User`s guide

E-Prime User’s Guide
Chapter 3: Critical Timing
Page 90
Figure 9. Timing diagram showing the effects of using event and cumulative timing modes on a stimulus presentation
sequence.
Event Mode Timing
In Event mode, delays in the onset of an event will not affect the specified duration of the event.
This results in a delay of the onset of all subsequent events due to the error, and an accumulation
of timing delay across events. For example, assume you wanted to present 5 stimuli in sequence
for 100ms each. If there was a timing error of 20ms before the second stimulus, and a 30ms
delay before the fourth event, the durations of all five events would be 100ms but the relative start
times would be at 0, 120, 220, 350, and 450ms. Note that the last display of the four “100ms
displays” actually occurs at 450ms from the first display rather then the expected 400ms due to
the cumulative timing error (see Figure 9).
Cumulative Mode Timing
In Cumulative mode, delays in the onset of an event result in an equivalent reduction in the
duration of the event, such that the cumulative timing error is minimized. For our example, the
delay of 20ms before the second event would cause a shortening of the duration of the second
event by 20ms. Thus, for Cumulative mode timing, delays of 20ms and 30ms before the second
and fourth events result in onsets of 0, 120, 200, 330, and 400. Note that the delay of the second
display does not change the start time of the third display, but rather changes the duration of the
second display. Even with two delays, the fifth display occurs at the expected onset time of
400ms after the first event (see Figure 9).
There are a few scenarios that can occur in an experiment which can temporarily defeat or limit
the effects of cumulative timing mode. First, if an unexpected timing error or delay is sufficiently
long enough (or equally if the duration of the stimulus presentation is sufficiently short enough)
there may not be enough time available during the presentation of the current stimulus to entirely
“absorb” the error. If this happens, any number of subsequent presentations may likewise have
their durations shortened until all of the cumulative error is accounted for. For example, a
stimulus presentation event with a specified duration of 14ms cannot possibly shorten its duration
to account for a delay of 20ms. In this case, the current stimulus will shorten its duration as much
as it possibly can and then subsequent stimuli will attempt to follow suit until all of the timing error
is absorbed and the presentation sequence eventually gets back in sync with the expected
stimulus onset times.