TLDR: The Precision of Sensory Evidence

In The Precision of Sensory Evidence, Scott Alexander discusses a new theory behind depression and other negative emotions. As someone who is higher in neuroticism, I’ve spent a lot of time thinking of ways to improve my mental wellbeing, but not as much on what might cause my predisposition to negative emotions in the first place.

It’s a thought-provoking read, but it’s also a bit difficult to parse. This is my attempt to distill the main ideas and my personal takeaways so I can revisit them in the future.


Our brain is a processing machine, constantly balancing between its assumptions of the world and new information taken in through our senses and experiences. The assumptions can be called our prior - our belief in what will happen prior to taking into account new information. The new information we take in can be called sensory evidence. Sensory evidence can be regular senses like sight, sound, touch, etc, but also “internal” senses like experiencing memories, and feeling emotions.

One recent paper posits that the brains of depressed and neurotic people tend to have stronger priors and weaker sensory evidence. In other words, they have higher confidence in their presuppositions about the world, and lower precision in their senses and perceptions 1.

When you have lower confidence in sensory evidence, you are more likely to defer to your prior, which in depressed people is “things will probably be bad.”

An example: You go on a date, but have a general expectation that things will go badly. The date goes fine. But your brain determines how it went based on a combination of evidence and priors. You downweigh the evidence that the date went fine in favor of your negative prior, and conclude that the date went badly. This reinforces your prior that “things will probably go badly”, and you sink into a deeper depression. A vicious cycle.

This model provides a framework that helps justify some less traditional treatments for depression. Mind-body activities such as meditation, yoga, tai-chi, and massage may help via strengthening awareness of sensations. Drugs like ketamine and psychiedlics may be effective because they reduce the strength of mental priors 2.

Some thoughts

What’s most interesting to me is how I can use this mental model of depression to help myself and others. One takeaway for me is to lean in to my meditation practice. When I’m feeling negative emotions, it would be interesting to try punctuating my experience with moments of vivid awareness, and to see how that affects the experience. I’m also interested in exploring ways to “weaken” my mental priors, but in ways that don’t require drugs.

Interestingly, the very act of reading and thinking about this essay makes me feel better. Perhaps the act of looking inward to observe the precision of my sensory evidence strengthened the precision of my sensory evidence. Now my brain hurts.

Some other examples that might fit in the model.

Gratitude Practice seems to help a lot of people, and essentially reminding yourself of the good things in your life and nudging your prior from negative to positive.

Comparing yourself to others is one of surest ways to feel bad and is essentially reminding yourself how your life is worse than you thought, changing your prior to be more negative.

ADHD or people prone to getting scatter brained may also have cloudier sensory input. When I’m lost in thought I don’t feel particularly in touch with my senses, both external and internal. This might explain a link between ADHD and depression (very speculative)

The Power of Positive Thinking might sound like some pseudo-science self-help B.S, but under this model it makes sense. If you believe that good things will happen, even if it’s not “True” or based on facts, then you are strengthening your mental prior in the positive direction. The next time something neutral happens to you, you’re more likely to interpret this as a positive, further strengthening your mental prior. This seems similar to the effect of having faith in something greater than yourself, and something many people find in organized religion.

Open questions

Why do people gravitate towards strong negative priors vs strong positive priors?

Or do they? Are there a similar number of people with strong positive priors? People who are so sure that the world is in their favor despite evidence to the contrary? Maybe this is what “Manic” symptoms resemble.

What would the characteristics of people in the other 3 quadrants of sensory precision and prior strength look like?

  1. Interestingly, the case for depressed people having lower precision sensory evidence is supported by several independent findings - depressed people actually see the world in less color, have a worse sense of smell, worse body awareness, and worse autobiographical memory.) ↩︎

  2. It kind of makes sense that the mechanism behind the stereotypical psychidelic experience of “seeing everything as if for the first time” would extend to weakening priors as well. ↩︎