SENSORY MEMORY IN A MODEL OF A CORTICAL COLUMN: NOISE, PHASE TRANSITIONS AND SIGNAL STATISTICS.

Abstract Sensory memory is the first step of a complex memorization process. Sensory informationis buffered in a ”sensory store” for a short period. Then part of it is transferred to workingmemory, where information can be actively processed and, eventually, through its rehearsal,can be memorized and stored in long-term memory. Sensory memory is characterized bya high capacity and a short temporal extension. In this work we show that such a tran-sient buffering of information is well captured in models of cortical microcircuits. Sensoryinformation stays for a short time in the perturbations it induced to the neural activity.Two fundamental characteristics of cortical columns appear to be useful to this pur-pose. The first one is an approximate balance of neuronal excitation and inhibition. Thesecond one is the sparseness of cells recurrent connections. Together, these two discriminantsensure a broad set of complex dynamical behaviours of the neural network along with thepresence of a phase of sustained non-oscillatory activity with very irregular spike trains inindividual neurons. We show that at the vicinity of a phase transition from a quiescent stateto a phase of chaotic spiking activity, global stimuli can be buffered in the ”macroscopic”neuronal activity. However if the sensory input only excites a subpart of the network, effi-cient information buffering is made using the detailed spiking activity of every neuron in thenetwork. The ”microscopic” structure of the network is therefore needed in order to buffersensory information.In most situations, addition of noise enhances the buffering performance and the compu-tational power of the network. This effect is known in the physical literature as ”stochasticresonance”. If the network has to achieve complex transformation of its inputs, stochasticresonance is shown to be an emergent property of the population of neurons.Both the very irregular spiking activity present in vivo and the balanced excitation andinhibition measured in cortical columns have a new functional relevance in being the neuronalsubstrate of an efficient sensory memory .v