An embedded subnetwork of highly active neurons in the neocortex.

<p>Unbiased methods to assess the firing activity of individual neurons in the <a>neocortex</a> have revealed that a large proportion of cells fire at extremely low rates (<0.1 Hz), both in their spontaneous and evoked activity. Thus, firing in neocortical networks appears to be dominated by a small population of highly active neurons. Here, we use a fosGFP transgenic mouse to examine the properties of cells with a recent history of elevated activity. FosGFP-expressing layer 2/3 <a>pyramidal cells</a> fired at higher rates compared to fosGFP<sup>−</sup> neurons, both in vivo and in vitro. Elevated activity could be attributed to increased excitatory and decreased inhibitory drive to fosGFP<sup>+</sup> neurons. Paired-cell recordings indicated that fosGFP<sup>+</sup> neurons had a greater likelihood of being connected to each other. These findings indicate that highly active, interconnected neuronal ensembles are present in the neocortex and suggest these cells may play a role in the encoding of sensory information.</p>