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BOLD response

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Functional Magnetic Resonance Imaging, also known as ƒMRI, is a neuroimaging technique that measures the blood oxygenation,also called BOLD, that accompanies neuronal activity. The blood oxygenation level dependent fMRI allows an image spatial resolution that is of the order of a few millimetres, which is very good, with a temporal resolution of a few seconds, which is relatively poor in comparison to other methods of neuroimaging[1]. In recent years, ƒMRI usage has increased exponentially, though the results are not actually as convincing as they might seem. Functional magnetic resonance imaging is hindered by some very serious theoretical and methodological problems outlined below.

The BOLD Response: A Fundamental Issue

When neurons fire or increase their firing rate, they draw on oxygen and various nutrients. The circulatory system of the brain reacts by sending the region that just fired more highly-oxygenated blood than is needed. This results in an increased blood oxygen level in the activated region. As a result of the magnetic characteristics of hemoglobin, oxygenated blood has a different magnetic signature than de-oxygenated blood. Therefore, with the right pulse sequence, an MRI scanner is able to detect this difference in blood oxygen level. In fMRI the resultant signal that is read is called blood oxygen level dependent (BOLD)[2]. That being said, the BOLD response possesses a number of concerns that must be attended to. Factors such as drugs, substances[3] and excitation[4] have been shown to increase BOLD response. Conversely, age and brain pathology[5] have been shown to decrease BOLD response. Furthermore, local differences in neurovascular coupling[6], and also attention[7] have been shown to modulate the BOLD signal. Haller and Bartsch[2] have also discussed how both hyperventilation and hypoventilation can modulate the BOLD response by altering the amount of carbon dioxide in the blood. This means that the same individual can have different BOLD activations depending on what type of respiratory state they are in (i.e. anxious and hyperventilating vs. relaxed). The abovementioned concerns expose the BOLD response as one of the fundamental issues pertaining to fMRI research.

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