Figure 1.

Basic properties of CA3 and DG neural firing and experimental procedure. (A–B) Recording location examples show tetrodes targeting CA3 (A) and DG (B). Because the transverse axis of the hippocampus is angled relative to the midline, the DG tetrodes targeted sites medial and posterior to the regions sampled by CA3 tetrodes. Scale bar equals 500 μm and arrows indicate the end of the tetrode tracks. Other tracks are visible that ended in adjacent sections. (C) One day of the experimental protocol consisted of three standard sessions interleaved with two cue-mismatch sessions. The mismatch angles depicted are 180° and 45°. (D–E) Putative cell types from CA3 (D) and DG (E) were differentiated by the mean firing rates (Hz; abscissa) and spike widths (ms; ordinate) of all well-isolated cells recorded in the first standard session of the day. For CA3 cells, two distinct groups were observed (putative principal cells with a mean firing rate < 10 Hz and putative interneurons with a mean firing rate ≥10 Hz). Three groups of cells were apparent in DG: (a) < 2 Hz, (b) 2–10 Hz, and (c) > 10 Hz. (F–G) The distribution of spatial information scores (Skaggs et al., 1996) from CA3 (F) was significantly higher than for DG (G) (Mann-Whitney U-test, Z = 3.1, p < 0.03). See Figure S6 for the information score distribution of the DG neurons that fired < 2 Hz.