The many layers of the Neocortex

Speaker:                     Randy M. Bruno

Department:             Neuroscience

Subject:                      The many layers of the Neocortex

Location:                    Erasmus MC Rotterdam

Date:                           05-10-2015

 Author: Katja Slangewal

Sensation, perception, finding patterns, conscious movement, reasoning, abstract thinking and solving problems, in all these processes the Neocortex is involved (see figure 1). Randy Bruno does research to tis part of the brain and he has found some very interesting processes. As he spoilers in the beginning of his seminar ‘the books were wrong’. The rest of the talk told us why.

Figure 2

Figure 1: The neocortex (from: http://www.lookfordiagnosis.com/mesh_info.php?term=Neocortex&lang=1)

The Neocortex is the upper part of the brain; it covers the inside and is made up of six layers. The most outer layer is layer I, going in it ends in layer VI. Information from the outside world is processed in the cortex as signals. These signals travel from the senses via neurons into the neocortex. Long time it was believed this signals arrive in the cortex in layer IV and are distributed over the rest of the layers afterwards. This was found by using the ‘follow the wires’ tactic. While looking to the images of neurons going into the neocortex it seems true, most dendrites end in Layer IV, this region is really dense. Though there are small (though obvious) dendrites in Layers V and VI as well. These dendrites were mostly ignored so far, but why are they there?

Different tests were done to find out. At first the reaction time of the different dendrites were measured. It was clear that layer IV responded a lot faster than Layer II and III. Though layer V and VI gave weird results, their reaction times were spread out. Some were faster than every single dendrite in layer IV and some were really slow. Also the connection between the different layers was tested. The connection probability gave a linear output depending on the depth, the deeper in the brain, the less the chance of finding a connection. Important is the number of known cells in the different layers, this also decreases when you get deeper. Combining this gives a maximum of connection just where axons enter layer V and VI. So now the question is how much of layer V and VI input derives from a direct pathway and how much from an indirect pathway? This was tested by deactivating layer IV and surprisingly 100% of the input goes via a direct pathway. This and some other tests conclude that every axon goes to layer IV as well as the border of layer V and VI, without the so called ‘sensory talk’ between the layers (See figure 2).

Figure 2:

Figure 2A: the old understanding of the signalling pathway. Figure 2B: the new understanding of the signalling pathway. (from: Christine M. Constantinople, Randy M. Bruno, Deep cortical layers are activated directly by Thalamus, Science: Volume 340, no. 6140, 28 June 2013, pages 1591-1594 http://www.sciencemag.org/content/340/6140/1591.full)

These results give raise to some questions. The upper layers and lower layers both get direct input; within milliseconds a signal is distributed all over the brain. This information hasn’t gone through the whole cortex before arriving in certain layers, so what does the cortex do? How can parts of the brain not communicate while being next to each other?  Some people believed the two layers are comparable with a hard-drive and its copy, though this is hard to believe. Maybe the upper layers and lower layers are just sensitive to different patterns.

This was tested as well. When measuring the signal, not only high spikes were seen, but also noise. Most of the experiments ignore the noise and focus on the high spikes, this time it was done the other way around and indeed some signals give more spikes in the lower layers than in the top layers and some reactions spikes are only seen in certain layers. So the layers are indeed sensitive to different patterns.

I really liked the seminar of Randy Bruno, it was relatively easy to follow and he had a good kind of humor included in his talk. He did use a lot of technical terms which I didn’t understand at the moment, though the core of his talk was clear and interesting. It taught me a lot about the working of the brain and I am looking forward to learn more about this process.

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