The Wing-Kristofferson (1973) model provides a
widely-accepted account of isochronous time production,
such as regular tapping. Each action is generated by a
central timing process followed by a motor delay, stages
whose durations are stochastically independent. It follows
that factors that influence the two stages selectively
(such as rate and type of movement) should have effects on
the interresponse-time variance that are additive.
We report a test of this prediction.