The subjective sensitivity to small changes in the contrast of a suprathreshold grating

Small periodic (0.25-40 Hz) changes in the contrast of suprathreshold, spatially sinusoidal (0.8-10.5 c/deg) grating test objects were generated on an oscilloscope screen. It was found that the minimum temporal modulation in spatial contrast which could be detected subjectively was approximately proportional to the mean contrast of the grating during the change, provided that the minimal spatial modulation of the grating was above threshold. The contrast increment threshold was dependent on both the grating spatial frequency and the temporal frequency of the modulation: for any fixed temporal frequency in the range 2-8 Hz it increased approximately linearly with spatial frequency; for fixed spatial frequency the threshold was minimal at temporal frequencies of a few Hz. The relevance of these results to the possible role of fast fluctuations in the control of accommodation response is considered.