Scanning the Brain
New technologies shed light on the brain's form and function.
Intensive instruction in reading improves how a child's brain works. In schizophrenia, key parts of the brain may not communicate well, making it hard to organize one's thoughts. And true love wouldn't be true without the neurotransmitter dopamine. We know all this and more thanks to neuroimaging, an increasingly sophisticated tool that sheds light — literally — on the human brain.
Doctors and scientists once had to wait until autopsy to examine the brain, and psychologists had to deduce from behavior where the brain was injured. Now they can study detailed three-dimensional images of the brain to spot problems, to understand what happens during tasks, thoughts and emotions and to assess the effectiveness of various treatments.
Current neuroimaging techniques reveal both form and function. They reveal the brain's anatomy, including the integrity of brain structures and their interconnections. They elucidate its chemistry, physiology and electrical and metabolic activity. The newest tools show how different regions of the brain connect and communicate. They can even show with split-second timing the sequence of events during a specific process, such as reading or remembering.
Psychologists employ these tools across the range of the discipline. Social cognitive neuroscientists, for instance, are capturing the psychological and neural processes involved in emotion, pain, self-regulation, self-perception and perception of others. Psychologists have used neuroimaging technology to demonstrate how white Americans, even those who report themselves free of prejudice, show differences in brain activity in the amygdala — a structure involved in emotional learning – when they look at pictures representing people of different racial groups. Positive emotions are also studied. Psychologists have compared functional images taken when students looked at pictures of their romantic partner versus pictures of an acquaintance. When students gazed at their beloved, two deep-brain areas that communicate as part of a circuit showed increased levels of activity. Those areas help to regulate the neurotransmitter dopamine, which floods the brain when people anticipate a reward.
Neuroimaging is also helping us understand how the brain develops from infancy through adulthood. Developmental neuroscientists study the neurobiological underpinnings of cognitive development. Combining functional measures of brain activity with behavioral measures, they explore how subtle early insults to the nervous system affect cognitive and emotional function later in life – for example, the effects of maternal illness or early childhood neglect on learning, memory and attention later in life. Imaging tools can pay off in the classroom, too: Using such tools, literacy experts have shown that a year of intensive, methodical reading instruction makes the brains of high-risk kindergarteners look and function like those of more skilled young readers.
To aid clinical treatments, psychologists are using functional imaging to get at the neural mechanisms involved in such difficult problems as post-traumatic stress disorder, phobias and panic disorder. For example, scans reveal that schizophrenia's diverse symptoms may result not from faults in single neural components but rather from differences in webs of neural connections. Scans similarly help researchers follow brain activity to assess whether various treatments change the underlying brain dysfunction.
Critics have argued that while neuroimaging may be flashy and exciting, its contributions to psychological theory are, so far, limited. Even with expensive technologies and tools, it turns out that understanding the brain isn't as clear cut as researchers might have hoped. In the early days of brain scanning, researchers often focused on locating regions for different emotions in the brain. Over time, it became evident that emotions don't map neatly onto specific brain regions, but rather stem from a complicated network of interconnected brain regions.
Despite these complexities, experts haven't given up on neuroimaging's promise. To the contrary, the field is thriving. In 2013, the National Institutes of Health joined with several other federal agencies to launch the Brain Research through Advancing Innovating Neurotechnologies (BRAIN) Initiative. The project is slated to invest $4.5 billion over 10 years in an effort to map brain circuits, understand patterns of electrical and chemical activity in those circuits and explore how their interplay creates cognitive and behavioral capabilities.
Researchers use a variety of neuroimaging tools to study the brain.
Computed tomography (CT) scans are oblique X-ray slices that show the density of brain structures. Magnetic resonance imaging (MRI) uses changes in electrically charged molecules in a magnetic field to form images of the brain. Both technologies are more precise than ordinary X-rays and can help find problems when people fall ill. These images also help researchers map the brain regions associated with different behaviors, often by studying people with specific brain injuries.
A form of MRI known as functional MRI (fMRI) has emerged as the most prominent neuroimaging technology over the last two decades. fMRI tracks changes in blood flow and oxygen levels to indicate neural activity. When a particular brain area is more active, it consumes more oxygen and blood flow increases. Early on, fMRI was typically used to map cognitive functions in different brain regions, such as labeling areas associated with visual perception, language or memory. As the technology was refined, fMRI researchers became able to characterize brain function at the level of neural processes.
Another imaging process called diffusion tensor imaging (DTI) uses a regular MRI machine to track how water molecules move in and around the fibers connecting different parts of the brain. DTI gauges the thickness and density of the brain's connections.
Other measures of brain activity include electroencephalography (EEG), which records the brain's electrical waves to detect abnormal activity, such as in seizures and sleep disorders, and positron emission tomography (PET) scans, which use radioactive tags to show which brain areas become active when someone performs a task. PET scans are also coming into use to aid diagnosis of Alzheimer's disease by identifying amyloid plaques in the brains of living people.
Initially, neuroimaging technologies were met with great enthusiasm by some researchers, and with strong skepticism by others. After more than two decades of research, many researchers now agree that while the tools have limitations, they also have great promise. While neuroimaging shouldn't replace older methods such as behavioral studies, most experts agree that it is a powerful tool in the scientist's toolkit.
Cited research and further reading
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American Psychological Association, August 2014