In Vivo Research Techniques

by j.w.gibson, m.s.

Advanced technology in body imaging has greatly increased the ability of researchers and clinicians to investigate cognitive functioning and diagnose brain dysfunctions. Two techniques that are currently enjoying widespread use are Functional Magnetic Resonance Imaging (fMRI) and Positron Emission Tomography (PET). These techniques are in vivo, that is, they do not require invasive intrusion in to the body to study the underlying mechanisms. In vivo techniques are advantageous because they can illuminate cognitive processing while it is happening, giving cognitive scientists a tangible picture of which parts of the brain function to produce certain cognitive activities.

Magnetic Resonance Imaging (MRI) has been used for more than fifteen years to analyze the soft tissue of the human body (Noll, 2001). MRI is an imaging technique that uses the property of Nuclear Magnetic Resonance (NMR) to differentiate between atomic nuclei with different magnetic properties (Ugurbil, 2001). Some types of atoms have nuclei that have an odd number of neutrons, odd number of protons, or perhaps both, which leads to a "net magnetic moment and will therefore be NMR active" (Noll, 2001). Hydrogen1 or simply a proton, is the most common type of nuclei used for NMR because of its high concentration in the human body (Noll, 2001). When these nuclei are subjected to a magnetic field they will align themselves parallel or anti-parallel to the static magnetic field (Ugurbil, 2001). Electromagnetic radiation is used to create discrete resonances in different chemical nuclei (Ugurbil, 2001). Imaging is then performed by assigning specific spatial encoding and contrast mechanisms to the resonance information (Noll, 2001). Colors or gray-scale coding are used to create a visual image of different tissue based on resonation differences.

Functional MRI (fMRI) is the application of MRI technology to investigate the physiologic changes that occur in brain tissue. For instance, fMRI is able to measure changes in "phosphorus metabolism and metabolic byproducts, blood flow, blood volume, and blood oxygenation" (Noll, 2001). Blood oxygen level dependent (BOLD) contrast is the most frequently used technique. Hemoglobin that is oxygenated has different magnetic properties than hemoglobin that has been deoxygenated by the metabolic processes of the cells. Since neural processing requires the use of neural cells, which in turn use oxygen, it is possible to identify which parts of the brain are more active during a specific cognitive task. However, the differences between magnetic resonances are typically so small that it is necessary to perform numerous trials and then subtract out the difference from the control and experimental data (Noll, 2001). Only then is there enough statistical difference to be superimposed onto brain images thus highlighting activated brain structures (Noll, 2001; Ugurbil, 2001).

Researchers have used fMRI to study a wide range of cognitive processing including the fundamental layout and relationship of brain structures during normal processing (Salvador et al., 2005) to dysfunctional processing (Pantano et al., 2005). However, most studies using fMRI do not utilize multivariate methods for analysis and so may not be realizing the full potential of fMRI data (Friman, Cedefamn, Lundberg, Borga, & Knutsson, 2001). New methods for detecting neural activity are constantly being developed, such as the Canonical Correlation Analysis (CCA) suggested by Friman et al. (2001). It is clear that MRI and especially fMRI have enormous potential as an investigative and diagnostic tool.

Positron Emission Tomography (PET) is an imaging technique that utilizes the properties of annihilation radiation that occurs when positrons are absorbed into matter (Riachle, 2001). Subjects are injected with a radioactive substance, called a radiotracer, which then flows through the heart and eventually to the brain. The decay of positrons produces gamma rays that can then be detected by machines. The patterns of gamma rays provides data, which is then processed by algorithm, to create an image (Montandon & Zaidi, 2002). Because PET relies on regional cerebral blood flow (rCBF), it is understood that more active areas of the brain will require more rCBF and thus have greater concentrations of the radiotracer. In the early days of PET it was realized that while both blood flow and metabolism could be accurately measured, blood flow provided a greater practical advantage because of it could be measured in less than a minute, and with a widely available radiopharmaceutical (H215O) (Riachle, 2001).

Researchers typically use a subtractive methodology for the measurement of thought processes. That is, they take a base measurement, for instance, how long it takes to respond to a light, and then subtract the difference in response times for responding to a particular color of light (Riachle, 2001). Using this same technique, researchers have been aided in the studying the relationship between certain types of brain disorders. For example, low serotonergic activity seems to be related to both Borderline Personality Disorder (BPD) and Major Depressive Disorder (MDD) (Oquendo et al., 2005). PET technology has helped researchers to demonstrate that both BPD and MDD are associated with unusual activity in the parietotemporal and anterior cingulate cortical regions (Oquendo et al., 2005).

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