This assumption is a primary conceit of most neuro research.
The evidence to support it is weak at best, contradictory at worst.
Increased firing without an attached metabolic effect, that is without a specific peptide/protein/chemical reaction, is spinning a wheel. It’s a pulse with no message.
We have a significant body of evidence that demonstrates increased activation coupled with heterogeneous functional result.
There’s a significant body of evidence which shows functional activity takes place without any electro-chemical gradient trigger.
The assumption that activation equates to functional differences is likely the reason MRI and EEG are so shitty with regard to consistent, replicable results. We’re assuming that they validate something they don’t.