Sensitivity to modulations of luminance and contrast in visual white noise: separate mechanisms with similar behaviour

Abstract

Human vision can detect spatiotemporal information conveyed by first-order modulations of luminance and by second-order, non-Fourier modulations of image contrast. Models for second-order motion have suggested two filtering stages separated by a rectifying nonlinearity. We explore here the encoding of stationary first-order and second-order gratings, and their interaction. Stimuli consisted of 2-D binary, broad-band, static, visual noise sinusoidally modulated in luminance (LM, first-order) or contrast (CM, second-order). Modulation thresholds were measured in a two-interval forced-choice staircase procedure. Sensitivity curves for LM and CM had similar shape as a function of spatial frequency, and as a function of the size of a circular Gaussian blob of modulation. Weak background gratings present in both intervals produced order-specific facilitation: LM background facilitated LM detection (the dipper function) and CM facilitated CM detection. LM did not facilitate CM, nor vice-versa, neither in-phase nor out-of-phase, and this is strong evidence that LM and CM are detected via separate mechanisms. This conclusion was further supported by an experiment on the detection of LM/CM mixtures. From a general mathematical model and a specific computer simulation we conclude that a single mechanism sensitive to both LM and CM cannot predict the pattern of results for mixtures, while a model containing separate pathways for LM and CM, followed by energy summation, does so successfully and is quantitatively consistent with the finding of order-specific facilitation.

The Effect of White-Noise Mask Level on Sinewave Contrast Detection Thresholds and the Critical-Band-Masking Model

“…We used the frame interleaving method to mask the test signal (Schofield & Georgeson, 1999;Solomon, 2000). The advantage of this method is that the test contrast can be varied independently of the contrast of the noise mask in which the test is embedded (Schofield, 1998).…”

“…When white noise is added, CDT increases with the noise power spectral density or level (N 0 ) (Pelli, 1981;Stromeyer & Julesz, 1972;Thomas, 1985), except for an initial decrease due to stochastic resonance (Blackwell, 1998), at the same time as the U-shaped contrast threshold curve progressively shifts upward and flattens out (Blackwell, 1998). When gratings are masked with high-level 2D white noise, the threshold curve becomes completely flat (i.e., CDT is independent of spatial frequency), at least for spatial frequencies below 10 c/deg (Rovamo, Franssila, & Näsänen, 1992;Schofield & Georgeson, 1999). In summary, white noise added to sinusoidal gratings increases their CDTs and changes the shape of contrast detection curves.…”

Foveal visual acuity is worse and shows stronger contour interaction effects for contrast-modulated than luminance-modulated Cs

“…Visual detection responses to second-order stimuli such as contrastmodulated (or CM) stimuli are valuable to study because they are thought to be processed by separate streams from first-order or luminance-modulated (LM) stimuli (Schofield & Georgeson, 1999;Allard & Faubert, 2006, albeit with cross-links between them (Ellemberg et al, 2004;Chung et al, 2007;Hairol & Waugh, 2010a,b).…”

Reduced sensitivity for visual textures affects judgments of shape-from-shading and step-climbing behaviour in older adults

“…Importantly the known deficiency for texture processing (Habak & Faubert, 2000) transfers into shape judgement tasks: our hypothesis is supported. While it is possible that the slight difference in base contrast between the binary and fine/coarse textures explained the differences in performance for some texture pairings, the effect of such a small contrast change is likely to be very much less than the difference observed here (Schofield & Georgeson, 1999). Further, this contrast difference cannot explain the difference in performance between the coarse and fine textures for the older adults as they had the same base contrast.…”

“…If either set of filters is compromised the stripes become harder to see. Thus a loss of sensitivity to higher spatial frequencies would effectively reduce the signal strength at the output of the first-stage filters and thus reduce sensitivity to the contrast modulations (Schofield & Georgeson, 1999). …”