Language and the brain

I'm taking a class on psycholinguistics right now, and the aspect of localization is absolutely fascinating. In the class we keep a weekly journal, so this is the stuff I've been thinking about in relation the brain and language.

It is extraordinary that there are areas of the brain known as language areas, and even more extraordinary there there are those who seek to map those connections at an even deeper and more micro level. The idea of mapping the cellular connections associated with something as seemingly abstract as language seems so daunting, and yet the technology that exists today, especially TMS, introduces the possibility of such a thing.

What makes language so difficult to study is that it is so much more than words; it is the meaning of those words, the application of that meaning, the physical nature of producing them ouside of the brain, and, most cryptic, producing them in the brain as well, as thoughts. The fact that Wernicke's area is at a crossroads of three of the lobes implies that this region is, above all else, a junction of the regions of the brain involved in a variety of types of sensations, perceptions and motor function. Language, as it is represented in its physical localization, can be seen as an expression of so many other processes taking place. To map such connections on a cellular or molecular level would be to scientifically understand one of the higher processes that makes us "human".

The relationship between lesions or disorders of particular brain areas and the functional deficits with which they are associated is quite interesting. They are both the behavioural nature of the deficit itself, while at the same time being an important aspect of neuroanatomical research and functional localization. Aphasia is often brought up in neuroscience textbooks and the literature in the context of localization, and how it is important to our understanding of the various aspects of language and how they are represented physically in the brain. In neuropsychology and psychology, it is both that and also a loss of language capability. There are cases such as K.H., the architect who, after undergoing surgery to remove a tumor from Broca's area, initially lost his ability to speak and comprehend both the oral and written word (though he later regained some of what had been lost).

One wonders how anyone could function with such an extreme language deficit. Aphasia must feel incredibly disorienting; to have had full use of the faculties of language and then to lose them must be a very frustrating experience. As language is so essential in the experience of human life, it must make the aphasic person feel disconnected and confused. I am not suggesting that one dwell upon the horrors of aphasia when studying it, but rather that the behavioural effects emphasize not only the localization of language function in the brain, as well as the importance of language in the human world. Aphasia is one of many examples which show that to study language is so interesting because it is to study mind, brain and behaviour, and often all at once.

The crazy present/future cellular mapping

I am fascinated by brain imaging and electrophysiological stimulation, mainly because of the way in which it allows us to have a more minute understanding of the processes involved in functioning. The more micro we can get with these sorts of tests, the more we can actually understand the cellular and molecular mechanisms governing mental processes and behaviours. I read an article recently in Nature ("Neuroscience: Illuminating the brain", Buchen, 2010) about optogenetic manipulation, which is the opening of ion channels through opsin proteins injected via a virus; light stimulation triggers these opsin proteins at the cell membrane to open ion channels. This is a really neat new way of determining, with a great deal of specificity, the cellular networks involved in certain processes, and even crazier than that, the individual activity of cells within those networks. At this time, the light source is implanted through an "optrode" (an optic fibre and an electrode) into the skull of the animal when done in vivo, so there is no application for this on humans. As such, the more tantalizing higher functions we so desperately desire to map are still out of reach; but the neural networks involved in processes such as spatial learning can be measured in mice and rats. Optogenetic manipulation is also used quite successfully in vitro, which is actually how it was discovered. In 2005, Karl Diesseroth and Ed Boyden at Stanford University inserted a light sensitive channel (channelrhodopsin2) from green algae into neurons in a growing dish. They exposed these neurons to a pulse of blue light, and the channels opened, flooding positive ions into the neurons and causing them to fire. Optogenetic manipulation is going to change the mapping of cellular networks in a huge way.

Another thing that's going to change neurological mapping is that crazy connectome! If you haven't seen the TED talk on it (Sebastian Seung: "I am my connectome"):



My problem with this, other than the fact that the talk itself comes off as being a motivational speech, is that it doesn't seem to take synaptic plasticity into account. In fact, what it really doesn't take into account are the individual differences that arise from synaptic plasticity; but most importantly, that we, living our lives, are constantly altering the strength and weakness of connections in our brain. These neural connections are ever-changing and plastic. With every trafficked AMPA, with every hippocampal place cell reorganized, our synaptic circuitry is changing. Yes, it is ever-so-slight, but when what you are measuring is cells, ever-so-slight is pretty huge.

On the other hand, the idea of it being used to visualize mental illness by identifying miswiring of individual connections is pretty wild.

References

Buchen, L. (2010). Neuroscience: Illuminating the brain. Nature 465, 26-28.