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Biological constraints on learning

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- [Lecturer] Researchers initially thought that all of learning could be narrowed down to these two basic processes of conditioning. Classical conditioning and operant conditioning. So they thought that all learning was based on these principles and it applied across the board. However, researchers soon discovered that there seemed to be biological constraints on learning. That animals seemed biologically prepared to learn different kinds of associations with different degrees of difficulty. And in the end what they discovered was that it was easier for animals to learn associations that are part of their natural world, but much harder to learn associations that are not. And I want to take a minute to look at one of the studies that proved this to be true. But first I want to take a minute to talk about taste aversion, and this is when an animal eats something and becomes sick and then learns to avoid that food because it's associated with that bad experience. And this is something that can happen to people too. In fact, it happened to me. I used to absolutely love cilantro, but I once got sick after eating something with a lot of it and now even the smell of it makes me want to gag. And if you have a food aversion like this, you may have noticed two things. The first is that they're incredibly powerful and really hard to get over. And the second one is that they don't always make sense. So I was eating a chicken dish with cilantro in it before I got sick. And even though I think it is probably a lot more likely that I got food poisoning from incorrectly prepared chicken than from the spice that was put on top of it, I wound up developing an aversion to the cilantro, and not the chicken. But more importantly, I want to note that my body connected the fact that I got sick with something that I ate. I didn't get an aversion to eating in restaurants with green leather seats, or wearing jeans, or eating at 7:30 p.m. My body seems to have naturally understood that nausea most likely resulted from the food that I consumed, and not any of the other factors that were going on around me at the same time. And this actually makes a lot of sense. Because we are in fact far more likely to get sick from the food that we eat, and not the type of pants that we're wearing. And because of this, that association is learned far faster than others. So now let's talk about how researchers actually went about studying this in a lab. So they started with a group of rats that they deprived of water and this was done in order to make them thirsty. These rats were then transferred to a cage where water is available, so of course they're going to drink. But this isn't everyday tap water, this water has been sweetened. It is sugar water, and prior to this these rats had only ever had tap water, so the sugar is really novel to them and they're pretty excited about it. And when the rats go to drink this very exciting sugar water a tone sounds and a light flashes, and these things all happen at the same time. So these rats actually belong to two different groups. For the first group, a tasteless, odorless substance has been added to the sugar water that's going to make them violently ill. It's gonna make them physically ill. The other group has not been given this substance. They just have plain sugar water, but sometime after they finish drinking it, about the same time the rats in our first group are becoming ill, the rats in our second group start getting electric shocks from the floor in their cage. And these are pretty painful shocks. It's not going to give them any long term damage, but they certainly aren't pleasant. And after this, some amount of time goes by where the rats are deprived of water again. And the rats are then put back into those same cages. They're put back in the same situation. So now half the rats in the first group, the group that got physically ill, and half the rats in the second group, the group that got an electrical shock, they're going to get sugar water again but this sugar water is presented without the light and the tone, so it has that same sweet taste but there's nothing external that happens when they drink. The other half of each of those two groups gets plain tap water, not sugar water. But now whenever they drink this tap water, they see the flashing light and they hear the tone sound just like before. So we wind up with these four groups. We have rats that were made sick in the first part, who are then given sugar water in the second part. Rats who were made sick in the original group who are then given plain tap water but with the external stimuli. We also have the rats that were given electrical shocks, and now they're being given sugar water again. And finally we have rats that were given those electric shocks and now they're being presented with tap water along with lights and sounds. So we have these four groups and our main question is which of these rats are going to develop an aversion, and what are they going to develop an aversion to? Okay, so let's look at this first group here. So these are the rats who are made violently ill after drinking the tainted sugar water and they were then exposed to the sugar water again in part two. Do you think these rats, these water deprived very thirsty rats will drink the sugar water after having been made ill by it? And the answer is no, they do not drink this water. They've developed an aversion to it and they're not going to drink the sugar water again, theoretically because they've made the connection that that is what made them sick the previous time. And all right, let me get a second color for my second group. These guys will be yellow. All right, so we know that the rats made the connection that drinking sugar water made them sick and so they didn't want to