The threshold of rectangular and annular increments was established on square-wave gratings of 1 foot lambert mean luminance. In the range of 5–18 cycles/degree the threshold fell as the spatial frequency of the background increased. The threshold change was similar for rectangles and annuli, but the exponent of the curve differed according to the relative orientation of the test stimulus. The threshold curve of rectangles was related to the human contrast sensitivity curve, and it was found that threshold is directly proportional to the contrast of the grating and effective contrast is a function of the spatial frequency of the pattern. In the range below 5 cycles/degree of the grating pattern the threshold dropped. This is attributed to the low-frequency decline in contrast sensitivity, but this decrease occurred at a higher spatial frequency than expected. Apparently orientation and size specificity in field adaptation is compatible with functions of cells established in the visual cortex of primates and humans.