Phase Precession
The hippocampus is one of many brain structures that can show a characteristic 4–12 Hz oscillation, theta rhythm, in an EEG recording. The oscillation has been observed in all mammalian species tested. In both rats and humans, it is associated with real or virtual movement through space.
When a neuron discharges, it can be said to fire in relation to the current phase of a theta cycle (0-360 degrees). When a rat enters a cell's place field, the cell will initially discharge when perisomatic inhibition is weakest. For theta recorded in the CA1 pyramidal cell layer, this approximately corresponds with the peak of the oscillation. On each following cycle as the rat progresses through the field, the cell will discharge at earlier and earlier phases, typically stopping just before the trough of the cycle (as recorded in CA1 stratum pyramidale). In other words, the place cell produces a rhythmic discharge of a slightly higher frequency than the ongoing theta oscillation.
Because place fields of different cells overlap, at any particular time the rat will be at different distances in different fields, so each place cell will fire at a different phase of theta, allowing the rat's position to be determined with good precision. This potentially provides an alternative temporal code for location. Phase precession also results in the compression of temporal sequences of place cell firing — a phenomenon believed to facilitate synaptic plasticity. There is evidence that phase precession is related to depolarization of the neuron, such that the firing rate and firing phase of the cell are tightly coupled, However, phase precession can also be robustly independent of firing rate in freely moving animals. This caveat of phase precession, which alludes to the potential neural mechanisms underlying it, requires further investigation before arriving at a definitive answer.
Read more about this topic: Place Cell
Famous quotes containing the words phase and/or precession:
“It no longer makes sense to speak of “feeding problems” or “sleep problems” or “negative behavior” is if they were distinct categories, but to speak of “problems of development” and to search for the meaning of feeding and sleep disturbances or behavior disorders in the developmental phase which has produced them.”
—Selma H. Fraiberg (20th century)
“But how is one to make a scientist understand that there is something unalterably deranged about differential calculus, quantum theory, or the obscene and so inanely liturgical ordeals of the precession of the equinoxes.”
—Antonin Artaud (1896–1948)