Neuronal networks in the inner parts of the brain have also adopted reader-initiated mechanisms for transient gains. For example, transfer of hippocampal information to the neocortex (the ‘reader’) during slow wave sleep can be initiated by the down-up transition of the neocortical slow oscillation (Buzsáki 1998; Isomura et al., 2006; Sirota and Buzsáki, 2005; Sirota et al., 2003), which can bias the spike content of hippocampal sharp wave-ripples (Battaglia et al., 2004; Ji and Wilson, 2007). In the waking brain, the direction bias works in the opposite direction. Now the dialogue is initiated by the hippocampus via theta-phase control of neocortical network dynamics (Sirota et al., 2008). As a result, the content of the temporally biased, self-organized gamma oscillations at multiple cortical locations can arrive to the hippocampus at the phase of the theta cycle when hippocampal networks (the ‘reader’) are in their most sensitive, plastic state (Huerta and Lisman, 1996). Exchange of information between different stages of the visual system appears to follow similar rules (Fries 2005; Womelsdorf et al., 2007), implicating a general rule for the reader-initiated transfer of neural messages.21
