The authors thank Pieter Schiphorst for his role in designing the setup and for providing the graphics of Figures 1 and 3, and Marnix Naber for helpful discussion and his contribution to Figure 6. The authors also acknowledge researchers and publishers for reusing Figure 1 and 6 from a published paper1.

This protocol has been approved by the Institutional Review Boards of Michigan State University.
1. Building the system
<ol>
	<li>Rationale
	<ol>
		<li>Prepare the mirror setup, a variant of the classic Wheatstone stereoscope28 illustrated in Figure 1, consisting of two mirrors positioned at a 45&#176; angle relative to the participant&#39;s midline. The mirrors reflect stimuli from two screens that are positioned at opposite ends of a table, facing each other.</li>
		<li>Seat a participant in front of the mirrors and have them view a different screen, reflected via a different mirror, with each eye. For best results, use a head rest for stabilizing the participant&#39;s head.</li>
		<li>Position an infrared-sensitive video-based eye tracker, including a camera and an illuminator, in front of the participant but behind the mirrors. The eye tracker is represented by a box in Figure 1. 
		NOTE: One challenge when trying to track the eyes in normal setups of this type, is that the eyes are blocked by the mirrors.</li>
		<li>Use two front-surface mirrors, often advertised as &#34;cold mirrors&#34; (incident angle: 45&#176;), that feature near-complete reflectance of visible wavelengths and near-complete transmission of near-infrared wavelengths (see Table 1 for detailed information about the mirrors). 
		NOTE: Such mirrors can be obtained via companies supplying optical equipment for scientific and industrial purposes, which usually list components like these as &#39;cold mirrors&#39; or as a type of &#39;dichroic mirrors&#39; (see more detail in Materials/Equipment Table).
		<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
			<tbody>
				<tr>
					<td colspan="2"></td>
					<td>Setup 1</td>
					<td>Setup 2</td>
				</tr>
				<tr>
					<td rowspan="3">Mirrors</td>
					<td>Dimensions</td>
					<td>10.10 &#215; 12.70 cm</td>
					<td>10.10 &#215; 12.70 cm</td>
				</tr>
				<tr>
					<td>Reflectance</td>
					<td>400 ~ 690 nm</td>
					<td>425 ~ 650 nm</td>
				</tr>
				<tr>
					<td>Transmission</td>
					<td>750 ~ 1200 nm</td>
					<td>800 ~ 1200 nm</td>
				</tr>
				<tr>
					<td rowspan="2">Eye Tracker</td>
					<td>Brand</td>
					<td>Research-end Eye Tracker</td>
					<td>Customer-grade Eye Tracker</td>
				</tr>
				<tr>
					<td>Transmission</td>
					<td>890 ~ 940 nm</td>
					<td>Around 850 nm</td>
				</tr>
			</tbody>
		</table>
		 
		Table 1. Details of two versions of the setup with which we have worked. 
		The eye tracker&#39;s transmission wave length range is covered by the mirrors&#39; transmission range at a 45&#176; incidence angle, but outside their reflectance range.</li>
	</ol>
	</li>
	<li>Structure of the Setup
	<ol>
		<li>Build the setup on top of a desk. Besides the mirrors and eye tracker, it consists only of three custom-built elements made of fiberboard (see Figure 2 for an assembly guide) and two flat-screen monitors on monitor-arms available from normal office supply stores.</li>
		<li>Fiberboard elements
		<ol>
			<li>Build the framework of the setup from three components of fiberboards: one central component and two reference boards on each side (see Figure 1 for general positioning, Table 2 for detailed dimensions, and Figure 2 for an assembly guide of each component). Paint all these pieces in matte black to reduce light scatter. 
			NOTE: The central component (see Figure 2B and 2D) holds the mirrors and eye tracker. Both are on the same plateau, thus keeping the eye tracker at participants&#39; eye level.</li>
			<li>Place the top element of this component such that it leaves 8 cm in depth in the front of the desk. Such an arrangement allows enough room for the participant&#39;s face when stabilized on the head rest and avoids condensation on the mirrors during expiration, while minimizing the distance between the participant&#39;s eyes and the mirrors to maximize the possible use of the participant&#39;s visual field.</li>
			<li>Position the two reference boards straight below the monitors (see Figure 1 for positioning and Figure 2 panels A and C for an assembly guide) for easy manual calibration of the screens. Note that the apparent offset in Figure 1 between screen and board is due to limited depth cues in the image; the boards are straight below the monitors on both sides.</li>
			<li>Exactly align the long horizontals with the edges of the desk, while the long verticals leave 4 cm beyond the front of the desk for ease of stabilizing a calibration board (see below) to these boards. The two small verticals will ensure the long vertical staying vertical as the reference for the monitors.</li>
			<li>Optionally, use a separate piece of fiberboard as a calibration board (see Figure 3). In this case, after obtaining an optimal position of a monitor, position the calibration board against the reference board and indicate the positions of both the reference board and the monitor on the calibration board while it is in place (in the example of Figure 3, wooden slats provide these indications).</li>
			<li>Whenever this desired monitor position is lost (accidentally or because other experiments require a different position), retrieve this position by using the markings on the calibration board to put the calibration board back in the same place relative to the reference board that has a fixed position on the desk. Move the monitor again to line up with the appropriate markings (see step 2.1.1. for details).
			<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
				<tbody>
					<tr>
						<td>Component</td>
						<td>Dimensions (cm)</td>
						<td>Number</td>
						<td>Remark</td>
					</tr>
					<tr>
						<td rowspan="4">Central Component</td>
						<td>80 &#215; 25 &#215; 2</td>
						<td>1</td>
						<td>Horizontal top</td>
					</tr>
					<tr>
						<td>23 &#215; 25 &#215; 2</td>
						<td>1</td>
						<td>Horizontal bottom</td>
					</tr>
					<tr>
						<td>21 &#215; 32 &#215; 2</td>
						<td>1</td>
						<td>Central vertical</td>
					</tr>
					<tr>
						<td>32 &#215; 25 &#215; 2</td>
						<td>1</td>
						<td>Front-facing vertical</td>
					</tr>
					<tr>
						<td rowspan="3">Reference Boards</td>
						<td>61 &#215; 11 &#215; 2</td>
						<td>2</td>
						<td>Long horizontal</td>
					</tr>
					<tr>
						<td>66 &#215; 29 &#215; 2</td>
						<td>2</td>
						<td>Long vertical</td>
					</tr>
					<tr>
						<td>11 &#215; 15 &#215; 2</td>
						<td>4</td>
						<td>Small vertical</td>
					</tr>
				</tbody>
			</table>
			 
