Questions Left Open
Because our understanding of the relationship between SMO and LFP is still incomplete, it leaves the question open: what is the timescale of phase modulation in the brain? The frequency of SMO and LFP consistently varies along the fronto-temporo-occipital axis, dominated by gamma in the occipital regions of the cortex, alpha in the frontal areas, and theta in the EC, hippocampal, and parahippocampal regions. In addition, gamma power is high and oscillations are phase-locked to hippocampal theta. Although hippocampal phase precession is defined relative to theta, we anticipate phase precession relative to gamma oscillations as well, while APs should be phase-locked to the intracellular gamma SMO. We also anticipate a similar relationship between EC theta and gamma. The phase modulation of spikes relative to alpha/theta LFP (Montemurro et al., 2008; Kayser et al., 2009) and relative to gamma LFP (Nadasdy and Andersen, 2009) in the visual cortex is still unclear. One of the most important questions is whether or not the interference principle would work at multiple timescales to allow information to be encoded relative to multiple frequency bands of ongoing oscillations and whether or not these frequency bands carry content-specific information. There is much to learn about the collective resonant property of the nervous system in the next few years that will complete our understanding of how the activity of millions of neurons is orchestrated, and this orchestration may happen in a much more deterministic fashion than the “noisy” brain models suggest.
Finally, as stated in the title, the phase-coding model suggests a critical revision of the concept of binding by synchrony. Accordingly, the key of preserving the integrity of the code across multiple stages of information transfer in the brain is the precise asynchrony of APs between neighbor neurons, as opposed to the zero-phase lag synchrony proposed earlier (Gray and Singer, 1989). We argued that a subtle but constant phase gradient of the propagating SMOs is critical for encoding and reconstructing the sensory information as well as to for performing different coordinate transformations on the sensory input to achieve context invariant object representations in the brain.