Mar 12 2018

The Brain and Predictive Coding

One way to learn about how a system functions is to examine how it fails. Historically much of our knowledge of the most complex system we know, the nervous system, derived from examining patients with neurological deficits and then examining their brain (prior to imaging, this meant at autopsy).

This process is particularly fascinating with the human brain because we don’t yet know all of the things that the brain does. Some brain functions are obvious, like vision or motor control, because they are conscious. But most of what the brain does is subconscious, and we have had to specifically learn that the brain even needs to do certain things, mostly by examining what happens when the brain fails to do those things.

For example, we take for granted that we move as much as we desire to more, no more or less. But this balance between desire to move and the resulting movement is not automatic. There is an entire system within the brain, the extrapyramidal system, that is a series of feedback loops that carefully modulate moment to moment the gain of voluntary movement (the relationship between input – desire to move, and output – movement). Parkinson’s disease results from a disruption in this circuit which causes the gain to be turned down, so people move less and can even freeze. Chorea (as in Huntington’s chorea) involves the gain being turned up, so people with this disease are constantly writhing.

There are many other amazing examples of things most people are not aware that their brains do, or even that they have to do them. There are circuits in the brain necessary for feeling that you occupy your body, that you own and control your various body parts, and that you are separate from the universe. Disrupt these circuits, and your reality changes.

Perhaps the most intensive background subconscious process of the brain is filtering input – sensory input as well as your own internal thoughts. This gets to the phenomenon of attention. What do we pay attention to, and what do we ignore? And this is just part of the process of constructing your stream of consciousness. Sensory and internal signals are not just filtered, they are compared to each other, and to our internal model of reality, our expectations.

This process works in both directions, meaning that what we perceive affects how we build our experience of reality, but then what we construct out of those perceptions reaches back and affects the perceptions themselves. Once our secondary visual cortex thinks you are looking at an elephant, for example, it communicates back down to the primary cortex and essentially says – make that look more like an elephant.

But there is yet another dimension to our brains construction of the reality we perceive – time. Partly because our brain processing lags slightly behind events in real time, our brains project what it thinks is going to happen slightly into the future. Otherwise, you would never be able to catch a baseball.

Further still we apparently predict how we think the world is going to behave, what neuroscientists call “predictive coding” (a term borrowed from computer science).  These constant predictions about what is coming next are built out of current events, but also the context of our current situation, and our expectations built out of our memories and internal models.

So, for example, if we are in a suburban setting after sunset and we hear crickets, that meets our expectations. There are soundscapes to different settings, and when they match expectations we don’t really notice them. They are considered informational “noise” in the background, not a signal that our brain needs to pay attention to. If we are in the middle of a city and we hear a car horn, we ignore it. If we are in the country and hear a car horn, we take notice.

In other words – there is a subconscious process that evaluates sensory input based upon expectations and decides whether or not something is noise or signal. This further means that our brains need to set the threshold of the filter. If we filter out too much, we miss important information. Filter out too little, and we are overwhelmed with noise.

Yet another layer to this process of determining what we should notice is detail vs gestalt. How much do we focus on the tiny details, vs getting the big picture? This appears to be a dynamic process, meaning the balance is also something the brain monitors and adjusts as necessary. We may pay attention to details at first, until we perceive the bigger patterns, and then we attend to the patterns and gloss over the details. The details become noise, the pattern becomes what is important.

As stated at the beginning of this article – pathology can offer us a window into the normal or typical functioning of the brain. Some researchers now hypothesize that some people with autism may have a different balance when it comes to attending to detail vs gestalt, or may have a decreased ability to engage in predictive coding – anticipating what should come next based on experience and internal models.

Science Magazine has a fascinating article about this hypothesis, which is worth a read.

The idea is that some types of autism may involve being perpetually surprised by what is happening around them because of a decreased ability to predict what will happen based on context and other cues. This may also come with a tendency to focus on details, and not be able to shift to focusing on the big picture and ignoring the details.

This could be why some people with autism like to have a lot of control over their environment, and to have rigid schedules. They want everything to be routine and predictable, because they have difficulty dealing with the background chaos of life.

There are also those who do not see any of this as a disorder, just as not typical. This is an interesting perspective, and I’m not sure there is any objective resolution to this debate. In this view there are different balances the brain can have, with different strengths and weaknesses. No one balance is “correct” or even optimal. There are just different trade offs. The most common balance (say, within two standard deviations) is considered “normal” by most people, but it is not necessarily normal, just the most common.

In fact, it’s a good thing that our population contains outliers with different strengths and weaknesses. It takes a lot of different talents to run our world.

In any case, it is fascinating to think about how our own brains work. We keep discovering new layers to how our brains function and create our experience of our own existence. I am always looking for that next thing I did not even know the brain does.

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