Today I was reminded of a very curious pattern of brain damage. When a specific bit of the right parietal lobe is damaged, people often end up with hemispatial neglect, that is, they can only attend to the right side of space. When the corresponding bit of left parietal lobe is damaged, hemispatial neglect does not often happen, and if it does, it typically resolves quickly. The ratio is around 2.5:1 cases of left side neglect to right side neglect, and the recovery of right side neglect is faster.

Some caveats of course; damage to a hemisphere can rarely produce neglect of the same side of space, and left-handed people tend to recover faster from hemispatial neglect, perhaps due to differences in how functions get lateralized. We don’t really know.

The inference from all this is that the left hemisphere attends to the right half of space, and the right hemisphere attends to both halves of space. This is a potentially shaky jump, but the asymmetry is still striking. It’s also not just a human thing, most animals have this too. It’s not even that the brain loses access to the data; if you have a person with hemispatial neglect and squirt cold water in their left ear, they will briefly be able to notice the left side of space again.

That such a gigantic asymmetry about a very basic aspect of our cognition was unremarked upon until literal brain damage got us to notice is wild, and does not bode especially well for certain models of introspection. I often collaborate with QRI, so I am naturally very curious about what introspection can tell us and under what conditions; this I think is a very nice example of the sort of thing I think a mature introspective practice should be able to tell you. In general, I think if some function of the brain is localized to or critically routes through some specific area, a good introspective practice should be able to tell you this.

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There are some ways the consequences of this asymmetry show up in regular consciousness. If you ask a healthy person to mark the midpoint of a horizontal line, they almost universally place it slightly left of center. This is called pseudoneglect, and curiously it is quite reduced (maybe gone? unclear) in left-handed individuals. Vertical lines do not have any asymmetry here that I was able to find.

There’s also the landmark test, where instead of having people mark the position of the midpoint you just ask if some point is left or right of the midpoint; people are more sensitive to marks that are slightly to the right of center than to the left. It’s also distinct from the line bisection, as some hemispatial neglect patients are fine with marking but have a worse time perceiving, and some vice versa.

This is all complicated by the effect getting stronger when lines are long and the effect diminishing, maybe even reversing, if the line is sufficiently small.

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In any case, really weird stuff, but very very interesting. My personal takeaway is to use this whole phenomenon as something of a target. I am working on developing introspective techniques that give me information about how my brain actually works, in large part because I want to use that information to get smarter. If some technique wouldn’t reveal this structure in how spatial perception typically operates, it may not be sufficient for my purposes.