OBJECTIVE Detection efficiency for flickering stimuli of constant duration decreases with increasing temporal frequency. Increasing frequency in this case also implies increasing number of flicker cycles. The current study was conducted to investigate whether this result could be due to the limited ability of the central detector to integrate flicker cycles. METHODS Flicker sensitivity was measured at 1 to 20 Hz in strong external temporal noise with increasing stimulus duration. RESULTS Sensitivity increased with stimulus duration in a nonsaturating manner up to the longest exposure times used, indicating probability summation. When expressed in terms of detection efficiency (eta) as a function of number of cycles presented (n) all data could be modeled as a single decreasing function of the form eta=0.29n(-0.70). CONCLUSIONS The results show that the number of cycles, not time, is the determinant of probability summation of flicker. The results are consistent with the idea that the central detector is a suboptimal matched filter spanning less than one cycle.