Modeling Cortical Activity: Cortical Columns

The aim of the talk is to introduce some notions about cortical modeling with a focus on cortical columns. In the first part, I introduce cortical columns from a biological point of view and point out why the notion of cortical column is complex. In the second part, I describe Jansen’s model of a single cortical column and discuss its adequacy to biological facts. The third and last part shows how a system describing a single cortical column, like Jansen’s, can be inserted in a model of cortical sheet, with the purpose of modeling large scale cortical activity. 1 Cortical Columns from the Biological Point of View It has been hypothesized that small vertical structures called cortical columns are the basic units of sensory and motor information processing in the cortex [1]. How can such a structure emerge from the complexity of the cortex? Many cortical neurons throw their axons and dendrites from the cortex surface to the white matter thereby forming the anatomical basis of the columnar organization in the cortex. In 1957, Mountcastle discovered a columnar organization in the cortex. With electrode recordings, he showed that neurons inside columns of 300 to 500 μm of diameter displayed similar activities. Those physiological units are usually called macrocolumns. Some of them are spatially well defined while some others are more difficult to distinguish from one another. What is the meaning of such units? Many experiments on somatosensory and visual cortices made it possible to relate physiological columns with sensory functions. In some cases the processing site for a given function is clearly defined like in rat’s sensory cortex where every whisker is associated with a sharply bounded cortical site in layer IV. In other cases, the information processing sites move continuously across the surface of the cortex when stimulation varies so that it is not possible to define a size for columns. It is the case of the orientation columns in the primary visual cortex [2]. 2 Jansen’s Model of a Single Cortical Column The model features a population of pyramidal neurons that receive excitatory and inhibitory feedback from inter-neurons residing in the same column and an