Source: Laboratory of Jonathan Flombaum—Johns Hopkins University
In the back of everyone's eye is a small piece of neural tissue called the retina. The retina has photosensitive cells that respond to stimulation by light. The responses of these cells are sent into the brain through the optic nerve, a bundle of neural fibers. In each retina there is a place somewhere in the periphery where the outputs from retinal cells collect and the bundled optic nerve exits to the brain. At that location, there is no photosensitivity-whatever light reflects from the world and lands in that position does not produce a signal in the brain. As a result, humans have a blind spot, a place in the visual field for which they don't process incoming stimuli.
However, people are not aware that they have blind spots; there is not an empty hole in the visual images in front of the eyes. So what do people see in their blind spots? The brain actually fills-in missing input based on the surroundings.
This video demonstrates how to find a person's blind spot, and how to investigate the mechanisms of perceptual filling-in.
1. Stimulus Design
- Open a blank white page in a slide editor (PowerPoint or Keynote will suffice).
- Near the right edge of the page, make a circle about as large as penny, when the slide is printed. This will be the fixation point.
- Near the left edge of the page, make a small star (or any other shape), so that when you print the slide, the shape is about the size of a penny This slide is the left eye blind spot stimulus.
- Duplicate the slide, and switch the positions of the fixation point and the shape. This makes the right eye stimulus.
- Print the stimuli.
- Figure 1 shows what these stimuli should look like when printed on an 8.5 x 11 piece of standard white paper.
Figure 1. Stimuli for finding the blind spot in each eye. The stimuli should each occupy an 8.5 x 11 in. piece of paper with the star about the same size as a penny, when printed.
2. Procedure for finding the blind spot
- Explain the procedure first for the left eye, then how to adjust it for the right eye. This procedure can be performed on one's self, or recited to an observer to perform.
- Hold the left eye stimulus sheet at arm's length with the shapes facing you.
- Now close or patch your right eye. With your left eye, fixate the circle on the right side of the page. You will still probably see the star on the other side.
- Slowly move the page to the left or to the right, up or down, closer and farther. At some point, the star will disappear from the corner of your eye.
- Once you have made the star disappear, move the page around in very small increments. You should be able to make the star appear and re-appear through small adjustments.
- The place where the star is when you cannot see it is your blind spot.
- Repeat the procedure with the right eye stimulus, closing your left eye this time.
3. Using the blind spot to study perceptual filling-in
- With slightly more complicated stimuli, the blind spot provides a powerful demonstration of how the brain creates, even at times invents, our perceptual consciousness.
- Generate three additional blind spot stimuli by placing the star shape among some kind of pattern-this can be as simple as a colorful background or within a continuous line segment. Figure 2 shows three example stimuli.
- Repeat the procedure in 2.2-2.7. This time though, ask the participant what they see when the star stimulus disappears.
Figure 2. Blind spot stimuli for demonstrating properties of perceptual filling-in. In these images, when the star occupies an observer's blind spot, their brains fill-in the missing stimulation to conform to the properties of the surrounding image.
To understand how the brain creates a perceptual experience—a representation of a person’s surroundings that involve sights—researchers may study an area in the visual field called the blind spot.
Normally, light reflected off of objects enters the eye, and is focused on the retina—a piece of neural tissue positioned at the back—where photosensitive cells exist and are stimulated by this light.
Their signals collect and leave the eye through a bundle of nerve fibers called the optic nerve, which then relays these responses to the brain.
When these signals reach the visual cortex, they are interpreted, resulting in the conscious experience of what images in the painting look like—including their shape, texture, and color.
However, the visual information the brain receives does not provide a complete picture of the painting; due to the anatomy of the eye, there are pieces missing. This is the result of photosensitive cells being absent from the region of the retina where the optic nerve exits to the brain.
Thus, any light that lands on this position does not produce a signal, which results in humans having blind spots for both of their eyes—positions in the visual field for which incoming stimuli are not processed.
