SURROUND EFFECTS ON THE SHAPE OF THE TEMPORAL CONTRAST-SENSITIVITY FUNCTION

The shape of the temporal contrast-sensitivity function at low tempora l frequencies is sensitive to the relative luminance of the test and t he surround. We show that this effect is due to greater sensitivity, i n different conditions, either to the internal luminance modulation in the test or to temporal changes in the spatial contrast at the edge o f the test. We measured temporal contrast sensitivity in tests at Vari ous luminance levels combined with surrounds at levels of higher, lowe r, or equal luminance as the test; compared the sensitivity for contra st modulation to luminance modulation at different temporal frequencie s; and compared temporal contrast sensitivity in uniform and textured surrounds of equal mean luminance. Temporal contrast sensitivity was s imilar on equiluminant steady and out-of-phase modulating surrounds, i ndicating that the measured sensitivity for small tests in equiluminan t surrounds is based on the detection of the temporal modulation of th e spatial contrast at the edge of the test field. For all temporal fre quencies, contrast sensitivity decreased as a monotonic function of th e absolute magnitude of the Michelson contrast between test and surrou nd. When small test fields of moderate to high intensities are embedde d in dark surrounds, the sensitivity at lower spatial frequencies is s imilar to the sensitivities measured for a large test and may reflect sensitivity for luminance modulation within the test. (C) 1997 Optical Society of America.