Hello world

This blog is about language related brain regions and neural network modeling of natural language processing. First I want to know more about the brain before going into the neural modeling business. I’ve lived long enough to see at least two, if not three, waves of neural networks “booms”. First, the Perceptron (Rosenblatt). Second, the Back-Propagation (Rumelhart, Hinton, Williams and possibly more preceding them), and now Deep Learning. The third wave will probably fade soon, unless we address the basic problems with current neural network models (eg. Marcus 2003 – more on this in a later blog) and learn more deeply from the biological brain. And most importantly, I don’t know about you but I want to know how the brain works, and the language may be a window to the human mind, granting a limited logical access to the dark, huge space of human psyche.

Ok, so the linguistic brain.  I want to start more specifically at the architecture of the cortical layers in general, since I find it’s a bit boring to start at Brodman’s area and sort of block diagram level, although we’ll eventually get there. The cerebral cortex consists of up to six layers, numbered from the cortical surface towards the deeper layers towards the brain stem. The naming varies but it’s easy to remember in four groups as I (molecular layer), II-III (external granular and external pyramidal layers), IV-V (internal granular and internal pyramidal layers), and VI (multiform layer).

The figure below is a diagram of cortical cells and connections taken from a review paper by Harris et al. (2013) and grossly simplified by picking up only prominent connections. According to the paper, the primary input feeds mainly to layer IV. The higher order input reaches layers I, V, VI.

Cells in the layer II/III projects to layer V and outputs to higher order and contralateral cortices.  The cells in layer IV projects to all layers but most strongly to layer II/III. The cells in layer V can be classified broadly into two.  The first class cells are located relatively shallow and project to layers II/II and ipsilateral and contralateral cortex and striatum. The cells that belong to he second cell type in layer V are the primary output cells. The layer VI includes at least two cell types. One has long horizontal axons. The other has slow corticothalamic projection that reaches reticular and sensory thalamus, and also projects to interneurons in layer IV.

The diagram might still look complicated, but it really isn’t if you pick up a path from one external input to one external output, for instance, higher order input to higher order output, which I would be interested if I want to model higher language related cortices. It at least looks like a good start, I hope.

References

Marcus GF. The algebraic mind: Integrating connectionism and cognitive science. MIT press; 2003.

Harris KD, Mrsic-Flogel TD. Cortical connectivity and sensory coding. Nature. 2013 Nov 7;503(7474):51-8.