Multiplication noise in the human visual system at threshold

Several kinds of light used in vision experiments produce photon statistics that are distinctly non-Poisson. Representative examples are light from a cathode-ray tube and an image-intensifier device. For the class of vision experiments in which the photon statistics play an important role, excess fluctuations produced by such light sources can alter the observed results and obscure the visual mechanisms being studied. They must therefore be accounted for in a proper way. We use the results of a Hecht-Shlaer-Pirenne type experiment, carried out with modulated Poisson light, to illustrate the point. Sensitivity and modulation depth, as well as sensitivity and reliability, are shown to be traded against each other. Finally, we demonstrate that number-state light, which is comprised of photons of an ideal kind, provides the ultimate tool for extracting information about the intrinsic noise distribution in the visual system at threshold. The state of the art in producing such light is discussed.This work was carried out at Columbia University and was supported by the National Science Foundation and the National Institutes of Health     

Signal/noise ratio at the visual recognition threshold of incomplete figures

We measured recognition thresholds of incomplete figure perception (the Gollin test). This test we regarded as a visual masking problem. Digital image processing permits us to measure the spatial properties and spatial frequency spectrum of the absent part of the image as the mask. Using a noise paradigm, we have measured the signal/noise ratio for Incomplete Figure. Recognition was worse with better spectral "similarity" between the figure and the "invisible" mask. At threshold, the spectrum of the fragmented image was equally similar to that of the "invisible" mask and complete image. We think the recognition thresholds for Gollin stimuli reflect the signal/noise ratio.     

An evaluation of sensory noise in the human visual system

It is assumed that the activity of a visual channel may be represented as V(t)=g(t)+(t), where g(t) is the deterministic response of the channel due to the presentation of a stimulus and (t) is the trajectory of a wide-sense stationary Gauss process. The stimulus is detected if the event V(t)>S for at least one t[0, T] occurs. Two approximations for the probability of this event are proposed, and it is demonstrated how they may be employed to estimate (i) the value of the second spectral moment ₂ of the noise process _t , where ₂ reflects the speed of the fluctuations of the trajectories _t , and (ii) the value of the internal threshold S. The commonly made assumption of peak — detection is shown to serve as a very good first approximation in particular if the channel is of transient type or — in case of detection by a channel of sustained type — if the stimulus durations are not too long.     

Influence of a 902.4 MHz GSM signal on the human visual system: investigation of the discrimination threshold

The proximity of a mobile phone to the human eye raises the question as to whether radiofrequency (RF) electromagnetic fields (EMF) affect the visual system. A basic characteristic of the human eye is its light sensitivity, making the visual discrimination threshold (VDThr) a suitable parameter for the investigation of potential effects of RF exposure on the eye. The VDThr was measured for 33 subjects under standardized conditions. Each subject took part in two experiments (RF-exposure and sham-exposure experiment) on different days. In each experiment, the VDThr was measured continuously in time intervals of about 10 s for two periods of 30 min, having a break of 5 min in between. The sequence of the two experiments was randomized, and the study was single blinded. During the RF exposure, a GSM signal of 902.4 MHz (pulsed with 217 Hz) was applied to the subjects. The power flux density of the electromagnetic field at the subject location (in the absence of the subject) was 1 W/m(2), and numerical dosimetry calculations determined corresponding maximum local averaged specific absorption rate (SAR) values in the retina of SAR(1 g) = 0.007 W/kg and SAR(10 g) = 0.003 W/kg. No statistically significant differences in the VDThr were found in comparing the data obtained for RF exposure with those for sham exposure.     

Statistical model of the inherent noise in the visual system

A concept of intrinsic noise of the visual system has been formulated. It has been shown that the intrinsic noise is realized in the form of the stochastic (from simple chance to complicated pithy) visual images. On the basis of the suggested concept in the limits of a normal approach, taking into account several obvious suppositions, two identical forms have been obtained of representations of probability density of own noise--the mathematical model of own noise. This model includes, as a particular case, the model of Swets, Tanner and Birdsall and has no inherent limitations.     

Biases in three-dimensional structure-from-motion arise from noise in the early visual system.

The projected pattern of retinal-image motion supplies the human visual system with valuable information about properties of the three-dimensional environment. How well three-dimensional properties can be recovered depends both on the accuracy with which the early motion system estimates retinal motion, and on the way later processes interpret this retinal motion. Here we combine both early and late stages of the computational process to account for the hitherto puzzling phenomenon of systematic biases in three-dimensional shape perception. We present data showing how the perceived depth of a hinged plane (''an open book'') can be systematically biased by the extent over which it rotates. We then present a Bayesian model that combines early measurement noise with geometric reconstruction of the three-dimensional scene. Although this model has no in-built bias towards particular three-dimensional shapes, it accounts for the data well. Our analysis suggests that the biases stem largely from the geometric constraints imposed on what three-dimensional scenes are compatible with the (noisy) early motion measurements. Given these findings, we suggest that the visual system may act as an optimal estimator of three-dimensional structure-from-motion.     

