A2 Only: Ways of studying the brain: scanning techniques, including functional magnetic resonance imaging (fMRI); electroencephalogram (EEGs) and event-related potentials (ERPs); post-mortem examinations.
Brain-scanning techniques (CAT, PET, and fMRI)
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 organise 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 behaviour 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.
Neuroimaging, or brain scanning, includes the use of various techniques to either directly or indirectly image the structure, function, or pharmacology of the brain. A wide variety of brain scanning techniques exist. These are used to provide biological data, rather than psychological. Scans have scientific purposes. They are commonly used to investigate for possible tumours, strokes or other abnormalities. However, they can be used as research methods too, such as aiding psychologists into understanding of how information is processed. Psychologists and scientists are also using brain scans as research methods, to investigate both normal differences between brains (such as differences between a male and female brain) and abnormal differences (such as differences between the brain of a murder and a non-murderer). Neuroimaging falls into two broad categories:
- Structural imaging, which deals with the structure of the brain and the diagnosis of large-scale intracranial disease (such as a tumor), as well as injury.
- Functional imaging, which is used to diagnose metabolic diseases and lesions on a finer scale (such as Alzheimer’s disease), and also for neurological and cognitive-psychology research. Functional imaging allows the brain’s information processing to be visualized directly, because activity in the involved area of the brain increases metabolism and “lights up” on the scan.
Here is a fantastic timeline of the history of neuroimaging. Click on the image below for a full-size PDF.
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 behavioural 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.
Key Types of Brain Imaging
The fMRI is a series of MRIs that measures both the structure and the functional activity of the brain through computer adaptation of multiple images. Specifically, the fMRI measures signal changes in the brain that are due to changing neural activity. In an fMRI, a patient can perform mental tasks and the area of action can be detected through blood flow from one part of the brain to another by taking pictures less than a second apart and showing where the brain “lights up.” For example, when a person processes visual information, blood rushes to the back of the brain, which is where the occipital lobe is located. FMRIs make it possible to show when things happen, how brain areas change with experience, and which brain areas work together. They have been used to study a wide range of psychological phenomena, including the neural activity of telling a lie, the differences between novices and experts when playing a musical instrument, and what happens inside our heads when we dream.
- Advantages: Revolution in brain scanning and mapping. Used for diagnoses, assistance in medical procedures, and as part of neuroscientific research. fMRIs are widely available and non-invasive. They do not use ionizing (harmful) radiation, making them superior to CT scans. Refinements have been made to improve existing MRI technology and continued future improvements are expected to continue in accordance with Moore’s law.
- Disadvantages: Can promote bouts of claustrophobia for certain individuals due to the fact that the scan is conducted in extremely tight spaces. May also cause hearing loss due to the loudness of the noise generated over an extended period of time. One of the major accusations aimed at neuroimaging is that fMRI (functional magnetic resonance imaging) data does not represent the neural processes themselves, which are of a more miniscule nature than scanners can capture, but rather the flow of oxygenated blood. The claim is that there is no ground for causal inference from blood flow to psychological processes.
A PET scan (positron emission tomography) is used to look at function. It studies brain activity levels and can be used to look for evidence of a stroke. It involves injecting a radioactive tracer into the bloodstream with a chemical used by the body, such as glucose, to see where most of the blood is flowing. The radioactive particle emissions (positrons) from the tracer give signals which are recorded so levels of activity in different parts of the brain can be detected. Greater levels of brain activity appear on the scan as different colours.
Participants are scanned in two conditions – when inactive (to provide a baseline measure) and when performing an activity. There difference between the two scans shows which part of the brain is being used. The PET scan allows the study of areas of activity within the brain when stimuli such as faces or names are shown; but also the study of memory and looking at sufferers of schizophrenia or epilepsy.
