tokens: The longest S1 - S2 interval at which S2 failed to capture ( effective refractory period ) was longer by 5 . 5 + / - 8 . 2 msec at twice threshold ( 0 . 05 less than p less than 0 . 10 ) and 5 . 5 + / - 3 . 5 msec at 5 times threshold stimulation ( p less than 0 . 01 ) when the basic drive and extrastimuli were delivered to separate sites .