Does the head follow the heart in matters of truth?

[Angrboda]

In case I wasn't clear, the heart/head dichotomy was just a metaphor for the dichotomy between non-rational and rational processes of belief formation and development.  In that sense, no, one could not be biased and still have the head do all the talking as it is a part of the definition of bias that it is not a rational process/influence.

So, if I understood you correctly, by "non-rational", you mean "driven by biases overruling logic/reason" rather than "emotional". So this means one can't be "biased for logic/reason" in this sense. Fair enough.

To be completely clear, I'm including both the effects of emotion and feelings as well as cognitive biases, as both are non-rational influences on our decision making and reasoning.