EPrime

EPrime Experiment Design

FMRI studies require collecting brain scans while an individual performs different tasks. It is important to synchronize the behavioral tasks with the scanning, so that we know precisely when an individual was doing each task.

The blood-oxygenation signal measured by fMRI is very weak and sluggish. Therefore, you want to carefully adapt your behavioral task to maximize the signal observed using fMRI. There is information about this in the fMRI training course.

EPrime is an excellent tool for designing behavioral experiments. When designing fMRI experiments, I generally recommend setting the ‘Timing Mode’ for every single event to ‘Cumulative’, rather than ‘Event’ mode. With event mode, when you ask the experiment to pause for 100ms, it will pause for 100ms plus the amount of time until the next screen refresh, so the actual delay will be randomly between about 100 and 117ms. With the cumulative mode, subsequent events compensate for the error in the previous event. In this mode, if the previous event was supposed to last 100ms but actually lasted 104ms, then the upcoming event will attempt to end 4ms earlier than specified. With cumulative mode, the experiment will always take longer than specified, and the amount of drift will vary from one participant to the next.

It is also important to ensure that the experiment starts precisely in time with the scanner. To do this, when the experiment initializes it should pause until the scanner sends a trigger pulse. The Siemens scanner will not reconstruct images collected during the first 3s of fMRI scanning (due to T1 effects), so if your TR is 2.2s, the first scanner trigger pulse will occur 4.4s after the scanner begins emitting loud sounds.

[EPrime settings]

Properties for synchronizing the experiment to start with the scanner’s trigger pulse. The optical pulse sent by the scanner simulates the 4th mouse button.

Notes:

  • Note that you should start an EPrime experiment
    using a TTL pulse from your scanner. This allows
    you to synchronize the presentation of stimuli
    with your scans. At the start of your experiment you will want an
    ImageDisplay or TextDisplay object that will pause until a scanner
    pulse is detected. You will want to open this object’s properties
    window and choose the “Input/Duration” tab. Set the duration to
    “(infinite)”, the “End action” to terminate. For the Devices, add the
    mouse and set the allowable input for this
    device to 4 (as the scanner will emulate the 4th mouse button being depressed). I also like to add a ‘Keyboard’ device, and set its
    allowable input to {F6}. This allows you to start the study without a
    pulse from the scanner by pressing Fn6.
  • The EPrime fMRI compatible response gloves emulate keyboard button presses. Therefore, you can use EPRime to record responses. The response for the right hand (from thumb to pinky) are mapped to 1,2,3,4,5, and left hand (from thumb to pinky finger) are mapped as 6,7,8,9,a.