PSYC 373 Lab Report 4: Brain rhythms and EEG
A good source of information: https://backyardbrains.com/experiments/eeg
The electroencephalogram (EEG) is a common technique that is often used to study
normal and abnormal brain activity, such as sleep cycles, responses to stimuli, and seizure activity.
Unlike other forms of imaging that take pictures of the brain, an EEG records the general electrical
activity of groups of neurons under the surface of the scalp and skull. As a result, EEG is considered a
direct measure of neural activity because it measures postsynaptic potentials generated by flowing
currents during excitation of dendrites of the cortical pyramidal neurons. When neurotransmitters are
released onto a cortical pyramidal cell, positive charge flows down the dendrite towards the soma,
leaving an area of relative negative electrical charge behind it. This relative negative charge is recorded
by various electrodes adhered to a person’s head. Because there are several electrodes on the head, the
electrical activity among the electrodes is compared, providing an overall EEG signal. An EEG’s signal has
low spatial resolution and high temporal resolution, meaning it does not produce a “fine” image like we
see in fMRI results, but it reflects changes in brain activity comparatively quickly.
Underlying activity of neurons, and therefore the EEG signal, can vary depending on one’s
current state, whether it is a state of sleep, attentiveness, panic, or relaxation. The EEG signal is often
described based on amplitude and frequency: the amplitude is the height of the wave, whereas the
frequency is the number of waves that occur in a period of time. When populations of neurons are firing
together, the EEG signal becomes synchronous which is characterized by large, rhythmic EEG waves with
high amplitude and low frequency. When populations of neurons are firing at different times (or
perhaps one population of neurons has relatively less firing than another), the EEG signal becomes
asynchronous which is characterized by small, irregular EEG waves with low amplitude and high
frequency. We see that EEG rhythms vary greatly and can correlate with states of behavior. In this lab,
we will focus on two states of behavior – having one’s eyes open and having one’s eyes closed.
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There are four primary frequency ranges that are defined as primary components of the EEG signal:
Wave pattern
Alpha
Beta
Theta
Delta
Frequency (Hz)
8 – 13
14 – 30
4–8
0.5 – 4
Typical behavioral state
Quiet, waking states
Highly active cortex; dreaming
Lighter sleep states
Deep sleep
In this report, we will be examining alpha and
beta waves, which are exhibited in awake states.
The difference between alpha and beta waves is
that beta waves are observed while the cortex is
highly active. As example of this is when an
individual has his/her eyes open (and therefore
is visually processing a scene) or is engaged in
mental math. Alpha waves, on the other hand,
are observed while the individual is awake but is
completely relaxed with his/her eyes closed.
There are other waves – theta and delta waves –
that are typically observed in sleep states, but
because we do not want students falling asleep
in this lab, we will not observe those today.
The figure on the right demonstrates the
different types of waveforms.
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Imagine you are a cognitive researcher interested in
studying brain activity in human subjects. You conduct
an EEG on a healthy control patient. Imagine you are
looking at the waveforms being conveyed by the scalp
electrodes for the following questions.
Questions:
1. What kind of waveform and frequency (Hz) would you see while your subject has his/her
eyes closed? Why?
Alpha waves – due to quiet, wakeful states; no stimulation for the eyes
2. What kind of waveform and frequency (Hz) would you see while your subject has his/her
eyes opened? Why?
Beta waves – stimulation to the eyes; high frequency
Could also be between alpha and beta
3. Imagine if your subject had his/her eyes closed but was engaged in a mental math task.
What kind of waveform would you see, and why?
Beta waves – eyes are closed with no visual stimulation, but there are higher cognition stimulation;
inputs of brain during higher order thinking
4. You are a sleep researcher observing EEG activity of human subjects. One of your participants is
in REM sleep, which is the stage characterized by rapid eye movements, dreaming, and atonia.
Why is this stage known as the paradoxical sleep stage? (Hint: Consult the diagram on Page 2 for
help.)
Has asynchronous waves with high frequency; it’s paradoxical because while you are asleep, the brain is
working quickly while the person dreams, so it looks like you’re awake based on the EEG alone; no
muscle activity
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