Olfaction & Gustation
These two terms refer to how we perceive flavour. Your olfactory bulb, at the top of your soft palate, detects chemicals in smells, while your gustatory system uses taste buds to detect flavour. Your gustatory system travels along interrelated neural pathways with your olfactory system. When brewing at high concentrations, bitter chemicals can be interpreted by your brain as harmful substances.
If you want to dive into the physiological side of flavour perception, here is a good place to start.
Note: One particularly helpful thing in this diagram is how it depicts aftertaste in action.