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Extending an attractor network with Martinotti cells and cortico-cortical long-range connections
(Department of Computational Biology, CSC, KTH / SU)
The first part of the talk would be about introduction of Martinotti cells in our attractor network. The population of pyramidal cells significantly outnumbers the inhibitory interneurons in neocortex, while at the same time the diversity of interneuron types is much more pronounced. One acknowledged key role of inhibition is to control the rate and patterning of pyramidal cell firing via negative feedback, but most likely the diversity of inhibitory pathways is matched by a corresponding diversity of functional roles. An important distinguishing feature of cortical interneurons is the variability of the short-term plasticity properties of synapses received from pyramidal cells. The Martinotti cell type has recently come under scrutiny due to the distinctly facilitating nature of the synapses they receive from pyramidal cells. This distinguishes these neurons from basket cells and other inhibitory interneurons typically targeted by depressing synapses. A key aspect of the work reported here has been to pinpoint the role of this variability. We first set out to reproduce quantitatively based on in vitro data the di-synaptic inhibitory microcircuit connecting two pyramidal cells via one or a few Martinotti cells. In a second step, we embedded this microcircuit in a previously developed attractor memory network model of neocortical layers 2/3. This model network demonstrated that basket cells with their characteristic depressing synapses are the first to discharge when the network enters an attractor state and that Martinotti cells respond with a delay, thereby shifting the excitation-inhibition balance and acting to terminate the attractor state. A parameter sensitivity analysis suggested that Martinotti cells might, in fact, play a dominant role in setting the attractor dwell time and thus cortical speed of processing, with cellular adaptation and synaptic depression having a less prominent role than previously thought.
The second part of the talk would focus on implementing cortico-cortical long-range connections. In our attractor network single-cell and recent functional magnetic resonant imaging (fMRI) studies have shown inter-hemispheric synchrony during both evoked and spontaneous activity that is abolished by removal of callosal axons. There is also abundant data on species-specific fiber diameters and conduction speed showing how larger brains during evolution compensated increase in distance by broadening the distribution of fiber-diameters. This work addresses how long-range synchrony is achieved in our cortical attractor network set-up with long inter-patch distances and properties of fibers running between the patches according to experimental data.