User`s guide

E-Prime User’s Guide
Appendix D: Display Presentation
Page A-55
dot is activated by an exponential function (see Figure 1 step 2), on an LCD the activation is a
square wave signal (on for a given intensity during the time of the refresh). The LCD still has a
refresh rate, which is the rate at which each dot is updated. Similar to the CRT, on the LCD the
middle dot would be updated 7.14ms after the first dot of the screen (on a 70Hz monitor). The
effect on the eye of a LCD and CRT display are indistinguishable.
Step 3 - Activating the Retina
The last step in the perception of stimuli on a computer monitor is the retinal response to the light
emitted from the screen. The chemistry and neural circuitry of the retina are such that the visual
system will integrate visual input over time. This is the reason that we cannot detect the raster
guns scanning the monitor. The retina will average the input and produce the perception of a
continuous display. For example, a stimulus which is to last for one refresh of the monitor will
produce the following results: The pixels on the screen which make up the stimulus will be
activated by the raster guns once, and then begin to decay. This decay process will take
approximately 5 milliseconds on a color display (see Figure 1, graph step 2) before the intensity is
below perceptible levels. On LCD, the dot will be on for the duration of the refresh. However, the
eye integrates that impulse for the next 80ms (see Figure 1, graph step 3). As an analogy, think
of what occurs when you see a photographer's electronic flash go off. Typically, a very brief flash
(microseconds) is presented and is perceived as a short (e.g., 100ms) flash. If you flash the
electronic flash 70 times a second, the retina would perceive the light as being continuous.
During a period of about 80ms, the eye is integrating the energy (e.g., a stimulus on for 5ms at 10
Lux is seen as intense as a stimulus on for 10ms at 5 Lux).
Example stimulus activation
Figure 2 provides an illustration
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of the time course of the raster display on each scan, and the
visual integration of the raster display. We want to display a ‘+’ (text mode) in the center of the
screen. Assume the monitor has a vertical retrace rate of 14.3 milliseconds. We will examine the
sequence of events, considering only the center pixel of the plus symbol. Execution of the display
command occurs at time t = 0. After about ten microseconds, the ‘+’ is written into the video
memory. At some point during the next 0 to 14.3 milliseconds, the raster guns will be aimed at
the center pixel on the screen (see Raster Display peaks in Figure 2). With the stimulus in the
center of the screen, the pixels for the ‘+’ are displayed after 7.14ms. As soon as the pixel has
been activated, it will be refreshed every 14.3 milliseconds. The retinal response begins as soon
as the pixel is activated for the first time, but the response is delayed in the sense that it must
build up over the next 80 milliseconds to reach a steady state (see Raster Based Visual
Activation Figure 2). At 200ms the ‘+’ is overwritten in video memory. When the raster guns
reach that point (at 207ms) where the ‘+’ was, the pixel is not refreshed. The retinal activation
begins to decay as soon as the pixel is no longer visible during the last refresh, and the visual
activation slowly drops off over the next 80 milliseconds.
There are four important aspects to note about the timing in the attached graph. First, there is a
variable delay of up to one refresh period from the time that the stimulus is written to the video
memory and the time that the center pixel is initially activated on the screen. The second feature
is the exponential decay of the light intensity of the center pixel between refreshes. Third, the
retinal response is slow and smoothes the bumps in the pixel activation. Fourth, the decay of
retinal activation is delayed, not instantaneous. If the stimulus is removed from video memory just
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Figure 2 is based on simulation and empirical studies reported in Busey, T. A., & Loftus, G. R., 1994,
Sensory and cognitive components of visual information acquisition, Psychology Review, 10, 446-469,
assuming a decay constant of 20ms.