SuperLab can present a stimulus with a duration as short as one refresh cycle. This is the shortest exposure time currently possible with computer hardware. In SuperLab, you can specify exposure times in milliseconds. This support note explains how to specify these exposure times so that they translate correctly into refresh cycles. But first…
It is a common misconception that updates to the computer display happen instantaneously. For example, if you want to draw five objects on the display with a 10 milliseconds interval between them, the misconception leads one to believe that the display will be updated like in the following timeline:
For most users running productivity software, games, and so forth, this misconception does not hurt anything and does not matter. But for psychology and vision researchers presenting stimuli for brief periods, it is important to understand that updates to the computer display happen periodically, and not instantaneously.
The rate at which these periodic updates happen is called the refresh rate. On LCD monitors, the most common refresh rate is 60Hz (hertz), meaning that they happen 60 times per second. By dividing 1000 milliseconds (1 second) by 60, we obtain a refresh cycle of 16.66 milliseconds. That is, changes can only happen in multiple of 16.66 milliseconds. In the example above, drawing five objects with a 10 milliseconds interval will actually yield a timeline as follows:
To summarize:
These constraints are a function of computer architecture and apply no matter which stimulus presentation software you use.
There are three primary considerations when wanting to present stimuli precisely and for brief durations:
Expressing Exposure Time in Milliseconds
The exposure time is specified as a time limit in the Event Editor’s Input tab. Keeping in mind that stimuli are presented at refresh cycle intervals, the proper approach is to set the exposure for less time and then let SuperLab round up to the next multiple of refresh cycles.
For example:
The table on the right provides a handy list of time limits for the first 10 refresh cycles.
On 60Hz Monitors, for
Set Time Limit to
1 refresh cycle
1 millisecond
2 refresh cycles
18 milliseconds
3 refresh cycles
35 milliseconds
4 refresh cycles
51 milliseconds
5 refresh cycles
68 milliseconds
6 refresh cycles
85 milliseconds
7 refresh cycles
101 milliseconds
8 refresh cycles
118 milliseconds
9 refresh cycles
135 milliseconds
10 refresh cycles
151 milliseconds
Last Revision: July 3, 2017
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