			Table 2. Details of the fiberboard components.</li>
		</ol>
		</li>
		<li>Monitors and Mirrors
		<ol>
			<li>Position the setup on top of a standard office desk.</li>
			<li>Mount two flat-screen monitors on standard monitor arms clamped to the side of the desk (clamping both the reference board and the desk). These arms allow translation in three dimensions as well as rotation in the plane of the screen. Conventional CRT-monitors are clearly also compatible with the setup, but would not afford the same flexibility in terms of positioning and repositioning.</li>
			<li>Mount the mirrors on mirror mounts that are sold for the purpose by the same suppliers that stock cold mirrors. Connect these mounts to the fiber board holding the mirror at participants&#39; eye level. Position the mirrors to touch at a 90&#176; angle in the center, right before the participant&#39;s nose.</li>
		</ol>
		</li>
		<li>Remaining elements 
		NOTE: Some experiments require participants not seeing the screens from the corner of their eyes, so that a direct line of sight to the screens (dashed lines in Figure 4A) should be avoided.
		<ol>
			<li>In that case, create &#34;blinders&#34; made of black cardboard and foam-padded hole straps painted in black, and attach them to the posts of the head rest (see Figure 4B). Adjust the blinders in height and angle to accommodate individual participants. If the wall in front of the participant has high reflectance, hanging a piece of black fabric will help remedy this.</li>
		</ol>
		</li>
	</ol>
	</li>
</ol>
2. Using the system
<ol>
	<li>Hardware calibration 
	NOTE: The purpose of calibration is to achieve satisfactory alignment of the two monitors for ease of fusion of the two monitors&#39; images for each participant. This can be achieved in two steps: hardware calibration (described here) and software calibration (described below).
	<ol>
		<li>When using a calibration board, as described above, align it with one of the reference boards, holding it in place with a C-clamp if needed, and then move the corresponding monitor to line up with the desired reference lines on the calibration board. The monitors should be parallel to each other, and each should be straight above its reference board.</li>
		<li>When using blinders, move them to the participant&#39;s eye level and rotate them slightly toward the midline, i.e. more inward, compared to the orientation of the monitors. Make sure that each eye can see the whole visual stimulus in the mirror without seeing any of it directly. Turning the blinders toward rather than away from the midline will minimize participants&#39; exposure to other visual input.</li>
	</ol>
	</li>
	<li>Software calibration
	<ol>
		<li>Since participants may vary in their eye position relative to the mirrors despite the use of a head rest, calibrate further before doing experiments. This part is most easily done in the software, i.e. without moving the setup&#39;s parts any further. There are two possible methods.
		<ol>
			<li>For the first, present a dot on each of the two screens in alternation, and instruct the participant to eliminate the perceived position change by moving the dot on one of the screens (or both in opposite directions).</li>
			<li>For the second method, instruct the participant to align the frames of experimental stimuli instead of two dots so that both eyes&#39; visual fields critical to the particular experiment are aligned.</li>
		</ol>
		</li>
		<li>After applying either method, center the stimuli in the experiment on the resulting on-screen positions. Other aspects of setting up displays and stimuli for dichoptic presentation in general can be found elsewhere5.</li>
	</ol>
	</li>
</ol>

