Yamaguchi’s interests in rat hippocampal theta began in 1985, at the same time as the start of visual pattern recognition study. Her question in the hippocampal study was
“Does theta rhythm coordinate information of daliy experience swith contexts or surroundings in the brain?”
In 1980s, experimental observation of theta rhythm and place cells were rich enough to enjoy the mystery. She elucidated simple synchronization model for hippocampal memory , while this dynamics was not enough to process experienced information including temporal sequence.
When she moved to Tokyo Denki University in 1993, O’Keefe and Recce reported theta phase precession. She was intersted in the paper, while thie obsercvation was too novel ans unclerar for her to develop concrete theory. It was in the short stay at University of Arizona that she noticed the mechanism of theta phase precession from the papr, Skaggs et al. 1996.
The simple idea is to assume oscillations in the entorhinal cortex, the entrance of the hippocampus, based on a popular principle of phase locking of nonlinear oscillations with naural frequency difference, under the hypothesis that theta phase dependent place cell firing concists of two components, uniform phase shift and the other at anti-phase in theta cycle.
She modified the former program of the hippocampal network and quickly found that it worked for memory formation. The second component in two component hyothesis is still unsolved.
This study is followed by CREST project (1999-2005) in Japan through collaboration with Bruce Mcnaughton and Carol Barnes, University of Arizona. At the laboratory of RIKEN BSI, the computational study based on theta phase precession has been extended to human memory formation function, as well as direct bridging between theories and rat experiments. A recent experimental report that neurons in the entorhinal superficial layer has theta phase precession by Moser’s group provides exciting coincidence with the theoretical studies.