There is a common exaggeration among fisherman about the big one that got away. Usually this requires a long tale of heroic proportions, detailing the fight to bring the trophy in, and how the prize fish escaped at the last moment. Always, there is a point in the story where the fisherman estimates the size of his catch, usually extending their arms wide to add emphasis and visual impact. The audience is usually skeptical. We all know the likely size of a fish, not to mention the unlikely size of one. Our conclusion? The fisherman is exaggerating. As is so often the case in life, people need to dress up real life events to make them more interesting. It is just part of telling a good story.

However, there is mounting evidence that the fisherman’s story may not be just storytelling. This is not to say that there are a lot bigger fish out there waiting to be caught than you ever imagined possible. Nor am I suggesting that fisherman are all honest about the size of their catches. What I am suggesting is that there is a lot more to the saying “seeing is believing”, than you might realize. The fact of it is we all have a bit of that fisherman in us.

To explain what I’m getting at, let’s take the fish and the need to tell a tall tale out of our example. Instead, why don’t we just ask a bunch of people to estimate their own body size. There are some nice examples of this approach, such as the recent work by Matthew Longo, who has spent years studying human perception and in particular something called “implicit hand maps”1.

You may not realize it, but your guesses about the size of body parts, and your body in general, aren’t very accurate. What is more interesting is that most people’s guesses are wrong in a similar way. Longo and other researchers have found that when we are asked to guess the length of a finger, or the width of our hand, we tend to err in a predictable manner. For example, we will generally estimate that our hand is wider than it actually is. When guessing the length of our fingers, we tend to see them as being shorter than they actually are.

These differences between our perception of the size of things, and their actual size can be remarkably vast. They are also surprisingly consistent. We all seem to make the same types of mistakes, and are off in our estimates by roughly the same amount. What Longo has determined is that there is not one, but two different sources for these distortions. The first of them is perceptual, and seems to involve an actual “spatial warping” of our body’s receptors and tissues, which results in a twisted image of what is being perceived. This seems to have part of its origin in the way our sense of touch works. In areas of our body where we have more sensory neurons, like our fingers or our lips and mouth, the distortions are the greatest. Essentially, we have a lot more information being generated in these body parts, and this also takes up more real estate in our brain where the “somatosensory” image of our body is represented. Depending on how our receptors are arranged in the area where the sensations arise, things may get stretched or compressed when we perceive them.

The other type of distortion is conceptual. A conceptual distortion arises when our idea of how large a body part is, or how different body parts are arranged in respect to each other is wrong. Conceptual distortions are involved when, for example, we guess that our fingers are shorter than they actually are, because we tend to place the location of our knuckles too far forward. This is an error in some assumptions about how our hand is put together, and reflects a problem that originates with what we think about our hand, as opposed to our perception of the feeling in our hand. Conceptual distortions tend to involve more higher order thinking about how things look, as opposed to how they feel. They are thought to explain why some people with eating disorders tend to over-estimate their body size when asked to choose between various images of different body sizes. The idea is that someone with anorexia is, for example, biased by the depiction of beauty as thinness in social media, and has a distorted concept of how they look relative to this.

This is where we get to the neat part of things. When we actually look at activity in the brain of someone who has anorexia, we see changes that do not correspond with the notion of conceptual distortion explaining the body image problem. In fact, based on imaging technology, there is actually less activity and functional connectivity in the visual cortex than normal, and the visual cortex is actually smaller than one would expect. Longo suggests that this may indicate that someone with an eating disorder is actually less able to process the visual appearance of their body in comparison to others, and is not relying on their visual sense of things to determine how they feel about their weight. Instead, they may be more attentive to perceptual feedback – the actual sensations of their body and body parts. This is not something that a person is consciously aware of doing. However, like the hand example, how we perceive things using our somatosensory system can severely distort our conception of how we actually look.

The idea that our brain could change to enhance the role that feelings and sensations play in our judgment is not new. It also helps to explain a lot of common errors that we make every day. For example, when I have a toothache and wander about in my mouth with my tongue, my tooth seems much larger and more significant than its actual dimensions justify. We may be astonished at how small the thorn or sliver that has been festering under our skin actually appears when it is finally extracted. And the fisherman can be excused for interpreting all the tugging and pulling and resistance in her line to equal a big fish. Along with the possibility that our sensory feedback may become more salient than our visual sense of things, it is well established that our executive thinking, which is responsible for our ability to reason things out, act with purpose, and inhibit impulses, tends to shut down when we get emotionally overwrought.

So in the end, the fisherman may be no different than someone with anorexia when it comes to misjudging the size of things. For that matter, we all may be more prone to seeing what we believe, and what we believe may be more strongly based on sensations and feelings than we realize.