IOI around 400ms was already described as the first fully-fledged "beat
specific" isotempo, as it was descibed as corresponding to the "first
sustainable rate of attention-shift" and the first pace that we can
imagine as a continuous row of thesis pulsations (like a 1/X bar).
No psychology textbook helped me disentangle the reasons why our brain
processes IOI 400ms (around) in this peculiar way – so I had to devise
my own experiments.
Whenever I felt unsure about producing with accuracy a IOI around 400ms
isotempo (that used to happen long ago, rest assured), I submitted it
to the zig-zag test:
"In order not to imply in the experiment ocular movements, we
shall imagine mentally any similar zig-zag trying then to pass from one
point to another (A, B, C, D, E, F, G, H etc.) is such a way that
consecutive stops do not create, from a temporal point of view,
subjective thesis-arsis relationships (nor binary neither compound) as
we should perceive any stop as experientially equal to any other stop.
Moreover, we shall make sure that each stop implies a thorough change
of the object of consciousness (i.e. a complete transfer of attention).
Upon these conditions, the next step is to find out (and measure), from
a psychophysical point of view, which would be the maximum rate of
passing from one point in the zig-zag to another. The result should
indicate an IOI value situated around 420-430ms. The absolute memory
(i.e. skipping the experiment just described, for that implies
relational memory) for this isochronous tempo can be achieved by
practice."
(Quoted from my article A Matter of Perspective)
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