Figure 2.

An example neuron in ACCd showing multiplicative modulation of memory traces by the epoch code. The colors in all panels denote trial epochs, following the format of Fig.1a. (a) The epoch code for an example neuron, i.e. the firing rate computed in twelve 250ms epochs within a trial and averaged over all trials (black squares, interpolated by the black line, broken during the saccade). Coloured disks correspond to the slopes fitted in panel c (error bars, ±SE); their correlation with the epoch coded quantifies the multiplicative modulation, and is referred to as the factorization index (FI=0.97 in this example). (b) The memory trace f of past rewards in the same neuron, up to five trials in the past. Coloured dots and error bars (±SE) show the results of the multiple linear regression model, Eq.(1), and the black line is the exponential fit (Eq.(2), continuous line, exponential ex(t); broken line, modulated envelope g·ex(t)). The parameters for the fit are shown (A, amplitude; τ, timescale). (c) The memory trace f (from panel b), plotted as a function of the exponential function ex. The lines are least squares fit, each line encompassing a particular epoch and all five trial lags. According to the factorization, the slopes should correspond to the epoch code, f = g·ex. The values of the slopes are plotted in panel a (coloured squares) and compared with the epoch code g(k).