Contrast masking was studied psychophysically. A two-alternative forced-choice procedure was used to measure contrast thresholds for 2.0 cpd sine-wave gratings in the presence of masking sine-wave gratings. Thresholds were measured for 11 masker contrasts spanning three log units, and seven masker frequencies ranging +/- one octave from the signal frequency. Corresponding measurements were made for gratings with horizontal widths of 0.75 degrees (narrow fields) and 6.0 degrees (wide fields). For high contrast maskers at all frequencies, signal thresholds were related to masking contrast by power functions with exponents near 0.6. For a range of low masking contrasts, signal thresholds were reduced by the masker. For the wide fields, high contrast masking tuning functions peaked at the signal frequency, were slightly asymmetric, and had approximately invariant half-maximum frequencies that lie 3/4 octave below and 1 octave above the signal frequency. The corresponding low contrast tuning functions exhibited peak threshold reduction at the signal frequency, with half-minimum frequencies at roughly +/- 0.25 octaves. For the narrow fields, the masking tuning functions were much broader at both low and high masking contrasts. A masking model is presented that encompasses contrast detection, discrimination, and masking phenomena. Central constructs of the model include a linear spatial frequency filter, a nonlinear transducer, and a process of spatial pooling that acts at low contrasts only.

https://courses.washington.edu/psy448/pdf/Legge_JOSA80.pdf

Contrast masking in human vision

A method of producing red-green and blue-yellow sinusoidal chromatic gratings is used which permits the correction of all chromatic aberrations. A quantitative criterion is adopted to choose the intensity match of the two colours in the stimulus: this is the intensity ratio at which contrast sensitivity for the chromatic grating differs most from the contrast sensitivity for a monochromatic luminance grating. Results show that this intensity match varies with spatial frequency and does not necessarily correspond to a luminance match between the colours. Contrast sensitivities to the chromatic gratings at the criterion intensity match are measured as a function of spatial frequency, using field sizes ranging from 2 to 23 deg. Both blue-yellow and red-green contrast sensitivity functions have similar low-pass characteristics, with no low-frequency attenuation even at low frequencies below 0.1 cycles/deg. These functions indicate that the limiting acuities based on red-green and blue-yellow colour discriminations are similar at 11 or 12 cycles/deg. Comparisons between contrast sensitivity functions for the chromatic and monochromatic gratings are made at the same mean luminances. Results show that, at low spatial frequencies below 0.5 cycles/deg, contrast sensitivity is greater to the chromatic gratings, consisting of two monochromatic gratings added in antiphase, than to either monochromatic grating alone. Above 0.5 cycles/deg, contrast sensitivity is greater to monochromatic than to chromatic gratings.

The contrast sensitivity of human colour vision to red‐green and blue‐yellow chromatic gratings.

We summarize the various strands of research on peripheral vision and relate them to theories of form perception. After a historical overview, we describe quantifications of the cortical magnification hypothesis, including an extension of Schwartz's cortical mapping function. The merits of this concept are considered across a wide range of psychophysical tasks, followed by a discussion of its limitations and the need for non-spatial scaling. We also review the eccentricity dependence of other low-level functions including reaction time, temporal resolution, and spatial summation, as well as perimetric methods. A central topic is then the recognition of characters in peripheral vision, both at low and high levels of contrast, and the impact of surrounding contours known as crowding. We demonstrate how Bouma's law, specifying the critical distance for the onset of crowding, can be stated in terms of the retinocortical mapping. The recognition of more complex stimuli, like textures, faces, and scenes, reveals a substantial impact of mid-level vision and cognitive factors. We further consider eccentricity-dependent limitations of learning, both at the level of perceptual learning and pattern category learning. Generic limitations of extrafoveal vision are observed for the latter in categorization tasks involving multiple stimulus classes. Finally, models of peripheral form vision are discussed. We report that peripheral vision is limited with regard to pattern categorization by a distinctly lower representational complexity and processing speed. Taken together, the limitations of cognitive processing in peripheral vision appear to be as significant as those imposed on low-level functions and by way of crowding.

Peripheral vision and pattern recognition: a review.

Humans can discriminate several million different colors and almost half a million different tones, but the number of discriminable olfactory stimuli remains unknown. The lay and scientific literature typically claims that humans can discriminate 10,000 odors, but this number has never been empirically validated. We determined the resolution of the human sense of smell by testing the capacity of humans to discriminate odor mixtures with varying numbers of shared components. On the basis of the results of psychophysical testing, we calculated that humans can discriminate at least 1 trillion olfactory stimuli. This is far more than previous estimates of distinguishable olfactory stimuli. It demonstrates that the human olfactory system, with its hundreds of different olfactory receptors, far outperforms the other senses in the number of physically different stimuli it can discriminate.