We are not aware of these regions, as our brains are capable of "filling-in" blind spots by extrapolating from our broader surroundings—like the flanking colors and textures.
Employing techniques that focus on the eyes’ blind spots, this video investigates the mechanisms by which the brain creates—and fills-in—perceptual experiences.
Not only do we explain how to design the stimuli and collect participants’ blind spot data, but we also explore how researchers use these methods to investigate the neural mechanisms behind, and diseases that affect, visual perception.
In this experiment, participants are first presented with simple shape-based stimuli—designed to locate their blind spots—followed by more complex ones to ultimately investigate how the brain fills-in missing portions of individuals’ visual fields.
The first type of stimuli—designed to locate participants’ blind spots—consists of a circle and star, both in the same color and positioned on opposite sides of a white piece of paper.
For a stimulus evaluating the left eye’s blind spot, the circle occurs on the right side of the paper. In contrast, for a right eye stimulus, the circle is positioned on the left side of the sheet.
Before viewing these images as part of the task, participants must place a patch over the eye that is not being tested—for example, the right eye for a left-focused stimulus—in order to avoid overlapping of the visual.
Participants are then instructed to hold the stimulus in front of them, and focus on the circle. Initially, they are likely to see both the circle and star, meaning that neither shape is positioned in a blind spot.
Participants then move the stimulus in a combination of directions: left or right, up or down, and closer or farther. This is continued until the full sheet of paper is still in view, but the star is reported as having disappeared.
The trick is that although the star remains on the sheet—the shape’s not physically erased—by shifting the stimulus, participants move it into the blind spot in their eye’s visual field.
As this cannot be supplemented by information from the patch-covered, opposing eye, the star perceptually vanishes.
To confirm the position of the blind spot, participants repeatedly move the paper in small increments, so that the star reappears and disappears.
Once the blind spots of both eyes have been located, "filling-in" tests are performed with more complicated stimuli.
In this case, the stars are placed in different settings—against a solid-colored background; among several uniform, colored shapes; or in the center of a colored rectangle—each of which constitutes a separate trial.
Respectively, these three types of stimuli are meant to look at how the brain perceptually approaches uniformity, patterns, and object continuity.
The same steps are performed as for blind spot-locating tests, but participants must report what they observe when the star disappears—for example, from the middle of a colored rectangle.
When the star is positioned in the blind spot, participants’ brains are expected to fill-in this lacking information based on the surrounding image. For example, they will likely report seeing a solid, continuous rectangle, given the local context.
To prepare for the experiment, use a slide-editing program to create the stimuli slides, which consist of different shapes approximately the same size and positioned on opposite sides. Create two sets for the left and right eyes: one group for finding the blind spot and the others for the filling-in trials.
Greet the participant when they arrive, and seat them at a table. Explain that for all stimuli they will be viewing, they should remain fixated on the circle.
To begin finding the blind spot, hand them the left eye stimulus sheet and an opaque cover. Instruct the participant to block their right eye and hold the paper at arm’s length, so that the circle and star are facing them.
Watch to ensure that they identify the position of their left eye’s blind spot. Repeat this procedure for the right eye: hand them a new stimulus sheet and ask them to cover their left eye.
Once the blind spots for both eyes have been located, allow the participant to complete the three filling-in trials for each eye.
After each trial, ask the participant what they observed when the star disappeared from their visual field, and record their responses.
To analyze the data, identify what participants most often reported seeing during filling-in trials when the star occupied either of their eye’s blind spots—in other words, when the star disappeared from view.
Notice that for stimuli where the star was on a yellow background, participants tended to observe a solid yellow space, which indicates that the brain expects uniformity in surface color and fills in missing blind spot information accordingly.
In contrast, a star positioned in a row of red circles was typically replaced by a circle of the same color and size, suggesting that the brain looks for patterns.
However, stars interrupting rectangles appeared filled-in with the same color as the rectangle itself, indicating that the brain expects object continuity.