<ol>
	<li>Brascamp, J. W., Naber, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Eye+tracking+under+dichoptic+viewing+conditions:+a+practical+solution.">Eye tracking under dichoptic viewing conditions: a practical solution.</a> Behav. Res. Methods. , 1-7 (2016).</li><li>Barendregt, M., Harvey, B. M., Rokers, B., Dumoulin, S. O. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Transformation+from+a+Retinal+to+a+Cyclopean+Representation+in+Human+Visual+Cortex.">Transformation from a Retinal to a Cyclopean Representation in Human Visual Cortex.</a> Curr. Biol. 25 (15), 1982-1987 (2015).</li><li>Held, R. T., Cooper, E. A., Banks, M. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Blur+and+Disparity+Are+Complementary+Cues+to+Depth.">Blur and Disparity Are Complementary Cues to Depth.</a> Curr. Biol. 22 (5), 426-431 (2012).</li><li>Julesz, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Foundations of cyclopean perception. xiv, (1971).</li><li>Carmel, D., Arcaro, M., Kastner, S., Hasson, U. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=How+to+Create+and+Use+Binocular+Rivalry.">How to Create and Use Binocular Rivalry.</a> J. Vis. Exp. (45), (2010).</li><li>Tsuchiya, N., Koch, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Continuous+flash+suppression+reduces+negative+afterimages.">Continuous flash suppression reduces negative afterimages.</a> Nat. Neurosci. 8 (8), 1096-1101 (2005).</li><li>Crick, F., Koch, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Consciousness+and+neuroscience.">Consciousness and neuroscience.</a> Cereb Cortex. 8 (2), 97-107 (1998).</li><li>Jiang, Y., Costello, P., Fang, F., Huang, M., He, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+gender-+and+sexual+orientation-dependent+spatial+attentional+effect+of+invisible+images.">A gender- and sexual orientation-dependent spatial attentional effect of invisible images.</a> Proc. Natl. Acad. Sci. 103 (45), 17048-17052 (2006).</li><li>Jiang, Y., Costello, P., He, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Processing+of+Invisible+Stimuli:+Advantage+of+Upright+Faces+and+Recognizable+Words+in+Overcoming+Interocular+Suppression.">Processing of Invisible Stimuli: Advantage of Upright Faces and Recognizable Words in Overcoming Interocular Suppression.</a> Psychol. Sci. 18 (4), 349-355 (2007).</li><li>Bahrami, B., Carmel, D., Walsh, V., Rees, G., Lavie, N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Spatial+attention+can+modulate+unconscious+orientation+processing.">Spatial attention can modulate unconscious orientation processing.</a> Perception. 37 (10), 1520-1528 (2008).</li><li>Smith, D. T., Ball, K., Ellison, A., Schenk, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Deficits+of+reflexive+attention+induced+by+abduction+of+the+eye.">Deficits of reflexive attention induced by abduction of the eye.</a> Neuropsychologia. 48 (5), 1269-1276 (2010).</li><li>Deubel, H., Schneider, W. X. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Saccade+target+selection+and+object+recognition:+Evidence+for+a+common+attentional+mechanism.">Saccade target selection and object recognition: Evidence for a common attentional mechanism.</a> Vision Res. 36 (12), 1827-1837 (1996).</li><li>Pastukhov, A., Braun, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Rare+but+precious:+Microsaccades+are+highly+informative+about+attentional+allocation.">Rare but precious: Microsaccades are highly informative about attentional allocation.</a> Vision Res. 50 (12), 1173-1184 (2010).</li><li>Reddi, B. a. J., Carpenter, R. H. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+influence+of+urgency+on+decision+time.">The influence of urgency on decision time.</a> Nat. Neurosci. 3 (8), 827-830 (2000).</li><li>Barbur, J. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Learning+from+the+pupil-studies+of+basic+mechanisms+and+clinical+applications.">Learning from the pupil-studies of basic mechanisms and clinical applications.</a> Vis. Neurosci. 1, 641-656 (2004).</li><li>Naber, M., Nakayama, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Pupil+responses+to+high-level+image+content.">Pupil responses to high-level image content.</a> J. Vis. 13 (6), 7-7 (2013).</li><li>Gilzenrat, M. S., Nieuwenhuis, S., Jepma, M., Cohen, J. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Pupil+diameter+tracks+changes+in+control+state+predicted+by+the+adaptive+gain+theory+of+locus+coeruleus+function.">Pupil diameter tracks changes in control state predicted by the adaptive gain theory of locus coeruleus function.</a> Cogn. Affect. Behav. Neurosci. 10 (2), 252-269 (2010).</li><li>Van Der Meer, E., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Resource+allocation+and+fluid+intelligence:+Insights+from+pupillometry.">Resource allocation and fluid intelligence: Insights from pupillometry.</a> Psychophysiology. 47 (1), 158-169 (2010).</li><li>Erkelens, C. J., Regan, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Human+ocular+vergence+movements+induced+by+changing+size+and+disparity.">Human ocular vergence movements induced by changing size and disparity.</a> J. Physiol. 379, 145-169 (1986).</li><li>Wismeijer, D. A., Erkelens, C. J., van Ee, R., M, W. e. x. l. e. r. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Depth+cue+combination+in+spontaneous+eye+movements.">Depth cue combination in spontaneous eye movements.</a> J. Vis. 10 (6), 25-25 (2010).</li><li>Takagi, M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Adaptive+Changes+in+Dynamic+Properties+of+Human+Disparity-Induced+Vergence.">Adaptive Changes in Dynamic Properties of Human Disparity-Induced Vergence.</a> Invest. Ophthalmol. Vis. Sci. 42 (7), 1479-1486 (2001).</li><li>Maiello, G., Harrison, W. J., Bex, P. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Monocular+and+Binocular+Contributions+to+Oculomotor+Plasticity.">Monocular and Binocular Contributions to Oculomotor Plasticity.</a> Sci. Rep. 6, (2016).</li><li>Rothkirch, M., Stein, T., Sekutowicz, M., Sterzer, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+direct+oculomotor+correlate+of+unconscious+visual+processing.">A direct oculomotor correlate of unconscious visual processing.</a> Curr. Biol. 22 (13), R514-R515 (2012).</li><li>Spering, M., Pomplun, M., Carrasco, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Tracking+Without+Perceiving+A+Dissociation+Between+Eye+Movements+and+Motion+Perception.">Tracking Without Perceiving A Dissociation Between Eye Movements and Motion Perception.</a> Psychol. Sci. 22 (2), 216-225 (2011).</li><li>Spering, M., Carrasco, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Acting+without+seeing:+eye+movements+reveal+visual+processing+without+awareness.">Acting without seeing: eye movements reveal visual processing without awareness.</a> Trends Neurosci. 38 (4), 247-258 (2015).</li><li>Piano, M. E. F., Bex, P. J., Simmers, A. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Perceptual+Visual+Distortions+in+Adult+Amblyopia+and+Their+Relationship+to+Clinical+FeaturesPerceptual+Visual+Distortions+in+Adult+Amblyopia.">Perceptual Visual Distortions in Adult Amblyopia and Their Relationship to Clinical FeaturesPerceptual Visual Distortions in Adult Amblyopia.</a> Invest. Ophthalmol. Vis. Sci. 56 (9), 5533-5542 (2015).</li><li>Wiecek, E., Lashkari, K., Dakin, S. C., Bex, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Novel+Quantitative+Assessment+of+Metamorphopsia+in+MaculopathyQuantitative+Assessment+of+Metamorphopsia.">Novel Quantitative Assessment of Metamorphopsia in MaculopathyQuantitative Assessment of Metamorphopsia.</a> Invest. Ophthalmol. Vis. Sci. 56 (1), 494-504 (2015).</li><li>Wheatstone, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Contributions+to+the+Physiology+of+Vision.--Part+the+First.+On+Some+Remarkable,+and+Hitherto+Unobserved,+Phenomena+of+Binocular+Vision.">Contributions to the Physiology of Vision.--Part the First. On Some Remarkable, and Hitherto Unobserved, Phenomena of Binocular Vision.</a> Philos. Trans. R. Soc. Lond. 128, 371-394 (1838).</li><li>Beach, G., Cohen, C. J., Braun, J., Moody, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Eye+tracker+system+for+use+with+head+mounted+displays.">Eye tracker system for use with head mounted displays.</a> 1998 IEEE Int. Conf. Syst. Man. 5, 4348-4352 (1998).</li><li>Gibaldi, A., Vanegas, M., Bex, P. J., Maiello, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Evaluation+of+the+Tobii+EyeX+Eye+tracking+controller+and+Matlab+toolkit+for+research.">Evaluation of the Tobii EyeX Eye tracking controller and Matlab toolkit for research.</a> Behav. Res. Methods. , 1-24 (2016).</li><li>Fox, R., Todd, S., Bettinger, L. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Optokinetic+nystagmus+as+an+objective+indicator+of+binocular+rivalry.">Optokinetic nystagmus as an objective indicator of binocular rivalry.</a> Vision Res. 15 (7), 849-853 (1975).</li><li>Leopold, D. A., Fitzgibbons, J. C., Logothetis, N. K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+Role+of+Attention+in+Binocular+Rivalry+as+Revealed+through+Optokinetic+Nystagmus.">The Role of Attention in Binocular Rivalry as Revealed through Optokinetic Nystagmus.</a> , (1995).</li><li>Zaretskaya, N., Thielscher, A., Logothetis, N. K., Bartels, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Disrupting+Parietal+Function+Prolongs+Dominance+Durations+in+Binocular+Rivalry.">Disrupting Parietal Function Prolongs Dominance Durations in Binocular Rivalry.</a> Curr. Biol. 20 (23), 2106-2111 (2010).</li><li>Robinson, D. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+Method+of+Measuring+Eye+Movemnent+Using+a+Scieral+Search+Coil+in+a+Magnetic+Field.">A Method of Measuring Eye Movemnent Using a Scieral Search Coil in a Magnetic Field.</a> IEEE Trans. Bio-Med. Electron. 10 (4), 137-145 (1963).</li><li>Kalisvaart, J. P., Goossens, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Influence+of+Retinal+Image+Shifts+and+Extra-Retinal+Eye+Movement+Signals+on+Binocular+Rivalry+Alternations.">Influence of Retinal Image Shifts and Extra-Retinal Eye Movement Signals on Binocular Rivalry Alternations.</a> PLOS ONE. 8 (4), e61702 (2013).</li><li>Fr&#228;ssle, S., Sommer, J., Jansen, A., Naber, M., Einh&#228;user, W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Binocular+rivalry:+frontal+activity+relates+to+introspection+and+action+but+not+to+perception.">Binocular rivalry: frontal activity relates to introspection and action but not to perception.</a> J. Neurosci. 34 (5), 1738-1747 (2014).</li><li>Duchowski, A. T., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Binocular+Eye+Tracking+in+Virtual+Reality+for+Inspection+Training.">Binocular Eye Tracking in Virtual Reality for Inspection Training.</a> Proc. 2000 Symp. Eye Track. Res. Appl. , 89-96 (2000).</li><li>Hayashi, R., Tanifuji, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Which+image+is+in+awareness+during+binocular+rivalry?+Reading+perceptual+status+from+eye+movements.">Which image is in awareness during binocular rivalry? Reading perceptual status from eye movements.</a> J. Vis. 12 (3), 5-5 (2012).</li><li>van Dam, L. C. J., van Ee, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Retinal+image+shifts,+but+not+eye+movements+per+se,+cause+alternations+in+awareness+during+binocular+rivalry.">Retinal image shifts, but not eye movements per se, cause alternations in awareness during binocular rivalry.</a> J. Vis. 6 (11), 3-3 (2006).</li><li>Maiello, G., Chessa, M., Solari, F., Bex, P. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Simulated+disparity+and+peripheral+blur+interact+during+binocular+fusionShort+Title??.">Simulated disparity and peripheral blur interact during binocular fusionShort Title??.</a> J. Vis. 14 (8), 13-13 (2014).</li><li>Vinnikov, M., Allison, R. S., Fernandes, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Impact+of+depth+of+field+simulation+on+visual+fatigue:+Who+are+impacted?+and+how?.">Impact of depth of field simulation on visual fatigue: Who are impacted? and how?.</a> Int. J. Hum.-Comput. Stud. 91, 37-51 (2016).</li><li>Tsuchiya, N., Wilke, M., Fr&#228;ssle, S., Lamme, V. A. F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=No-Report+Paradigms:+Extracting+the+True+Neural+Correlates+of+Consciousness.">No-Report Paradigms: Extracting the True Neural Correlates of Consciousness.</a> Trends Cogn. Sci. 19 (12), 757-770 (2015).</li><li>Naber, M., Fr&#228;ssle, S., Einh&#228;user, W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Perceptual+Rivalry:+Reflexes+Reveal+the+Gradual+Nature+of+Visual+Awareness.">Perceptual Rivalry: Reflexes Reveal the Gradual Nature of Visual Awareness.</a> PLOS ONE. 6 (6), e20910 (2011).</li></ol>

