Linking strings of fundamental assemblies requires readers with longer time integration abilities. In addition to the membrane time constant of single neurons, multiple other time integrators are present in the brain. NMDA receptors operate at the time scale of tens to hundreds of milliseconds (Monyer et al., 1992). Time-integration of cell assemblies at the subsecond to seconds time scale can be performed by metabotropic glutamate receptors (Nakanishi, 1994; Conn and Pinn, 1999), GABAB receptors (Deisz and Prince, 1989) and slow afterhyperpolarization-associated conductances (Lancaster and Adams, 1986). Another time integration mechanism at this time scale, and at the level of a single neuron rather than a synapse, is the spiking history-dependence of spike threshold. After a burst or train of spikes but even after a single spike, the spike threshold increases measurably for tens to hundreds of milliseconds, independent of the synaptic inputs (Henze and Buzsáki, 2001; Mickus et al., 1999). Reader mechanisms of spiking activity at longer time scales may be exemplified e.g., by the autonomic nervous system and the 0.1 Hz periodicity of the brain’s ‘default networks’ (Raichle et al., 2001).
