Each of our eyes normally sees a slightly different image, and the brain combines these two images into a single coherent representation. However, when the eyes are presented with images that are sufficiently different from each other, a pattern of perceptual alternations transpires, one image dominates awareness while the other is suppressed. Dominance alternates between the two images typically every few seconds.
This perceptual phenomenon is known as binocular rivalry. To create a binocular rivalry stimulus, each eye is presented with a different image at the same perceived location. This video describes several inexpensive and straightforward ways to create and use binocular rivalry.
Hi, I am David Carmel. I'm a postdoc in Department of Psychology at New York University. Hi, I am Mike Arca and I'm a graduate student in the Department of Psychology at Princeton University.
In this video, we will describe a few cheap and straightforward ways to create and use binocular rivalry. Binocular rivalry is considered useful for studying perceptual selection and awareness in both human and animal models because unchanging inputs into each eye leads to alternations in visual awareness. There are several ways of creating binocular rivalry, but newcomers to the field are often uncertain which method would best suit their specific needs.
Our goal here is to describe the advantages and disadvantages of the most common methods. So let's get started. Binocular rivalry can be induced by presenting each eye with an irreconcilable different image at corresponding retinal locations instead of a single combined image, what observers normally see is a pattern of perceptual alternations in which each eye's image dominates conscious perception for a certain period while the other image is suppressed.
Periods of dominance and suppression reverse periodically with only brief durations of mixed perception. Binocular rivalry can be used to examine several important questions such as at what stages within the visual processing hierarchy do neural events correlate with conscious experience? How does the brain resolve competition between stimuli and choose which one to bring into awareness?
What aspects of an image that is suppressed from awareness can nonetheless be processed and how can such processing affect behavior? There are several simple methods for creating a binocular rivalry display. In this video, the most popular methods are outlined, including each method's, advantages and shortcomings.
Before going into the methods, we must address the issue of maintaining stable virgins during binocular rivalry experiments. Before going into specific methods for creating rivalry, it's important to mention the issue of stable virgins, which is a vital consideration in all the methods that will be described. Normally, our eyes turn or make virgins movements in a way that makes the same fixated image fall on each fovea.
However, successful virgins depends on each eye seeing the same things. If each eye is presented with an entirely different image, virgins will be disrupted because the brain won't have sufficient information to decide on the correct virgin's angle. This may disrupt binocular rivalry as the two images may not fall on corresponding retinal locations.
Therefore, in addition to the different images, the display should contain elements that are identical for both eyes. This enables the eyes to maintain a stable gaze despite the difference between the rivaling elements of the images. Usually virgin stabilizing identical elements include a fixation point at the center of the rivaling images and a frame around the images.
The frame can be either uniform or textured and can have any shape as long as it is identical in both eyes. Uncorrelated horizontal eye movements are more likely than vertical ones. Therefore, a textured bar on either side of each image can be used instead of a full frame.
Finally, in some studies, a frame may be undesirable. For example, if the experiment requires that stimuli appear on a uniform background, in such cases, it's possible to use NUNUs lines or an image that appears further out from the stimulus such as dartboard rings. Now let's take a look at how to induce binocular rivalry.
Here we will review three inexpensive and straightforward options, including red blue goggles, the mirror stereoscope and prism goggles using chromatic goggles, either red, blue, or red green is a popular method preferred by many researchers because it's the easiest and cheapest to implement all one needs is a pair of cellophane goggles available at many toy stores. Here we will use red blue goggles set up by preparing one image that is displayed solely by the monitor's blue gun and another that is displayed at the same location on the screen solely by the red gun. Each of the lenses will only pass one of the images, so the two different images will fall on corresponding retinal locations in the two eyes and start to rival each other.
The two images should contain identical information like a frame or fixation point to ensure stable virgins. These identical elements should be in a color that both lenses will let through, such as black or white. Note that this technique has nothing to do with color vision.
The colors are simply used to segregate the two images each to one eye. This should work even for observers with abnormal color vision. The advantages of using red blue goggles include the equipment is very inexpensive and stimuli are very easy to prepare.
Red blue goggles can easily be used with all neuroimaging methods, including MRI. And finally, red blue goggles do not require head stabilization or individual adjustment of the viewing device for each observer. The disadvantages include each image can only contain shades of a single color, so no chromatic stimuli.
The lenses are not perfect, so there will always be some bleed through and each eye will see some of the other eye's image. This creates a problem for claiming that the suppressed image was entirely unseen. Bleed through can be reduced by using more than one filter.
For example, wearing two goggles, one on top of the other, and finally, red blue goggles do not work well with most current eye trackers. Now let's move on to the mirror Stereoscope method. Mirrors can easily be set up to deliver a different image to each of the observer's eyes.
First, prepare two different images that have some identical elements and display them side by side on a monitor. A mirror stereoscope is easy to construct to do so all one needs is four mirrors and stands to mount them on position. Two mirrors so that each is near one eye and at a 45 degree angle to that eye's line of viewing, use a chin rest to stabilize the location of the observer's head.
Place another mirror on either side of each of the first two mirrors facing the stimuli at a 45 degree angle so that each image falls on a corresponding location in each eye. The dissimilar images should now rival each other. In most cases, it is convenient to use one of several commercially available stereoscopes.
