by measuring the ratio of hemoglobin with and without oxygen, the fMRI is able to map changes in cortical blood and infer neuronal activity.
Measurements using fMRI are made by having a person lie on his back inside a large magnet and radio frequency device, which measures changes in blood oxygen levels. Initially, a structural image of the brain is created (see Figure 2.15). In contrast, a structural image (MRI), like an X-ray, shows the anatomy of the brain but does not reflect activity. However, a reduction in brain volume is seen in a variety of disorders including schizophrenia. These measures can be determined from the MRI.
Blood flow measurements in the brain using fMRI are made by having a person lie on his back inside a large magnet and radio frequency device.
© iThinkStock/Snowleopard1
Brain activity can be determined with the fMRI, or functional MRI. A common procedure for showing brain activity is to take a baseline in which the patient just relaxes. Following this baseline period, the patient performs specific tasks. The fMRI response recorded during the task is subtracted from that during the baseline period. This shows which specific areas of the brain are involved in performing a task. This information is then placed on the structural MRI image of the brain as shown in Figure 2.16. The color used reflects the amount of activity seen in a particular brain area. As you will see throughout this book, fMRI has been used with almost every disorder discussed. You can also compare one group of individuals with another. For example, Figure 2.17 shows that women with post-traumatic stress disorder (PTSD) activate different areas of the brain (the amygdala and insula) when processing emotional information compared with women without PTSD (Bruce et al., 2013).
Figure 2.15 Magnetic Resonance Imaging (MRI) Shows the Anatomy of the Brain
Source: ©iStockphoto.com/CGinspiration.
Diffusion Tensor Imaging
It is also possible to use the MRI magnet to measure cortical connections in the brain, which is referred to as diffusion tensor imaging (DTI). DTI is available with most MRI imaging systems (see Thomason & Thompson, 2011, for an overview of DTI and psychopathology). It is a procedure for showing fiber tracts (white matter) in the brain. This information can then be visualized by color coding it as shown in Figures 2.18 and 2.19. This allows one to map the white matter connections in the brain. In these figures, the connections between different parts of the brain can be seen.
Figure 2.16 Functional Magnetic Resonance Imaging (fMRI)
Source: © Zephry/Science Source.
Developmentally, after-infancy measures of white matter suggest a linear development until a person is in her thirties. Following a plateau, these gradually decline with age. Using DTI, it is possible to map the mild cognitive impairment seen in dementia and the more severe impairment seen in Alzheimer’s disease. Disconnections are seen between the major areas involved in memory such as the hippocampus and the temporal lobes (Stebbins & Murphy, 2009). As would be expected, this loss of connectivity is greater in Alzheimer’s than in mild cognitive impairment. Individuals with schizophrenia also exhibit problems with cortical connections (Phillips et al., 2011). It is also possible to compare the structure of pathways in the brain between humans and other primate species (Wedeen et al., 2012).
Spatial and Temporal Resolution
There are a number of trade-offs that researchers must consider when choosing a brain imaging technique (see Table 2.2 for pros and cons of using the different techniques). It begins with the research question one is asking. If you wanted to know if the areas of the brain associated with memory, such as the hippocampus, are larger or smaller in those with PTSD, then you would want a measure of structure. If you wanted to know if those with autism quickly viewed different emotional faces in a different way, then you would want a measure that reflects changes in brain processes.
Figure 2.17 Brain Activation Differences in the Amygdala and Insula
Source: Reprinted from Steven E. Bruce, Katherine R. Buchholz, Wilson J. Brown, Laura Yan, Anthony Durbin, & Yvette I. Sheline, Altered Emotional Interference Processing in the Amygdala and Insula in Women With Post-Traumatic Stress Disorder, NeuroImage: Clinical, Vol. 2, pp. 43–49, Copyright © 2013, with permission from Elsevier.
Figure 2.18 Mapping White Matter Connections in the Brain Measured With Diffusion Tensor Imaging (DTI).
Source: © Zephyr/Science Source
Figure 2.19 Mapping White Matter Connections in the Brain Using Color Coding
Source: Thomas Schultz (2006), Wikipedia.
One important question is how fast a particular technique can measure change. This is referred to as temporal resolution. EEG and MEG, for example, can measure quick changes in the brain on the millisecond level. PET, on the other hand, can only record changes that take place in a period of a few minutes or more. Another consideration is spatial resolution—that is, what size of brain area a technique can measure. PET and fMRI are better able to pinpoint the location of activity in the brain, whereas with EEG it is less possible to know specifically where in the brain activity came from. The relationship between spatial and temporal resolution is shown in Figure 2.20.
Table 2.2
Neuroethics
When we read in the news about discoveries in the neurosciences, they are often presented in an optimistic manner. We are told they will help us treat medical disorders or learn more about how we think and feel. This is true. However, traditionally, societies have based codes of conduct and the law on observable behaviors. An important question that is currently being asked is who should have access to data and scans of your internal processes. LENS: Neuroethics: Ethical Considerations When Using Neuroscience Techniques examines the field of inquiry that is asking these questions. It is referred to as neuroethics.
Figure 2.20 Spatial and Temporal Resolution of Imaging Techniques
Source: Meyer-Linderberg (2010, p. 194).
Cultural Lens
Using Brain Imaging to Understand Culture
A traditional healer in the Shona village in Harare, Zimbabwe.
Robert Fried/Alamy
For thousands of years, humans have traveled and been fascinated by the diversity of human behavior and thinking around the world. Historically, those interested in psychopathology and neuroscience research have focused more on the universality of human processing rather than the diversity found in different cultures. This is beginning to change with an integration of human diversity and neuroscience perspectives on human behavior and