This study examined the detectability of flicker for small long-wavelength foveal test stimuli centered within larger long-wavelength surround stimuli. Flicker visibility was evaluated as a function of surround and test illuminance and as a function of test wavelength, of the time elapsed following test or surround onset, and of surround dimensions. Consistent with prior flicker threshold-versus-illuminance results [ Vision Res. 26, 917 ( 1986)], flicker threshold decreased abruptly once the surround illuminance became sufficiently great. However, as test illuminance was increased above flicker threshold, flicker again vanished. Flicker reappeared at still higher test illuminances, as middle-wavelength-sensitive (M-) cone-mediated flicker threshold was exceeded. Meanwhile, the time required for the surround to render flicker visible increased at a rapidly accelerating rate with decreasing surround illuminance; it increased at a more sporadic rate with increasing test illuminance. At bright enough surround illuminances, flicker did not vanish with increasing test illuminance. These and other results are compatible with a framework derived from previous dark-adaptation data8 [ Vision Res. 32, 1975 ( 1992)]. In that framework the test stimulus itself induces losses of flicker sensitivity by sufficiently perturbing retinal response during states or stages of adaptation that fail to cause spectrally antagonistic processes to redress that perturbation adequately. The relevant adaptation processes, which can require minutes, involve an adaptation pool that includes (and is affected by) the test stimulus.
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