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. Aftertaste is a synonym for retronasal olfaction i.e. backwards-nose-smelling
When strength becomes overpowering or insipidly weak, the transparency of flavour is lost. Transparency, when balanced, is a positive — a balanced transparent coffee has easily recognised flavour associations. At extremely high TDS concentrations, the transparency is lower due to our senses being overwhelmed. In the coffee world, the overwhelming majority of quality assurance tasting is conducted between the 1–1.5% TDS range. We think this is due to the flavour transparency found within this range.
Espresso, however, is an unusually strong drink in our diet. Coffee scientist, Chris Hendon, observes in a recent article from The Conversation, “There are a limited number of technologies that achieve 8–10% concentrations, the espresso machine being the most familiar.” This is not to say an espresso won’t perhaps convey an intense blueberry flavour; espresso beverages have around ten times more TDS than filter, so each sip has ten times more volatile gases available for your olfactory system to detect.