Neuroimaging vs. other methods

Transcription

1 BASIC LOGIC OF NEUROIMAGING fmri (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 1

2 BOLD Time Course fmri Setup Subtraction Method Let s be a bit more specific 2

3 Slice Terminology Slice Thickness e.g., 6 mm SAGITTAL SLICE Number of Slices e.g., 10 VOXEL (Volumetric Pixel) In-plane resolution e.g., 192 mm / 64 = 3 mm 3 mm 6 mm IN-PLANE SLICE 3 mm Activation Statistics Functional images ~2s fmri Signal (% change) ROI Time Course Time Condition Statistical Map superimposed on anatomical MRI image Matrix Size e.g., 64 x 64 Field of View (FOV) e.g., 19.2 cm Time Region of interest (ROI) ~ 5 min Example Sex Sells Risk Knutson, B., Wimmer, G. E., Kuhnen, C. M., & Winkielman, P. (2008). Nucleus accumbens activation mediates the influence of reward cues on financial risk taking. NeuroReport, 19, Emotional Influence in Nevada Of course, sex works! But how does it work? 3

4 Cold and dry view: Influence via activation of evaluative concepts Gamble? YES! Hot and juicy view: Influence via activation of corereward processes Gamble? YES! success, risk, fun, play, etc Sexy Gambles Sexy Gambles: Procedure 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 noninformativeness of affective pictures Dependent Measures Behavior (% gambles) fmri of reward system Sexy Gambles: Behavioral Results % of risky choices out of total choices Risky Choice Shifts to Risk Positive Neutral Negative Positive pictures enhance risky choice and shifts to risky choice 4

5 reward system Activation differences in Nucleus Accumbens (Nacc) Positive vs. Negative Stimuli Anticipation of High vs. Low Risk Shfit Conjunction (ps<.01) 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 RESPONSE 5

6 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? 6

7 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 Terminology SAGITTAL SLICE Slice Thickness e.g., 6 mm VOXEL (Volumetric Pixel) In-plane resolution e.g., 192 mm / 64 = 3 mm 3 mm 6 mm IN-PLANE SLICE 3 mm Number of Slices e.g., 10 Matrix Size e.g., 64 x 64 Field of View (FOV) e.g., 19.2 cm 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. 7

8 Does this matter? YES! Closing Science is a quest to understand causal mechanisms. Theory 1 vs. Theory 2 vs Theory 3 Picking out the highest correlation out of thousands of voxels leads to overestimation 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. Highest correlations in social neuroscience are often obtained in this manner: Simple solutions exist to avoid these problems! 8