Self-Motion and the Hippocampal Spatial Metric

A. Terrazas
2005 Journal of Neuroscience  
Self-motion signals are sufficient for animal navigation ("path integration") and for updating hippocampal location-specific firing. The contributions of ambulatory, vestibular, and optic self-motion signals to CA1 unit activity and EEG were studied while rats either walked or drove a car between locations on a circular track (referred to as WALK and CAR, respectively) or experienced pseudomotion, in which the animal was stationary and the environment was rotated (WORLD). Fewer pyramidal cells
more » ... er pyramidal cells expressed place fields during CAR and those that did exhibited substantially larger place fields. The number of theta cycles required to traverse a place field increased, whereas the slope of the theta phase of firing versus position function was reduced. The presence and/or location of place fields were not well correlated between conditions. These effects were even more accentuated during WORLD. These results are not explainable by a simple "smearing out" of place fields but, in terms of size of place fields relative to the track size, are comparable with what would be observed if the track circumference was reduced and the animal moved around it at a correspondingly slower speed. Theta (and its 14 -18 Hz harmonic) power were dependent on velocity, but the gain of this function was substantially reduced during CAR and WORLD, again as if the rat were moving more slowly. The spatial scale over which the hippocampal population vector is updated appears to be derived primarily from the gain of a self-motion velocity signal with approximately equal components derived from ambulation, vestibular, and optic-flow signals. (35):8085-8096 • 8085 implantation methods, and the parallel recording technique have been described in detail previously (Wilson and McNaughton, 1993; Gothard et al., 1996) . Surgery Six male Fischer 344 rats (age range 10 -18 months; Charles River Laboratories, Wilmington, MA) were used as experimental subjects. Animal care, surgical procedures, and killing were performed in accordance with the National Institutes of Health and University Animal Care guidelines for the use of vertebrate animals in research. The animals were housed individually in Plexiglas containers and maintained on a reversed 12 h light/dark cycle in a colony room. Subjects had access to water ad libitum and were weighed, handled, and checked for overall health daily. After surgical recovery (ϳ5 d), animals were maintained at 75-80% of their ad libitum weights until the conclusion of the experiments. During electrode implantation, rats were anesthetized using sodium pentobarbital anesthesia (Nembutal, 40 mg/kg, i.p.; Abbott Laboratories, Abbott Park, IL) and placed in a stereotaxic frame (David Kopf Instruments, Tujunga, CA). Small doses of supplemental methoxyflurane (Metofane; Pitman-Moore Pharmaceuticals, Chicago, IL) were administered by inhalation as required to maintain a state of deep anesthesia throughout the surgery. Just before insertion of the HyperDrive, bilateral stimulation electrodes were implanted with their tips aimed at the medial forebrain bundle (MFB). In two of the animals, two additional electrodes were implanted contralaterally, aiming for the hippocampal fissure and alveus/ stratum oriens, respectively. This was done to facilitate optimal differential recording of the theta component of the EEG. The leads of these additional electrodes were later connected to the HyperDrive circuit board and then attached to the dental acrylic mass surrounding the HyperDrive post. Immediately after surgery, 30,000 U of penicillin (Bicillin; Wyeth-Ayerst, Philadelphia, PA) was administered intramuscularly to each hindlimb, and the animals were allowed to recover for 4 -8 h in an incubator. After recovery from anesthesia, 25 mg of acetaminophen (Children's Tylenol Elixir; McNeil, Fort Washington, PA) was administered orally for analgesia. The rat was then placed in a clean cage with a surgical sheet. An additional 2.7 mg/ml acetaminophen dissolved in the drinking water was provided for 3 d after surgery.
doi:10.1523/jneurosci.0693-05.2005 pmid:16135766 fatcat:htvl3q2ykng27cvb2jg6cos5cm