The human brain when fully engaged can burn a whopping amount of calories. Throw in other factions of the human brain - memory, creativity, thought and analysis, derivations, imagination and visualization, multitasking, mathematics and logic, left side use, right side use, Cortex engagement, and the burn factor can shoot the roof.
This is obviously less and even minuscule for the less fortunate less engaged people with less brain power (who often go through life using a mere 2% of brain capacity) and far more in people who have learned to use substantial and significant amounts of their brain.