Parameter estimation of the threshold time function in the neural system

An algorithm for parameter estimation is presented for the neural system model. Because of its firing mechanism analogous to that of the model based on the first time crossing problem, this problem is solved numerically for our model according to the results of Kostyukov et al. (1981). We propose the algorithm that estimates the parameters of the model considering the equivalence between the probability density function of the 1st crossing time and that of the interspike interval, which is derived from the interspike interval histogram by making use of the spline function technique. The ability of the algorithm is ensured by the application to the simulated interspike interval data. The parameter estimation is carried out also for the practical neural data recorded in the cat's optic tract fibers in both the spontaneous and the stimulated cases. These applications will show the effectiveness of the algorithm in practical cases.     

Work of the human visual system. I. Adequate visual stimulus]

It is shown that the adequate stimulus permitting to detect the presence of colour differentiation in the visual field is the change of relative space-time differences of light actions in different retinal points. Differences only in space or only in time are not sufficient for perception.     

Work of the human visual system. II. Color]

In the first part of this work [1] the author presented the expression for so called adequate visual stimulus. This expression describing the change of relative space - time differences of light actions in different retinal points defines the conditions necessary for producing visual sensations. In this paper author makes a statement that the quality of visual sensation (the percieved colour) can be evaluated by integrating the expression for the adequate visual stimulus both in time and in space. The integration in space is fulfilled starting from the extreme periphery of the retina, which is usually illuminated by scattered light averaged over the whole visual field. Human visual system works in such a way that the light action in the extreme periphery plays a role of the unit for scaling of visual sensations.     

Brightness generation in the human visual system

In this study we show that it is possible to derive experimentally measured Colour-Brightness Signals from 3 fundamental sensitivity functions. The cortical Colour-Brightness Signal is computed from the weighted summation of all afferent colour difference signals, cortically decoded. Large retinal bipolar signals are limited by ganglion cell nonlinearities. To account for colour adaptation, we use as a model principle a signal-adaptative gain regulation of the receptors. A logarithmic transformation of receptor signals is assumed. The summation of blue receptor signals which varies as a function of the adaptation level is described by a multiplication factor where <1.     

Coding of stimulus movement parameters in cat visual system

The principal component analysis of matrices composed of spike numbers generated by visual neurons of cats in response to motion of simple and complex stimuli revealed vector encoding. Responses of detectors of moving dot direction and detectors of oblique line orientation are encoded independently in V1 and V2 cortices by excitation of two cardinal neurons. Each pair of these neurons generates sine and cosine functions. Responses of detectors in the association cortex selective to specific orientation of moving stripes depend on the activity of four cardinal neurons which sum up the excitation incoming from the direction and orientation channels.     

Work of the human visual system. III. Color perception space]

In the previous parts of this work the author presented thet expression for percieved colour: Ei=k1n(ai/ao) where k -- is the coefficient of proportionality which have dimension of sensation and ai, ao -- are the light actions in some retinal point and in the extreme periphery correspondingly. Here the author describes some consequences from this expression and gives a new conception about the space of colour sensations.     

Influence of priors in Bayesian estimation of genetic parameters for multivariate threshold models using Gibbs sampling

Simulated data were used to investigate the influence of the choice of priors on estimation of genetic parameters in multivariate threshold models using Gibbs sampling. We simulated additive values, residuals and fixed effects for one continuous trait and liabilities of four binary traits, and QTL effects for one of the liabilities. Within each of four replicates six different datasets were generated which resembled different practical scenarios in horses with respect to number and distribution of animals with trait records and availability of QTL information. (Co)Variance components were estimated using a Bayesian threshold animal model via Gibbs sampling. The Gibbs sampler was implemented with both a flat and a proper prior for the genetic covariance matrix. Convergence problems were encountered in > 50% of flat prior analyses, with indications of potential or near posterior impropriety between about round 10 000 and 100 000. Terminations due to non-positive definite genetic covariance matrix occurred in flat prior analyses of the smallest datasets. Use of a proper prior resulted in improved mixing and convergence of the Gibbs chain. In order to avoid (near) impropriety of posteriors and extremely poorly mixing Gibbs chains, a proper prior should be used for the genetic covariance matrix when implementing the Gibbs sampler.