So in a previous post I discussed what EEG or brain waves are, now I will discuss how we interpret them. When you record EEG or brain waves it looks something like this:
So, that is what brain waves look like - but how do neuroscientists interpret the squiggly lines? To understand that you have to think in terms of frequency. When we say frequency - we essentially mean how many times per second a waveform or signal oscillates. If you think of televisions - most people know now that a 120 Hz television is better for watching sports than a 60 Hz television. Why is that? A 60 Hz television draws the image on the screen 60 times a second which seems like a lot. However, is something is moving very fast it actually might be moving at a rate such that you see blur or gaps in the signal because you need to draw is faster to see the complete motion. Thus, a 120 Hz television draws an image 120 times a second which removes the blur. Here is a video that explains the concept.
But what about brain waves? If you look at the EEG signal above you will see that the signal oscillates, or goes up and down, and thus it has a frequency. This stems from the fact that the firing of neurons is not constant but differs in intensity and amplitude over time - think of your heart rate speeding up and what that would look like on a ECG. We are starting to understand that different frequencies of neural activity reflect different neural processes. For example, when you are concentrating on solving a complex problem we see an increase in brain activity in the 4 to 8 Hz range over the front of the scalp, the prefrontal cortex. Note, most human brain activity is between 1 and 40 Hz. So, when we record frequency we use a mathematical technique called the fourier theorem to find out how much brain activity there is at each frequency (e.g., at 1 Hz, 2 Hz, 3 Hz - read more HERE).
So, the FFT (or Fast Fourier Transform) tells us how much activity there is at each frequency interval. If you take a snap shot of EEG, say over 1 minute, and run a FFT on the EEG data you get a "brain state" - the frequency spectra of the EEG signal. We, as neuroscientists, are starting to understand that different brain states reflect different things. For example, monks during meditation typically have a brain state in which we see more gamma (32+ Hz) activity (more on monks and EEG HERE). Typically, as opposed to talking about a specific frequency neuroscientists talk about frequency ranges: delta (1 to 4 Hz), theta (4 to 8 Hz), alpha (8 to 15Hz) beta (8 to 32 Hz), and gamma (32+ Hz). Wikipedia covers the topic reasonably well HERE.
Brain waves, or more properly termed - the human electroencephalogram (EEG) - are a physical representation of the electrical activity in your brain. Indeed, electricity plays a key role in the function of your brain. As a starting point, let's look at what a neuron is.
Neurons are the building blocks of your brain. Peripheral afferent neurons bring signals from your muscles or other sensory areas to your brain. Peripheral efferent neurons carry signals to your brain, a vast number of interneurons - neurons connected to other neurons - give rise to all of your brain's activity - from feeling of fear and hunger to the complicated decisions you make.
Neurons communicate with each other via action potentials. The video below gives a good overview of what an action potential is.
In short, the action potential is the electrical signal generated by a neuron to communicate with other neurons. When the action potential reaches the end of the axon - it causes a release of neurotransmitter (a chemical compound) that crosses the small gap between the neurons and the next neuron where it binds, or joins, with the next neuron. That process results in a small electrical potential, or voltage, called an excitatory or inhibitory post synaptic potential. If a large number of neurons are grouped perpendicular to the surface of your head then electrodes on the scalp can measure the voltage activity, the excitatory or inhibitory post synaptic potentials - or the EGG / "Brain waves".
For comparison, think of ECG or the electrical activity of your heart. Electrodes places on your chest can detect the electrical activity that is needed to contract your heart. The "Flat line" we all fear is the result of there being no more electrical activity to contract the heart muscle.
ECG is similar in a sense - it is the electrical activity of millions of neuron s firing in your brain. For example, when you see an image, think of a friend, or get hungry, millions of neurons "fire" to create the pattern of thought. We can measure this neural activity with EEG.
Last year I spoke at the Centre for Biomedical Research's Cafe Scientifique and gave a talked entitled "Why We Do the Dumb Things We Do: The Neuroscience of Human Decision Making". You can find a podcast of the talk here
I love telling people what I find fascinating about the human brain. For instance, there is a subset of people in the world who have "prosopagnosia", or face blindness. These people can see most things normally and typically have normal functional lives - however, they have a special form of blindness wherein they cannot see faces. This is really hard to describe, but imagine looking at a face and simply not seeing what is there - they can see the rest of the body and can hear the person's voice - but they literally cannot see the persons face! Here are a couple of You Tube clips about prosopagnosia.
Prosopagnosia stems from damage to a very specific part of the brain the Fusiform Face Area.
Damage to this region results in prosopagnosia - or face blindness. If the damage is quite extensive, like the second video, the person in question is also blind to other animate objects. People with extensive damage to the inferior temporal cortex can have visual form agnosia - or a deficit in the ability to see shapes and objects. For example, someone with visual form agnosia would not only be face blind but they also would not be able to tell the difference between a circle or a square.
I really enjoyed being on the morning show with Pamela McCall this morning talking about our trip to Nepal!
Listen to our morning talk here.