Even the brain has middle management
Many cars these days come with a dashboard-mounted GPS device that allows you to see where your car is on the map. For most of us, this has seriously reduced how often we get lost. There are two different ways of viewing the relationship of your car to the map.
In the first, the picture of the car on the map is always facing forward – just like the car itself. When you turn the car, the map turns around the car’s image. True north rotates when you use this method – when the car is moving north, north is ‘up’ on the screen, but when the car is moving south, north is ‘down’ on the screen. This is because the map is displayed relative to you. Most people find this a useful way of looking at things, because you can easily figure out when you need to turn right or left – the screen looks exactly like what you, the driver, are seeing.
In the second display option, North is North. Period. End of story. When the car is headed north, what you see on the screen matches what you see in real life. But when the car is headed south, you must mentally rotate the map in order to have it match what you are seeing (a neuroscience feat of the parietal cortex that has been amply investigated, and which we will return to someday). The point we’re making here, however, is that this isn’t a relative way of seeing the map, but rather an ‘absolute’ one.
In the previous post (the brain is a river) we explored how neuroscience talks about the brain in a relative way when it talks about information flowing upstream or downstream. What’s up and what’s down depends on where you are in the brain, and whether you are looking at information flowing into or out of that area. This is analagous to the first viewing mode on a GPS – the brain changes relative to the scientist’s view of it.
But just as true north matters sometimes when you’re driving, ‘true’ upstream matters in the brain. And here’s where neuroscience transitions from the river metaphor to the corporate one.
In the corporate model of brain function, certain parts of the brain are ‘higher up’ in the chain of command than others. Specifically, parts of the brain closest to the sensory periphery are treated the same way voters are treated by the political system, or cashiers are treated by a large department store. They are the ‘bottom’ – the ‘grass roots’ – the most numerous, but individually the least powerful, people in the system. The parts of the brain that most directly influence movement – or as we call it in neuroscience, motor behavior – are treated the same way as the President of the country, or the CEO of the department store. They are the ‘top’, the executive, or as George Bush would say, ‘the decider’.
These categories are not relative. They don’t depend on what part of the brain you’re looking at. The sensory periphery is always the bottom, and the motor planning regions are always the top. Which, naturally enough, leaves a large amount of brain in the middle. And here’s where things get interesting.
In our last Neuroself 101 posting we talked about the river analogy of brain function. Using the river analogy, if the amygdala is sending information to the prefrontal cortex, an executive and evolutionarily recent region of the brain, that information is moving downstream; but if it is sending information to the visual cortex, a sensory and evolutionarily ancient region, that information is also moving downstream. But in the bureaucracy model, that would not be the case. The amygdala is middle management The amygdala is ‘above’ the visual cortex but ‘below’ the prefrontal cortex in the grand scheme of things, because it is closer to controlling the motor cortex than the visual cortex is, but not as close as the prefrontal cortex.
And here comes the payoff point. Finally. The amygdala can receive both ‘top down’ influences, as when the prefrontal cortex sends it a message, but can also received ‘bottom up’ influence, from, say, the visual cortex.