Speaker: Randy M. Bruno
Department: Neuroscience and Zuckerman Institute of Columbia University
Subject: The many layers of the Neocortex
Location: Erasmus MC
Date: 5 October 2015
Author: Gabriele Kockelkoren
The neocortex is the top layer of the cerebral hemispheres, 2-4 mm thick, and made up of six layers, labelled I to VI (with VI being the innermost and I being the outermost). It is involved in higher functions such as sensory perception, generation of motor commands, spatial reasoning, conscious thought, and in humans, language. In Figure 1 the neocortex is shown. Each of the three brains is connected by nerves to the other two, but each seems to operate as its own brain system with distinct capacities.
Figure 1: The brain can be subdivided into three main parts: the neocortex, the limbic brain and the brain stem. Source: https://bookofthrees.com/triune-brain/
The conventional model states that axons coming from the thalamus reach to layer four of the neocortex. The cells of the neocortex transduce the signal to layers II and III, which subsequently activate layers V and VI. The latter layers function as feedback mechanisms to the outer world.
Randy Bruno is particularly interested in the whisker system of rodents and sensory perception. This system resembles the sensitive human skin. Axons from the thalamus are projected into every layer of the cortex. Bruno and colleagues observed that axons from the thalamus branch into layer IV, and in some cases in layer V and VI. Randy Bruno’s lab intends to analyse how and why this occurs? What is the function of this shortcut?
The first step in answering these questions relies in recording the reaction of neurons from each layer after applying a stimulus. Rats were used to do these experiments. By analysing the results, it has been found that cells in the IVth layer respond prior to cells in layer II and III. This is in line with the conventional concept of the signal processing pathway. However, in half of the tracked neuron cells derived from the Vth layer cells depolarised at the same time or even before the layer IV cells. Concerning the other half of the layer V cells, these reacted after layer IV, but prior to cells coming from layer II and III.
Furthermore experiments have been conducted with mice to see the relevance of the IVth layer compared to the Vth layer. A patch was inserted in the IVth layer of the neocortex, which registers all activity and action potential in the layer. Additionally a pipe filled with ACSF (artificial cerebrospinal fluid) is inserted in layer IV. Hereafter the ACSF is switched for an anaesthetic, lidocaine. This results in no action potential. The same procedure is done for a patch in layer V (ACSF is still in layer IV). Lidocaine is used to set all action potential to zero in layer four and the behaviour of cells in layer five was observed. The signal was normal, indicating that the layer five response does not require layer four.
After thus finding, Bruno’s group dedicated research to knowing the difference in the function of cells in different layers of the neocortex. Therefore they looked at the response of mice neurons by applying stimuli to the whiskers of mice in different directions. The neurons they were interested in, do not respond often to the stimuli, therefor they solely looked at the voltage and not at peaks above the threshold. This enabled finding a pattern of stimuli which is required to get a maximum response. For layer IV, V and VI cells the optimized stimulus gave a better response than a random stimulus. For the layers II and III there was no difference in response.
I liked Mr. Bruno’s talk very much, as it showed research that goes against common ideas and this is very valuable to see. It was difficult to understand many technical terms in the presentation, as I lack the knowledge of this medical jargon. Therefor it is very difficult to interact in discussions in neuroscience related talks.