Now that we understand a little bit about the complicated architecture of the brain, we can finally start to appreciate the beautiful complexity of the world it creates. Everything we see, hear, smell, taste, and touch is filtered through the lens of the brain, refracted through our thoughts and past experiences, and reconfigured to create our unique perception of reality. Because of this complicated system of translating sensation to perception, we never really interface directly with reality. Most of the time, the image that perception creates is fairly accurate—or at least we mostly agree on it. But like any complicated system, there are holes and pitfalls in perception that can alter the way we view the world and how we communicate our reality with others. Each of us is locked inside our own controlled hallucination that may or may not be representative of reality.
One of the reasons why perception is so complicated is that it is so interconnected. No sensation happens in a vacuum. A seemingly simple sensation like the taste of something sweet gets tangled up with the object’s smell, what it looks like, the texture of it on your tongue, and the sound it makes when you bite down. All of those complex sensations inform how you perceive the object’s taste, along with sensations from the ambient environment. The brain integrates this complex web of sensory information and attempts to organize it into a complete experience of the world. This data-driven, computational form of perception is called bottom-up processing. Bottom-up processing is the more rational side of perception, building reality up from individual sensory inputs.
Taste is actually a great example of how massive amounts of different sensory stimuli converge to create perception. Your sense of taste is intimately connected with your sense of smell—to the point that foods tend to taste bland when you can’t smell them. While taste as a sensation comes from the interaction of chemicals on your tongue, the experience of flavor is a holistic experience that relies on the taste, smell, look, sound, and feel of the food you are eating. Additionally, your perception of smell is closely tied with memory, and the emotions evoked by certain memories associated with a smell can bleed into your experience of flavor. So eating something your mother made you as a child may evoke fond nostalgia that makes the food taste even better. Or eating something that you ate during a stomach flu ten years ago may make you feel disgusted or nauseous even if the food is perfectly well prepared. This added layer of complexity is one of the reasons why taste is such a subjective experience.
As if the constant integration of millions of sensations isn’t complicated enough, perception also simultaneously goes the other direction. While your brain is rapidly sorting through and computing sensory information, it also uses its existing experience to predict the outcome. This form of perception, called top-down processing, utilizes knowledge, memory, and context to impose the most likely reality onto the data provided. Top-down processing is the reason why humans tend to perceive patterns in sensory data even when none exist—our brains try to impose a sense of order onto reality to make the computation easier. Evolutionarily, top-down processing meant the difference between staring dumbly at a shadow in the forest while trying to determine what it could be versus booking it in the other direction because whatever it is, it’s probably not friendly. While this type of processing is necessary in order to quickly understand reality, its reliance on assumption and pattern matching can easily lead to misperception.
Illusions take advantage of our brain’s desire to impose patterns on visual or auditory cues. Unintelligible sounds can take the form of a coherent sentence at the merest suggestion. Ambiguous lighting in a photograph can spark a worldwide debate over the color of a dress. Vision is especially susceptible to misperception due to the sheer number of assumptions our brains need to make to interpret sight. Ambiguity in the depth, color, relative distance, lighting, or grouping of objects in our field of vision can force our brain to make radical assumptions.
Another way that your brain sorts through the constant stream of sensory data is with selective attention and adaptation. Just like a frenzied and harried mother being pulled in six different directions by her family, the brain has to prioritize certain stimuli over others. The sensations that are considered “background noise”—usually because they are consistent and innocuous—get pushed to the back of your consciousness. For example, where are your feet right now? You probably (hopefully) know the answer to that question, but did you really know a minute ago? A minute ago, you were likely devoting very little attention to the position of your feet. Unless one of your feet fell asleep or suddenly became a home for hundreds of fire ants, there was probably nothing new or interesting happening with your foot. It’s a waste of your brain’s resources to continuously process the sensory information telling you that your foot is on the ground. So, your brain just stops responding to that stimulus. This sensory adaptation is the reason why you don’t spend the whole day feeling your clothes on your body. It’s also the reason why you can get used to sensations that are initially uncomfortable (like jumping in a chilly pool). If the sensation is consistent, and it isn’t going to harm you, then your brain shifts attention to more important things.
But even sensory adaptation and attention are influenced by our cognitive state and memories. You can learn over time to focus on certain sensations and ignore others—like how chronic pain sufferers can learn to divert attention from their pain. And your selective attention can be influenced by memories of similar sensations. If you were unfortunate enough to sit with your foot in a swarm of fire ants once, then the next time you go outside, you will likely have more awareness of sensations on your foot. So to really understand our sense of perception, we need to take a closer look at these other more complex factors—learning, memory, and cognition.
But more on that next week! For now, check out last month’s series on the architecture of the nervous system and brain. Comment on this post or email me at contact@anyonecanscience.com to let me know what you think about this week’s blog post and tell me what sorts of topics you want me to cover in the future. And subscribe below for weekly science posts sent straight to your email!
