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(Investigative Ophthalmology and Visual Science. 2002;43:3131-3135.)
© 2002 by The Association for Research in Vision and Ophthalmology, Inc.

Noise Frame Duration, Masking Potency and Whiteness of Temporal Noise

Heljä Kukkonen1, Jyrki Rovamo1, Kristian Donner2, Marja Tammikallio2 and Antti Raninen2

1 From the Department of Optometry and Vision Sciences, Cardiff University, Cardiff, Wales, United Kingdom; and the 2 Department of Biosciences, University of Helsinki, Helsinki, Finland.

PURPOSE. Because of the limited contrast range, increasing the duration of the noise frame is often the only option for increasing the masking potency of external, white temporal noise. This, however, reduces the high-frequency cutoff beyond which noise is no longer white. This study was conducted to determine the longest noise frame duration that produces the strongest masking effect and still mimics white noise on the detection of sinusoidal flicker.

METHODS. Contrast energy thresholds (Eth) were measured for flicker at 1.25 to 20 Hz in strong, purely temporal (spatially uniform), additive, external noise. The masking power of white external noise, characterized by its spectral density at zero frequency N0, increases with the duration of the noise frame.

RESULTS. For short noise frame durations, Eth increased in direct proportion to N0, keeping the nominal signal-to-noise ratio [SNR = (Eth/N0)0.5] constant at threshold. The masking effect thus increased with the duration of the noise frame and the noise mimicked white noise. When noise frame duration and N0 increased further, the nominal SNR at threshold started to decrease, indicating that noise no longer mimicked white noise. The minimum number of noise frames per flicker cycle needed to mimic white noise decreased with increasing flicker frequency from 8.3 at 1.25 Hz to 1.6 at 20 Hz.

CONCLUSIONS. The critical high-frequency cutoff of detection-limiting temporal noise in terms of noise frames per signal cycle depends on the temporal frequency of the signal. This is opposite to the situation in the spatial domain and must be taken into consideration when temporal signals are masked with temporal noise.




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J. Rovamo, H. Kukkonen, A. Raninen, and K. Donner
Efficiency of Temporal Integration of Sinusoidal Flicker
Invest. Ophthalmol. Vis. Sci., November 1, 2003; 44(11): 5049 - 5055.
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Copyright © 2002 by the Association for Research in Vision and Ophthalmology