TL;DR
Psychological theories often deal with mental states.
Mental states are hard to objectively measure.
Let's use brain-decoding to measure them.
So difficult.
Example
Theory: people don't like waiting for things because future outcomes are not vividly imagined.
Test: check if people's imagination vividness is decreased when they think of the future.
Difficulty: how do you measure 'imagination vividness'?
Solution: build a brain decoder of imagination vividness.
Result: when people see future rewards, their imagination vividness is low as measured by the brain decoder.
Paper: A neural signature of the vividness of prospective thought is modulated by temporal proximity during intertemporal decision making. [PDF]
Ongoing challenges
1) Constructing a high-dimensional brain model is hard.
We don't have that many brain images
It takes a long time to build models
It's hard to interpret the model
Paper: Fast construction of interpretable whole-brain decoders. [PDF]
2) Is our decoder working properly?
How do we know if our decoder is predicting the mental state that we want and not something else?
If our decoder is confounded, how do we fix it?
Paper: Distinguishing deception from its confounds by improving the validity of fMRI-based neural prediction. [PDF]