Border enhancement is observed in a flickering bright-dark field. It is found that both the dark and the bright portions of the enhanced region increase in extent from the border during certain flicker frequencies, but not symmetrically about the border. This asymmetry is described by plotting the ratio of the dark and bright enhanced regions against stimulus frequency. The ratio fluctuates with frequency, the amplitude of the fluctuations being greatest at low frequencies. The foveal region stimulated by the border has some effect on the ratio functions. They cannot be mimicked by ratios obtained from enhancement responses to low and high luminances on either the dark or bright side of the border. It is suggested that these flicker-induced asymmetries are best understood in the context of separate neural systems for the perception of darkness and brightness.
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