drink it the second time, but remember that that's not the only thing that happened. The rats also heard this tone beeping and they also saw this light flashing, and so of course it's also possible that that led to them being ill. And that's what we're testing in the second group. So these are rats who were made sick the first time, and now when they go to drink the water, which is now plain tap water, they're going to hear that tone and see that light again. Do you think that this is going to be enough to stop them from drinking the water when they're thirsty? So will these rats show the same type of aversion to the external stimuli as they did to the sugar water? And it turns out that they don't. They do not develop an aversion and they will drink, and why might this be? Well it's actually the exact same reason as before in the example about myself and my aversion to cilantro. I developed an aversion to food and not other external stimuli. And it's the same thing with these rats. In a rat's natural world, you get sick when you eat something unpleasant. They don't get sick like that when it's a certain temperature or a certain type of day or even after an injury. So even though in this case the rats got sick after being exposed to both sugar water and these external signals, they only learned to associate getting sick with a novel food and not with a novel environmental factors. All right, so now let's talk about the rats who got shocked instead of being made sick. So these are rats who also drank sweet water and saw the novel environmental stimuli so they heard the beeping and they saw the light flashing, but then instead of getting sick a short time later they actually get electrical shocks to their feet. And then a short time later, they're returned to these cages and they're either given sweet water without these environmental stimuli, or they're given tap water but with the beeping and the flashing lights. So which of these two groups do we think is going to develop an aversion and what are they going to develop an aversion to? And let's actually start talking about group four over here. So these are the rats who were given tap water on the second part, and also heard the beeping and saw the lights. So now our question is, will these rats refuse to drink the tap water in the presence of the light and the sound? And it turns out that the answer is yes, they do develop an aversion. They do not drink the water. And let's talk about why this might be because this might seem a little bit weird. It makes sense to us that the rats who were made sick by the water wouldn't drink that water a second time, but why would the rats after getting shocked on their feet develop an aversion to these novel environmental signals? So obviously electric shocks are not really something that a rat's going to come across in its daily life, but it is possible that it's going to get physically injured in some way. And when a rat does get injured in the real world it is most likely due to the fact that it interacted with something in its environment, and not because it ate something strange. And as you might have been able to guess by now, other rats in group three, so the rats that are shocked in the first part of the experiment and then given sugar water in the second part, they don't show this aversion, because as I said eating food in the real world doesn't really result in physical injury, and so these rats don't immediately associate the two. And so what this study showed, and the reason why this study was so important, is that it showed that not all associations were equal. That animals didn't naturally make associations in all circumstances. Instead, the rats were learning to avoid the things that would have been dangerous to them in the real world, so when they got sick to their stomachs, they associated that with food. And when they got shocked when their feet were injured, they learned to associate that with an environmental change. And so contrary to what the behaviorists thought, what Pavlov and Skinner thought, the rats seemed to have come into the world prepared to learn certain things. Prepared to learn that if you get physically ill it's probably because of something you ate, and if you get injured it's probably because of something that's in your environment. And what this means is that we have some kind of biological predisposition. We are predisposed through evolution to learn some associations better than others. There is also other evidence that there might be a biological evolutionary influence on learning. I think a great example of this is phobias, or more specifically what we do and don't develop phobias of. We are much more likely to develop phobias to things that were threatening to our ancestors than we are to develop phobias of things that might actually injure us in the modern world. So it's fairly easy for someone to develop a phobias for snakes or spiders or heights, but very unlikely that someone's going to develop a phobia to cars and electrical outlets. And really you are much more likely to be hurt by a car than by a snake these days. Much, much more likely but learning to avoid things like snakes and spiders would have conferred an evolutionary advantage to your ancestors. In much the same way that it would have been adaptive to learn to avoid foods that made you sick. But compared to snakes and spiders, cars are still very new. So new that even though they are actually a threat, there's no biological predisposition to avoid them. So really the takeaway from all of this is that associations that are adaptive, those that confer some kind of evolutionary advantage, are learned much faster than associations that don't rely on those biological principles. And because of this, because of these biological predispositions, we know that behaviorism can't provide the whole story. That learning can't simply be reduced down to classical and operant conditioning.