The authors have nothing to disclose.

We present a step-by-step guide for the construction and use of an experimental setup that allows simultaneous tracking of the both eyes and dichoptic presentation of visual stimuli. In many situations where dichoptic stimulation is used, the critical issue preventing effective eye tracking is that the mirrors for dichoptic presentation block the sight of video-based eye trackers. This is resolved here by using infrared-transparent mirrors and an infrared-sensitive eye tracker. This setup allows researchers of 3D vision, interocular suppression or clinical research to collect high quality eye tracking data while using large, arbitrarily colored stimuli.
This setup can be modified based on the experimental needs. If the stimuli from both eyes are small enough5 to fit onto a single screen, four mirrors with one screen might be enough to achieve eye tracking with dichoptic presentation. In that case, two more front-surface mirrors (infrared-transparency not required) would be placed peripheral and parallel to the current mirrors, reflecting visual stimuli on the screen to the current mirrors (see reference 5 for placement of mirrors in a standard mirror stereoscope).
There are a few limitations of this experimental setup. One is the potential visual contamination of the illuminator of the eye tracker mentioned in the representative results. Secondly, if the color of the visual stimuli is irrelevant, anaglyph glasses might be a better choice in terms of the cost, especially if it is not critical that separation of the two eyes&#39; images is not always complete when using anaglyph glasses.
Compared to techniques that rely on non-optical recording directly from the eyes, for instance electro-oculography31,32,33 and the scleral coil technique19,34,35, the proposed method is less invasive and allows pupillometry. On the other hand, some participants have eyes that are difficult to capture using video-based eye recording, so in those cases direct recording methods are preferred. Our method should also be compared to other methods that do rely on a visual signal. For example, eye tracking can be achieved with goggle systems that have cameras integrated in the eye pieces36 or the head mounted displays37. Goggle systems have the benefit that they do not require participants to stay still but the spatial and temporal resolution of such systems can be low compared to the proposed method. It is also possible to do video-based eye recording through the lenses of anaglyph glasses (e.g. red-green or red-blue goggles)20,38,39, which has the drawback of limiting the colors that can be used in the visual stimuli shown to the participant. Separation of the eyes&#39; images can also be achieved using polarized stereo glasses30 or active stereo shutter glasses22,40,41. Such methods are easier to implement than the proposed one but the visual stimulation quality may suffer from stereoscopic crosstalk.
One group successfully used a setup combining a standard 4-mirror stereoscope with an eye tracker24,25 by tracking one eye through a gap between the mirrors. Aside from allowing only monocular eye tracking, this method has the drawback that recording through this gap limits the size of the mirrors used and, therefore, the field of view, and that it requires very specific positioning of the eye tracker. As a result, the setup routine can take up to 20 min (Miriam Spering, personal communication, May 7, 2017). In comparison, the proposed method allows a field of view of more than 40 degrees, tracking of both eyes, and it takes around 10 min to finish the whole calibration process.
There is a trend in research involving interocular suppression to use pupillary and oculomotor responses in addition to, or in replacement of, the traditional button press responses36,42,43. For one thing, eye dynamics might reveal unconscious processing, while button presses typically signal subjective awareness24,25. Moreover, relying on eye responses can prevent potential confounds associated with manual responses in experiments26,33. Our setup provides an ideal solution for those wishing to pursue this combination of interocular suppression and eye tracking.

