Peter Zuidema

Spatial and temporal summation in the human dark-adapted retina

A coherent set of absolute-threshold data is presented for circular flashes with a diameter of 5-343 min of arc, a flash duration of 32-1000 msec, and at eccentricities between 7 and 50 deg in the temporal retina. A reduction in the flash interval from 4 to 1 sec causes a threshold elevation for eccentricities exceeding 15 deg for all other stimulus parameters. It is shown that local adaptation affects the measurements significantly, especially when long-lasting stimuli and large eccentricities exist. The results can be described with the help of a quanta-coincidence model if adaptational properties are included.

A mechanistic approach to threshold behavior of the visual system

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.

Some Applications of Hierarchical Image Processing Algorithms

Within the past few years considerations concerning the structure of the human retina and the perceptual system have led to increasing interest into hierarchically organized image description schemes (1). Such a hierarchical organization allows for controlling the simultaneous processing of information, on low levels of resolution, by information that itself was acquired on a high level of resolution. For example, the investigation of a certain object in an image can be established in an efficient way by determining the spatial location of that object on low resolution levels, followed by detailed analysis on higher resolution levels. In these and related techniques the resolution decreases with increasing height in the hierarchy and vice versa. As a side-effect the amount of noise may be reduced, or superfluous details suppressed. A degree of similarity of spatial distributions can be obtained from a comparison of their descriptions throughout a range of resolution levels. The greater the resolution range in which spatial distributions match, the greater is the degree of similarity between these distributions.

Increment thresholds with various low background intensities at different locations in the peripheral retina

A set of increment threshold data as a function of test-flash diameter, background luminance, and retinal eccentricity is presented. It is shown that for low background intensities the results can readily be described by simple transformations of flash diameter and background luminance: The threshold is independent of eccentricity if the quotient of diameter and eccentricity is constant and if the flash is presented on a background for which the product of background luminance and the square of eccentricity is constant. At an eccentricity of 50 deg, Riccos law is violated: A small stimulus has a threshold 10 times as high as a large stimulus. On the basis of results found by other investigators for smaller eccentricities, it is concluded that the receptive field size at 50 deg of eccentricity is more than 10 deg (for low background luminances). For eccentricities smaller than 50 deg, a data analysis is given in order to derive an appropriate measure of the size of the sample units. This analysis shows that with increasing background luminance the decrease in the size of the sample unit is steplike rather than gradual.

Detection of light and flicker at low luminance levels in the human peripheral visual system. I. Psychophysical experiments

The interaction between summation and adaptation mechanisms near the absolute threshold of vision is studied. In this paper, results are presented of measurements of both light detection and flicker detection for circular flashes with a diameter of 5.7-480 min of arc, a flash duration of 5-1000 msec, a period of 0-8000 msec, and at eccentricities of 7 and 40 deg in the temporal retina. It is confirmed that the estimates of the summation times obtained from the light-detection-threshold energy as a function of the period and from the light-detection threshold as a function of the flash duration are similar; these estimates depend on the stimulus size and eccentricity. It is suggested that two summation mechanisms can be distinguished, one for the preprocessing and one for the detection mechanism. The summation time of the first mechanism is estimated to be about 100 msec; that of the latter, about 500 msec. At 40-deg eccentricity, decreasing the period of presentation leads initially to a rise in the light-detection threshold, pointing to adaptational phenomena. The flicker threshold is determined by the energy per flash (for brief flashes); when the flash duration exceeds 100 msec, the intensity of the flash is the important parameter. The flicker threshold depends strongly and in an irregular way on the stimulus size because of lateral inhibition effects.

A quanta coincidence model for absolute threshold vision incorporating deviations from Ricco's law

Abstract In this paper a quanta coincidence model of absolute threshold vision is presented, the properties of which include not only Riccos law and Pipers law, but also the deviations from Riccos law which were recently found ( Martinez, Sturr and Schmalback, 1977 : Scholtes and Bouman, 1977 ). Furthermore, it is shown that an apparent paradox for double spot measurements ( Sakitt, 1971 ) can be solved if. in the retina, discrete summation areas are assumed instead of summation as a function of the distance between quantum absorptions.

Detection of light and flicker at low luminance levels in the human peripheral visual system. II. A mechanistic model

A model describing the processing of quantal effects at low luminance levels is evaluated with respect to generally known experimental results for the temporal domain. The model consists of two sequential stages: a square-root scaler and a simple integrate-to-threshold detector. It is shown that with this model the experimental behavior of the absolute light threshold in relation to the flash duration and to the interval for periodic stimuli can be correctly described. When the detection mechanism is changed to a mechanism that detects a fixed minimum number of changes per time unit, the model can describe experimental data obtained at low luminance levels on the increment and decrement thresholds, the modulation transfer function, and the flicker experiments with varying light-dark ratio. From the data fitting it is concluded that in the near-peripheral visual field, adaptation processes have typical time constants of the order of 100 msec, whereas in the far periphery these processes have typical time constants ranging from 50 to 5000 msec, depending on stimulus size. The sampling time of the visual system for the above mentioned stimuli lies between 350 and 1500 msec.

Noise Limited Performance Of The Visual System.

We present results of recent experiments concerning the processing of signals in the visual system under conditions limited by both natural (Poissonean) and artificial white noise. The results for natural noise limited conditions show that at any location in the visual field sampling apertures of different size are simultane-ously present. The dominant aperture size decreases with increasing irradiance. In the temporal domain it appeared that presentation of the signals over a longer period than 1 sec. did not improve the performance anymore. In the second part results of experiments with static and moving artificial noise are presented. It appears that the visual system is capable of processing almost all of the information in the image in order to set its threshold limited by noise. Detection of coherent movement of details within dynamic white noise can be performed with a signal to noise ratio of 1%. Models which describe these phenomena are discussed and their possible impact on image processing is suggested.