Collectively, these results indicate that the brain creates perceptual experiences based on the context—either uniformity, pattern-based consistency, or continuity—of its surroundings.
Now that you know how to design a blind spot-based experiment to investigate human visual perception, let’s explore other ways researchers apply this technique.
Up until now, we focused on typical blind spots that result from the position of the optic nerve in the retina.
However, there are other types of abnormal blind spots, referred to as scotomas, which stem from retinal damage or disease, such as macular degeneration.
In such cases, researchers have found that when individuals were shown stimuli spaced in a low-density array, the dot appearing in the scotoma region was perceived as missing. In contrast, with a high-density array, fewer dots were reported as being absent, suggesting that the brain is able to fill-in certain patterns even when damage exists.
Finally, much work is aimed at identifying the areas of the brain involved in creating perceptual experiences.
By pairing blind spot filling-in stimuli with fMRI technology, researchers were able to pinpoint regions in the visual cortex responsible for processing blind spots in the visual field.
Importantly, when stimuli were placed in a blind spot, the associated visual cortex neurons responded as if they were receiving external signals, even though they actually obtained no input from the retina.
In other words, these cells responded as if what participants perceived in a blind spot—what the brain created to fill-in this region—was an actual external stimulus.
Collectively, this work suggests that neurons in the early part of the visual system are directly involved in constructing perceptual experience.
You’ve just watched JoVE’s video exploring how blind spots can be used to gain insight into the brain’s creation of perceptual experience. By now, you should know how to generate different types of blind spot stimuli, and collect and interpret "filling-in" data. You should also have an idea of how researchers study the mechanisms and neuroanatomy behind blind spot supplementation.
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Figure 3. Perceptual reports: What participants report seeing (shown in the right column) when the star in the left column images disappears in the blind spot. The reports reveal several principles of how the brain creates perceptual experience. Most broadly, in all cases, the brain fills-in the blind spot as the most likely content given the local context.
Figure 3 visualizes what people report seeing when shown the stimuli in Figure 2, that is, what they report seeing once the star disappears because it occupies their blind spot.
The reports revealed several principles of how the brain creates perceptual experience. The top stimulus revealed that the brain tends to expect uniformity in surface color, thus filling-in the blind spot to match the yellow background in the remainder of the image. The middle stimulus revealed that the brain looks for patterns, here causing perceptual experience to include a red circle in the blind spot in order to complete the prevailing pattern in the region. The bottom stimulus revealed that the brain implements expectations about object continuity. When the star occupies the blind spot, observers see a solid green rectangle, filling-in the missing middle portions of that object. Thus in general, the brain fills-in experience with the most likely stimulus to occupy the blind spot. Whatever is most likely given the context is what the brain causes people to perceive.
Applications and Summary
Because the brain fills-in perception within the blind spot, one application involves studies that seek to identify the brain areas involved in producing conscious experience. For example, for a long time, researchers were uncertain whether the earliest part of human visual cortex, called V1, was involved directly in the production of conscious experience. To address this issue, researchers used fMRI (Functional Magnetic Resonance Imaging) to measure neural responses in the monocular regions of V1 that mapped to the blind spot.1 In other words, they looked at neural regions that only received input from one of the eyes, and by mapping the blind spots of their participants, they were able to look at neural responses specifically when direct stimulation was absent, because the stimulus was placed in the relevant eye's blind spot. What should happen? Would the neurons not fire because they were not receiving any signal from the outside world? Instead the neurons responded as though they were being stimulated by what the person perceived in that location-they responded as though the images that the brain filled-in were actually producing external stimulation. This suggested that these neurons in very early parts of the visual system are not only involved in transmitting signals received from the retina, but also in constructing perceptual experience itself.
- Tong, F., & Engel, S. A. (2001). Interocular rivalry revealed in the human cortical blind-spot representation. Nature, 411(6834), 195-199.