Electroencephalography (EEG) is used to show brain activity under certain psychological states, such as alertness or drowsiness. It measures the electrical activity of the brain by recording from electrodes placed on the scalp. The resulting traces are known as an electroencephalogram (EEG) and represent an electrical signal from a large number of neurons. Electroencephalography (EEG) is used to show brain activity in certain psychological states, such as alertness or drowsiness. It is useful in the diagnosis of seizures and other medical problems that involve an overabundance or lack of activity in certain parts of the brain. EEGs are frequently used in experimentation because the process is non-invasive to the research subject. The EEG is capable of detecting changes in electrical activity in the brain on a millisecond-level. It is one of the few techniques available that has such high temporal resolution.
To prepare for an EEG, electrodes are placed on the face and scalp. After placing each electrode in the right position, the electrical potential of each electrode can be measured. According to a person’s state (waking, sleeping, etc.), both the frequency and the form of the EEG signal differ. Patients who suffer from epilepsy show an increase of the amplitude of firing visible on the EEG record. The disadvantage of EEG is that the electric conductivity—and therefore the measured electrical potentials—may vary widely from person to person and also over time, due to the natural conductivities of other tissues such as brain matter, blood, and bones. Because of this, it is sometimes unclear exactly which region of the brain is emitting a signal.
A famous psychological study using EEG is Dement & Kleitman’s study of REM vs NREM sleep, which you can read about here.
- Advantages: Collects real-time data of the brain’s electrical activity in various regions to the nearest millisecond. Can be used to assess neuroelectrical abnormalities within the brain at a low cost. May be used as an adjunct diagnostic tool for neurological conditions and is especially helpful for diagnosing epilepsy. It is non-invasive, painless, and can be combined with other forms of brain scanning such as fMRI or PET. Is also a helpful research tool in the field of neuroscience where it provides high resolution without exposure to radiation or magnetic fields.
- Disadvantages: Limited to collecting electrical activity unless combined with another scan. Doesn’t measure activity below the cortex and cannot pinpoint neural activation of regions within the brain nor determine neurotransmission. It sometimes takes awhile for an individual to connect to an EEG machine with electrodes and various pastes to keep them in place.
The EEG proved to be a useful source in recording brain activity over the ensuing decades. However, it tended to be very difficult to assess the highly specific neural process that are the focus of cognitive neuroscience because using pure EEG data made it difficult to isolate individual neurocognitive processes. Event-related potentials (ERPs) offered a more sophisticated method of extracting more specific sensory, cognitive, and motor events by using simple averaging techniques. By recording small potential changes in the EEG signal immediately after the presentation of a sensory stimulus it is possible to record specific brain responses to specific events. This method is called Event-Related Potentials (ERPs) and is one of the classic methods for investigation of psychophysiological states and information processing.
Typical electrode montage for EEG/ERP recordings (mid-picture) and examples of EEG tracings separated for different frequency bands (left picture) and ERP tracings to repeated auditory stimuli (right picture).
ERPs can be reliably measured using electroencephalography (EEG). The EEG reflects thousands of simultaneously ongoing brain processes. This means that the brain response to a single stimulus or event of interest is not usually visible in the EEG recording of a single trial. To see the brain’s response to a stimulus, the experimenter must conduct many trials and average the results together, causing random brain activity to be averaged out and the relevant waveform to remain, called the ERP.
The random (background) brain activity together with other bio-signals and electromagnetic interference (e.g., line noise, fluorescent lamps) constitute the noise contribution to the recorded ERP. This noise obscures the signal of interest, which is the sequence of underlying ERPs under study.
Physicians and neurologists will sometimes use a flashing visual checkerboard stimulus to test for any damage or trauma in the visual system. In a healthy person, this stimulus will elicit a strong response over the primary visual cortex located in the occipital lobe, in the back of the brain.
ERP component abnormalities in clinical research have been shown in neurological conditions such as:
- Parkinson’s disease
- multiple sclerosis
- head injuries
- obsessive-compulsive disorder
Studies of the postmortem human brain have become an increasingly essential element of the effort to understand the neurobiology of psychiatric disorders, especially in light of advances in our knowledge of functional brain circuitry. Studying the postmortem human brain is going straight to the source – what you see is a real snapshot of brain chemistry. Considering this point, any findings that come from the postmortem human brain are valuable and insightful.
