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

We propose a model of how humans sense the velocity of moving images. The model exploits constraints provided by human psychophysics, notably that motion-sensing elements appear tuned for two-dimensional spatial frequency, and by the frequency spectrum of a moving image, namely, that its support lies in the plane in which the temporal frequency equals the dot product of the spatial frequency and the image velocity. The first stage of the model is a set of spatial-frequency-tuned, direction-selective linear sensors. The temporal frequency of the response of each sensor is shown to encode the component of the image velocity in the sensor direction. At the second stage, these components are resolved in order to measure the velocity of image motion at each of a number of spatial locations and spatial frequencies. The model has been applied to several illustrative examples, including apparent motion, coherent gratings, and natural image sequences. The model agrees qualitatively with human perception.

Model of human visual-motion sensing

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.

Chapter 9 Visual adaptation and retinal gain controls

Perceptual visual quality metrics: A survey

A dynamic model based on electrophysiological findings is presented for information processing in the visual system. The visual system behaves as an optimal encoder both of information perturbed by Poisson noise at low luminances and of noise-distorted images at suprathreshold level. The basic elements of the model, are: (1) a first layer of square-root scalers mainly performing noise reduction, where each scaler consists of two leaky integrators and a comparator; (2) a layer containing a light detector consisting of one leaky integrator and a comparator, and an increment/decrement detector consisting of two leaky integrators and a comparator. The results of simulations of threshold behavior are given. The authors show that all generally known psychophysical facts can be described with this model. When spatial interaction between neighboring basic elements is introduced, the effects of these interactions spread over a large area, thus changing properties of the total network. So far, this extensive effect has only been proved with phenomenological models. Possible applications of this model in image processing are proposed.

A mechanistic approach to threshold behavior of the visual system

On threshold mechanisms for achromatic and chromatic vision.

In t roduc t ion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126 Scotopic and Pho top ic Cone Vision . . . . . . . . . . . . . . . . . . . . . t 28 H u m a n O m m a t i d i a . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131 Ret ina l Color Signals . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 Photop ic and Scotopic Rod Vision . . . . . . . . . . . . . . . . . . . . . . 139 General Coincidence Concept and Visual A d a p t a t i o n . . . . . . . . . . . . . . 142 The De Vries-Rose Law and Webe r ' s Law for Visual Disc r imina t ion . . . . . . . 143 Square Root Coincidence Scalers: De Vries-Rose Machines . . . . . . . . . . . 146 Linear Coincidence Scalers: W e b e r Machines . . . . . . . . . . . . . . . . . t48 Neural Q u a n t u m De tec to r . . . . . . . . . . . . . . . . . . . . . . . . . 150 Mesopic Vision . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152 Color Discr imina t ion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154 Quanta l Effec ts in the Electrophysiology" of Vision . . . . . . . . . . . . . . 155 Adap t ing Scaler Act ions of Synap t i c Junc t ions . . . . . . . . . . . . . . . . t 58 Q u a n t u m Spike Rat ios and Threshold Vision in E lec t rophys io logy . . . . . . . 162 W h a t the H u m a n E y e Tells the H u m a n Brain . . . . . . . . . . . . . . . . 165 S u m m a r y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . t 68 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170

Psychophysical basis of coincidence mechanisms in the human visual system.

Motion coherence detection as a function of luminance level in human central vision

An apparatus is described that measures automatically the response of the eye to an accommodation stimulus. It enables one to record the change of the lens power, of the pupil size, and of the eye position simultaneously from one eye without impeding that eye from performing its normal visual tasks. The measuring technique is based on the properties of the light beam coming out of the eye from an ir light spot that is projected on the retina. The outcoming beam is split into two parts. The extent of convergence of the beam is a measure for the lens power. The sum of and the difference between the intensities of the two parts of the light beam are measured to determine, respectively, the pupil size and the position of the eye with regard to the axis of symmetry of the apparatus.

Maarten A. Bouman

Papers

A mechanistic approach to threshold behavior of the visual system

On threshold mechanisms for achromatic and chromatic vision.

Psychophysical basis of coincidence mechanisms in the human visual system.

Motion coherence detection as a function of luminance level in human central vision

An accommodometer: an apparatus for measuring the total accommodation response of the human eye.