Scientific background

The cerebral cortex is organized horizontally in 5 or 6 layers and vertically in so-called cortical columns (Fig. 1). The columnar architecture is not only a hallmark of sensory and motor cortical areas, but also of "higher" neocortical areas such as the prefrontal cortex (Krimer and Goldman-Rakic, 2001;Mountcastle, 1997;Hubel and Wiesel, 1977). It has been suggested that the microcircuit underlying a cortical column represents a most powerful framework for the processing of subcortical and intracortical information and that the cerebral cortex consists of repeated copies of the same fundamental circuit (Douglas and Martin, 2007). It has been further proposed that even minor disturbances in the columnar microcircuitry may have a pronounced impact on cortical processing and may cause long-term neurological deficits such as schizophrenia or autism (Amaral et al., 2008;Lewis and Gonzalez-Burgos, 2006;Walsh et al., 2008). Therefore, an understanding of the neocortical column in its structural, functional and developmental aspects is of fundamental interest in basic and clinical neurosciences.

 

 

Fig. 1: The basic structure of a neocortical column and its cellular elements as published in 1978 by János Szentágothai (The neuron network of the cerebral cortex: A functional interpretation. Proc. Roy. Soc. London Ser. B 201: 219-248). To understand the cortical column in its cellular connectivity, spatio-temporal dynamic  neuronal network activity, behaviourally relevant activation pattern and its development is the aim of the current proposal for a binational Swiss-German Research Unit.

 

          Although the structural and functional properties of a cortical column have been intensively studied in a variety of mammalian species since the pioneering studies of Vernon Mountcastle (Mountcastle, 1957) and David Hubel and Torsten Wiesel (Hubel and Wiesel, 1962), the "canonical circuit" of a cortical column and the interaction between different columns is still largely unknown (Douglas and Martin, 2007). Over the last years we also learned that understanding neocortical information processing does not only require a detailed knowledge of the synaptic circuitry at the single cell level (Helmstaedter et al., 2007;Schubert et al., 2007;Cruikshank et al., 2007), but also an in-depth analysis of the columnar network activity at the population level (Berger et al., 2007;Ferezou et al., 2007;Kerr et al., 2005).

          The rodent barrel cortex offers unique opportunities for studying sensory processing in a cortical column and to correlate whisker-related behaviour with neuronal activity in a well-defined cortical map (Lübke and Feldmeyer, 2007;Petersen, 2007;Diamond et al., 2008). The sensory information from the whiskers is transmitted in a highly ordered topographic manner to the primary somatosensory cortex (Fig. 2). Here, the thalamocortical afferents arising from one single whisker of the contralateral snout,  project primarily to layer IV and neocortical modules of 300-500 µm in diameter process this information. These "discrete cytoarchitectonic units" have been termed barrels by Woolsey and van der Loos (Woolsey and Van der Loos, 1970) and were in addition shown to be the functional units in which the first processing of the sensory input takes places (Lübke and Feldmeyer, 2007;Schubert et al., 2007;Petersen, 2007). The rodent barrel cortex offers a number of additional unique advantages to study the mechanisms underlying the organization, plasticity and development of a neocortical column: (i) The barrel-related cortical column can be easily identified in vivo (Berger et al., 2007;Ferezou et al., 2007) and in unstained brain slices in vitro (Feldmeyer et al., 2006;Schubert et al., 2006), (ii) the sensory periphery can be manipulated in various ways and trimmed whiskers regrow (for review Feldman and Brecht, 2005), (iii) specific neuronal cell types located in a selected cortical layer of a well-defined cortical column of mouse barrel cortex can be targeted by genetic manipulation of specific genes (for review Aronoff and Petersen, 2008), and (iv) the monitoring and manipulation of single neurons in vivo (Brecht et al., 2004).