- The first reference signal is variable and is synchronized to the low-frequencies generated by sequences within the hippocampus; it is used primarily for sequence learning. There is one reference signal of this type per sequence.
- The second reference signal has a much higher frequency than the first and is fixed. It is used to record the relative intervals between signals within a sequence. One such signal can serve a huge number of sequences.
It turns out that in-vitro and in-vivo experiments on rats brains reveal three types of oscillations in the hippocampus: theta (4–10 Hz), gamma (20–80 Hz) and high-frequency (~200 Hz). Personally, I think that both theta and gamma oscillations should just be categorized as low-frequency oscillations. This would give us two distinct types of oscillations: low-frequency and high-frequency. I suspect that theta frequencies are generated mostly in connection with visual signals whereas gamma frequencies are associated with auditory signals.
It is almost 1-AM where I am right now (Texas). I just thought I would post this before I go to sleep. Later.