Visual Search (VS)

Fig. 2 Visual search stimuli with interest area sizing and white search array constrain

Overview

The Visual Search task measures mean reaction time to find a target amongst distractors. A target (red-apple) flys from a randomly selected corner of the screen into a random location within the circular array, where there are distractors (red apple slices, and/or whole blue apples). Trials vary between 5, 9, or 13 total distractors, and between 1 or 2 types of distractors (red apple slices, blue whole apples, or both). When the participant gazes to the target (red whole apple), the apple rotates and an applause is played. The target red apple and the distractors are blue apple are both 180*180px (3.1*3.1o) images, and the red apple slice is 45*270px (0.8*4.6o), as shown in Figure 3. When the participant gazes to the target (red whole apple), the apple rotates and an applause is played.

Conditions

Mismatch type

Colour mismatch

Whole red apple amongst whole blue apples

Slice mismatch

Whole red apple amongst red apple slice

Conjunction mismatch

Whole red apple amongst whole blue apples and red apple slices

Number of distractors

5, 9 or 13 distractors

Interest Areas

The interest areas for the apples are 220px in diameter. They are this small in order to have non-overlapping interest areas. The apple slice interest areas are 65*290px, note that the 0 and 12 IA locations do overlap substantially, so there are no trials with both 0 and 12 location apple slice distractors. There are interest areas for all displayed images including the distractors. This may be informative to analyze eye movements to distractor features as a factor.

Gaze Triggers

The two gaze triggers in this experiment are low latency invisible boundary triggers. The minimum duration for the fly-in apple and the target apple gaze triggers are both 50ms. As with the Gap overlap task, the reaction time for the search time is taken from the first sample within the invisible boundary that fired (50ms later).

Classifiers

A trial is deemed valid if the search target IB trigger fires producing an RT value. There experiment plays until the participant completes 26 valid trials. If the participant does not gaze to the target apple within 2000ms, the trial is recycled. If the participant does not complete 26 valid trials within 48 trials, the experiment ends.

Animations

Each trial begins with a red target apple (180*180px) which flies from either the top left or right corners over 750ms. It then remains in the center of the screen with a 220px diameter interest area, until gaze is within the interest area region for 50ms or 2000ms elapses (the trial is immediately recycled, and the attention getter is shown). All trials that show a search array present a reward animation, which is the target apple rotating clockwise 180 degrees for 500ms then anti-clockwise 180 degrees (returning upright) for 750ms.

Event Messaging

Each trial begins with the message APPLE_FLY_IN and is followed by GAZE_TO_ATTENTION_APPLE (or NO_GAZE_TO_APPLE after the 2000ms timeout). When the fixation image is shown, the message DISPLAY_FIXATION is written and after 750ms delay the message DISPLAY_SEARCH is written. If the participant’s gaze triggers the target IA, GAZE_TO_TARGET is written (or TIMEOUT if not after 4000ms). When the reward animation begins DISPLAY_REWARD is written, and the trial ends with DISPLAY_BLANK when the final blank screen is displayed.

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 Marker	Red Apple Slices			Blue Whole Apples			Conjunction			DIN Event EGI	DIN Event EYELINK
APPLE_FLY_IN	5	9	13	5	9	13	5	9	13	DIN3	4
	da5s	da9s	datc	da5a	da9a	datc	da5c	da9c	datc
GAZE_TO_ATTENTION_APPLE	5	9	13	5	9	13	5	9	13
	ga5s	ga9s	gatc	ga5a	ga9a	gatc	ga5a	ga9a	gatc
NO_GAZE_TO_APPLE
DISPLAY_FIXATION	5	9	13	5	9	13	5	9	13	DIN2	2
	df5s	df9s	dftc	df5a	df9a	dftc	df5c	df9c	dftc
DISPLAY_SEARCH	5	9	13	5	9	13	5	9	13	DIN3	4
	ds5s	ds9s	dstc	ds5a	ds9a	dstc	ds5c	ds9c	dstc
GAZE_TO_TARGET	5	9	13	5	9	13	5	9	13
	gt5s	gt9s	gttc	gt5a	gt9a	gttc	gt5c	gt9c	gttc
TIMEOUT
DISPLAY_REWARD	5	9	13	5	9	13	5	9	13	DIN2	2
	dr5s	dr9s	drtc	dr5a	dr9a	drtc	dr5c	dr9c	drtc
DISPLAY_BLANK	5	9	13	5	9	13	5	9	13
	db5s	db9s	dbtc	db5a	db9a	dbtc	db5c	db9c	dbtc