functional MRI. 3 Tesla Siemens Timm Trio with 8 channel coil will be used for event-related fMRI, with single-shot, T2*-weighted sequence, (TR 2000 ms, TE 30 ms, flip angle 90°, FOV 220 x 220 mm, 36 contiguous axial oblique slices, 3.4 mm isotropic voxels). A structural MRI (MP-RAGE) will be acquired for normalization of EPI images. Pre-processing will include spatial realignment, slice timing correction, spatial normalization to T1 template, 8mm smoothing kernel. The GLM will be used with HRF convolved with a series of delta functions, and regressors for Task events, for each combination of Modafinil and Placebo and either Cue or Target. Errors/no-response trials are modeled separately. The functional connectivity analysis will use an LC-seeded beta series method; each event modeled uniquely and the time series of betas (segregated by task/treatment condition) correlated with the seed at every voxel. The LC will be localized in EPI images with a mask in MNI space, derived from voxels meeting these criteria: A) located in rostrodorsal pons, adjacent to 4th ventricle/sylvian aqueduct, restricted to an anatomically-defined dorsal pontine mask (e.g., not extending rostrally into the mesencephalon or superiorly into the 4th ventricle; and B) showing significant association with in-scanner pupil diameter, using this as a parametric regressor in GLM of BOLD time series, in an independent, healthy sample without treatment.