Do decision processes have separate access to the individual Fourier c
omponents of a complex pattern? Subjects discriminated between plaid p
atterns formed by multiplying or by summing vertical and horizontal sq
uare-wave gratings. Such patterns differ in two ways: (1) product plai
ds contain distinctive off-axis Fourier components; (2) adjacent lumin
ances are related by equal ratios in the products and by equal differe
nces in the sums. Performance on this task was compared with the separ
ately measured unmasked detection thresholds of the off-axis Fourier c
omponents and with performance in control tasks which provided only th
e non-Fourier cues. Performance in the plaid discrimination was worse
than predicted from the unmasked detection thresholds, but better than
predicted from the control tasks. The results argue that Fourier comp
onents are accessible, but that masking reduces their usefulness.