-
Essay / Strengths and Weaknesses of Electroencephalography and Magnetic Resonance Imaging
Functional magnetic resonance imaging (fMRI) and electroencephalography (EEG) are two measures adopted to examine the activities of the human brain. The first uses a magnetic field to detect magnetic changes in the blood and the second uses electrodes placed on the human skull to detect electrical potentials. The main advantages and disadvantages of these two measures are present in what they measure, their temporal resolution, their spatial resolution and the efficiency of their data. Comment from Emma Soneson: This is a good introductory paragraph: I can tell that you followed the "funnel" structure we talked about and created a thesis sentence that gives an overview of what you go write in the essay. Well done. Say no to plagiarism. Get a tailor-made essay on “Why Violent Video Games Should Not Be Banned”? Get the original essay Functional MRI (fMRI) measures hemodynamic changes after increased neuronal activity. “fMRI requires an MRI scanner, a high rate of image acquisition, and specialized pulse sequences to measure localized brain activity.” (Voos & Pelphrey, 2013, p.2) This describes the basic framework for the operation of functional magnetic resonance imaging, which includes an MRI scanner to detect blood deoxyhemoglobin contrast, rapid acquisition of the images produced and programs called pulse sequences to send instructions to the scanner hardware to “turn certain hardware on or off at certain times” (Huettel, Song & McCarthy, 2009, p.42). Currently, the most common fMRI technique is the Blood Oxygenation Level Dependent (BOLD) technique, which uses magnetic resonance to detect the direction of blood flow through the change in blood oxygen content. Ogawa, Lee, Nayak, and Glynn (1990) first discovered the intrinsic BOLD contrast mechanism. To describe the mechanism generally, when a region of the brain is active, neurons in that region communicate information with each other through electrical impulses – synaptic potentials and action potentials. This activity requires energy, which is provided by glucose and oxygen. As a result, high levels of oxyhemoglobin occur in active areas, leading to an increase in the surrounding ratio of oxyhemoglobin to deoxyhemoglobin. To meet the demand for energy and oxygen, oxygenated blood flows into the area and displaces deoxygenated blood (Ogawa, Lee, Nayak, & Glynn, 1990). The principle of MRI is that when people lie in an MRI scanner, the protons in the people's bodies tend to align with the magnetic field (Huettel, Song & McCarthy, 2009). These protons then rotate around the axis of the field (Huettel, Song & McCarthy, 2009). In oxygenated blood, protons organize themselves and rotate at the same rate (Bekinschtein, 2019). In contrast, protons in deoxygenated blood are not as organized, because deoxygenated hemoglobin molecules have magnetic field gradients that change the rotational speed of the neighboring diffusing hydrogen nucleus. The local magnetic field is thus affected and the variation in the magnetic field can be detected by the long TE gradient echo (echo time) pulse sequences used in fMRI. Thus, the direction of oxygenated blood flow can be detected by MRI scanners through the difference between the MRI signals produced by oxygenated and deoxygenated blood. Comment from Emma Soneson: Since you are clearly explaining the quote in the previous sentence here, you are not required to include thetwo. Comment from Emma Soneson: Good understanding Comment from Emma Soneson: Almost word for word. Electroencephalography (EEG) is a non-invasive method for detecting neuronal activities in the brain by measuring the brain's electrical fields. Electrodes placed on the human scalp record voltage potentials, which result from current flow “in and around neurons” (Biasiucci, Franceschiello & Murray, 2019, p. R80). The main potential EEG measurements concern electrical activities associated with postsynaptic dendritic currents generated in cells of the cortical pyramid. This neuronal activity is called “primary current” (Denes Szucs, lecture notes). “An excitatory postsynaptic potential at an apical dendrite will translate locally into an intracellular current source. At the soma, there will be an intracellular current sink and an extracellular current source. These source-sink configurations are also known as current dipoles” (Biasiucci, Franceschiello & Murray, 2019, p. R80). Brain tissue, fluid and the skull act as a conductive medium in which electrical waves propagate. This propagating electrical activity is called “secondary current” (Denes Szucs, lecture notes). This current is also captured by EEG electrodes placed on the human scalp. EEG can only detect a portion of all the varieties of electrical activity taking place in the brain. However, detected electrical signals may include physiological electrical activity, "such as heart, eye, and other muscular activity" and ambient noise "such as computer screens and other electrical equipment, power lines." (Biasiucci, Franceschiello & Murray, 2019, p. R80). After describing two methods separately, we will compare their weaknesses and strengths. A disadvantage of fMRI is that “the BOLD signal cannot provide information about the directions of information flow.” (Voos & Pelphrey, 2013, p.3) That is, fMRI cannot identify the exact sequential neuronal activities in the information transmission process, because BOLD is a neurometabolic signal and has a delay with respect to the activities neuronal. Also due to the delay, the temporal resolution of fMRI is coarse, on the order of a few seconds. (Shah, Anderson, Lee and WigginsIII, 2010) In contrast, EEG directly measures neuronal activity in real time by “measuring the electrical activity of neuronal cell assemblies on a sub-millisecond time scale” (Michel , Murray, Lantz, Gonzalez, Spinelli and Grave De Peralta, 2004). Therefore, EEG has a much higher temporal resolution than fMRI. Another limitation of fMRI is that because the BOLD signal relies on a relatively slow vascular response, a neuron's inhibitory and excitatory inputs from other neurons will sum up. and therefore, the BOLD signal in the neuro will appear zero, because the two inputs will cancel each other out (Voos & Pelphrey, 2013, p.3). Although EEG does not have such a problem, unfortunately, EEG faces the problem of measured signals. on the scalp surface do not directly indicate the location of active neurons in the brain, due to the ambiguity of the underlying static electromagnetic inverse problem (Helmholtz, 1853; Michel, Murray, Lantz, Gonzalez, Spinelli et Grave De Peralta, 2004). “Many different source configurations can generate the same distribution of potentials and magnetic fields on the scalp.” (Michel, Murray, Lantz, Gonzalez, Spinelli and Grave De Peralta, 2004) Therefore, maximum activity at certain electrodes does not certainly mean that the generators.2004.06.001