Under normal viewing conditions each of our eyes receives a slightly different visual input. This input is then processed to produce one coherent, three-dimensional representation of the world. Dichoptic presentation, the practice of independently controlling the input presented to each of the two eyes, thus enables researchers to study how humans reconstruct a three-dimensional representation from two two-dimensional retinal images2,3,4. In addition, if the two eyes&#39; images are too dissimilar, this interocular combination fails, and observers instead report perception of only one of the images at a time while the other remains suppressed, in phenomena such as binocular rivalry5 and continuous flash suppression6. Researchers of such interocular suppression, too, use dichoptic presentation, in this case to examine questions related to topics like the neural locus of awareness7, perceptual selection8,9, and unconscious processing10.
Gaze and pupil dynamics are recorded for multiple purposes in research on human behavior and perception. Gaze direction can inform about, for instance, attention allocation11,10,13 and decision making14, while pupil size can reveal aspects of visual processing15,16, task engagement17, or fluid intelligence18.
Combining eye tracking with dichoptic presentation is useful in research into, for instance, three dimensional (3D) perception19,20,21,22 or ocular responses to visual input during interocular suppression23,24,25. For example, eye movements have been found to reveal unconscious processing without subjective perception during continuous flash suppression23. Clinical visual researchers can use the ability to track both eyes during dichoptic presentation to investigate ocular diseases that affect the two eyes asymmetrically, for example, to monitor the monocular and binocular visual distortions occurring in amblyopia26 and maculopathy27.
We recently described a setup1 that allows for the combination of high quality video-based eye tracking and dichoptic stimulation with little limitation on the size or color of the stimuli, and we evaluated its performance. Below we will summarize the construction and use of this setup.

<table><tbody><tr><td>Mirrors in Setup 1</td><td>Edmund Optics</td><td>&amp;nbsp;#64-452</td><td>dimensions 10.10 &amp;times; 12.70 cm; Reflectance: 400 ~ 690 nm; Transmission: 750 ~ 1200nm</td></tr><tr><td>Mirrors in Setup 2</td><td>Edmund Optics</td><td>Item discontinued</td><td>dimensions 10.10 &amp;times; 12.70 cm; Reflectance: 425 ~ 650 nm; Transmission: 800 ~ 1200nm</td></tr><tr><td>Other Mirror Option</td><td>Edmund Optics</td><td>#62-634</td><td>dimensions 12.50 &amp;times; 12.50 cm; Reflectance: 425 ~ 650 nm; Transmission: 800 ~ 1200nm</td></tr><tr><td>Eye Tracker in Setup 1</td><td>SR Research Ltd., Mississauga, Ontario, Canada</td><td>Eyelink 1000</td><td>Transmission: 890 ~ 940 nm</td></tr><tr><td>Eye Tracker in Setup 2</td><td>The Eye Tribe Aps, Copenhagen, Denmark</td><td>Eye Tribe (item discontinued)</td><td>Transmission: around 850 nm</td></tr></tbody></table>

how to build a dichoptic presentation system that includes an eye tracker

The presentation of different stimuli to the two eyes, dichoptic presentation, is essential for studies involving 3D vision and interocular suppression. There is a growing literature on the unique experimental value of pupillary and oculomotor measures, especially for research on interocular suppression. Although obtaining eye-tracking measures would thus benefit studies that use dichoptic presentation, the hardware essential for dichoptic presentation (e.g. mirrors) often interferes with high-quality eye tracking, especially when using a video-based eye tracker. We recently described an experimental setup that combines a standard dichoptic presentation system with an infrared eye tracker by using infrared-transparent mirrors1. The setup is compatible with standard monitors and eye trackers, easy to implement, and affordable (on the order of US$1,000). Relative to existing methods it has the benefits of not requiring special equipment and posing few limits on the nature and quality of the visual stimulus. Here we provide a visual guide to the construction and use of our setup.