Each observer's eyes are a little different, so in placing an observer in front of the display, it may be necessary to adjust the mirror's angles to obtain stable virgins. When using a mirror stereoscope, it is important to make sure that each eye can only see the image it's supposed to, and that this image is only seen at the location where it rivals the other image. In many cases, each eye will also have a line of vision to the other eye's image.
To block this undesired line of vision, place a divider. For example, a sheet of cardboard extending from the stereoscopes midline between the observer's eyes toward the center of the display in such a way that it'll block the line of vision to the other eye stimulus. An additional problem that may occur is that each eye might see the image it is supposed to see twice, once through the mirror, and once again directly.
This will cause an additional image of each stimulus to appear next to the location where rivalry occurs. To avoid this, adjust the relation between the images location and the observer's distance from the screen. In order to make these adjustments before the experiment begins, prepare an image showing only the parts of the display that are identical on both images and uses to set the stereoscope up for each observer before displaying the rivalry stimulus.
The advantages of using mirror stereoscopes are that separate images allow for the use of chromatic stimuli. The images are completely separate and cannot bleed into each other. Stimulus preparation is easy and simple.
Any two pictures presented side by side can rival each other. And finally, stereoscopes can be used in combination with eye tracking. The disadvantages include stereoscopes only allow for presentation of fairly small stimuli because only half of the visual field can be used to present each image.
Stereoscopes cannot easily be used in an MRI scanner as this would require all elements of the stereoscope to be non-magnetic, and the setup would also have to incorporate the additional tilt of the mirror through which stimuli are normally viewed in the scanner. And finally, stereoscopes require head stabilization and individual adjustment for each observer. Now let's see how to use prism goggles.
The prism goggle method is a variation on the idea of the stereoscope using goggles in which the lenses are prisms instead of mirrors. As with a mirror, stereoscope images are presented side by side on a monitor. Prism lenses can be purchased from any commercial optic supplier along with plastic frames.
Each of the prisms bends light making objects that are off to the side seem to be straight ahead. Two, such prisms oriented in opposite directions act in the same way as a mirror. Stereoscope, would they create the illusion that two images that are in fact physically side by side overlap in space?
Note that when using prism goggles, you still need to use a divider as each eye can see the other eye's image. However, the distance and size of the display do not need to be adjusted because each image only has one line of vision to each eye. The advantages and disadvantages of prism goggles are similar to those of mirror stereoscopes with one big difference.
It is easy to use prism goggles in an MRI scanner as they can be made of plastic and are more compact than a mirror stereoscope. Now let's see how to ensure complete suppression of the images to address research questions concerning the processing of the suppressed image. During binocular rivalry experiments, a strong form of rivalry known as continuous flash suppression or CFS for short is most suitable to create a CFS stimulus.
Present a relatively low contrast image to one eye. This will be the suppressed image. Present a high contrast, rapidly changing image to the other eye.
This will be the dominant CFS mask to be maximally effective. The CFS mask should change at a rate of between 10 and 20 hertz. CFS can be induced using all the rivalry methods described earlier in this video when using a mirror stereoscope or prism goggles.
CFS mask composed of many small, colorful elements are highly effective. However, A CFS mask composed of gray scale elements can be effective too when using red blue goggles. The CFS mask can be composed of many elements that are all the same color.
If you watch this video through red blue goggles, you will probably get a lot of bleed through because of the video compression it went through to maximize the chances of complete suppression. Adjust the contrast level of the suppressed image before the experiment begins. Alternations, the dominant and suppressed image are usually gradual and can be rather slow, meaning that quite a bit of the viewing time is taken up by mixed phases.
The specific form of mixed phases varies between observers and for different stimuli. Shown here are two common forms of mixed phases. First, in piecemeal rivalry, a mixed phase can consist of the suppressed image gradually becoming dominant through an increasing number of dominant patches across the stimulus.
Second, a mixed phase may also occur through a wave of dominance sweeping across the image. To induce such a wave, introduce a contrast increment to a specific part of the suppressed image. Binocular rivalry alternations occur at random independent duration intervals.
This means that the duration of the last dominance interval does not predict how long the next one will be. If dominance durations are divided into bins with an equal width. A histogram showing how many dominant durations of each length occurred tends to be WellFit by a skewed distribution known as a gamma function.
The effects of experimental manipulations on durations and rivalry tend to manifest themselves in the shape of the best fit gamma function in each condition. Many different dominance durations will occur, but the probability of these might be altered by the manipulation factors such as the two images. Low level features affect the relative durations of their dominance and suppression period.
For example, if the two images differ, in contrast, the higher contrast image will have longer dominance durations leading to a best fit gamma distribution with a greater median. Additionally, different observers may yield different gamma distributions for the same stimulus set. It is possible to use the parameters of the gamma function as dependent variables in an experiment, but the relationship between these parameters and the shape of the distribution is not readily transparent.
Therefore, a more accessible central tendency measure can be useful. However, because the gamma distribution may be highly skewed, the median duration rather than the mean is more often representative of the results. Using the median of a non Gaussian distribution also means that unless there is a large number of data points, the relevant statistical tests should be non-parametric.
In this video, we've described the nature of binocular rivalry, several methods for creating it, and what considerations must be taken into account when this is used. We Hope you find our introduction helpful in employing this fascinating phenomenon.
