Article Figures & Data
Figures
- Figure 1.
The DNMP task. A, Schematic diagram of the DNMP task. For each trial, the sample phase began when the rat entered the maze from the holding pedestal (blue oval) via a moveable ramp and ended when the rat entered either the right or left reward zone, depending on which reward arm was available. The rat then returned to the base of the stem (orange shaded region) and was held there for a 10 s delay period. The choice phase began when the delay barrier was removed. For the choice phase, the rat was allowed to choose between a left or right lap, with reward provided only for traversals to the reward arm opposite to that visited on the sample lap. The rat then returned to the holding platform for an intertrial interval of 10–20 s, during which the experimenter set up for the next trial. Rats were given 20–40 trials per day for 8–10 d. The trials were given in pseudorandom order, with approximately equal numbers of right–left and left–right trials given in a daily session. B, Average percentage of correct trials for the four rats across the first 8 d of training. Rats performed well above chance even on the first day of the task. Spatial correlates of hippocampal neurons were analyzed only for sessions in which the percentage of correct trials was 80% or greater (error bars indicate SEM).
- Figure 2.
Firing characteristics of a choice-selective neuron. A, Unit firing locations for sample (red dots) and choice (blue dots) phases of the task superimposed on the position data (gray traces). B, Distribution of waveform heights in two-dimensional cluster cutting space, with each dot representing the relative spike heights recorded on two channels of a tetrode. Cluster boundaries of the unit in A are outlined in green. Inset, All waveforms (top) and average waveform (bottom). C, Interspike interval histogram, autocorrelogram, and time versus maximum height plot for the unit in A. Notice the bursting pattern in the ISI histogram (top), the peak at t = 0 in the autocorrelogram (middle), and the stability across the recording session (bottom).
- Figure 3.
Examples of units showing trial-stage selectivity. For each cell, the position data (gray traces) and spike locations are shown for left sample and right sample laps (red dots) and left choice and right choice laps (blue dots). The numbers beside each plot indicate the stem firing rate for each lap type. The left column contains choice-selective neurons, and the right column contains sample-selective neurons. The DIphase is shown above each plot.
- Figure 4.
Distribution of discrimination indices for all stem-firing neurons. The area of the maze that was used for firing rate comparisons is indicated by the diagram to the right of each plot. A, Comparison of firing rates on the maze stem between sample and choice laps (DIphase). Cells with a DI >0 fired more robustly on sample than on choice laps, and cells with a DI <0 fired more robustly on choice laps than on sample laps. Notice the high proportion of cells with DI values near 1 and −1 and approximately equal numbers of sample-selective and choice-selective units. B, Distribution of discrimination indices (DIturn) for all stem-firing neurons comparing right-turn and left-turn trials. Although a small number of units showed turn selectivity, many neurons were not sensitive to turn direction as indicated by the clustering of DI values around 0. C, Distribution of DIphase values comparing firing rates between sample and choice laps in the reward zones, in which sensory and behavioral variables did not differ between sample and choice laps. Notice the similarity to the distribution in A.
- Figure 5.
Linear forward shift in the firing fields toward reward zones. A, Examples of the time course of the firing field location from four separate firing fields over the first 15 trials of the session (red dots, spike locations; gray traces, position data). The trials are grouped in blocks of three. B, ΔCOM shift for all 50 firing fields. There was a significant linear relationship between ΔCOM and trial (r = 0.64; p < 0.01). Notice that there was also a backward ΔCOM shift over the first two trials of the session [t (paired) = 2.79; p (one-tailed) = 0.004].
Tables
Rat Number of sessions at 80% correct or higher Number of units Number of stem-firing units 1 3 33 3 2 4 32 10 3 7 110 22 4 6 165 42 Firing location Number of units Delay zone 45 Stem 87 Return arms 35 Reward arms 17 Pedestal/ramp 58 Sparse firing 87 Interneurons 11 -
Sparse firing neurons emitted <100 spikes in a session. Pyramidal cells were distinguished from interneurons based on the shape of the interspike interval histogram, waveform shape, and firing rate (see Materials and Methods).
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- Table 3.
Main effects and interactions of context-sensitive factors from the three-factor ANOVA
Trial phase effects Number of units (percentage of total)b Number of units excluding bisected fields (percentage of total)c Turn direction effects Number of units Number of units excluding bisected fields Trial phase 38 (49.4) 20 (45.5) Turn direction 3 (3.9)a 2 (4.5) Trial phase × turn direction 11 (14.3) 6 (13.6) Trial phase × turn direction 11 (14.3) 6 (13.6) Trial phase × stem segment 5 (6.5) 2 (4.5) Turn direction × stem segment 0 (0) 0 (0) - Table 4.
Mean firing field size for each of the four lap types and mean percentage of the trajectory covered by the field
Lap type Mean field size (cm) Percentage of trajectory (total length was 155 cm for sample laps and 280 cm for choice laps)a Left sample laps 138.57 ± 29.62 86% Right sample laps 134.51 ± 51.26 89% Left choice laps 134.51 ± 51.26 48% Right choice laps 140.71 ± 38.75 51% -
↵aNote that choice lap trajectories were longer than sample lap trajectories because choice laps included the return arm bins.
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