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.
Thanks for watching!
