Figure 12.

Cells that take conjunctions of simple functions of space and time generate a rich set of firing correlates. a, b, Conjunctions of boundary vector cells generate canonical place cells with fields that move when the dimensions of the environment are changed. Each of the four panels gives a firing rate map for four different environments of different sizes. a, A cell taking the product of input from one boundary vector cell coding for distance to the northern wall and a cell coding for distance from the western wall. b, Input from three boundary vector cells was combined. The first cell peaks in its firing 2 units from the northern wall, the second cell peaks firing 4 units from the western wall, and the third peaks 6 units from the eastern wall. The firing rate of the unit shown is the firing rate of cell 1 times the sum of cells 2 and 3. c, Firing rate map for a simulated conjunctive time cell/place cell for different periods of time. The simulated cell's firing rate is given by the product of two boundary vector cells (Case III) and a time cell (Case I) coding for an event at the beginning of the session. The first firing rate map is for 0–2000 units of time; the second map is for 2000–4000 units of time. Because the temporal history is scale-invariant, the units of time are arbitrary except insofar as the time scale is long enough to allow the path to thoroughly sample positions.