When asked to tap a comfortable beat, the steady tempo produced will likely fall in the vicinity of 600ms. This organic beat can be referred to as a SPONTANEOUS TEMPO.
SPONTANEOUS RHYTHMICIZATION refers to how we hear a series of steady beats. Though the clicks may be identical and evenly temporally spaced, we tend to hear them in groups of twos, threes, or fours. There is a certain presupposition that what may be unmusical clicks are grouped in a musically sensible, rhythmic way.
A great example of this is the “Met and Mot” dialogue in the Bamberger reading (“Coming to Hear in a New Way”.)— Mot’s diagram in particular reflects spontaneous rhythmicization. Though Met’s diagram, based solely on long and short note values, is the empirically correct way to notate the series of beats, Mot’s diagram perhaps makes more musical sense, as he spontaneously grouped the series of beats he perceived aurally into groups of four.
I found this paper quite informative on the topics of spontaneous tempo/ subjective rhythmicization/ metricization: