Effects of simple and complex surrounds on temporal contrast sensitivity

Purpose When the mean luminance of a small foveally fixated target matches the luminance of the surround, the temporal contrast sensitivity at low temporal frequencies is markedly improved in comparison to "dark" surrounds. This effect has generally been attributed to light adaptation to the surround, rather than to the surround acting as a perceptual reference point. We tested these hypotheses by using uniform and complex surrounds. Methods Temporal CSFs were measured using 2IFC staircase procedures. A 1 deg central disk, mean luminance 25 cd/m², sinusoidally flickering at 0.25, 0.5, 1 or 2 Hz, was embedded in various surrounds: Exp.1 Three spatially uniform surrounds: "dark", "equiluminant" and "light" (0, 25 and 50 cd/m²). Exp. 2 Surrounds filled with random binary texture of contrast from 0.1 to 1.0, mean luminance equal to the test. Exp. 3 Binary random texture surrounds with components at (0, 25), (12.5, 37.5), and (25, 50) cd/m². Results Exp. 1 At low frequencies, sensitivity was markedly higher on the equiluminant surround, and equally low for dark and light surrounds, ruling out the light adaptation hypothesis. Exp. 2 Sensitivity decreased as a function of the spatial contrast of the surround. At a surround contrast of 1.0, sensitivity was identical to that for the light and dark surrounds, further ruling out mean adaptation level as a factor. Exp. 3 Despite two of the textures having half the squares at the test mean level, sensitivities were similarly decreased by all three backgrounds of equal spatial contrast. Conclusions The increased sensitivity on the uniform equiluminant surround might be due to the availability of a reference level relative to which the center at different phases of modulation appears different, but is not due to light adaptation. Textured surrounds affect sensitivity proportional to the internal contrast, and not as a function of mean level or by providing partial reference points.