When most people think about how vision works, they think of the eyes as being like a camera that sends pictures into the brain which tries to make sense of what the eyes are seeing. In this paradigm, the brain is a passive recipient of the information from the senses. But the latest theories from neuroscience give the brain a much more active role in vision and the other senses.
The prevailing theory, known as predictive coding, is that the brain constructs a model of the world and it continuously sends messages to the visual cortex to ask “Is my model accurate?”. Is there still a dog chasing the ducks? Is the woman walking down the street still walking down the street? Is the ball still heading towards the goal? The visual cortex responds that yes, the dog is still chasing ducks and yes, the woman is still crossing the street and yes, the ball is still headed towards the goal.
The predictions made by the brain help us to know that the dog is still chasing the ducks even when the dog runs behind an object that hides it briefly and the brain is able to predict when the dog will emerge from the other side. Most things that happen in the world have a certain predictability: objects usually don’t vanish for no reason and a ball on a certain path will usually continue on that path unless something intervenes. If the visual cortex has information that contradicts the brain’s model — if the dog did not emerge from behind the object — the brain gets a little surprise and tries to figure out a new hypothesis for what is happening. It updates its model and makes new predictions.
I don’t know much about how the olfactory cortex works but I imagine it’s not too different to the visual system. If you suddenly smell dog treats, the brain decides there must be dog treats nearby and continually asks the olfactory cortex to confirm “Do you still smell dog treats?”… “How about now?” When the smell goes away, the brain updates its model and stops asking about the dog treats.
Now, consider that something went wrong in the brain and it reports that it can smell dog treats all the time. How would we explain this?
One possible explanation is that the olfactory cortex — or the part of the insular lobe that integrates smells into the brain’s model of the world — is experiencing a seizure. Seizures are sudden bursts of electrical activity in the brain that temporarily affect how it works. A seizure in the olfactory cortex might magic up the idea of the smell of dog treats and tell the brain, “We smell dog treats!” even if there are no dog treats within smelling distance.
Another possible explanation is that the brain’s olfactory model of the world says “Hey! We were smelling dog treats before. Are we still smelling them?” And the olfactory cortex says “I dunno. I have a glioma. Could be anything!” And the brain says “Probably still dog treats then.” Or maybe the dog-treat-detector is just stuck in the on position.
I smell dog treats ALL THE TIME. My coffee smells of dog treats. My salmon smells of dog treats. Sometimes the smell changes: on Sunday, I had liquorice tea all day; last week, it was dry-roasted peanuts. I guess it’s possible that I am experiencing epileptic seizures — sudden bursts of electrical activity — ALL THE TIME but I think it’s more likely that my olfactory cortex is failing to update my brain’s olfactory model because it has been damaged in some way by the tumour that is growing inside.
I wish there were some kind of diagnostic test to tell whether or not my brain is experiencing seizures.
My epilepsy nurse says that I should continue taking the anti-seizure meds and see if the seizures stop (narrator: the seizures have not stopped) and add another anti-seizure medication to see if that makes them stop (narrator: the seizures have still not stopped). Today she suggested adding a third anti-seizure medication. I said I’d rather not add any more medications until I have some indication that I am actually having the seizures that they are intended to treat. I wish I knew more about neuroscience so that I could argue my case more confidently.
My textbook — Principles of Neural Science — arrives tomorrow.