Sensitivity to modulations of luminance and contrast in visual white noise: separate mechanisms with similar behaviour.
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.
Schofield
A J
AJ
School of Psychology, University of Birmingham, Edgbaston, UK. a.j.schofield@bham.ac.uk
Georgeson
M A
MA
D015350:Contrast Sensitivity / Q000502:physiology*; D003463:Cues; D006801:Humans; D008027:Light; D008297:Male; D008433:Mathematics; D008954:Models, Biological; D010364:Pattern Recognition, Visual / Q000502:physiology*; D012684:Sensory Thresholds / Q000502:physiology