There’s one issue that I don’t have an answer yet: how would the visual system detect “height”? Could we presume there is a spatial engine that needs to be taught first, and then linked to this phobia?Or would it make sense to have a straight link to a spatial predictive system instead, and if the system would predict that there’s some uncertainty in if the agent suddenly needs more space to maneuver, and then that space is instead occupied with a void? At least *I* cannot look up when the fear of heights triggers, and get a sudden sensation of vertigo: I need to know where the closest brace-point is when I know falling might be imminent.
The visual system wouldn’t detect the abstract concept of height, and that would be the brain’s job to figure out by being primed on when the thing triggers and what else correlates with it.
I imagine the visual system would detect visual depth from binocular vision. Babies learn this in the first few months. It is one of the things that cause them distress when it gets activated in the brain. I don’t know the research papers, but these might be starting pointers:
https://www.beltz.de/fileadmin/beltz/leseproben/978-3-621-27926-0.pdf (picture 2.2, German)
https://www.thewonderweeks.com/babys-mental-leaps-first-year/ (week 26)
So visual depth you have without much learning—or with other priming steps ahead of that; I understand these are well researched). What is left is the vertical component, and I guess that it comes from the vestibular system. Looking down + visual depths = height trigger.
It is funny that you mention the need to grasp something, and maybe that is the hard-wired cue: Close the hand.
If I understood correctly, babies cannot focus their eyes properly for the first two months, and this may indicate they are learning some universal 3D-spatial models into their heads, as a prerequisite for many of the other instincts they have as later developmental windows. So there has to be some thread of signals that string this system to the later affects/instincts, such as the fear of heights. It is also funny to relate the ability of many ungulate babies ability to walk immediately on birth, meaning there has to be some seriously robust set of instincts that coordinate this for them. This blurs the … requirements… between instinctual and learned coordination, but I believe in the end all cortex-having brains would benefit from moving away from instincts and into learned models.
Related: Gene for upright walk in humans discovered: https://www.theguardian.com/science/2008/jun/02/genetics.medicalresearch