After the calibration described in the protocol, we performed a calibration-validation procedure without problems with the mirrors in place. The effectiveness of the method is clearly illustrated by Figure 5, which shows the camera&#39;s image (using a research end eye tracking system) with the mirrors in place. The two sets of parallel lines along participants&#39; nose and the lines above the eye brows are the edges of the mirrors but, nevertheless, the face is as clear inside of that frame as it is outside. This highlights the lack of signal loss at the wavelengths recorded by the camera. A formal evaluation previously showed pupil size, saccade, and smooth pursuit results to be equivalent with mirrors and without mirrors1. We describe a representative part of that evaluation.
A short experiment was conducted with only one mirror in place to compare the results with and without the mirror. The participant made saccades to different locations on the screen. The eye tracker did not miss any samples for either eye. The average correlations in the horizontal gaze angle and vertical gaze angle were 0.99 (see Figure 6).
How much does it cost?
In a laboratory that already has standard eye-tracking materials such as an eye tracker, a head rest, and monitors, the approximate price of the additional components would approach US$1,000. This price compares favorably to some alternatives such as goggle systems29 at the time of publication (2017). Mirrors: $400; mirror holders: $150; fiberboard, glue, etc.: $100; monitor arms: $300. The cost of an eye tracker may range from $100 to over $25,000 depending on factors like the precision and sampling rate (see more options in 30).
How well does it work for different eye trackers?
Two types of infrared eye trackers were previously evaluated in terms of the quality of the eye data1. They are a desktop-mounted research-end eye tracker and a consumer-grade eye tracker, each in combination with a slightly different mirror pair (see Table 1 for details). The product specifications suggest that both trackers should work well with this setup, and this is corroborated by the published evaluation1. More options for eye trackers can be found in 30.
How to avoid interference of the eye tracker&#39;s infrared illuminator?
The wavelength of light transmitted by the eye tracker&#39;s infrared illuminator extends into the visible range. Participants can therefore sometimes see the red array or dots through the mirrors, especially during the calibration-validation procedure when the screen is mostly black. The severity of the concern depends on the particular experimental design, e.g. avoidance of using the color red in the stimulus will decrease the possibility of potential confusion. In addition, experimenters can increase the background luminance so that the red dots are hardly visible, and some eye trackers allow the illuminator power to be turned down. Moreover, in cases where the stimulus of interest covers a relatively small part of the screen, the illuminator can be moved not to overlap with this part.
What is the maximum size of the field of view?
The current setup could cover more than 30 degrees in visual angle both vertically and horizontally.
How long does it take to build the setup and calibrate each participant?
Building the system takes about a day if all the materials and equipment are available. It takes less than 10 min to calibrate each participant on both the dichoptic presentation and eye tracking system.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/56033/56033fig1.jpg" /> 
Figure 1. Schematic illustration of the setup. 
The setup is on an office desk, and the participant is seated at the desk and looking into a different mirror with each eye. Although not strictly necessary, best results are obtained by supporting the participant&#39;s head with a head rest mounted on the side of the table. (Note that the apparent offset between screen and board on the right side is due to limited depth cues in the image). The figure has been adapted from 1. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/56033/56033fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/56033/56033fig2.jpg" /> 
Figure 2. An assembly guide of the reference boards (panels A and C) and the central component (panels B and D). 
Panels A and C only show the reference board that is on the left side of the setup; the reference board on the right side is the mirror image of the left one, i.e. with the small vertical boards pointing away from the midline. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/56033/56033fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/56033/56033fig3.jpg" /> 
Figure 3. Calibration board. 
In this example, wooden slats take a role that can also be performed by drawn lines. A vertical slat and a horizontal slat trace around the corner of the monitor when it is in the correct position. Another vertical slat near the bottom of the board aligns with the short side of the reference board (long vertical board) when it is in the correct position. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/56033/56033fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/56033/56033fig4.jpg" /> 
Figure 4. Demonstration of the blinders. 
The blinders prevent a direct line of sight to the screens (dashed lines). <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/56033/56033fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/56033/56033fig5.jpg" /> 
Figure 5. Frame of the camera&#39;s view during dichoptic presentation, faintly showing the edges of the mirrors but otherwise showing no obstruction due to the mirrors.
<img alt="Figure 6" class="xfigimg" src="/files/ftp_upload/56033/56033fig6.jpg" /> 
Figure 6. Data collected from representative participants using the research-end eye tracker during a saccade task. 
The vertical dashed lines indicate changes in target position. The figure has been adapted from 1.

Watch this Scientific Journal Video about How to Build a Dichoptic Presentation System That Includes an Eye Tracker at JoVE.com

How to Build a Dichoptic Presentation System That Includes an Eye Tracker

We recently proposed a method that allows dichoptic visual stimulus presentation and binocular eye tracking simultaneously1. The key is the combination of an infrared eye tracker and the corresponding infrared-transparent mirrors. This manuscript provides an in depth protocol for initial setup and everyday operation.

The presentation of different stimuli to the two eyes, dichoptic presentation, is essential for studies involving 3D vision and interocular suppression. ...

how-to-build-dichoptic-presentation-system-that-includes-an-eye

Nanyang Technological University

Research

JoVE Journal

Behavior

8.4K Views.  Michigan State University. We recently proposed a method that allows dichoptic visual stimulus presentation and binocular eye tracking simultaneously1. The key is the combination of an infrared eye tracker and the corresponding infrared-transparent mirrors. This manuscript provides an in depth protocol for initial setup and everyday operation.

Video: How to Build a Dichoptic Presentation System That Includes an Eye Tracker

Using Eye Movements to Evaluate the Cognitive Processes Involved in Text Comprehension

The present article describes how to use eye tracking methodologies to study the cognitive processes involved in text comprehension. Measuring eye movements during reading is one of the most precise methods for measuring moment-by-moment (online) processing demands during text comprehension. Cognitive processing demands are reflected by several aspects of eye movement behavior, such as fixation duration, number of fixations, and number of regressions (returning to prior parts of a text). Important properties of eye tracking equipment that researchers need to consider are described, including how frequently the eye position is measured (sampling rate), accuracy of determining eye position, how much head movement is allowed, and ease of use. Also described are properties of stimuli that influence eye movements that need to be controlled in studies of text comprehension, such as the position, frequency, and length of target words. Procedural recommendations related to preparing the participant, setting up and calibrating the equipment, and running a study are given. Representative results are presented to illustrate how data can be evaluated. Although the methodology is described in terms of reading comprehension, much of the information presented can be applied to any study in which participants read verbal stimuli.

The present article describes how to use eye tracking methodologies to study the cognitive processes involved in text comprehension. Descriptions of eye tracking equipment, how to develop experimental stimuli, and procedural recommendations are included. The information presented can be applied to most any study using verbal stimuli.

The present article describes how to use eye  tracking methodologies to study the cognitive processes involved in text  comprehension. Measuring eye ...

Eye Movement Monitoring of Memory

Explicit (often verbal) reports are typically used to investigate memory (e.g. "Tell me what you remember about the person you saw at the bank yesterday."), however such reports can often be unreliable or sensitive to response bias 1, and may be unobtainable in some participant populations. Furthermore, explicit reports only reveal when information has reached consciousness and cannot comment on when memories were accessed during processing, regardless of whether the information is subsequently accessed in a conscious manner. Eye movement monitoring (eye tracking) provides a tool by which memory can be probed without asking participants to comment on the contents of their memories, and access of such memories can be revealed on-line 2,3. Video-based eye trackers (either head-mounted or remote) use a system of cameras and infrared markers to examine the pupil and corneal reflection in each eye as the participant views a display monitor. For head-mounted eye trackers, infrared markers are also used to determine head position to allow for head movement and more precise localization of eye position. Here, we demonstrate the use of a head-mounted eye tracking system to investigate memory performance in neurologically-intact and neurologically-impaired adults. Eye movement monitoring procedures begin with the placement of the eye tracker on the participant, and setup of the head and eye cameras. Calibration and validation procedures are conducted to ensure accuracy of eye position recording. Real-time recordings of X,Y-coordinate positions on the display monitor are then converted and used to describe periods of time in which the eye is static (i.e. fixations) versus in motion (i.e., saccades). Fixations and saccades are time-locked with respect to the onset/offset of a visual display or another external event (e.g. button press). Experimental manipulations are constructed to examine how and when patterns of fixations and saccades are altered through different types of prior experience. The influence of memory is revealed in the extent to which scanning patterns to new images differ from scanning patterns to images that have been previously studied 2, 4-5. Memory can also be interrogated for its specificity; for instance, eye movement patterns that differ between an identical and an altered version of a previously studied image reveal the storage of the altered detail in memory 2-3, 6-8. These indices of memory can be compared across participant populations, thereby providing a powerful tool by which to examine the organization of memory in healthy individuals, and the specific changes that occur to memory with neurological insult or decline 2-3, 8-10.

