how do non drug induced Visual Hallucinations work ? what happens in the brain that makes the eyes see something that isn’t there?
In: 598
Seeing involves more than just the eyes. It involves the brain too. And not just one part of the brain but a series of different parts of the brain. Each part of this system does a lot of processing and interpreting of the image to turn it in to something the brain can recognise.
Vision involves both ‘bottom-up’ and ‘top-down’ processing. Bottom-up processing is where light enters the eyes and your visual system does it’s best to turn this in to information that the brain can use. Top-down processing is where your brain tells your visual system what it expects to see. This means that after all this work the visual system does to produce an image, you are more likely to see what you expect and less likely to see what you don’t expect to see.
We can also say we can train our visual system to see certain things based on our experiences. So, for example, a hunter can become really good at recognising the shapes of different animals through this top-down processing. Someone who plays with Lego can become really good at recognising the different shaped blocks.
By extension, some visual hallucinations seem to be the result of what people expect to see, based on their previous experiences, through this top-down processing. This means that our ordinary thoughts, hopes and fears from our minds can appear in front of us as things that we see. In this way, hallucinations as a result of top-down processing will always be meaningful to the person experiencing them.
You don’t experience the sensation of vision with your eyes, you experience it with your brain. You can experience visual stimuli in the absence of eyes, but you can not experience visual stimuli with intact eyes and without a visual area in the brain.
Visual hallucinations aren’t as much a case of seeing something that isn’t there, but seeing what your visual area generates in the absence of input from the eyes. This can be due to any number of reasons involving mental illness, epilepsy, near-death experiences, trance experiences, substances (especially psychedelics) and it is also known to occur in blind people.
Think of it like this: The brain is constantly generating experiences and imagery. The information from your eyes doesn’t generate an image of the outside world, it restricts the amount of freedom your brain has to generate visual experiences. Without certain restrictions, it has more freedom to generate different experiences/images.
For a wonderful overview of this idea: **Defining and measuring hallucinations and their consequences – what is really the difference between a veridical perception and a hallucination? Categories of hallucinatory experiences (Pages: 23-45) –
Jan Dirk Blom** in: Collerton, D. (ed), 2015. *The Neuroscience of Visual Hallucinations.* Chichester: Wiley-Blackwell.
You’re actually always hallucinating to some extent. You have a blind spot in both of your eyes where the optic nerve attaches to the retina. Your visual cortex just fills in what it thinks should be there. Mood can also play a role in how you see things and how attractive a person or object may appear to you. Or have you been sick lately with a fever? Sometimes even that feels like being on drugs visually. Your brain is creating everything you see. Not your eyes.
Mine are caused by a part of my brain not having enough electricity, so my brain “fills in the gaps” with information it assumes is correct. The brain is very good at assuming things or ignoring things it feels is irrelevant. For example, we don’t notice our nose all the time. We can also tell where our nose is with our eyes closed. Sometimes, the brain fails at that and assumes the wrong thing. For me, it’s not getting enough context and assumes something familiar, like my mom’s voice when I’m at home, or my roommates voices in my apartment. Sometimes two parts are assuming things at the same time so things get more complicated.
Sometimes, the brain assumes you are experiencing an emergency. That’s a panic attack. When you’ve been in an extremely stressful situation, sometimes the brain gets stuck in that single event. If something sounds even close to gunfire, a vet might “overreact” because the brain assumes that it’s back in that single situation and tries to survive. You get the panic reaction, and also the “familiar” things that I hear and see with my brain filling in the gaps.
The one time I tried a hallucinogen it seemed as if what happened was I just started seeing this sort of harlequin pattern of colors that’d spin kaleidoscopically in front of my vision – sort of transparently. The colors weren’t visible at the start of the trip, but became visible later. The way they moved made it apparent to me that the visual disturbances I’d been seeing – bird foot prints in the asphalt road, muck running down walls, walls billowing, shapes pushing walls out as if something was behind them – were just that color cylinder’s spinning motion, except somehow overlaid on what I saw without being colored.
Must all be just the brain misfiring because it gets confused and try to interpret the issue by making it appear as if I were seeing foot prints and shapes moving.
The top answer is great, just wanted to add that we all see things that aren’t there every night, in our dreams. The images in our dreams play out through our visual cortex as if our eyes were open. Even beyond vision, our brains generally don’t make much distinction between imagining doing something and actually doing it. This is why studies have found that visualizing exercising an immobilized injured limb helps prevent muscle atrophy during healing. I highly recommend the book The Body Has a Mind of Its Own by Matthew and Sandra Blakeslee, full of fascinating science about the link between the mind and the body, but written in a totally accessible way.
Drugs are what we call external chemicals that alter your internal chemistry. There are plenty of not drug things that also alter your internal chemistry such as stress and a person’s specific phenotype. I have had chronic migraines since birth and i get visual effects that look like streaks of brightness and darkness throughout my vision. When i was in a much more stressful situation i would get dark spots in the corners of my vision and during the rare truly awful migraines my vision will warp and/or different spots will shrink down to points.
You know how you see things in your dreams?
It’s sort of like that but while you’re awake, your brain is basically creating dream images that it meshes in with your visual information.
I get hallucinogenic seizures if I don’t take medication twice a day. It’s almost like being in a bad dream, you know it shouldn’t be real but it’s scary AF. Cops were sure I was high but I wasn’t, got thrown in a facility for 12 days until they figured it out. Def the brain, I wish it were more of an exact science but it’s just not.
Latest Answers