fMRI Methods

Lecture 12 – Random stuff…

Correlation/Covariance

The idea with correlation is that two variables are related, enabling you to predict one from the other.

Implies, but does not prove causality.

Tricky business:

Rat

e of

aut

ism

Rate of atheists in the population

Correlation with behavior

1. Explain between-trial variability in behavior (reaction time, accuracy, memory, decision making variables, fear, bravery) with response amplitude.

2. Explain between-subject variability in behavior with response amplitude.

In both cases: look for brain areas that predict behavior. Places where the extent of behavior is encoded by strength of response (e.g. fear in Amygdala).

Example

Mirror system responses in autism:

Dapretto et. al. Nat. Neurosci 2006

Spurious correlations?

Personality measure reliability ~ 0.8fMRI measure reliability ~ 0.7

Correlation between the two should be < 0.74

Spurious correlations?64,000 voxels and multiple subjects…

Lack of independence

Solutions/Approaches

1. Test for correlations in ROIs defined based on other criteria.

2. Split your data into two halves and show reproducibility.

Examples

Identify ROI that responds to faces using one scan.

Determine whether responses are correlated with face familiarity in this ROI and not in other ROIs using data

from another scan.

Say a word about correlations in rest scans

Planning experiments

Number of subjects and trials - power analysis

Caveats and proper controls – Signal and noise, statistical interpretation.

Auditory experiments

Issues with sound presentation.

Auditory system and language.

To the lab!