Posts Tagged ‘schizopohrenia’

The Brain’s Secret Powerhouse That Makes Us Who We Are

July 7, 2018

The title of this post is identical to the title of an article by Caroline Williams in the Features section of the 7 July 2018 issue of the New Scientist. The cerebellum is tucked beneath the rest of the brain and only a tenth of its size. In the 19th century phrenologists, who examined the shape of the skull to determine a person’s character, declared the cerebellum to be the root of sexual desire. They thought, the larger the cerebellum, the greater the likelihood of sexual desire.

During World War 1, the British neurologist Gordon Holmes noticed that the main problems for men whose cerebellum had been damaged by gunshot wounds had nothing to do with their sex lives and everything to do with the fine control of movements, ranging from a lack of balance to difficulties with walking, speech, and eye movements. From then on, the cerebellum was considered the mastermind of our smooth and effortless motions, with no role in thinking.

In the mid 1980s when brain imaging came along researchers noted activity in the cerebellum while people were lying still in a brain scanner and thinking. Unsure as to why this was occurring it was explained away as the neural signature of eye movements.

It took until the 1990s that it became undeniable that something else was occurring. Reports emerged describing people who had clear damage to their cerebellums but no trouble with movement. They experienced a host of emotional and cognitive issues, from depression to attention problems and an inability to navigate.

By this time, advances in neuroscience made it possible to trace long-range connections to and from the cerebellum. It was found that only a small proportion of the cerebellum was wired to the motor cortex, which is the brain region involved in making deliberate movements, explaining why movement was unaffected for some people with a damaged cerebellum. The vast majority of the cerebellum connects to regions of the cortex that are involved in cognition, perception, language and emotional processing.

A review of maps of the cerebellum built from functional MRI brain scans confirmed that all major cortical regions have loops of connections running to and from the cerebellum. The cerebellum has conversations with different areas of the cortex: taking information from them, transforming it and sending it back to where it came from.

One of the more unexpected connections was with the prefrontal cortex, which lies far from the cerebellum at the front of the brain and has long been considered the most advanced part of the brain. This region is in charge of abilities such as planning, impulse control, and emotional intelligence. It is disproportionately large and complex in humans compared with our closest species.

Robert Barton, an evolutionary neuroscientist at Durham in the UK says that when compared to primate brains, he found there is something special about the ape cerebellum, particularly our own. Throughout most of mammal evolution, the cerebellum increased in size at the same rate as the rest of the brain. But when apes split off from other primates, something strange began to happen. The ape cerebellum had a runaway growth spurt, becoming disproportionately larger than it evolved in the lesser apes. In our own brains the cerebellum is 31% larger than you would expect scaling up the brain of a non-ape primate. And it is jam-packed with brain cells, containing 16 billion more than you would anticipate finding if a monkey brain were enlarged to the size of ours. By strange coincidence, there are 16 billion neurons in the entire cortex. Neurons are particularly energy hungry cells, so this represents a huge investment of resources of the kind the brain wouldn’t both with without good reason.

Barton suspects that what started this unlikely growth spurt was the challenge of moving a much larger body through the trees. While small primates can run along the branches even gibbon-sized apes are too heavy to do the same, at least without holding on to branches above. This led apes to switch to swinging through the branches, known as brachiation, which in turn made the ability to plan ahead a distinct advantage. Barton says, “Brachiation is a relatively complex locomotor strategy. It involves fine sensory motor control, but it also involved a need to plan your route so that you can avoid accidents.”

To be able to plan a route, it helps to be be able to predict what is likely to happen next. To do that, you need to make unconscious adjustments to the speed, strength and direction of your movements on the fly.

Neuroscientists believe that the cerebellum achieves this by computing the most likely outcome based on previous experience using so-called forward models. Once it has these models in place through learning, it can then update and amend them depending on what is happening now. Narebder Ramnani, a neuroscientist at Royal Holloway University in London says, “Forward models respond very quickly because they allow the brain to generate what are likely to be the correct movements without waiting around for feedback.”

The leap in motor skills that came with brachiation and forward planning doesn’t completely explain the vast increased in the size of the cerebellum. Vineyard-like rows of bushy neurons called purkinje cells are linked by parallel fibers coming from the senses and vertical climbing fibers, which are thought to carry error messenger with which to update the internal model.

This structure is copied and pasted across the entire cerebellum with processing units set up like banks of computers, spitting out predictions all day long. Unlike the cortex, the structure of the cerebellum looks exactly the same regardless of where you look or which part of the cortex it is connected to. The only distinction is that different “modules” connect to different parts of the cortex.

Ramona says, “This suggests that whatever kind of computation that the cerebellum is carrying out for the motor regions of the brain, it is likely to be doing much the same for the cognitive and emotional regions too. And if the cerebellum is learning to automate rules for movements, it is probably doing likewise for social and emotional interactions, which it can call up, adapt and use at lightning speed.

Barton believes that having the ability to learn, plan, predict and updates was a key movement in our evolution, opening up a whole new world of complex behaviors. At first, these behaviors revolved around planning sequential movements to reach a goal, such as adapting twigs as a tool for termite fishing. But eventually thinking unhooked from movement, allowing us to plan our behaviors without moving a muscle. Barton thinks that being able to understand sequences could have allowed our ancestors to decode the gestures of others, setting the stage for the development of language.

The idea that the cerebellum makes and updates forward models contribute to the understanding of how the brain builds a picture of the word around us. The brain makes sense of the cacophony of sensory information with which it is bombarded by using past experience to make predictions that it updates as it goes along. With its forward planning capabilities, the cerebellum plays a more important role in the general working of the brain than we thought.

This new thinking strongly suggests that the cerebellum is involved in everything from planning to social interactions, and has a role in a range of conditions. For example, differences in how the cerebellum and the prefrontal cortex are connected are thought to affects the ability of people with Attention Deficit Hyperactivity Disorder (ADHD) to focus.

Schizophrenia is commonly linked with cerebellum changes, which could result in an inability to balance internally generated models of reality with sensory information entering the brain.

There is some hope that giving the cerebellum a boost using a type of brain stimulation called transcranial magnetic stimulation could help. A clinical trial is under way for schizophrenia.

This stimulation could even do us all some good; a recent study found that applying it to the cerebellum of healthy volunteers improved their ability to sustain attention.