Adaptation, afterimages and cone saturation

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At low background intensities foveal increment thresholds obtained against briefly presented backgrounds are mediated by the initial appearance of the background and increment fields, and at high intensities by a short-term homochromatic afterimage. Masking the afterimages shows that the thresholds mediated by the initial image are a continuously accelerating function of background intensity, whereas afterimage thresholds follow a power function with an exponent of around 0.9. This pattern of results is obtained whether the eye is dark-adapted or light-adapted to a fixed level; however, on a log-log plot, the increment-threshold functions obtained in the light-adapted eye are shifted along a 45° line with respect to those obtained in the dark-adapted eye. This suggests that adaptation reduces neural response to lights as if multiplying their intensities by a factor between 0 and 1. A final experiment allows rejection of the hypothesis that decision factors (rather than adaptation processes) are responsible for the transient changes in thresholds observed immediately after the onset of intense backgrounds.