Chapter 4 The Imaged Brain

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?

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

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

Basic of MRI Physics

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

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 http://upload.wikimedia.org/wikipedia/ commons/8/82/DTIsagittal-fibers.jpg.

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

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

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

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

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

PET v. fMRI

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

Peterson et al. (1988): PET Study

Peterson et al. (1988): PET Study

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

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

Event v. Block Designs

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)

Which Design?

Analysing Functional Imaging Data

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

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

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

Disagreements Between Imaging and Lesion Studies

Disagreements Between Imaging and Lesion Studies (cont.)

Disagreements Between Imaging and Lesion Studies (cont.)

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

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)

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