3.4—windows to the brain: measuring and observing brain activity

Question 1

3.4 Learning Objectives

Answer 1

• know the key terminology associated with measuring and observing brain activity.
• understand how studies of animals with brain lesions can inform us about the workings of the brain.
• apply your knowledge of neuroimaging techniques to see which ones would be most useful in answering a specific research question.
• analyze whether neuroimaging can be used to diagnose brain injuries.

Question 2

Focus Questions

Answer 2

• how can lesions help us learn about the brain?
• how can we make sense of brain activity as it is actually occurring?

Question 3

Lesioning

Answer 3

• lesioning: a technique in which researchers intentionally damage an area in the brain (a lesion is abnormal or damaged brain tissue).
• the logic of studying people who’ve suffered brain damage is that if a person has a certain part of their brain damaged and is unable to perform a particular task, it’s assumed the damaged structure plays a role in that behaviour.
• however, researchers have no control over what gets damaged so each individual will be unique (i.e. no controlled studies), and it’s difficult to isolate the effects of damage to one brain area when several are damaged.
• sham group: the control subjects that go through all of the surgical procedures aside from the lesion itself in order to control for the effects of stress, anesthesia, and the annoyance of stitches.
• e.g. researchers hypothesized that the hippocampus was vital for spatial learning.
  
  • they lesioned the hippocampus on both sides of the brains of one group of rats and performed sham surgery on the others.
  • rats put into a Morris Water Maze (container filled with opaque fluid) and had to find a platform.
  • rats with lesions to the hippocampus showed impairment in learning the location of the platform.

Question 4

Transcranial Magnetic Stimulation (TMS)

Answer 4

• transcranial magnetic stimulation (TMS): a procedure in which an electromagnetic pulse is delivered to a targeted region of the brain.
• the pulse temporarily disrupts brain activity, allowing us to study healthy human volunteers.
• interestingly, if a weaker electromagnetic pulse is delivered, TMS can be used to stimulate a brain region. (figure 3.33)
• e.g. participants were given TMS during a gambling task tended to show more cautious, risk-aversive behaviour.
• TMS has also been used to stimulate underactive areas associated with depression.

Question 5

Structural Neuroimaging

Answer 5

a type of brain scanning that produces images of the different structures of the brain.

Question 6

Computerized Tomography (CT Scan) | Structural Neuroimaging

Answer 6

• computerized tomography: (or CT scan) is a structural neuroimaging technique in which x-rays are sent through the brain by a tube that rotates around the head.
• the x-rays pass through denser tissue (like grey matter) at a different rate than less dense tissue (like fluid in the ventricles).
• a computer calculates the differences for each image that’s taken to crate a 3D image.

Question 7

Magnetic Resonance Imaging (MRI) | Structural Neuroimaging

Answer 7

• magnetic resonance imaging: (or MRI) is a structural imaging technique in which clear images of the brain are created based on how different neural regions absorb and release energy while in a magnetic field.
• step 1: brain (or other body part) is placed inside a strong magnetic field, causing the protons of H atoms to spin in the same direction.
• step 2: a pulse of radio waves is sent through the brain, and is absorbed by the atoms in the brain which knocks them out of their previous position (aligned with the magnetic field).
• step 3: the pulse of radio waves is turned off, allowing the atoms to become aligned again; but as they do, they released the energy absorbed during the pulse.
• different tissues release different amounts of energy; computers are used to calculate these differences to create a detailed 3D image.
• MRIs produce more detailed images than CT scans, but a CT scan is first used just in case the patient has metal in their brain.

Question 8

Diffusion Tensor Imaging (DTI) | Structural Neuroimaging

Answer 8

• diffusion tensor imaging: (or DTI) is a form of structural neuroimaging allowing researchers or medical personnel to measure white-matter pathways in the brain.
• most head injuries cause the brain to twist around the skull, resulting in white-matter pathways connecting the different brain areas that are torn.

Question 9

Functional Neuroimaging

Answer 9

• functional neuroimaging: a type of brain scanning that provides information about which areas of the brain are active when a person performs a particular behaviour.
• tradeoff between temporal resolution (how brief a period of time can accurately be measured) and spatial resolution (a clear picture of the brain).

Question 10

Electroencephalogram (EEG) | Functional Neuroimaging

Answer 10

• electroencephalogram: (or EEG) measures patterns of brain activity with the use of multiple electrodes attached to the scalp.
• has fantastical temporal resolution because it measures the firing of neurons every millisecond.
• used to detect when patients with epilepsy are having a seizure; sudden spike in activity (neuronal firing) in one or more brain areas.
• researchers are interested in how brain responses differ for different types of stimuli; but how do you link EEG output with your stimuli?
• event potentials: (or ERPs) take note of exactly when a given stimulus was presented to the participant.
  
  • EPRs allow the researcher to examine the EEG readout for a brief period of time (1-2 seconds) after the appearance of the stimulus.
  • researchers can then collect the average brain responses for different types of experimental trials.
  • limited because they can’t tell you where activity is taking place.
• researchers can look at the size of the peaks and valleys (waveforms) to determine whether there was a difference in the amount of brain activity in response to the different stimulus types.
• if a patient (e.g. someone with multiple sclerosis) was missing an expected waveform, the neurologist could conclude that a particular region of their brain was not functioning normally.

Question 11

Magnetoencephalography (MEG) | Functional Neuroimaging

Answer 11

• magnetoencephalography: (or MEG) is a neuroimaging technique that measures the tiny magnetic fields created by the electrical activity of nerve cells in the brain.
• like the EEG, MEG records the electrical activity of nerve cells milliseconds after it occurs, allowing researchers to record brain activity at nearly the instant a stimulus is presented.
• cannot provide a detailed picture of the activity, so can’t tell you where it happened.

Question 12

Positron Emission Tomography (PET) | Functional Neuroimaging

Answer 12

• positron emission tomography: (or PET) is a type of scan in which a low level of a radioactive isotope is injected into the blood, and its movement to regions of the brain engaged in a particular task is measured.
• works under the assumption that active nerve cells use up energy at a faster rate than less active cells, causing more blood to flow to the active areas.
• greatest strength is that they show metabolic activity of the brain.
• allows researchers to measure the involvement of specific types of receptors in different brain regions while people perform a task.
• limited because they take about 2 minutes to gather data, so you can’t see movement-by-movement activity of the brain.
• also only usually used on men due to radioactivity (women may be in early stages of pregnancy).

Question 13

Functional Magnetic Resonance Imaging (fMRI) | Functional Neuroimaging

Answer 13

• functional magnetic resonance imaging: (or fMRI) measures brain activity by detecting the influx of oxygen-rich blood into neural areas that were just active.
• used to study sensory processes, memory, social behaviour, psychological disorders, and disorders of consciousness.
• when the brain works, it uses oxygen, so the body must send oxygen-rich blood to the brain while pumping deoxygenated blood away; these two types of blood have different magnetic properties, which can be measured.
• in one study, participants looked at happy and fearful faces in an fMRI.
• seeing faces lit up an area in the bottom right hemisphere known as the fusiform gyrus.
• fearful expressions activated the amygdala, and happy faces activated a wide network of structures in the frontal lobes.
• fMRIs are limited we can see that the activity is correlated, but we don’t know if it’s the cause.
• as well, just because an area lights up doesn’t mean it’s necessary.