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. 1997 Aug 1;17(15):5900–5920. doi: 10.1523/JNEUROSCI.17-15-05900.1997

Fig. 1.

Fig. 1.

Activity packet on a chart constructed from the experimental data of Wilson and McNaughton (1993). The rat was randomly foraging for food in a 62 × 62 cm box. Parallel recordings of ∼100 hippocampal cells, 36 of which showed robust activity, were taken during the running session. The whole population of recorded place cells is symbolically distributed in the box, each cell being placed at the center of its place field. In the present paper, this planar arrangement of place cells is called achart. A fuzzy snapshot of a momentary firing rate distribution over the chart is taken every 50 msec. The rat’s position and orientation are marked on each snapshot, and all of the snapshots are superimposed so that all rat position marks are aligned at the center. The resultant average distribution is shown on the figure; therefore, the plot can be viewed as a typical momentary distribution of the firing rate over a chart (in allocentric coordinates). Units on horizontal axes are centimeters. The animal is located at thecenter of the square and is moving to theleft and toward the viewer. The total number of P cells (presumably CA1–CA3 pyramidal cells) in a rat’s hippocampus is of the order of 3 × 105 (Amaral et al., 1990). From empirical studies, for a typical recording environment of ∼1m2, a given P cell has a probability of ∼0.3 of having a place field. Thus, the density of units on a typical chart can be estimated as ∼105m−2. The variance of the distribution shown on the figure is ∼0.15 m, which is consistent with the observation that ∼10−2 of all P cells fire at a given location. The averaged activity packet seems to have two “subcomponents.” In fact, the real activity packet oscillates between these “subcomponents” with the theta frequency (Fig.9C) (also see Skaggs et al., 1995) and therefore has smaller variance. We performed after-processing of the experimental data as described above with various data selection: right turns only versus left turns only, high velocity versus low velocity, high acceleration versus low acceleration, etc. The results suggest (within the error of measurement) that the shape of the activity packet does not depend on velocity, acceleration, future trajectory of motion, or theta frequency. This result will be presented in more detail elsewhere.

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