Monday, January 20, 2020

Top-Down and Bottom Up Perceptions, WRT Schizophrenia and Autism

 An older article from Scott Alexander over at Slate Star Codex, about the predictions at various levels of your brain of what you are likely to perceive, and how that interacts with your sensory data, resulting in your actual perception. I miss a lot over there, because it takes more intellectual work than I am willing to put in day after day. I like my intellectualism easy.  But when someone else references his stuff I go over and buckle down.  I am always gratified by what I find.  Steve Sailer called Alexander the greatest public intellectual to emerge in the 2010's. 
Corlett, Frith & Fletcher (2009) (henceforth CFF) expand on this idea and speculate on the biochemical substrates of each part of the process. They view perception as a “handshake” between top-down and bottom-up processing. Top-down models predict what we’re going to see, bottom-up models perceive the real world, then they meet in the middle and compare notes to calculate a prediction error. When the prediction error is low enough, it gets smoothed over into a consensus view of reality. When the prediction error is too high, it registers as salience/surprise, and we focus our attention on the stimulus involved to try to reconcile the models. If it turns out that bottom-up was right and top-down was wrong, then we adjust our priors (ie the models used by the top-down systems) and so learning occurs.
He examines the possibilities that schizophrenia is a problem of increased bottom-up signalling with decreased top-down signalling, and autism a disorder of not tolerating even small differences between top-down and bottom-up's conclusions and refusing to shake hands.
They argue that autism is a form of aberrant precision. That is, confidence intervals are too low; bottom-up sense-data cannot handshake with top-down models unless they’re almost-exactly the same. Since they rarely are, top-down models lose their ability to “smooth over” bottom-up information. The world is full of random noise that fails to cohere into any more general plan.
In the final section, Alexander sees some weaknesses in both possibilities.

If you like that sort of thing, I think this is his followup.

1 comment:

Unknown said...

Great links - this is clearly a very big onion, so to speak, and I sense that we have only uncovered the first few layers. The human body is immensely more complex than any of us truly realize.

In my own experience with moderate anxiety I found dietary changes and nutritional supplements (but not drugs) that seemed to improve the way my brain handled events. But I also found that I couldn't really make a lot of progress until i spent about 18 months working with a counselor on psychological aspects of what I was experiencing.

These articles provide some explanation for my experience - chemistry changes help improve the signalling and handshakes; and the counseling is required to help reset bad "priors". I imagine this is old news to those in the profession but it's a bit of an eye-opener to me.

BTW, I have been reading your site now for a couple of years and am enjoying working through the older posts. Keep on keeping on.