Noise Frame Duration, Masking Potency and Whiteness of Temporal Noise

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 (E th ) 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 N 0 , increases with the duration of the noise frame. RESULTS. For short noise frame durations, E th increased in direct proportion to N 0 , keeping the nominal signal-to-noise ratio [SNR ϭ (E th /N 0 ) 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 N 0 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 detectionlimiting 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. (Invest Ophthalmol Vis Sci. 2002;43: 3131-3135) A dding white temporal noise to a temporal signal allows the investigation of "central" aspects of temporal contrast detection beyond the initial optical, retinal, and neural filtering, The masking power of external noise is described by its spectral density. The spectral density of white spatiotemporal noise is the product of the spatial dimensions of the noise checks, duration of each noise frame, and root mean square (RMS) contrast of noise squared. 6,8 Spatial parameters are held constant for purely temporal noise, and its spectral density can be expressed simply as the product of the duration of the noise frame and RMS contrast squared. 1,2,9 The masking effect of white temporal noise is thus dependent on its contrast and the duration of each noise frame. The frame rate of the display limits the temporal bandwidth of noise. Hence, so-called white noise always has a specific high-frequency cutoff, beyond which it is no longer white. Previous research indicates, however, that noise reduces detectability only at the frequencies of the signal and its immediate neighborhood. In the present study, we determined the lowest high-frequency cutoff-that is, the minimum bandwidth of temporal noise that still acts as white in masking sinusoidal flicker (1.25-20 Hz) by determining the longest noise frame duration that still mimics white noise. METHODS Stimuli were generated under computer control on a 16-in. red-greenblue (RGB) monitor (Flexscan T57S; EIZO Nanao, Cypress, CA) with Fast Phosphor P22 used in white mode and driven by a graphics board (Millennium II Powerdesk; Matrox, Dorvoli, Montreal, Canada) that generated 640 ϫ 480 pixels, each 0.47 ϫ 0.47 mm 2 in size. The frame rate of the monitor was 132 Hz. The amplitudes and frequencies of flickering stimuli were checked with a phototransistor (TIL81; Texas Instrument, Dallas, TX). No attenuation of amplitude was measured at 30 Hz. Even if there were any contrast attenuation at shorter noise frame durations (down to 1 ⁄ 132 seconds), signal and noise are similarly affected at each temporal frequency so that SNR remained unchanged. The average luminance of the display was 50 cd/m 2 . To increase the number of gray levels available, the red, green, and blue outputs of From th