Part of the problem involved with grappling with leading issues in neuroscience is that even the level of complexity involved is, well, complicated.
There is the obvious, “in your face”, sort of complexity based upon the sheer volume of possibilities: coming to terms with the vast number of neurons in the brain, determining how to go about isolating one specific protein out of countless thousands, and doggedly following each one of them down their unique thousand-fold biochemical pathways.
But then there is a much more subtle sort of complexity, the sort that is not immediately obvious, but appreciating its depth has the power to launch us on a journey towards many fascinating possibilities.
Take vision, for example. Nothing could be more natural than blithely declaring that what we call sight involves the brain appropriately processing the visual information that is received by our eyes. Which is true, of course, as far as it goes. But dig a little bit deeper and a much more sophisticated picture starts to emerge.
Duke neuroscientist Jennifer Groh has spent the vast majority of her research career doing just that: looking to unravel the inherent, and often overlooked, complexity of how our brains develop an understanding of where we are.
“The photoreceptors in our eyes give us a representation of where visual information is, where objects are in the world, and that frame of reference depends on where the stimuli are with respect to the array of photoreceptors. In other words, it depends on where the stimuli are with respect to your eyes.
“Well, we can move our eyes – and we do. In fact, we move them a lot, and we move them really fast. That’s a lot of eye motion that the brain has to deal with: it has to assemble the snapshots that are taken by the photoreceptor array at each of the different positions that your eyes might be looking.”
That’s complicated enough, but that’s only the beginning. Now think about the way the brain integrates a wide range of other sensory input – vision, hearing and touch into one coherent whole.
“If you then extend this problem to include some of the other senses, like the auditory system, it’s important to first recognize that, of course, sounds aren’t affected by how the eyes are moving. A sound that is located on one particular side will arrive in that ear first and will be slightly louder in that ear than the other. The brain has to compare the signals arriving in one ear with the signals arriving in the other to compute the angle that the sound is coming from.
“In general, then, the auditory system is computing sound location based on cues that are fundamentally anchored to the head, while the visual system is computing visual locations based on cues that are fundamentally anchored to the orientation of the eyes. So every time your eyes move, you’re yanking your visual scene around to some new position with respect to your auditory scene.”
Well, alright, you might say. It’s complicated. Very, very complicated, even – and perhaps unexpectedly so. But then, when it comes to the brain, lots of things are complicated – speech, language, regulating our emotions, playing the cello. What makes our sensory processing so different?
Well, it might not be, of course. But then, perhaps how we process sensory information about the world around us is somehow different. Maybe the effort required for the brain to understand where we are in space does somehow play a more pre-eminent role than other brain functions, particularly given the neural hardware dedicated to the task at hand.
“There’s an awful lot of the brain that has been identified as carrying some kind of information that’s relevant to these kinds of processes. There’s a lot of the brain that responds to visual information, there’s a lot that responds to sound, there’s a lot that responds to touch, there’s a lot that’s involved in controlling movements – and movements are essential to understanding how to combine information across these different sensory systems.
“If you were to say that all of those brain structures are only doing those things, there wouldn’t be that much left for doing the things that concern, say, what makes us smart. How come most of the brain isn’t involved in language, for example?
“It turns out that, if you look at areas of the brain that seem to be involved in language, or memory, or attention, or planning, or motivation, there’s a lot of overlap between the structures that are implicated in those processes and the structures that are involved in sensory and motor processing.”
And that, Jennifer suspects, might provide a clue to a longstanding evolutionary mystery. After all, why should such overlaps exist at all? Why is there, for example, such a well-documented link between memory and space? What could be possibly causing that?
“A general problem in evolution is to envision how simple events, like the mutation of an individual gene, can produce an organism that functions better than the other organisms that don’t have that mutation – because usually, when you tweak something, you make it worse. Intermediate states in the course of evolution are often hard to envision.
“One thing that may be happening is that modules in the brain might be duplicated through a fairly simple set of mutations so that you might take a structure that’s working well, and maybe one small change means that you now have two of those. And if you now have two, and the one before was sufficient, you find yourself with an extra that can be used for something that you weren’t originally doing.
“The reason why so much of our neural architecture might seem related to our perceptions of space, then, is that nature has used those spatial processing systems as a template for a sort of evolutionary copying machine to create a multitude of other high-level networks that can eventually develop into systems for things like logic and language.”
That sounds pretty complicated, too, I must admit. But that doesn’t mean it’s wrong.
Howard Burton, firstname.lastname@example.org
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