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BASIC LOGIC OF NEUROIMAGINGfMRI

(functional magnetic resonance imaging)

Bottom line on how it works:

Adapts MRI to register the magnetic properties of oxygenated and deoxygenated hemoglobin, allowing real-time blood flow to be imaged.

Neuroimaging vs. other methods Hemoglobin

Brilliant Insight! From Stimulus to BOLD

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BOLD Time CoursefMRI Setup

Subtraction Method Let’s be a bit more specific

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Slice Thicknesse.g., 6 mm

Number of Slicese.g., 10

SAGITTAL SLICE IN-PLANE SLICE

Field of View (FOV)e.g., 19.2 cm

VOXEL(Volumetric Pixel)

3 mm

3 mm6 mm

Slice Terminology

Matrix Sizee.g., 64 x 64

In-plane resolutione.g., 192 mm / 64

= 3 mm

Statistical Mapsuperimposed on

anatomical MRI image

~2s

Functional images

Time

~ 5 min

Time

fMRISignal

(% change)

ROI Time Course

Condition

Activation Statistics

Region of interest (ROI)

Example

Knutson, B., Wimmer, G. E., Kuhnen, C. M., & Winkielman, P. (2008). Nucleus accumbensactivation mediates the influence of reward cues on financial risk taking. NeuroReport, 19, 509-513.

Sex Sells Risk

Emotional Influence in Nevada Of course, sex works!

But how does it work?

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“Cold and dry” view: Influence via activation of evaluative concepts

“success, risk,

fun, play”, etc

Gamble?YES!

“Hot and juicy” view:

Influence via activation of core-reward processes Gamble?

YES!

Sexy Gambles

All males, Paid $20 per hour

$10.00 cash endowment. Plus their earnings (positive or negative) from gambling during the task.

54 trials

Task learned well before the scanner

Fully informed (and convinced) about non-informativeness of affective pictures

Sexy Gambles: Procedure

Dependent Measures

Behavior (% gambles)

fMRI of “reward system”

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35

40

45

50

55

60

65

70

Risky Choice Shifts to Risk

PositiveNeutralNegative

Sexy Gambles: Behavioral Results% of risky choices out of total

choicesPositive pictures enhance risky choice and shifts to risky choice

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“reward system”

Conjunction (ps<.01)

Anticipation of High vs. Low Risk Shfit

Positive vs. Negative Stimuli

Activation differences in Nucleus Accumbens (Nacc)

Mediation via “juicy” mechanisms?Anticipatory NAcc activation partially mediates the

influence of positive stimuli on subsequent shifts to the high risk option

(t-scores above paths, *p<.025, †p<.05; one

BIG PROBLEM IN PSYCHOPHYSIOLOGY:REVERSE INFERENCE

Example: Perils of Reverse Inference:New York Times Story on “Politics and the brain”

ORIGINAL

http://www.nytimes.com/2007/11/11/opinion/11freedman.html

RESPONSE

http://www.nytimes.com/2007/11/14/opinion/lweb14brain.html

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DISGUST = INSULA THEREFORE INSULA = DISGUST

BIG PROBLEM IN PSYCHOPHYSIOLOGY:REVERSE INFERENCE

Psychological state -> Physio. Change , Thus if Physio. change then Psychological state must

have occurred.

Polygraph: lying -> GSR, thus GSR = lying fMRI: mentalizing = OFC activity, thus OFC activity =

mentalizing. EMG: Happiness-> smile, thus smile->happiness

BUT: GSR = anxiety, OFC = complexity, Smile = politeness

What is the mapping between psychological and physiological domain? (cacioppo)

Many to one?

One to one?

One to Many?

Many to many!

What is the relative probability of an physiological activation given psychological event (and vice versa)

POLDRACK

MORE PROBLEMS IN PSYCHOPHYSIOLOGY:INFLATED CORRELATIONS

Many papers report very high correlation between physiological response and some behavioral measure.

Why? And How?

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Correlation between activity in a ventral MPFC cluster and ADI-R social score

r = 0.94(88% of variance!)

Correlations between the right posterior STS and ADI-R

f

Slice Thicknesse.g., 6 mm

Number of Slicese.g., 10

SAGITTAL SLICE IN-PLANE SLICE

Field of View (FOV)e.g., 19.2 cm

VOXEL(Volumetric Pixel)

3 mm

3 mm6 mm

Slice Terminology

Matrix Sizee.g., 64 x 64

In-plane resolutione.g., 192 mm / 64

= 3 mm

They did what?Why running so many correlations ‘guarantees” some very high results?

The number of tests run statistically guarantees that regardless of the true correlations present, some will be very high.

Analogy: A new test of ‘coolness” to 12 people. The test has 1,000 questions. A subset or one of these questions will correlate highly with your external

criterion. SOME of this is just due to favorable noise (accidental lining up). Thus, there is an inflation. (i.e., this question would not correlate as

highly in the next sample).

Your “winner” in the correlation race, is not as strong/fast as he seems.

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Does this matter? YES!

Science is a quest to understand causal mechanisms. Theory 1 vs. Theory 2 vs Theory 3

Misleadingly high correlation give us false leads about what mechanisms/locations are important. E.g., mirror neuron area in autism vs. other areas?

Medical conclusions and implications for treatment.

Closing

Picking out the highest correlation out of thousands of voxels leads to overestimation

Highest correlations in “social neuroscience” are often obtained in this manner:

Simple solutions exist to avoid these problems!