Chapter 3: Observing the Brain
Multiple Choice Questions (1-10)
1. Neuroimaging techniques such as MRI, fMRI, and MEG have changed our approach to the brain in what way?
a. Neuroimaging provides a completely direct measurement of brain activities b. Neuroimaging provides a way to study the activity of individual neurons c. *Neuroimaging permits functional studies of brain areas, as well as the
connections between them.
d. Neuroimaging has not basically changed the study of the brain
2. What is a disadvantage of studying individuals with brain injuries?
a. *Brain injuries are typically not limited to a specific brain function b. It is difficult to find an individual who has a brain injury
c. Brain injuries are often easily healed
d. Brain functions are too complex to be studied in individuals with brain injuries
3. Which of the following methods has the best spatial resolution? a. Electroencephalography (EEG)
b. Magnetoencephalography (MEG) c. *Magnetic resonance imaging (MRI) d. Lesion studies
a. The whole brain can be represented by a few spikes b. It is a noninvasive procedure
c. Cells can be determined as excitatory or inhibitory based on single spikes d. *It is the most precisely localized recording method
5. Which of the following is correct about fMRI and PET?
a. fMRI and PET are both direct measures of brain activity
b. fMRI is a direct measure of brain activity while PET is an indirect measure c. fMRI is an indirect measure of brain activity while PET is an indirect measure d. *fMRI and PET are both indirect measures of brain activity
6. How do fMRI and MRI differ?
a. fMRI images the full brain while MRI images a specified portion
b. *fMRI images functional brain activity while MRI images anatomical images c. fMRI images the frontal lobe while MRI images the entire brain
d. fMRI images the final development of the brain, while MRI images developing brain
7. What is the key principle of BOLD fMRI? a. *Active brain areas consume oxygen
b. Communication between distinct hemispheres occurs c. Cortical regions interact in a feedforward manner d. Some cortical neurons have myelinated axons
8. What is a concern when using an experimental design that compares an “active” stage to one at “rest”?
a. If a person is unable to sleep, it is impossible to achieve a resting state b. Individuals that exercise regularly will have less difficulty in the “active”
requirements
c. *The brain is not truly “at rest” in the absence of an experimental task, since people are likely to be thinking of other things
d. What one person considers to be active, another may regard as restful
9. Methods such as tell researchers precisely where activity is happening while the methods of reflect more precisely when it is happening.
a. EEG and MEG, MRI and PET b. EEG and MRI, MEG and PET c. *MRI and PET, EEG and MEG d. MEG and PET, EEG and MRI
10. A key benefit of the advent of brain imaging techniques is that they allow us to a. Investigate causational relationships between cognitive processes and brain
activity
b. *Investigate aspects of cognition that were previously impossible to observe directly, such as brain areas that are active for seen versus imagined stimuli c. Record direct signals of neural activity for differing kinds of stimuli
d. All of the above
Short Answer Questions (1-3)
1. What is a voxel and what does it measure in the brain?
Individual brains therefore need individual images. MRI and CAT scans are used to take a snapshot of the three-dimensional brain at any particular moment. Figures 3.4 and 3.5 in the text show the smallest unit imaged using MRI: a “voxel.” The actual size of a voxel varies depending upon factors such as the resolution of the MRI scanner, the size of the brain being scanned, and the brain region being scanned. A typical voxel for a T1-weighted scan is about one cubic millimeter (mm3). If it is from the cortex, a single voxel may contain tens of thousands of neurons. Figure 3.6 shows a brain navigation program with a screenshot of the standard coordinate system used in most MRI research.
Comparing brain responses on a voxel-by-voxel basis may tell investigators about the neural functions of comparatively small regions of the cortex and subcortical structures.
2. Discuss the difference between structural MRI and functional fMRI? How are each used?
Magnetic resonance imaging (MRI) is the predominant anatomical imaging device used in hospitals, clinics, and research laboratories today. It's emergence only a few decades ago revolutionized both clinical and research brain investigations. MRI scanners contain strong magnets that range between 0.5-Tesla (T) up to 7-T (5000- 70,000 gauss) and beyond. MRI utilizes the magnetic properties of the tiny cells -- in the body and in the brain -- to render sharp,
precise images. A hydrogen nucleus (a single proton) in a cell is present in water, which in turn is present in all body tissues. MRI scanners make use of the magnetic properties in protons to align them using the strong magnetic field that is present in the MRI scanner (see Figure 3.7 in the text). The protons are rotated by the radio waves in the magnetic field of the scanner and detected by coils designed to collect information across the three dimensions of the area of the body or brain being scanned.
The result is a detailed 'picture' of soft tissue, bones, cartilage, etc., in whatever is being scanned.
Currently, the most popular method of studying human cognition is functional MRI (fMRI) (see Figures 3.19 and 3.20 in the text) and especially the kind that measures the oxygen level of the local blood circulation (called BOLD, for blood-oxygen level dependent activity).
When neurons fire, they consume oxygen and glucose and secrete metabolic waste products. An active brain region consumes its local blood oxygen supply, and as oxygen is consumed, we can see a small drop in the BOLD signal. In a fraction of a second, the loss of regional oxygen triggers a new influx of oxygen-rich blood to that region producing a recovery of the signal. However, as the compensatory mechanism overshoots, flooding more oxygenated blood into the area than is needed, and the signal rises high above the baseline. Finally, as unused oxygen-rich blood flushes out of the region, we can see a drop in the BOLD signal back to the baseline (Figure 3.21).
Thus, as the oxygen content of blood produces changes, we can measure neural activation indirectly. The BOLD signal comes about six seconds after the onset of neuronal firing. The relationship between neural activation and the BOLD fMRI signal is shown in Figure 3.22. Note
that these studies frequently use a block design, where a certain task is cycled on and off and the BOLD signal is measured across the ON and OFF blocks. Other experimental methods are also used in order to refine the contrasts between experimental conditions and their corresponding functional brain responses (Figure 3.23).
3. What is a single unit recording? What is the 'unit' it records?
Single unit recording entails the recording of individual 'units' -- neurons -- in the brain. Single neurons have electrical and magnetic properties, like electrical batteries. We can measure the charge left in a battery and the amount of work it can do for us. We can also measure its
magnetic field, as well as its chemistry and overall structure. Hubel and Wiesel (1962) recorded single feature-sensitive cells in the visual cortex of the cat—an achievement for which they received a Nobel Prize in 1981. More recent work has focused on recording from single neurons, clusters of neurons, and grids within the cortex using electrodes and grids of differing sizes (see Figures 3.10 and 3.11 in the text). Like every method, electrical recording of neuronal firing has its limitations, but it continues to be a major source of information.
