remaining pairs fail the ISI test
     or (2) the connection strength drops below a (heuristic) cutoff of 0.01.

     The ISI test needs an idea of the expected refractory period to test
     for violations; as a default, it uses 1.5 times the width of the
     waveforms in the SPIKES structure.  To override this, define the
     value SPIKES.OPTIONS.REFRACTORY_PERIOD with units of seconds (e.g.,
     set to 0.0017 to indicate an expected refractory period of 1.7 msec).

     The SPIKES.HIERARCHY.TREE outpu