1. Chapter 4
The Imaged Brain

2. Brain Reading?
To what extent can we tell what someone is thinking by monitoring momentary changes in their brain?
What do techniques such as fMRI actually measure?
What are the limitations and positives of the various methods?

3. Structural Imaging CT related to X-ray absorption
Figure shows CT and two types of MR image

4. Structural Imaging (cont.)
Advantages of MRI over CT
Does not use ionizing radiation
Better spatial resolution (e.g. folds of individual gyri)
Better discrimination between white matter and gray matter
Adapted for use in fMRI

5. Basic of MRI Physics

6. Linking Structure to Function
Voxel-based morphometry (VBM)
Measures white and gray matter density in each voxel
Can look for differences between groups or correlations with cognitive measures

7. Linking Structure to Function (cont.)
Diffusion Tensor Imaging (DTI)
Measures white matter organization
based on limited diffusion of water molecules in axons
Image by Thomas Schultz from
commons/8/82/DTIsagittal-fibers.jpg.

8. Functional Imaging
Neural activity consumes oxygen as well as generating electrical signals
In order to compensate for increased oxygen consumption, more blood is pumped into the active region
PET measures the blood flow in a region, whereas fMRI measures the blood oxygenation
The time taken for this response is slow (several seconds) and so functional imaging has a poor temporal resolution, but a good spatial resolution
This is the complementary profile to ERPs

9. Positron Emission Tomography (PET)
Measures local blood flow (rCBF)
Radioactive tracer injected into blood stream
Tracer takes up to 30 seconds to peak
Catherine Pouedras/Science Photo Library

10. Functional Magnetic Resonance Imaging (fMRI)
Over the last ten years functional magnetic
resonance imaging (fMRI) has overtaken PET scans in functional imaging experiments.

11. Functional Magnetic Resonance Imaging (fMRI) (cont.)
Does not use radioactivity, but directly measures the concentration of deoxyhemoglobin in the blood
This is called the BOLD response (Blood Oxygen Level Dependent contrast)
The change in BOLD response over time is called the hemodynamic response function and it has a number of distinct phases (not to be confused with the ERP waveform, which is completely unrelated)
The Hemodynamic Response Function peaks in 6–8 seconds. This limits the temporal resolution of fMRI

12. Functional Magnetic Resonance Imaging (fMRI) (cont.)
Hemodynamic response function (change in BOLD signal over time)

13. PET v. fMRI

14. What Does it Mean to Say a Brain Region is "Active"?
The brain has a constant supply of blood and oxygen; if it didn’t, it would die
This means we cannot literally stick someone in a scanner and read their thoughts (because the whole brain would look active)
In order to infer functional specialization, one needs to compare RELATIVE differences in brain activity between two or more conditions
This involves selecting a baseline or comparison condition
A region is "active" if it shows a greater response in one condition relative to another
If the experimenter chooses inappropriate conditions the regions of activity will be meaningless (junk in, junk out) – functional imaging isn’t foolproof

15. Peterson et al. (1988): PET Study

16. Peterson et al. (1988): PET Study

17. Factorial Designs Why can’t we tickle ourselves?
Bottom diagram adapted from Blakemore et al. (1998). Copyright © 1998 Elsevier. Reproduced with permission.

18. Parametric Designs
Different brain regions show different response profiles to different rates of word presentation. Adapted from Price et al. (1992), and Friston (1997).

19. Event v. Block Designs

20. Event-related designs
If stimuli need to be presented randomly
If stimuli are infrequent
If conditions defined by participant (e.g. correct/incorrect trials; presence of an hallucination - see right)

21. Which Design?

22. Analysing Functional Imaging Data

23. Analysing Functional Imaging Data
Smoothing
Increases signal:noise ratio
Compensates for individual differences in anatomy

24. Interpreting Functional Imaging Data
Inhibition v. Excitation (not to be confused with activation/deactivation)

25. Interpreting Functional Imaging Data (cont.)
Necessity v. sufficiency (i.e. is there a causal relationship between activity and behavior/cognition?)

26. Disagreements Between Imaging and Lesion Studies

27. Disagreements Between Imaging and Lesion Studies (cont.)

28. Disagreements Between Imaging and Lesion Studies (cont.)

29. Lie Detection
Traditional polygraph measures bodily response (e.g. sweating, heart rate), but what if someone doesn’t feel guilty?
Brain is the organ that creates the lie
Anterior cingulate cortex active when asked to generate false answers to questions relative to truthful ones (e.g. “where was your last vacation?”)
BUT, not necessarily active if lie is memorized in advance

30. Is Brain Reading Possible?
Most experiments manipulate cognitive processing and measure changes in brain response
Can the reverse be done? In other words, can the brain response be used to infer what people are thinking?
Brain response can be used to predict which object is seen or imagined from a limited set (e.g. face v. house v. cats v. shoes) with ~90% accuracy
However, can only be done if experimenter pre-tests the participants on these items (to know where to look in that particular person)

31. Limits on Brain Reading
Each brain is subtly different anatomically
Always need a baseline for comparison―can’t infer from brain activity alone
May be able to measure general contents of thought (e.g. a face) rather than specific contents (e.g. Tony Blair’s face)
Not easy to distinguish between conscious and unconscious cognition just from looking at the brain