The resulting NU7441 cost contrast-response function had a much steeper slope than that measured in the focal cue condition, and did not fit well the measured contrast-response functions in any of the visual areas (V1, r2 = 0.58, Figure 5D, blue curve; V2, r2 = 0.63; V3, r2 = 0.63;

hV4, r2 = 0.64; average across observers), nor for any observer (observer 1, r2 = 0.63; observers 2, r2 = 0.59; observer 3, r2 = 0.64; average across visual areas). Allowing the standard deviation (σ) and the baseline response (b) to be adjusted for the focal cue condition ( Figure 5E) resulted in good fits to the contrast-response functions for each visual area ( Figure 5F; V1, r2 = 0.89; V2, r2 = 0.85; V3, r2 = 0.89; hV4, r2 = 0.83; average across observer) and for each individual observer (observer 1, r2 = 0.90; observer 2, r2 = 0.77; observer 3, r2 = 0.91; average across visual areas). For V1, the best-fit value of the sensory

noise standard deviation (σ) was 0.085% signal change for the distributed cue and 0.016% signal change for the focal cue condition. The best-fit value of the baseline response (b) was 0.34% signal change for the distributed cue and 0.55% signal change for the focal cue condition. Thus, Metformin supplier there was no evidence for a change in the response gain of the fMRI responses, only for a change in the sensory noise standard deviation and baseline response parameters. A similar result was found for each visual area and observer; the ostensible effect of the focal cue was to decrease sensory noise and increase the baseline response.

whatever These two model parameters were fit separately for the distributed cue and focal cue conditions for each visual area and each observer. The average σ value for the distributed cue (σd) condition was 0.064% ± 0.02% and 0.016% ± 0.01% for the focal cue (σf) condition. The ratio of σd to σf was significantly greater than 1 in all observers and visual areas (p < 0.01, bootstrap test; see Supplemental Experimental Procedures: Statistical Tests in Individual Observers) and implied approximately a 4-fold reduction in sensory noise (Figure 6A). The average b value increased from 0.58% ± 0.02% for the distributed cue condition to 0.74% ± 0.04% for the focal cue condition ( Figure 6B, bd and bf, respectively). The difference between bd and bf was significantly different from zero in all observers and visual areas (p < 0.05, except for hV4 in one observer, p = 0.38, bootstrap test). The approximately 400% reduction in σ between the distributed and focal cue conditions could be due to a decrease in early signal-to-noise ratio, to greater inefficiencies in “reading out” the sensory signals, or to a combination of the two. Monkey electrophysiology experiments have shown that attention can reduce sensory noise, but not by such a large amount.