Top-down enhancement is driven by sensory retuning.

A sketches minimal SDT model consisting of front-end filter (grey box) followed by additive internal noise (black random trace pointing to + symbol); sensitivity may be enhanced by reducing internal noise (red trace), sharpening filter around congruent (thick blue line) and/or incongruent orientation (thin blue line). B plots rich/poor log-ratios for internal noise estimates (red) and projected sensitivity from filter estimates (blue) returned by psychophysical reverse correlation (plotting conventions similar to Fig 2F). Aggregate perceptual filters are shown in C-D for rich vs poor locations on top-down (C, green versus red) and bottom-up (D, solid versus open) maps. Congruent/incongruent orientations are indicated by orange/magenta vertical lines (0 and π/2 on x axis). Error bars show ±1 SEM. Lines show fits from 2 Gaussian functions of opposite sign centred on congruent/incongruent orientations (for visualization only). Shading in C plots ±1 SD across simulations from gain-control model (inset), consisting of 2 front-end filters oriented along congruent (left icon in inset) and incongruent (right icon) orientations. Model simulations for red/green shading were generated by red/green-tinted front-end filters (transition indicated by blue arrows). Data for this figure is available from S2 Data. SDT, signal detection theory.