Steady State Visual Evoked Potentials (SSVEP)¶

Overview¶

The SSVEP task is a visual task that presents 1 second videos of circular random pattern images alternating between 180º rotated copies at rates of 6Hz or 15Hz. There is a jittered blank screen of either 800, 900, 1000, 1100, or 1200 ms between the offset of the fixation animation and the onset of the stimulus train. A small, red fixation cross is constantly displayed in the center of the screen throughout the trial. Before each trial, a small, a fixation animation (throbbing clock) is displayed at the center of the screen, with the red cross overlaid on top.

Conditions¶

6Hz: The circular random pattern images alternate between 180º rotated copies at a rate of 6Hz
15Hz: The circular random pattern images alternate between 180º rotated copies at a rate of 15Hz

Blocks / Trials¶

The experiment conists of 4 blocks (3 mandatory, 1 optional) of 40 trials each (20 per condition). If 3 blocks are completed, 120 trials will be completed (60 per condition). If 4 blocks are completed, 160 trials will be completed (80 per condition).

Animations¶

Fixation (looming clock): At the onset of a trial the fixation is 0*0px and over 300ms increases in size to 350*350px (looming). Following this the Central Stimulus pulses, increasing/decreasing between 234*234px to 350*350px at 3Hz (throbbing).
Fixation (spinning clock): At the onset of a trial the fixation is sized 234*234px and spins clockwise at 1.5Hz.
Fixation (throbbing star): At the onset of a trial the fixation is 0*0px and over 1 second increases in size to 234*234px (throbbing).
Steady state visual evoked stimuli: A 1-second videos of circular random pattern images alternating between 180º rotated copies at rates of 6Hz or 15Hz. The stimuli are 200 pixels wide.

Interest Areas¶

An interest area is placed around the SSVEP stimuli, and is used to determine whether the participant is looking at the stimuli or not, determining whether the trial is valid or not.

Gaze Triggers¶

The SSVEP task does not use gaze any triggers. The stimuli will be presented regardless of whether the participant made gaze to the stimuli or not. This is out of consideration for maintaining the steady state of the stimuli. However, eye-tracking data is collected and can be used assess the validity of trials during offline analyses.

Classifiers¶

To aid the researchers running the study with real-time information, classifiers in the experiment check for certain behaviors required for a valid trial. A trial is deemed valid if:

Gaze is on stimulus for at least 600ms during the trial.

Valid trials are added to a count to the valid sum for that type of trial shown on the Eyelink Host PC.

Event Messaging¶

Experiment Builder will send experiment event codes to both the Eyelink and EGI acquisition systems, and the event code patterns differ slightly:

EGI acquisition¶

EGI Netstation does not support the same event messaging as the Eyelink Host PC, as event codes are generally restricted to 4 characters. The table below shows the corresponding event codes for the EGI Netstation acquisition, and the DIN event triggered by the photo-diode on the screen for each event.

Event Description	EGI Event		DIN
Condition	6Hz	15Hz
Jittered ISI	`N.A.`	`N.A.`
Fixation animation onset	`fvsr`	`fvsb`
Jittered ISI	`N.A.`	`N.A.`
Onset of 1-second video	`sv06`	`sv15`	`DIN2`

In addition to the event codes above, the following codes are also sent to the EGI Netstation acquisition system, but generally are not needed for offline analysis:

Description	EGI Event
Fixation animation onset	`fvsr`, `fvsb`, `fvct`, `fvcr`
begin task	`VBeg`
Netstation misc event	`TSYN`
display start menu	`dstr`
display break menu	`dbrk`
display end screen	`dend`