Hippocampus
The hippocampus is a curved structure beneath the neocortex, 2 to 3 mm below the brain surface in mouse and 3 to 4 mm in rat, with a layered organization of its own (CA1, CA3, dentate gyrus, subiculum) and field potentials that identify it in a recording.
The signals are the reason it is a common target. Theta oscillation at 6 to 12 Hz in the local field potential during movement, and sharp-wave ripples at 150 to 250 Hz during rest and sleep, are read live from the field potential across the sites and confirm that the shank is in the hippocampus before any unit is sorted; the amplitude and phase profile of theta across the layers marks the CA1 pyramidal layer on a linear array. Place cells in CA1 and CA3, and grid cells in medial entorhinal cortex, are the single-unit signals.
Two probe numbers follow. The subfields span 1 to 2 mm, so the site span decides whether one shank records CA1, CA3 and dentate together, and a fine site pitch separates the layers; the edge (Buzsáki) layouts were designed for recording units in the hippocampal pyramidal layers. The target depth sets the shank length, at least 3 mm in mouse and 4 to 5 mm in rat, with a chronic implant usually on a Hybrid or CM package. Coordinates are set from bregma; the neocortex above it is under cortical layers.
Typical values
| Depth below surface | 2 to 3 mm (mouse) · 3 to 4 mm (rat) |
|---|---|
| Subfields | CA1, CA3, dentate gyrus, subiculum |
| Theta | 6 to 12 Hz |
| Sharp-wave ripples | 150 to 250 Hz |