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Support   SuperLab    High Precision Presentation of Stimuli

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…

What Is A Refresh Cycle?

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:

  • Updates to the screen can happen only in multiples of the refresh cycle
  • The minimum exposure time for a stimulus is one refresh cycle

These constraints are a function of computer architecture and apply no matter which stimulus presentation software you use.

Specifying Exposure Time in SuperLab

There are three primary considerations when wanting to present stimuli precisely and for brief durations:

  • Specifying the exposure time correctly; more on this below.
  • An event that specifies an exposure time for the stimulus needs to be followed by another event to erase the screen, otherwise the stimulus will remain on the screen for longer than wanted.
  • Be aware that erasing the screen adds one refresh cycle. This is a factor only when presenting two or more successive stimuli. In that trial design, we recommend that all visual stimuli have the same size in pixels. This way, presenting a stimulus superimposes and erases the previous one, allowing you to turn off the “Erase screen” option in SuperLab.


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:

  • To present a picture for precisely 1 refresh cycle, or 16.66ms on a 60Hz monitor, in SuperLab you should set the time limit to just 1ms. The presentation time will be rounded up to 16.66ms.
  • To present a picture for 2 refresh cycles, set the time time to (1 x 16.66ms ) + 1ms. That’s 17.66ms, so you rounded it up to 18ms.
  • To present a picture for N refresh cycles, where the duration of the cycle is L, set the time limit to ( (N – 1) x L ) + 1ms.

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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