Gap Overlap (GO)

Fig. 3 Gap-Overlap stimuli and interest area placement and sizing.

Overview

The Gap Overlap task measures reaction times from a central stimulus to a peripheral stimulus. A central stimulus is first presented (spinning clock), followed by a peripheral stimulus (Cloud), either 803px eccentricity to the left or right. After the participant gazes to the peripheral stimulus (or if the trial times-out), A reward animation (star) is presented at the same place as the peripheral stimulus. Sounds are played when the central and peripheral stimuli are presented. The color of the background screen is randomly selected before the session begins, and remains the same for all trials.

Conditions

Baseline condition

the central fixation disappears simultaneously with the presentation of the peripheral stimulus

Gap condition

the central fixation disappears 200 ms before the presentation of the peripheral stimulus

Overlap condition

the central fixation disappears 200 ms after the presentation of the peripheral stimulus

Interest Areas

The Central, Peripheral and Reward stimuli all have corresponding interest area (IA) regions: Non-overlapping elliptical resources with a diameter of 450px, shown as red circles in Fig. 3. This produces a 108px buffer around the stimulus. There is also a 700*1080px IA that detects gaze to the wrong side of the screen (shown in Fig. 3). This informs the trial classifier, as gaze to the wrong side of the screen invalidates the trial.

Gaze Triggers

Gaze behaviors are classified by low-latency Invisible boundary (IB) gaze triggers, that fire near immediately after gaze is detected within the IA. To begin a trial, the participant’s gaze must stay within the 450px diameter IA around the Central stimulus for 500ms. The IB trigger for the ‘wrong side’ of the screen IA will fire immediately if any gaze is allocated within that IA. Gaze must be within the Peripheral Stimulus IA region for 50ms to trigger this IB. This small delay in firing makes sure that gaze is kept within the target IA before it is classified as such.

Note

The Saccadic RT from the central to peripheral stimuli is calculated using the first gaze sample within peripheral stimulus IA, not first gaze sample that meets the 50ms threshold.

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:

1. The saccade RT value is less than 1200ms.

2. The saccade RT is greater than the 100ms.

3. There was is gaze to the wrong side of the screen.

Valid trials are added to a count to the valid sum for that type of trial shown on the Eyelink Host PC ("Gap", "Overlap", and "Baseline").

Animations

Central Stimulus (CS)

At the onset of a trial the Central Stimulus 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). Once gaze is detected, the Central Stimulus sized 234*234px spins clockwise at 1.5Hz.

Peripheral Stimulus (PS)

Following the onset of the Peripheral Stimulus, gaze to it triggers a rotation animation. The Peripheral stimulus sized at 234*234px rotates clockwise at 1.5Hz. Finally,

Reward Stimulus (RS)

the reward animation starts at 234*234px, reduces to 0px as it rotates clockwise at 1.5Hz over 1 second (the reward duration).

Event Messaging

Because the task is designed to be run on both the Eyelink and EGI acquisition systems, Experiment Builder will send experiment messages to both systems. The messages sent to the Eyelink Host PC and to the EGI Netstation acquisition will differ slightly.

Eyelink Acquisition

Throughout the recording sequence of the trial, messages are sent to the Eyelink Host PC to mark key onsets, and are written to the Eyelink EDF file. The trial begins with a message "CS_ONSET" when the CS begins to loom. When gaze is detected to the CS a message "GAZE_TO_CS" is written, directly followed by "CS_SPIN" when the CS beings spinning for the 600ms (+ jitter value). This is followed by "ONSET_200MS" to signify the 200ms period when the CS either remained or left the screen, and then "ONSET_PS" when the PS is displayed. If gaze is oriented to the wrong side, a message "GAZE_TO_WRONG_SIDE" is written, if gaze is to the PS, then "GAZE_TO_PS" is written, followed by "REWARD_ONSET" when the reward begins. The trial ends with a "DISPLAY_BLANK" message.

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.

Eyelink Event	EGI Event			DIN EGI	DIN EYELINK
Condition	Overlap	Gap	Baseline
"CS_ONSET"	dfoc	dfbc	dfgc	DIN2	2
"GAZE_TO_CS"	gfoc	gfbc	gfgc
"CS_SPIN"	dsoc	dsbc	Dsgc	DIN3	4
"ONSET_PS"	dtoc	dtbc	dtgc	DIN2	2
"GAZE_TO_PS"	gcoc	gcbc	gcgc
"GAZE_TO_WRONG_SIDE"	geoc	gebc	gegc
"REWARD_ONSET"	droc	drbc	drgc	DIN3	4
"DISPLAY_BLANK"	dboc	dbbc	dbgc