Introduction
What a lot of people forget or just don’t understand is that unlike the traditional ‘computer/problem-solver/integrator’ analogy, the brain is highly context-dependent. The way it reacts to the outside world, and in fact the way it perceives it, is highly dependent on both exteroceptive (the world outside) situations and interoceptive(mood, motivation, etc) responses. This is a function of the limbic system. Which is one of the most fascinating parts of the brain.

It’s one of the weaknesses of most facets of psych that they don’t take into account how the wiring and reactions of the limbic system can radically change the way we think, without us even knowing about it. I’m not going to get hugely detailed today because I have a final in four hours, and because that would bore everyone, including me.

Instead, I’ll give a quick and dirty overview and integrate it with a story about my ancient dog that illustrates my main point dramatically.

Neuroanatomy of the Limbic System
First, a few pictures.

(From the wiki page I linked to earlier–Limbic System in red)
This is a real simplistic image, but it allows us to consider the gross brain anatomy and introduce a couple of evolutionary considerations.

The yellow, blue, and green areas collectively make up the brainstem. Each of them has a different function: the yellow and blue areas are called the medulla oblongata and the pons, respectively. The green is called the cerebellum. The yellow and blue areas are arranged basically like the spinal cord on steroids, and handle a lot of the basic aspects of physiological maintenance. They do the breathing and the intestinal movement and the heart and all that. You can actually cut off or destroy most of the brain structures above them and still have a live animal…just one that can’t do anything (essentially a coma). The green area is completely different from these guys. It handles coordination and making sure your movements are smooth, certain, and accurate. People with strokes often have this area affected, which is why they might shake when they move (intention tremor), have trouble walking (ataxia), or be unable to adjust their movements in the middle of an action. The brainstem is about as basic as a brain can get: You can live with it, but not really do much. Some of the non-vertebrate chordates seem to just have a brainstem and nothing above it.

The tan part of the brain that’s surrounded by the red limbic system is what’s called the diencephalon or sometimes the basal ganglia. This area of the brain serves as a relay station between the tan stuff outside (the cortex–stuff that does the ‘thinking’) and the ‘lower stuff’ below. Basically, it filters and gets sensory information to the right place to perceive it; it also modifies the info just a bit. And after the cortex is done sensing, integrating, and responding, it takes the cortex’s signal, processes it again, and sends it to the right places in the brainstem and spinal cord to get the motor response we want.

The basal ganglia are pretty complex and can do a little behaving of their own. ‘Motor programs’ such as throwing a punch or walking are found there. It also helps with keeping your head on straight. Two degenerative diseases that affect the basal ganglia are Huntington’s and Parkinson’s. Huntington’s preferentially attacks a basal ganglia nucleus called the caudate, which is important for cognition. This is why Huntington’s patients develop dementia in addition to the chorea (involuntary movements). Parkinson’s, on the other hand, attacks the Substantia Nigra, which help in the initiation and cessation of movement. This is why Parkinson’s patients shake when they’re still, but have a hard time moving on purpose; it’s like a faintly heard radio station. The static is almost as loud as the signal.

The limbic system is one of the phylogenetically oldest parts of the cerebral cortex; even the way it’s constructed at a cellular level is more primitive than the rest of the cortex (but more complex than lower structures). The oldest part of the limbic system is called paleocortex or archicortex, while the newer parts are called allocortex. All vertebrates have well-developed limbic lobes, but many don’t have much–if any–of the cortex surrounding it (the outer tan stuff). This is why goldfish seem so stupid. And why rats are less complex than cats, which are less complex than monkeys. Less of what’s called neocortex.

Archicortex has 3 layers of neurons that interact with each other in a complex network. Allocortex has 3-5 layers that can be kind of indistinct. And neocortex has 6 well-defined layers. Your emotions start in the paleocortex, move through the allocortex and then on into the neocortex. We’ll look at that in detail after a couple more pictures…

Here’s a schematic of how these structures talk to each other:

(click for larger)

And here are a couple of drawings:

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This last one is probably the best. It not only shows the structures but how they connect.

(clicky)

Stuff comes from the sensory cortices (the unshaded cortical parts toward the back and sides of the brain) into the Hippocampus (the archicortex) and Amygdala. Sensory information travels to the hippocampus and amygdala through the parahippocampal cortex (allocortex)The hippocampus is the organizer for sensing and remembering details about the outside world (exteroceptive information) while the amygdala handles emotional context (interoceptive information).

Together, they send information to the anterior cingulate gyrus (which is part of the neocortex) through the thalamus which is part of the relay system we talked about earlier. The cingulate gyrus then projects back to the hippocampus and amygdala through the parahippocampal cortex. Making Papez Circuit.

