Illusions pose some of the most puzzling questions of all about how our brains receive then process information and most importantly of all, how we learn.
For centuries, people thought the brain was for ‘cooling the blood’. Later it was thought that a brain’s intelligence was determined by its size and that someone's personality could be predicted by the bumps on their skull. Humans have made more mistakes about their own brains than any other organ. Even today, every new discovery seems to raise more questions than it answers.
We may have learnt more in the past 10 years than in the previous 1000 years put together. However, we still know very little about so many aspects of our brains including how we sleep and dream, how genes influence personality, how the brain develops and grows, what causes diseases, how different areas of the brain can share responsibility for a certain task and how memories are stored. The biggest puzzle of all is how consciousness is created by, and linked to, this crumpled looking organ in our heads.
The human brain in adulthood weighs an average of 2% of the person’s body weight. The normal range is 1-2 kg. Nowadays we know that brain size does not determine intelligence, nor does the actual number of cells. The major players in the brain are nerve cells called neurons. An average human brain peaks at around 100 billion neurons, nearly 20 times the number of people on Earth. The key to our intelligence is not the number or size of these neurons but the connections between them.
Brain development in a foetus is first concentrated on making neurons with not much networking going on. After birth however, the rate of neuron creation slows down while the rate of connection skyrockets. It is normal for each neuron to become connected to between 10 000 and 100 000 others. Over time, neuron creation stops altogether then actually goes into reverse as nerve cells gradually die. The brain can still be finetuning its internal network well into our twilight years.
Each neuron has just three main components: a cell body called the soma which is like a central processing facility, a long thin projection called an axon which transmits signals and a branching collection of short finger–like projections called dendrites which receive the signals. Inside the neurons, the signals move as electrical impulses. Where the neurons connect to each other the signals are transformed into chemical reactions. This junction between neurons is called a synapse.
Messages travel at different speeds depending on the type of neuron. For example, a signal from the brain telling the left foot to move must first traverse a synapse between the brain and a spinal chord motor neuron. The signal traverses another synapse at the base of the spinal cord and travels along a peripheral motor neuron which connects with muscles which move the foot. The brain requires many synaptic transmissions in the thought processes leading up to the decision to move the left foot.
Each synaptic transmission is comparatively slow, requiring about half a millisecond each. The nerve impulse moving down the motor neuron in the spinal cord travels at 70-80 m/s. Finally the impulse is delivered to the muscle through the peripheral motor neuron at a speed of 120 m/s or 430 km/h.
What is perhaps most interesting is that, because so many synaptic transmissions take place in making a decision, it takes longer to think about moving a muscle than it does to actually move it. Reflex actions are much quicker than deliberate actions because they do not require thought. Here only one synaptic transmission takes place (in the spinal cord) between the sensory neuron and the peripheral motor neuron, thus bypassing the brain. The sensation may actually reach the brain after the motor reflex action has taken place!
Just a few years ago, scientists thought that neurons worked on an ‘all-or-nothing’ basis like computer circuits. This would have meant that neurons are only ever on or off and that they have no individual ability to carry more detailed information. Now we know our neurons are far more advanced than this. As well as sending a signal, neurons can carry information about the strength and qualities of the signal and influence the type of reaction they generate in other neurons. One neuron can even modulate the signal being sent between two other neurons. In fact, the communication between neurons accounts for the majority of the brain’s activity.
On average, only 10% of the inputs to a neuron come from the rest of the body or from lower parts of the brain’s hierarchy of operations. The rest is taken up by an enormous network of relationships with fellow neurons, all constantly providing feedback to each other ‘ the brain’s round-the-clock conversation with itself.