Exponent of the latency of brightness power functions in the fovea and periphery of the visual field.

RT and perceived brightness against log luminance, covering the range between 3.4 and - 1.6 log cd/m2 in steps of 1 log unit, were measured at the fovea, 20 degrees, and 40 degrees in the periphery. The RT and brightness data were fitted to the RT and brightness power function, respectively. It was found that (a) asymptotic RT increased as eccentricity increased and that (b) the RT exponent decreased as retinal eccentricity increased: Estimated mean RT exponent was found to be approximately - .31 in the fovea, but - .25 and - .17 in the 20 degrees, and 40 degrees periphery, respectively. Further, (c) the brightness exponent tended to increase as eccentricity increased. The RT exponent in the fovea was in agreement with the exponent for perceived brightness (.36); however, the RT exponent in the periphery was about 1/2 to 2/3 as small as that in the fovea. This implies that as luminance changes equal RT contour changes as a function of eccentricity. That is, RT in the periphery does not primarily depend on the target's brightness but its luminance.