Saturday, August 24, 2013
Functional Images of the Brain
A variety of methods exist for obtaining functional images of the brain. Cat (or CT) scans use x-rays and produce medium-resolution images especially useful for imaging stroke, tumors and some types of dementia (Breedlove, Watson, & Rosenzweig, 2020). MRIs provide images that are higher resolution than CT scans, without danger of radiation. PET scans produce images of functional processes in the body by utilizing radioactive substances that collect in organs and reflect light rays which are reproduced with the aid of a computer (Breedlove, Watson, & Rosenzweig, 2010).
Although previously misunderstood as a relatively simple measurement of neurons' electrical signals, functional magnetic resonance imaging (fMRI) measures changes of blood flow in the brain as a result of an increase in the neurons' activity. Active neurons require an increased amount of blood (and oxygen concentration), and this increase produces a signal in the fMRI scan (Wood & Wehrli, (1999). (Recall, blood has iron in it, and so reacts to a magnetic field.) Although the visual result of functional image scans makes the brain appear to light up in specific areas and under certain conditions, it does not (light up). These images do portray increases in neural activity, but the increase is because of blood flow rather than electrical impulse (Logothetis, Pauls, Augath, Trinath, & Oeltermann, 2001).
Logothetis, Pauls, Augath, Trinath, and Oeltermann (2001) advise caution when interpreting fMRI images. It is possible that statistical analysis and methods utilized to make such interpretations may significantly underestimate neural activity generated by specific behaviors or tasks undertaken during the scanning (Logothetis et al., 2001). Although the fMRI is used to "draw provocative conclusions about the neural mechanisms of cognitive capacities" (p. 869), its users must understand that the human mind is less modular than some individuals believe (Logothetis, 2008). Further, because the fMRI portrays mass activity of the neurons in a specific brain area, the idea of interpreting accurately exactly what the fMRI signals mean, takes on complex implications, and perhaps, complicates the conclusions that are routinely drawn (Logothetis, 2008). The images are unmistakably complex. Further, the limitations of functional scanning do not exist in the fMRI technology; they exist because of the brain's complex circuitry and organization (Logothetis, 2008). Shortcomings aside, fMRI technology continues to evolve and offer invaluable insight into the human brain.
To a lay person, I would explain that the functional imaging measures functional processes in the body. When imaging the brain, the activity appears to glow or light up on the scan images, and technicians, trained to understand the meaning of the images, interpret them. The fMRI technology has limitations, but it is the most accurate tool currently available, in many cases, to determine what goes on in the human brain (Logothetis, 2008).
Breedlove, S. M., Watson, N. V., & Rosenzweig, M. R. (2010). Biological psychology: An introduction to behavioral, cognitive, and clinical neuroscience. (6th ed.) Sunderland, MA: Sinauer Associates, Inc. Publishers.
Logothetis, N. K., Pauls, J., Augath, M., Trinath, T. & Oeltermann, A. (2001). Neurophysiological investigation of the basis of the fMRI signal. Nature 412, 150 - 157. doi:10.1038/35084005
Logothetis, N. K. (2008). What we can do and what we cannot do with fMRI. Nature, 453(7197), 869-878. doi: 10.1038/nature06976
Wood, M. L. & Wehrli, F. W. (1999). Principles of magnetic resonance imaging. In Magnetic resonance imaging (3rd ed., pp. 1-14). Baltimore, MD: Mosby-Year Book.