Raster plot of the working memory model output. A raster plot of the working memory model output (left panel) illustrates the concept of working memory span. Each line of asterisks is the activity of a single neuron with each asterisks indicating an action potential occurred at that time for the neuron. For simplicity we only show 10 excited pyramidal neurons, 10 unstimulated pyramidal cells and 10 GABA interneurons. At time 2 sec, a current (equivalent to a 'memory' stimulus) is injected into the attractor neurons (lowest ten lines) and these neurons then fire a sustained burst for a certain period until the stochastic noise starts to deteriorate the attractor pattern. The memory span is the time in seconds over which at least 80% of the attractor neurons fire within a similar time period (right panel).