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.     

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     

Some results on global stability of a predator-prey system

In this paper we derive some results to ensure the global stability of a predator-prey system. The results cover most of the models which have been proposed in the ecological literature for predator-prey systems. The first result is very geometric and it is very easy to check from the graph of prey and predator isoclines. The second one is purely algebraic, however, it covers the defects of the first one especially in dealing with Holling's type-3 functional response in some sense. We also discuss the global stability of Kolmogorov's model. Some examples are presented in the discussion section.Works partially supported by the National Science Council of the Republic of China     

Study on the transient characteristic in the human visual system using masking experiments

In this paper the visual masking effect is interpreted on the basis of the transient characteristic in two dimensional neuronal networks. The study investigates the suitability of the effect for use as a measurement method. It is shown that the stimulus distribution in space can be scanned at different points in time and that various dynamic characteristic values of the system can be measured.This paper was supported by the DFG with grant no. Se 251/13; Prof. W. von Seelen was in charge of the project     

Plasticity and stability of the visual system in human achiasma

The absence of the optic chiasm is an extraordinary and extreme abnormality in the nervous system. The abnormality produces highly atypical functional responses in the cortex, including overlapping hemifield representations and bilateral population receptive fields in both striate and extrastriate visual cortex. Even in the presence of these large functional abnormalities, the effect on visual perception and daily life is not easily detected. Here, we demonstrate that in two achiasmic humans the gross topography of the geniculostriate and occipital callosal connections remains largely unaltered. We conclude that visual function is preserved by reorganization of intracortical connections instead of large-scale reorganizations of the visual cortex. Thus, developmental mechanisms of local wiring within cortical maps compensate for the improper gross wiring to preserve function in human achiasma.     

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.     

The sensitive period in the development of the human visual system

The human visual system is the most sensitive to the deprivation of the object vision up to the age of 7, when the amblyopia produced by congenital or traumatic cataract develops in all cases and is "relatively" sensitive during the period from 7 to 15 years, when probability of amblyopia development and its degree are determined by the age of cataract appearance and duration of its existence. Visual deprivation taking place after 15 years does not lead to ambliopy. The data on visual evoked potentials (VEP) obtained during stimulation of the ambyopic and the second, intact eye are used in discussion of the neurophysiological mechanisms of the unilateral visual deprivation and of informative importance of VEP.     

Adequate system for studying translation initiation on the human retrotransposon L1 mRNA in vitro

The L1 retrotransposon codes for a unique bicistronic mRNA, which serves as a transposition intermediate and as a template for the synthesis of two proteins. According to preliminary data, the translation of both cistrons is initiated by a noncanonical mechanism. The L1 mRNA was translated in rabbit reticulocyte lysate (RRL), a standard system widely used to study the eukaryotic mechanisms of protein synthesis. Translation yielded not only the expected products, but also several products of aberrant translation initiation on internal AUG codons. Such products are not generated during in vivo translation of the L1 mRNA. When RRL was supplemented with a cytoplasmic extract of HeLa cells, the aberrant products were not synthesized, while the first cistron was translated with the same efficiency. The efficiency of translation of the second cistron became substantially lower, corresponding to the situation in vivo. These and other experiments clearly demonstrated that the new combined system RRL + HeLa is far more adequate for studying the mechanisms of translation initiation than the standard RRL system.     

Contrast invariance in the human lateral occipital complex depends on attention

The human visual system has a remarkable ability to successfully operate under a variety of challenging viewing conditions. For example, our object-recognition capabilities are largely unaffected by low-contrast (e.g., foggy) environments. The basis for this ability appears to be reflected in the neural responses in higher cortical visual areas that have been characterized as being invariant to changes in luminance contrast: neurons in these areas respond nearly equally to low-contrast as compared to high-contrast stimuli. This response pattern is fundamentally different than that observed in earlier visual areas such as primary visual cortex (V1), which is highly dependent on contrast. How this invariance is achieved in higher visual areas is largely unknown. We hypothesized that directed spatial attention is an important prerequisite of the contrast-invariant responses in higher visual areas and tested this with functional MRI (fMRI) while subjects directed their attention either toward or away from contrast-varying shape stimuli. We found that in the lateral occipital complex (LOC), a visual area important for processing shape information, attention changes the form of the contrast response function (CRF). By directing attention away from the shape stimuli, the CRF in the LOC was similar to that measured in V1. We describe a number of mechanisms that could account for this important function of attention.     

