User`s guide

E-Prime User’s Guide
Chapter 3: Critical Timing
Page 82
Display Sequence Interaction with Refresh Cycle
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 7 14 21 28 35 42 49
Time (ms)
Relative Display Intensity
Time of
Write to
Video
Memory
+ ***
+
CAT
***
***
Time of Pixel Activation
Figure 5. Presenting displays for less than a refresh cycle produces partial displays. The arrows on the bottom show
writing of the stimuli "+", "CAT" and "***" to video memory at 0, 5, and 10ms respectively. The subject sees “CAT” at
in at 35ms. Note that the “+” is overwritten by “CAT” before it is ever displayed to the
subject because it is not refreshed before it is overwritten.
Note that if the position of the stimuli are changed slightly the subject sees an entirely different
sequence of displays. For example, if the stimuli were presented closer to the top of the screen
instead of in the center, the subject would see only the “+” and “***”. This occurs because at the
initial refresh, the stimulus “+” was presented, and the co
in the same location as the “+”. The display of the stimulus (i.e., “CAT” at the top of the display)
is not possible until the next refresh cycle, at which point the command has already been given to
draw the mask in the same location. Stimulus display changes that are faster than the refresh
cycle are not always presented to the subject.
The third consequence of the refresh cycle is that specifying changes in stimuli at a regular period
of time generally results in displays of varying durations. If the display duration is specified to be
200ms, and that value is not an exact multiple of the refresh rate, the duration will vary up to one
refresh cycle less than or greater than the desired time. For example, if the refresh rate is 73.1Hz
(a refresh duration of 13.67ms) and the specified display duration is 200ms, the subject will see
durations that are either 191.3ms (14 refreshes) or 205.0ms (15 refreshes). Since the requested
duration (200ms) is 63% of the distance between the onset times of the two refresh cycles (191.3
and 205.0), 63% of the refreshes will be at 205ms and 37% at 191ms.
Difference in Duration
= Specified Duration – Lower Bound Duration
Percentage of Refreshes
= Difference in Duration / Refresh Duration
Difference in Duration = 200ms – 191.3ms = 8.7ms
Percentage of Refreshes = 8.7ms / 13.67ms = 0.63 = 63%