Confusing, but what it all boils down to is that the hippocampus and amygdala are what help us store memory, recognize similar situations, and, in the case of the amygdala, tell the brain how to feel about that. The emotional side of things is what we’re most concerned with today, so we’ll talk about the amygdala a bit more. The anterior cingulate cortex is what turns the amygdala’s emotional response into a guiding behavior for the frontal cortex (the part that does the thinking). The anterior cingulate is important in motivation; lesions to it result in people that understand that something scary is happening, but don’t respond to it. The amygdala also projects to the nucleus accumbens. If you’ve ever read about the neurobiology of addiction you’ve probably heard of this structure. It’s the one that dopamine activates. It’s our ‘reward center’. So together the areas the amygdala projects to provide motivation (anterior cingulate) and reward (nucleus accumbens). One encourages you to do something, the other one congratulates you on achieving it.

These areas in turn project to the frontal cortex, the area we think of as the seat of cognition, personality, and consciousness. In other words, any time you think about anything, your conscious thoughts are being unconsciously modified by the limbic system. This is evolutionarily important because it allows the brain to be efficient and speedy. It makes sure you’re more motivated to do things that’ll help you avoid dying or help you make more babies. It rewards you for doing something that doesn’t get you killed or helps you so you’ll do it next time.

Instead of spending equal time dealing with things in the environment, it lets you ignore crap like dirt and leaves and focus on the jaguar approaching you and the pretty little thing that just smiled at you. But, of course, it does all this without actually letting you know.

The Story
My dog has been going through a rough patch. Breathing heavy, coughing up a little phlegm. Didn’t touch her food all day yesterday. But she ate everything else she was given; fruits, pudding, bread, whatever. Mom was freaking out. Dad was worried. I was at school taking a test and knew nothing of her going off her feed. So I get home, see the full bowl, and then I started freaking out. Called mom. She told me the dog had something to eat.

So I figured she had a viral infection. Would explain the phlegm and the thirstiness and the heavy breathing. And if you’ve ever had a lingering respiratory viral infection (like RSV), you might have noticed a change in your sense of smell, even if your nose wasn’t plugged up (anosmia). We have really bad noses but really good tongues. Dogs are the reverse. Flavor is mainly to do with your nose, believe it or not. So with her nose all blocked up, that food wouldn’t mean much to her.

Little brown pellets are just little brown pellets. They go through the hippocampal (exteroceptive) system but completely fail to activate the amygdala. The amygdala doesn’t tickle the cingulate, so the frontal cortex doesn’t care too much either. So my dog has no interest in a food that to her aging eyes looks just like gravel. That slightly off-putting odor, on the other hand, sends her into a frenzy. She remembers eating it with fondness as the amygdala grabs interoceptive memories of her nucleus accumbens rewarding her for eating. Her cingulate gyrus motivates her to go right after that food. The motivational memory was tied to the smell, and only the smell.

But that doesn’t explain the fact she’d eat her various ‘treats’. Well, actually it does. To a dog, when their master gives them something from his hand, it’s literally a reward. And that nucleus accumbens fires right up. So Shelly has generalized taking food from my hand to mean she’s going to be rewarded when the food hits her taste buds. Her amygdala and hippocampus together recognize me holding something in my hand and holding it out to her as food in my hand. Her amygdala adds the memory of the ‘reward’. It tells the cingulate gyrus which promptly kicks her frontal cortex in the pants, sending her questing after the tidbit. Of course, in her senility, usually she misses and chews on my finger first.

We’d tried everything to get her to eat. Brought the bowl to her, held her upside down and stuck her nose in it, threw it at her. At one point I was considering chewing on it like you do with a baby to convince them that Gerber’s isn’t as nasty as it looks.

I was explaining the reward and motivation thing to my little bro, when he said “Doofus, then hold her dog food in your hand like it’s a treat. She’ll start eating it then and probably keep eating.” Sure enough, it worked. Once I’d gotten her to take the ‘treat’ from my hand, the taste memories kept her motivated.

Final Thoughts
Same object, different responses. Not because my dog’s thoughts changed but because which thoughts she was enabled to perceive were different. The subconscious mind is some powerful stuff, and while we do have some control over it, we only get that way by understanding the way our brains work in the first place. The challenge of psych and mental health research in the coming decades is to understand how to work with these subconscious processes as much as any conscious ones. Cognitive Behavioral Therapy is all fine and good, but if the problem isn’t necessarily cognitive (as limbic system could be argued to be), it won’t have much of an effect.