yet read the study showing that the J-cells and their strong sensitivity to upward movement are unique (from what we can tell) in the mouse retina (ie there are not similar cells with different orientations that respond strongly to motion in other directions). In light of that my previous assumption is obviously wrong. My guess now is that the function of the J-cells is to detect big land-based things (relative to the mouse) rapidly moving directly toward the rat, Assuming the ground is level and both the big thing and the rat are on the ground, the big thing would be moving upward in the mouse’s visual field, especially at closer ranges. The faster and the more directly it is moving toward the rat, the faster the upward motion in the visual field. Anything significantly larger than the rat should cause this upward motion even at a modest distance. Since the majority of predators that prey on mice are larger than the mouse (I think?), this would be a good system for detecting an approaching predator, particularly one that is running directly toward the mouse (or leaping toward it, or pouncing, etc).