as evoked potentials (EP), they show electroencephalography (EEG) activity in relation to a particular event
evoked potentials (EP): also known as event-related potentials (ERPs), they show electroencephalography (EEG) activity in relation to a particular event
Evoked Potentials
Event-related potentials (ERPs), also known as evoked potentials (EP), show EEG activity in relation to a particular event. Imagine taking a continuous EEG signal during which a picture or tone is presented to an individual a number of times. If we were to take the EEG in the half-second following the stimulus presentation and average these together, we would have the brain response to the stimulus (see Figure 2.13).
The waveform of the ERP is described in terms of positive and negative peaks and the time elapsed from the stimulus presentation. Thus, a P300 waveform is a peak in the ERP in the positive direction occurring 300 milliseconds after the stimulus presentation. Based on early recording equipment characteristics, positive peaks are often shown pointing downward and negative peaks upward. For simplicity, P300 is sometimes referred to as P3, since it represents the third positive peak following a stimulus presentation. Thus, one sees both N1 or P3 as well as N100 or P300 in the literature.
Evoked potentials offer a view of cognitive and emotional processing that takes place in the brain outside of awareness. They are also useful in groups such as infants who cannot respond verbally. In one study, evoked potentials were recorded from 7-month-old infants as they saw faces with emotional expressions. A stronger reaction was seen at around 400 milliseconds when they saw a fearful face as opposed to a happy face (Taylor-Colls & Pasco Fearon, 2015). A common use of evoked potential research in terms of mental disorders has been to show how cognitive and emotional processing differs for those with a disorder and those without. Evoked potentials have also been used to distinguish those with schizophrenia from those without (Laton et al., 2014). In addition, evoked potentials have shown that children with ADHD (attention deficit/hyperactivity disorder) show different types of evoked potential components from those with autism spectrum disorder (Tye et al., 2014).
Figure 2.12 Wavelet Analysis Associated With Seeing the Dalmatian Dog
Source: Reprinted from Catherine Tallon-Baudry & Olivier Bertrand, Oscillatory Gamma Activity in Humans and Its Role in Object Representation, Trends in Cognitive Sciences, Vol. 3, pp. 151–162, Copyright © 1999, with permission from Elsevier.
magnetoencephalography (MEG): brain imaging technique that measures the small magnetic field gradients exiting and entering the surface of the head that are produced when neurons are active
Magnetoencephalography
Magnetoencephalography (MEG) measures the small magnetic field gradients exiting and entering the surface of the head that are produced when neurons are active. It uses a SQUID (superconducting quantum interference device) to detect small magnetic activity that results from the activity of neurons. As shown in the photo, the person simply puts his head in a device that contains magnetic sensors.
Figure 2.13 Evoked Potentials Are Created by Averaging Periods of EEG
Source: Reprinted from Steven J. Luck, Geoffrey F. Woodman, & Edward K. Vogel, Event-Related Potential Studies of Attention, Trends in Cognitive Sciences, Vol. 4, pp. 432–440, Copyright © 2000, with permission from Elsevier.
MEG signals are similar to EEG signals but have one important advantage. This advantage stems from the fact that magnetic fields are not distorted when they pass through the cortex and the skull. This makes it possible to be more accurate in terms of spatial location of the signal with MEG. For example, youth with bipolar disorder show greater activation in the frontal gyrus and less in the insula following negative feedback than do control participants (Rich et al., 2011).
positron emission tomography (PET): a brain imaging technique that measures the blood flow in the brain that is correlated with brain activity
Positron Emission Tomography
Positron Emission Tomography (PET) is a measure related to blood flow in the brain that reflects cognitive processing. PET systems measure variations in cerebral blood flow that are correlated with brain activity. It is through blood flow that the brain obtains the oxygen and glucose from which it gets its energy. By measuring changes in blood flow in different brain areas, it is possible to infer which areas of the brain are more or less active during particular tasks. Blood flow using PET is measured after participants inhale, or are injected with, a tracer (a radioactive isotope) that travels in the bloodstream and is recorded by the PET scanner (a gamma ray detector). Figure 2.14 depicts a PET scan in which individuals with schizophrenia show less metabolism in the frontal lobes as compared with healthy controls (Buchsbaum & Haier, 1987).
Magnetoencephalography measures brain activity by measuring small magnetic fields produced in the brain.
National Institutes of Mental Health
The general procedure is to make a measurement during a control task that is subtracted from the reading taken during an experimental task. Although it takes some time to make a PET reading, which reduces its value in terms of temporal resolution, it is possible to determine specific areas of the brain that are active during different types of processing. Since PET can measure almost any molecule that can be radioactively labeled, it can be used to answer specific questions about perfusion, metabolism, and neurotransmitter turnover.
Figure 2.14 Comparing Positron Emission Tomography Scans
Source: Buchsbaum, M. S., & Haier, R. J. (1987), Functional and Anatomical Brain Imaging: Impact on Schizophrenia Research, Schizophrenia Bulletin, 13(1), 115–132.
Some of PET’s main disadvantages include expense; the need for a cyclotron to create radioactive agents; the injection of radioactive tracers, which limit the number of experimental sessions that can be run for a given individual; and limited temporal resolution. Due to risks associated with exposure to the radioactive tracer elements in a PET study, participants typically do not participate in more than one study per year, which limits the degree to which short-term treatment efficacy can be studied. With the development of fMRI, PET is no longer the technique of choice for research studies in psychopathology.
However, PET does offer an advantage for studying specific receptors such as dopamine receptors in the brain, which are particularly active in those with an addiction or inactive in those with Parkinson’s disease. Another study used PET to examine serotonin in those with social anxiety disorder (A. Frick et al., 2015). An overactive serotonin system was found at the synaptic level in those with social anxiety disorder as compared to matched controls.
functional magnetic resonance imaging (fMRI): a brain imaging technique that measures increased blood flow in active areas of the cortex by determining the ratio of hemoglobin with and without oxygen
diffusion tensor imaging (DTI): procedure that uses the magnetic resonance imaging (MRI) magnet to measure fiber tracts (white matter) in the brain
Functional Magnetic Resonance Imaging
Functional magnetic resonance imaging (fMRI) is based on the fact that blood flow increases in active areas of the cortex. Specifically, hemoglobin, which carries oxygen in the bloodstream, has different magnetic properties before and after oxygen is absorbed.