The human brain simply may not be wired up to deal with lots of different levels of value. A series of psychological experiments, many dating back to the 1950s, shows that we cannot distinguish between more than about five degrees of, well, almost anything: sweetness in a solution; saltiness; the pitch of a note; brightness; the intensity of an electric shock; the length of a line; or the pungency of a smell. The details vary, but the level of consistency is surprising.
Practice does not help. Neither, surprisingly, does varying the gaps in the scale: it’s no easier to distinguish five sounds between “very loud” and “very quiet” than between “fairly loud” and “fairly quiet”. Some people have perfect pitch and can transcend these limits when it comes to musical tones, but there seem to be few other exceptions. No wonder so many reviews use a scale of one to five stars.
Nick Chater, a psychologist at University College London, argues that the human brain doesn’t have an internal scale for these stimuli, nor for “utility” or “value”. Instead the brain makes comparisons: that light was brighter than the previous light. We can just about wrap our minds around the idea of “much brighter” by comparing a recent gap in brightness with some previous gap in brightness.
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