Samples of the filtered letters. The spatial frequency is given underneath each letter. The frequency increases with the increase in the number of details seen by the human eye. The contour/outlines have the highest frequency which facilitates the viewing of the correct shape of the letter. Source: Chung, S.T., Legge, G.E., & Tjan, B.S. (2002). Spatial-frequency characteristics of letter identification in central and peripheral vision. Vision Research, 42