EEG (Electroencephalogram) Explained transcript

let's talk about eeg something that may be distantly familiar to you eeg stands for electroencephalogram blessing electro refers to electricity encephalo refers to the brain and gram refers to something written so together an electroencephalogram just refers to something that measures electricity in the brain and records it eeg can be recorded from electrodes outside the skull non-invasively or from electrodes being placed inside the brain by a neurosurgeon highly invasively this video will focus on scalp eeg so how could this technology actually work eeg exploits something called a dipole a dipole is two different charges that are separated by a distance so if we stick electrodes bits of metal that conduct electricity here and here we can generate a readout of the difference in charge between these two areas so why does your brain have areas with different charges hold my beard your brain is one big electric circuit boards made up of neurons interestingly for a neuron that's resting the inside of the neuron has a negative charge relative to outside the neuron about minus 17 millivolts neurons can communicate with each other by neurotransmitters diffusing across this space called a synapse once neurotransmitters reach the next neuron they bind to receptors where one of two things can happen something called an excitatory postsynaptic potential or something called an inhibitory postsynaptic potential what are those an excitatory postsynaptic potential is where neurotransmitters binding causes positively charged particles called ions to flow into the neuron making the inside of the neuron more positive this makes this area the extracellular space outside the neuron relatively more negative because this area is now more negative the distant extracellular space at the other end of the neuron becomes relatively more positive this means we have two different charges separated by a distance we have a dipole but it's also possible to have a dipole the other way around from an inhibitory postsynaptic potential here the neurotransmitter binding causes negative ions to flow into the neuron making the inside of the neuron more negative making the extracellular space relatively more positive making this distant extracellular space more negative so excitatory postsynaptic potentials and inhibitory post-synaptic potentials create differences in charge dipoles at the opposite ends of the neurons but the dipole for a single neuron is really really small and not fabulously detectable but luckily in the cerebral cortex just below the scalp there are billions of large neurons that lie parallel to each other perpendicular to the scalp apologies for the less beautiful drawing but these large neurons are called pyramidal neurons clusters of these pyramidal neurons are usually stimulated together giving them individual dipoles which summate to form a larger charge just below the scalp now we have this area of the brain being one charge and this area of the brain being another charge a larger detectable dipole so we can stick an electrode here and here and measure the difference in charge between these two areas this is exactly what an eeg signal is it's the electrical difference between a pair of electrodes on two different parts of the brain created as a result of clusters of pyramidal neurons being excited and inhibited a typical eeg sees 32 electrodes placed over the brain and we need a way to pair up these electrodes how do we do this one approach is to compare every electrode to the same electrode this produces something called a common reference montage where each horizontal line shows the electrical difference from each electrode to the same electrode this can be identified from the left hand side of the montage another approach is to compare every electrode to the electrode next to it this produces something called a bipolar montage where each horizontal line shows the electrical difference from each electrode to the one next to it both of these approaches and some others are incredibly useful for understanding brain activity with lots like years of training you can learn to notice certain patterns in all of the squiggly lines just fyi i most certainly don't have this ability you can look at the amplitude of the signal which is how much it goes up and down from this midline you can also look at the frequency of the signal which is the number of waves in a given amount of time measured in hertz eeg patterns are commonly described in terms of the frequency of the signal with five key bands sleep produces delta waves which have a frequency of up to four hertz deep relaxation produces theta waves which fall between 4 and 8 hertz passive attention creates alpha waves which are between 8 and 13 hertz and active mind shows beta waves which are 13 to 35 hertz and high concentration or problem solving can produce gamma waves above 35 hertz so hopefully watching this video you're producing some gamma waves hopefully not delta waves establishing the frequency of the signal helps highly specialized clinicians to identify abnormal brain patterns this can aid the diagnosis of different diseases so if they see spikes like this in the eeg signal called ieds these can indicate potential epilepsy if they see increased slow wave activity this can one of potential damage or disease in the brain there's also an emerging area of research using machine learning and deep learning to analyze eeg montages this is actually something i do well i try anyways thank you so much for watching let me know if you have any questions and i'll see you in the next one