Figure 3. Time coding and spatial coding Aa | Design of a non-spatial task in which time cells were observed. Rats learned to associate each of two visually distinct objects with one of two cups of scented sand (for example, the green block was associated with the lemon-scented cup and the purple sphere was associated with the nutmeg-scented cup). In each trial, rats approached and sampled one of the two objects and, after a fixed (10 s or 20 s) delay, were exposed to one of the two odours. If the odour matched the object (for example, lemon odour following a green block), the rat had to dig in the scented sand to retrieve a reward. If the odour did not match the object (not shown), no reward was available in the odour cup and the rat did not dig. Ab | Two examples of cells that ‘re-timed’. On the left, a cell that progressively lost its time field as the delay was elongated from 5 s, to 10 s, to 20 s. On the right, a cell that did not fire when the delay period was 10 s but that did fire (at about t = 6–7 s) when the delay period was 20 s. Ac | The spatial activity pattern (place field) of the cell whose raster plot is shown on the right in part Ab. Activity is plotted for the entire delay period (top) and for the first ten successive seconds of the delay period in each trial block. Note that the cell fired at a particular location but only during the sixth to seventh second of the 20 s delay period (middle column) and not in trials with a 10 s delay period (left and right column; also see the normalized firing rates over time in the trace at the bottom). Ba | Design of a spatial alternation task. Rats ran on a treadmill in between alternating left turns and right turns on a T-maze (paths indicated by black and grey arrows). Bb,Bc | The firing patterns of a hippocampal neuron shown in raster plots (top), firing rate histograms (middle) and normalized firing rate (bottom) across a range of treadmill speeds. When firing is plotted according to time elapsed (Bb), the cell fired at about 14 s into treadmill running regardless of speed. Plotting the same data according to distance travelled (Bc) reveals that the cell fired at different distances depending upon speed. These experiments showed that hippocampal neurons can encode time and space conjointly (A) or can encode time only and not spatial dimensions (B). Parts Ab and Ac reprinted from Neuron, 71, MacDonald, C. J., Lepage, K. Q., Eden, U. T. and Eichenbaum, H. Hippocampal “time cells” bridge the gap in memory for discontiguous events, 737–749, © (2011), with permission from Elsevier. Parts Bb and Bc reprinted from Neuron, 78, Kraus, B. J., Robinson II, R. J., White, J. A., Eichenbaum, H. and Hasselmo, M. E. Hippocampal ‘time cells’: Time versus path integration, 1090–1101, © (2013), with permission from Elsevier.