The direct study of the postmortem human brain provides several essential elements in the study of psychiatric disorders that are not currently, and that are not likely to be in the foreseeable future, available through other approaches. For example, although in vivo neuroimaging studies provide insights into critical areas (e.g., regional patterns of functional activation and volumetric changes across time) that cannot be assessed in postmortem material, only postmortem studies provide the resolution required for the characterisation of psychiatric disorders at the level of populations of neurons and the specific neural circuits that they form. That is, at present, disease-related alterations in local and distributed neural circuits, and at the cellular, synaptic and molecular levels, can only be detected through the direct study of brain tissue. Similarly, as susceptibility genes are identified for psychiatric disorders, postmortem tissue studies will provide an indispensable means for determining how those genetic liabilities are converted into altered expression of gene products.
- Advantages: only postmortem studies provide the resolution required for the characterisation of psychiatric disorders at the level of populations of neurons and the specific neural circuits that they form.
- Disadvantages: It may be difficult to obtain clinical information on an individual prior to post-mortem examination; for example of individuals who die through suicide. Information is also limited to patients who have been seen by the medical profession. One way around this can be through “psychological autopsies” which involve structured interviews of surviving relatives and friends and directed questioning of health care providers. Here is a more in-depth article on the disadvantages of post-mortem studies.
Criticisms of Neuroimaging Studies
- They do not directly measure neural activity. For example, fMRI doesn’t measure brain activity directly, it only measures blood oxygen.
- Different groups of researchers adopt different statistical and methodological strategies.
- Another charge against neuroimaging is that brain images do not explain but only localise psychological processes. Even though it is true that studies of localisation of certain psychological functions are still published occasionally, most of modern neuroimaging research is not exclusively concerned with finding an area activated when faced with a certain task.
- Also, neuroimaging allows for determining which processes require coordinated activity of different parts of the brain: functional connectivity analysis reveals which parts are working together when faced with a given cognitive task. Since most psychological processes are caused by coordinated activity of several brain areas, this provides insight into the psychological process itself and not only into which parts of the brain are active during its course.
Brain images that we usually see are comprised of thousands of 3-D cubes called “voxels”. Each voxel contains millions of cells, hence brain activity in fMRI is represented in voxel and not in neuronal level. When we test, for example, 50.000 voxels and set significance level to exactly one twentieth (0.05), we can expect 2.500 voxels to cross the level of significance by chance alone. However, if we make the significance level too low, we will not be able to observe any difference at all. This problem is known asmultiple comparisons problem, and it is a serious shortcoming in statistical analysis of neuroimaging data.
One humorous illustration of the problem is an experiment where a dead salmon was placed in an MRI (magnetic resonance imaging) scanner and instructed to think about the emotions experienced by people in photographs that were shown. With significance level set below 0.001, certain parts of dead salmon’s brain were found to engage in perspective-taking activity. However, authors of the publication also devised a solution which did not yield the same results. This shows that the problem itself is an obstacle that is to be overcome by methodological refinement – a natural step in any scientific field.
Here is a summary of the dead salmon study.
Transcranial magnetic stimulation (TMS) is a recent innovation in brain imaging. In TMS, a coil is held near a person’s head to generate magnetic field impulses that stimulate underlying brain cells to make someone perform a specific action. Using this in combination with MRI, the researcher can generate maps of the brain performing very specific functions. Instead of asking a patient to tap his or her finger, the TMS coil can simply “tell” his or her brain to tap his or her finger. This eliminates many of the false positives received from traditional MRI and fMRI testing. The images received from this technology are slightly different from the typical MRI results, and they can be used to map any subject’s brain by monitoring up to 120 different stimulations. This technology has been used to map both motor processes and visual processes (Potts link at bottom of TMS). In addition to fMRI, the activation of TMS can be measured using electroencephalography (EEG) or near infrared spectroscopy (NIRS).
Here is a brilliant timeline of landmarks in functional brain imaging.
Here is a great example of brain scanning in action: The Love Competition