Temporal sensitivity, defined as the reciprocal of the minimum exposure duration required for a grating to be detected, was measured for gratings with constant contrasts in a range of spatial frequencies. Temporal sensitivity functions were found to be given by curves with maximum sensitivity at 0.7 approximately 0.8 C/deg. Two criteria of detection were used: the detection of any change on the screen and the detection of patterns. The difference between temporal sensitivities under the two criteria was large at low spatial frequencies but was small at high spatial frequencies.