Eye movement monitoring (or eye tracking) reveals where in space the eyes linger, when and for how long. Here, we demonstrate how eye tracking can be used to investigate the integrity of memory in multiple participant populations, without requiring verbal, or otherwise explicit, reports.

Explicit (often verbal) reports are typically used to investigate memory (e.g. "Tell me what you remember about the person you saw at the bank ...

Neuroscience

VisualEyes: A Modular Software System for Oculomotor Experimentation

Eye movement studies have provided a strong foundation forming an understanding of how the brain acquires visual information in both the normal and dysfunctional brain.1 However, development of a platform to stimulate and store eye movements can require substantial programming, time and costs. Many systems do not offer the flexibility to program numerous stimuli for a variety of experimental needs. However, the VisualEyes System has a flexible architecture, allowing the operator to choose any background and foreground stimulus, program one or two screens for tandem or opposing eye movements and stimulate the left and right eye independently. This system can significantly reduce the programming development time needed to conduct an oculomotor study. The VisualEyes System will be discussed in three parts: 1) the oculomotor recording device to acquire eye movement responses, 2) the VisualEyes software written in LabView, to generate an array of stimuli and store responses as text files and 3) offline data analysis. Eye movements can be recorded by several types of instrumentation such as: a limbus tracking system, a sclera search coil, or a video image system. Typical eye movement stimuli such as saccadic steps, vergent ramps and vergent steps with the corresponding responses will be shown. In this video report, we demonstrate the flexibility of a system to create numerous visual stimuli and record eye movements that can be utilized by basic scientists and clinicians to study healthy as well as clinical populations.

Neural control and cognitive processes can be studied through eye movements. The VisualEyes software allows an operator to program stimuli on two computer screens independently using a simple, custom scripting language. The system can stimulate tandem eye movements (saccades and smooth pursuit) or opposing eye movements (vergence) or any combination.

Eye movement studies have provided a strong foundation forming an understanding of how the brain acquires visual information in both the normal and ...

Eye Tracking Young Children with Autism

The rise of accessible commercial eye-tracking systems has fueled a rapid increase in their use in psychological and psychiatric research. By providing a direct, detailed and objective measure of gaze behavior, eye-tracking has become a valuable tool for examining abnormal perceptual strategies in clinical populations and has been used to identify disorder-specific characteristics1, promote early identification2, and inform treatment3. In particular, investigators of autism spectrum disorders (ASD) have benefited from integrating eye-tracking into their research paradigms4-7. Eye-tracking has largely been used in these studies to reveal mechanisms underlying impaired task performance8 and abnormal brain functioning9, particularly during the processing of social information1,10-11. While older children and adults with ASD comprise the preponderance of research in this area, eye-tracking may be especially useful for studying young children with the disorder as it offers a non-invasive tool for assessing and quantifying early-emerging developmental abnormalities2,12-13. Implementing eye-tracking with young children with ASD, however, is associated with a number of unique challenges, including issues with compliant behavior resulting from specific task demands and disorder-related psychosocial considerations. In this protocol, we detail methodological considerations for optimizing research design, data acquisition and psychometric analysis while eye-tracking young children with ASD. The provided recommendations are also designed to be more broadly applicable for eye-tracking children with other developmental disabilities. By offering guidelines for best practices in these areas based upon lessons derived from our own work, we hope to help other investigators make sound research design and analysis choices while avoiding common pitfalls that can compromise data acquisition while eye-tracking young children with ASD or other developmental difficulties.

Eye tracking has long been used to study gaze patterns in typically-developing individuals, but recent technological advancements have made its use with clinical populations, including autism, more feasible. While eye-tracking young children with autism can offer insight into early symptom manifestations, it involves methodological challenges. Suggestions for best practices are provided.

The rise of accessible commercial eye-tracking systems has fueled a rapid increase in their use in psychological and psychiatric research. By providing a ...

Medicine

Eye Tracking During Visually Situated Language Comprehension: Flexibility and Limitations in Uncovering Visual Context Effects

The present work is a description and an assessment of a methodology designed to quantify different aspects of the interaction between language processing and the perception of the visual world. The recording of eye-gaze patterns has provided good evidence for the contribution of both the visual context and linguistic/world knowledge to language comprehension. Initial research assessed object-context effects to test&#160;theories of modularity in language processing. In the introduction, we describe how subsequent investigations have taken the role of the wider visual context in language processing as a research topic in its own right, asking questions such as how our visual perception of events and of speakers contributes to&#160;comprehension informed by&#160;comprehenders&#39; experience. Among the examined aspects of the visual context are actions, events, a speaker&#39;s gaze, and emotional facial expressions, as well as spatial object configurations. Following an overview of the eye-tracking method and its different applications, we list the key steps of the methodology in the protocol, illustrating how to successfully use it to study visually-situated language comprehension. A final section presents three sets of representative results and illustrates the benefits and limitations of eye tracking for investigating the interplay between the perception of the visual world and language comprehension.

The present article reviews an eye-tracking methodology for studies on language comprehension. To obtain reliable data, key steps of the protocol must be followed. Among these are the correct set-up of the eye tracker (e.g., ensuring good quality of the eye and head images) and accurate calibration.

The present work is a description and an assessment of a methodology designed to quantify different aspects of the interaction between language processing ...

