Figure 4. Serotonin neuron firing rates correlate with expected uncertainty on slow and fast timescales.

(A) Action potential raster plots for an example neuron with a significant correlation with expected uncertainty during the go cue aligned to cue onset (left) and outcome (second lick, right) and ordered by increasing ε.

(B) Activity of the example neuron in (A) averaged within terciles (increasing values of ε represented by darker hues) of ε and aligned to the go cue (CS, gray rectangle) and outcome.

(C) The t-statistics across all neurons from a linear regression, modeling firing rates during the inter-trial interval as a function of ε(t). Blue bars indicate neurons with significant regression coefficients.

(D) Population Z-scored firing rates plotted as a function of ε(t). Inset shows population split by positive and negative correlations. Main plot combines these neurons by “sign-flipping” positively correlated firing rates (also used in E and F). Pie chart shows ratio of significant neurons (blue).

(E) Within-trial dynamics of expected uncertainty (ε(t), ε(t + 1), top row) aligned to go cue (CS, left column) and outcome (right column) across all significant neurons. Scale bar, 0.5 Z score. Gray curve: Response time (RT) distribution (cut off at 1 s).

(F) The Z-scored firing rates of serotonin neurons split by e(f) tercile. Scale bar, 0.5 Z score.

(G) Example dynamics of ε(t) estimated from behavior and neuronal firing rates.

(H) Log-log plot of the expected uncertainty update rate (ψ) from the firing rate model for each neuron and from the behavioral model derived from simultaneous choice behavior. See also Figure S4.