(a–c) Spike count correlations obtained from neuron pairs in the homogeneous network with bottom-up correlations but no top-down inputs (a), with top-down inputs and no bottom-up correlations (b), and in the heterogeneous network with both top-down inputs and bottom-up correlations (c). The matrix entries ρ(ti, tj) are correlation coefficients of the spike counts in time bins ti and tj (count window T=250 ms) averaged over cell pairs. Insets: correlations ρ(ti, tj) versus time lag ti−tj from two instants of the stimulus interval (dashed diagonals in a). Bottom-up correlations were generated by non-replicate stimuli. (d,e) Time-course of correlations in the full model (c) shows large amplitude sustained instantaneous correlations (d; main diagonal in c), caused mainly by stimulus fluctuations (see a), and slowly rising lagged correlations (e, blue dashed diagonal in c), caused by top-down inputs (see b). (f,g) MT correlations (n=32 neuron pairs at coherences −3.2, 0, +3.2%) show similar time-courses as the model: instantaneous correlations (f) do not change significantly over time whereas lagged correlations (g) increase significantly (regression line slopes −0.0092, s−1 and 0.018 s−1, with P=0.718 and P=0.046, respectively; permutation tests). The weak non-monotonic trend of the instantaneous correlations (fast-rise + slow-decay) shown in f can be partly due to the similar trend displayed by the evoked rate (not shown). (h) Slopes of lagged correlations ρkk'(ti, ti+1) for individual MT pairs versus the mean late CP of the two corresponding neurons (correlation R=0.27, P=0.02). (i) Average early and late CP for pairs with positive (red) and negative (black) slopes of lagged correlations (early: 0–1 s, late: 1–2 s; T=1,000 ms). (j) Average CP time-course for cell pairs showing rising (black) and decaying (red) lagged correlations (T=250 ms). Error bars indicate s.e.m. Thick horizontal lines mark periods of significant difference (P<0.05, permutation test).