/pdf/humans-can-discriminate-more-than-1-trillion-olfactory-21uwe9dbda.pdf

Humans can discriminate more than 1 trillion olfactory stimuli

The perception of contrast was measured in humans by a technique of subjective contrast-matching, and was compared with contrast sensitivity as defined by threshold measures. 2. Contrast-matching between different spatial frequencies was performed correctly (especially at frequencies above 5 c/deg) despite the attenuation by optical and neural factors which cause large differences in contrast thresholds. 3. Contrast-matching between single lines of different widths was also veridical, and was not limited by the spatial integration (Ricco's Law) present at threshold. Adaptation to gratings altered the appearance of lines, and this could be best understood in Fourier terms. 4. The generality of these results was shown by matching the contrast of pictures which had been filtered so that each contained a one octave band of spatial frequencies. 5. Within the limits imposed by threshold and resolution, contrast-matching was largely independent of luminance and position on the retina. 6. Six out of eleven astigmatic observers showed considerable suprathreshold compensation for their orientation-specific neural deficit in contrast sensitivity. 7. These results define a new property of vision: contrast constancy. It is argued that spatial frequency channels in the visual cortex are organized to compensate for earlier attenuation. This achieves a dramatic 'deblurring' of the image, and optimizes the clarity of vision.

https://physoc.onlinelibrary.wiley.com/doi/pdf/10.1113/jphysiol.1975.sp011162

Contrast constancy: deblurring in human vision by spatial frequency channels.

Functional organization of the peripheral retina: sensitivity to periodic stimuli.

Wavelength discrimination was measured from 480 to 660 nm using wavelength-modulated square-wave gratings. When adjacent grating bars are matched in brightness, wavelength-difference thresholds in all regions of the spectrum increase with spatial frequency within the range of 2.4–19.9 cycles per degree (cpd). Unlike luminance-modulated gratings, no sign of low-frequency attenuation was found. Wavelength differences alone do not yield good visual acuity. Small brightness mismatches between adjacent grating bars generally improve wavelength discrimination and cause low-frequency attenuation in the wavelength modulation vs spatial-frequency function so that it resembles conventional luminance modulation vs spatial-frequency functions.

Wavelength discrimination measured with square-wave gratings.

Hue discrimination is poor when measured with fine gratings in which alternate bars differ in wavelength but not in luminance, or with small test objects that differ in wavelength from their background. Hue discrimination improves if a small luminance contrast is added. This effect is strongest for small test objects, and decreases as the size of the test object increases, or as the spatial frequency of the grating decreases, so that luminance contrast has little effect on hue discrimination measured with a 2° bipartite field. When measured with equiluminous square-wave gratings, an observer’s sensitivity to small wavelength differences increases monotonically as spatial frequency is decreased. When a luminance contrast is included, maximum sensitivity occurs when the grating has a spatial frequency of about 3 cycles/deg, and decreases at higher and lower spatial frequencies, so that its shape resembles the luminance modulation transfer function of the visual system. Hue-discrimination performance is better when measurements are made with the 3-cycles/deg grating with luminance contrast than with a conventional bipartite field.

Influence of luminance contrast on hue discrimination

The size of human visual receptive fields was measured by two methods while the observer accommodated on near and distant targets. In one method, interference patterns that were not affected by the state of accommodation of the eye were formed on the retina and used to measure the modulation-sensitivity function of the visual system without the influence of its optics. The spatial frequency to which the observer was most sensitive, which is related to receptive-field size, was not affected by changes of accommodation or convergence. Visual acuity also remained constant when accommodation was changed. In the second experiment, receptive-field size was estimated by determining the size of a superimposed background that most effectively masked a small test flash. As in the first experiment, viewing distance did not appear to influence receptive-field size. It is concluded that size constancy is not a result of changes in receptive-field dimensions.

C. R. Cavonius

Papers

Functional organization of the peripheral retina: sensitivity to periodic stimuli.

Wavelength discrimination measured with square-wave gratings.

Influence of luminance contrast on hue discrimination

Invariance of visual receptive-field size and visual acuity with viewing distance.

A Technique for Testing Visual Function in the Presence of Opacities