Efficiently Recording the Eye-Hand Coordination to Incoordination Spectrum

The objective analysis of eye movements has a significant history and has been long proven to be an important research tool in the setting of brain injury. Quantitative recordings have a strong capacity to screen diagnostically. Concurrent examinations of the eye and upper limb movements directed toward shared functional goals (e.g., eye-hand coordination) serve as an additional robust biomarker-laden path to capture and interrogate neural injury, including acquired brain injury (ABI). While quantitative dual-effector recordings in 3-D afford ample opportunities within ocular-manual motor investigations in the setting of ABI, the feasibility of such dual recordings for both eye and hand is challenging in pathological settings, particularly when approached with research-grade rigor. Here we describe the integration of an eye tracking system with a motion tracking system intended primarily for limb control research to study a natural behavior. The protocol enables the investigation of unrestricted, three-dimensional (3D) eye-hand coordination tasks. More specifically, we review a method to assess eye-hand coordination in visually guided saccade-to-reach tasks in subjects with chronic middle cerebral artery (MCA) stroke and compare them to healthy controls. Special attention is paid to the specific eye- and limb-tracking system properties in order to obtain high fidelity data from participants post-injury. Sampling rate, accuracy, permissible head movement range given anticipated tolerance and the feasibility of use were several of the critical properties considered when selecting an eye tracker and an approach. The limb tracker was selected based on a similar rubric but included the need for 3-D recording, dynamic interaction and a miniaturized physical footprint. The quantitative data provided by this method and the overall approach when executed correctly has tremendous potential to further refine our mechanistic understanding of eye-hand control and help inform feasible diagnostic and pragmatic interventions within the neurological and rehabilitative practice.&#160;

Cerebral injury can damage both ocular and somatic motor systems. Characterization of motor control post-injury affords biomarkers that assist in disease detection, monitoring, and prognosis. We review a method to measure eye-hand movement control in health and in pathologic incoordination, with look-and-reach paradigms to assess coordination between eye and hand.

The objective analysis of eye movements has a significant history and has been long proven to be an important research tool in the setting of brain ...

Simultaneous Eye Tracking and Single-Neuron Recordings in Human Epilepsy Patients

Intracranial recordings from patients with intractable epilepsy provide a unique opportunity to study the activity of individual human neurons during active behavior. An important tool for quantifying behavior is eye tracking, which is an indispensable tool for studying visual attention. However, eye tracking is challenging to use concurrently with invasive electrophysiology and this approach has consequently been little used. Here, we present a proven experimental protocol to conduct single-neuron recordings with simultaneous eye tracking in humans. We describe how the systems are connected and the optimal settings to record neurons and eye movements. To illustrate the utility of this method, we summarize results that were made possible by this setup. This data shows how using eye tracking in a memory-guided visual search task allowed us to describe a new class of neurons called target neurons, whose response was reflective of top-down attention to the current search target. Lastly, we discuss the significance and solutions to potential problems of this setup. Together, our protocol and results suggest that single-neuron recordings with simultaneous eye tracking in humans are an effective method to study human brain function. It provides a key missing link between animal neurophysiology and human cognitive neuroscience.

We describe a method to conduct single-neuron recordings with simultaneous eye tracking in humans. We demonstrate the utility of this method and illustrate how we used this approach to obtain neurons in the human medial temporal lobe that encode targets of a visual search.

Intracranial recordings from patients with intractable epilepsy provide a unique opportunity to study the activity of individual human neurons during ...

Exploring Infant Sensitivity to Visual Language using Eye Tracking and the Preferential Looking Paradigm

We discuss the use of the preferential looking paradigm in eye tracking studies in order to study how infants develop, understand, and attend to the world around them. Eye tracking is a safe and non-invasive way to collect gaze data from infants, and the preferential looking paradigm is simple to design and only requires the infant to be attending to the screen. By simultaneously showing two visual stimuli that differ in one dimension, we can assess whether infants show different looking behavior for either stimulus, thus demonstrating sensitivity to that difference. The challenges in such experimental approaches are that experiments must be kept brief (no more than 10 min) and be carefully controlled such that the two stimuli differ in only one way. The interpretation of null results must also be carefully considered. In this paper, we illustrate a successful example of an infant eye tracking study with a preferential looking paradigm to discover that 6-month-olds are sensitive to linguistic cues in a signed language despite having no prior exposure to signed language, suggesting that infants possess intrinsic or innate sensitivities to these cues.

Eye tracking studies using a preferential looking paradigm can be used to study infants&#39;&#160;emerging understanding of, and attention to, their external visual world.

We discuss the use of the preferential looking paradigm in eye tracking studies in order to study how infants develop, understand, and attend to the world ...

A Methodology for Capturing Joint Visual Attention Using Mobile Eye-Trackers

With the advent of new technological advances, it is possible to study social interactions at a microlevel with unprecedented accuracy. High frequency sensors, such as eye-trackers, electrodermal activity wristbands, EEG bands, and motion sensors provide observations at the millisecond level. This level of precision allows researchers to collect large datasets on social interactions. In this paper, I discuss how multiple eye-trackers can capture a fundamental construct in social interactions, joint visual attention (JVA). JVA has been studied by developmental psychologists to understand how children acquire language, learning scientists to understand how small groups of learners work together, and social scientists to understand interactions in small teams. This paper describes a methodology for capturing JVA in colocated settings using mobile eye-trackers. It presents some empirical results and discusses implications of capturing microobservations to understand social interactions.

Using multimodal sensors is a promising way to understand the role of social interactions in educational settings. This paper describes a methodology for capturing joint visual attention from colocated dyads using mobile eye-trackers.

With the advent of new technological advances, it is possible to study social interactions at a microlevel with unprecedented accuracy. High frequency ...

Assessing Binocular Central Visual Field and Binocular Eye Movements in a Dichoptic Viewing Condition

Macular degeneration typically results in heterogeneous binocular central visual defects. Currently available approaches to assess central visual field, like the microperimetry, can test only one eye at a time. Therefore, they cannot explain how the defects in each eye affect the binocular interaction and real-world function. Dichoptic stimulus presentation with a gaze-controlled system could provide a reliable measure of monocular/binocular visual fields. However, dichoptic stimulus presentation and simultaneous eye-tracking are challenging because optical devices of instruments that present stimulus dichoptically (e.g., haploscope) always interfere with eye-trackers (e.g., infrared video-based eye-trackers). Therefore, the goals were 1) to develop a method for dichoptic stimulus presentation with simultaneous eye-tracking, using 3D-shutter glasses and 3D-ready monitors, that is not affected by interference and 2) to use this method to develop a protocol for assessing central visual field in subjects with central vision loss. The results showed that this setup provides a practical solution for reliably measuring eye-movements in dichoptic viewing condition. In addition, it was also demonstrated that this method can assess gaze-controlled binocular central visual field in subjects with central vision loss.

Presented here is a protocol for assessing binocular eye movements and gaze-controlled central visual field screening in participants with central vision loss.

Macular degeneration typically results in heterogeneous binocular central visual defects. Currently available approaches to assess central visual field, ...