User`s guide

E-Prime User’s Guide
Chapter 3: Critical Timing
Page 81
Pixel Intensity During a Display Sequence
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0 7 14 21 28 35 42 49
Time (ms)
Relative Display Intensity
+
CAT
***
+
CAT
***
Time of Pixel Activation
Time of
Write to
Video
Memory
Figure 4. Pixel activation of center of display for “+” presented at 0ms, “CAT” presented at 14ms, and “***” presented
at 28ms. The subject actually first sees “+” at 7ms, “CAT” at 21ms, and “***” at 35ms.
The arrows at the bottom of the figure illustrate when the text is written to the video diplay card.
The exponential decay functions show what is actually displayed to the subject’s eyes when the
experiment is presented using a CRT monitor. The delay in the activation is due to the vertical
refresh cycle. The intensity change is due to the phosphor decay of the CRT monitor after the
pixel was activated once per refresh cycle. Note that LCD monitors do not experience the
phosphor decay effect because of the technology they use to maintain the image. When using
LCD monitors the update of the image data on screen is still subject to the concept of a refresh
cycle, but once updated the image is does not decay but is rather continuously illuminated via
florescent light or similar technology. For this reason typical refresh rates for LCD monitors are
often lower (e.g., 60-75Hz) as compared to those of CRT monitors (e.g., 60-180Hz+).
There are three important consequences of the display having a fixed and non-varying refresh
cycle. First, you cannot put up or remove a display at any time. Displays may be presented or
removed only when the next refresh cycle comes along. For example if the refresh cycle is 14ms,
displays may be changed at delays of 0, 14, 28… milliseconds from the time of the vertical blank
that initiated the display sequence. The actual display of the stimuli that are half way down the
screen will be halfway through the refresh cycle, or at 7, 21, and 35ms.
Second, displays that are not a full refresh cycle in duration may possibly not be visible, and this
varies depending on the position of the stimulus. For example, let us consider the same
experiment used above but this time let us change the display specifications so that the fixation
“+” is presented at the time of the first vertical blanking event, the probe word “CAT” is written
5ms later, and finally the mask (“***”) is written 5ms after the probe word. Given this stimulus
timing specification, the “+” will actually never be presented to the subject. The video monitor
would present “CAT” at 7ms and “***” at 21ms (see Figure 5). This is because the “+” would be
overwritten by ”CAT” before it is every refreshed (e.g., the first refresh of the pixels in the center
of the screen occur at time 7ms, but the write of “CAT” occurs at time 5ms which is 2ms prior to
the refresh of the pixels in that area of the screen).