I'm blind to the fact that I'm blind
Did you know you have a huge hole in your vision? It's always there, in both eyes, and it's called the blind spot. It is caused by the junction of a bundle of fibers in the back of the eye called the optic nerve. Instead of having a huge blank spot in your field of vision, your brain does some quick acrobatics to fill in the visual scene so it doesn't look choppy or incomplete.You might have done this as a kid, but it still blows my mind as an adult. Look at the plus sign on the left side if the screen. Close your left eye, and look straight at the cross. Without moving your eye, move your head closer to your screen until the little person on the right disappears. Crazy, right?!
The blind spot is interesting because you don't perceive that you have a hole in your vision. Your brain takes information from the surrounding regions and integrates that to complete your visual perception. But does that type of integration happen on a level of cognition that's higher than visual perception? You bet it does! On to the next lie.
What the H?
Without any particular of context, what is this a picture of? Is it a picnic table? Is it a poorly painted parking lot?
What if I told you it was a letter? Now what do you think it is?
Take a look at the lines in the context below.
Take a look at the lines in the context below.
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THE CAT is a nice demonstration of the mind's influence on perception at the level of higher-order symbols (e.g., letters). We call the brain's influence on perception top-down processing. An expectation, or the current context, can completely determine the perceptual experience of an ambiguous stimulus.
Another great example of top-down processing is to look at something you absolutely know the color of in the broad daylight. For example, you know that a vine-ripe tomato is a brilliant shade of red; however, when the sun has set, go back outside and look at your tomatoes again. Are they still bright red? Your brain says "yes," but it is lying to you. If you didn't know you were looking at your tomatoes in the dark, you would see those plump little fruits as dark red, or even black [2].
Fun stuff also starts to happen when the brain attempts to integrate across different senses.
A McGurk Says What?
A really mind-bending demonstration of the integrative power of the brain is called the McGurk Effect. Instead of my trying to explain what it is, take a look at this short video and experience it for yourself (see especially seconds 0:30 - 1:30).When you think about it, your brain has to do something fairly tricky to pull off this lie. It has to pick out a noise-producing event from the visual system and bind it to the thing that's actually causing the sound. The McGurk Effect is interesting because the visual information comes into direct conflict with the auditory information. To resolve the difference, the brain has to do something. It resolves the discrepancy by giving superiority to the visual information and tweaks what it thinks it hears; hence, you hear "fah" instead of "bah" when the lips form the visual cues of "fah."
The STEM Connection
One STEM application of top-down processing relates to "the philosophy of science" and the importance of collecting data to help us learn about the world around us. Given that our brains lie to us on a regular basis, it is important to be open to the possibility that what we see in the world around us is NOT the truth, the whole truth, and nothing but the truth. Science is all about discovery, and to make new discoveries we need to make careful observations and find ways to test whether the world is really as it appears to be, according to what are brains are telling us.To demonstrate the impact of top-down processing on scientific discoveries, suppose it is 1473, and you are looking up at the stars. You have been taught, like many generations before you, that the Earth is firmly planted in the center of the universe, and all of the stars rotate around us. In fact, that makes perfect sense because, when you look at the heavens above, the stars spin and we stay put. How could it be any other way?
Then comes along Nicolaus Copernicus, with Galileo Galilei not far behind. These astronomers had the audacity to suggest that the Earth is, in fact, not the center of the universe. Instead, they suggested that sun could be at the center of our own solar system, and that the Earth rotates around that. Sacrilege! Based on their extensive documentation of planetary movement in the night skies, they concluded that the data simply did not conform to the theory that the Earth was the center of the universe. Given how entrenched people were in the prior worldview, however, it took hundreds of years for the ideas proposed by Copernicus and Galileo to be embraced as the new truth.
When introducing students to the idea of the scientific method, illustrating the influence of top-down processing on our perception and understanding of the world around us might help convince them that collecting and analyzing data is not just something you make them do to kill time between lunch and study hall. To really get them fired up about the need to collect data to advance science, have them discuss questions like these:
- Is it possible for a scientist to be an "impartial" observer?
- What sorts of cognitive biases does the observer need to be aware of?
- How can he or she overcome (or at least minimize) them?
In closing, your mind is constantly pulling off some really complicated tricks. As we discussed, the brain fills in missing information with each movement of the eye, in real time. That strikes me as a computationally heavy task, in a minimal amount of time. In other words, perception is nothing short of amazing, even when the results happen to be a lie.
Share and Enjoy!
Dr. Bob
For More Information
[2] This is called the Purkinje Effect, and even knowing about the illusion doesn't make it any less powerful. You have to reduce the effects of top-down processing by eliminating any of the surrounding context. One way is by looking through a paper roll tube and just look at the object of interest. Then, you can start to appreciate that the colors have indeed shifted.
[3] An interesting math problem would be to calculate the distance between the fovea, which is the center of your vision, and your blind spot. The retina contains the highest density of light-sensitive photoreceptors at the fovea, and the blind spot, naturally, has the fewest. So you could set this up as a nice application of using the Pythagorean Theorem to calculate distances that you can't measure directly (in other words, you can't have students sticking rulers in their eyes).