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.     

The human visual system is optimised for processing the spatial information in natural visual images

A fundamental tenet of visual science is that the detailed properties of visual systems are not capricious accidents, but are closely matched by evolution and neonatal experience to the environments and lifestyles in which those visual systems must work. This has been shown most convincingly for fish and insects. For mammalian vision, however, this tenet is based more upon theoretical arguments than upon direct observations. Here, we describe experiments that require human observers to discriminate between pictures of slightly different faces or objects. These are produced by a morphing technique that allows small, quantifiable changes to be made in the stimulus images. The independent variable is designed to give increasing deviation from natural visual scenes, and is a measure of the Fourier composition of the image (its second-order statistics). Performance in these tests was best when the pictures had natural second-order spatial statistics, and degraded when the images were made less natural. Furthermore, performance can be explained with a simple model of contrast coding, based upon the properties of simple cells in the mammalian visual cortex. The findings thus provide direct empirical support for the notion that human spatial vision is optimised to the second-order statistics of the optical environment.     

Segmentation of moving images by the human visual system

 New segments appearing in an image sequence or spontaneously accelerated segments are band limited by the visual system due to a nonperfect tracking of these segments by eye movements. In spite of this band limitation and acceleration of segments, a coarse segmentation (initial segmentation phase) can be performed by the visual system. This is interesting for the development of purely automatic segmentation algorithms for multimedia applications. In this paper the segmentation of the visual system is modelled and used in an automatic coarse initial segmentation. A suitable model for motion processing based on a spectral representation is presented and applied to the segmentation of synthetic and real image sequences with band limited and accelerated moving foreground and background segments. Received: 1 August 1995/Accepted in revised form: 25 February 1997     

A ratio model of perceived speed in the human visual system

The perceived speed of moving images changes over time. Prolonged viewing of a pattern (adaptation) leads to an exponential decrease in its perceived speed. Similarly, responses of neurones tuned to motion reduce exponentially over time. It is tempting to link these phenomena. However, under certain conditions, perceived speed increases after adaptation and the time course of these perceptual effects varies widely. We propose a model that comprises two temporally tuned mechanisms whose sensitivities reduce exponentially over time. Perceived speed is taken as the ratio of these filters' outputs. The model captures increases and decreases in perceived speed following adaptation and describes our data well with just four free parameters. Whilst the model captures perceptual time courses that vary widely, parameter estimates for the time constants of the underlying filters are in good agreement with estimates of the time course of adaptation of direction selective neurones in the mammalian visual system.     

Synthesis of visual rhodopsin in a cell-free translation system. I. Influence of the structure of the synthetic bovine visual opsin mRNA on its translational efficiency

Influence of structural changes in nontranslated regions and translation initiation site of the in vitro synthesized bovine opsin mRNA on its translational efficiency in the wheat germ cell-free system has been studied. It is shown that level of the opsin synthesis up to 30 micrograms per 1 ml of translational mixture can be attained by optimizing structure of 5'-nontranslated region.     

Motion-based analysis of spatial patterns by the human visual system

BACKGROUND: It is known that the visibility of patterns presented through stationary multiple slits is significantly improved by pattern movements. This study investigated whether this spatiotemporal pattern interpolation is supported by motion mechanisms, as opposed to the general belief that the human visual cortex initially analyses spatial patterns independent of their movements. RESULTS: Psychophysical experiments showed that multislit viewing could not be ascribed to such motion-irrelevant factors as retinal painting by tracking eye movements or an increase in the number of views by pattern movements. Pattern perception was more strongly impaired by the masking noise moving in the same direction than by the noise moving in the opposite direction, which indicates the direction selectivity of the pattern interpolation mechanism. A direction-selective impairment of pattern perception by motion adaptation also indicates the direction selectivity of the interpolation mechanism. Finally, the map of effective spatial frequencies, estimated by a reverse-correlation technique, indicates observers' perception of higher spatial frequencies, the recovery of which is theoretically impossible without the aid of motion information. CONCLUSIONS: These results provide clear evidence against the notion of separate analysis of pattern and motion. The visual system uses motion mechanisms to integrate spatial pattern information along the trajectory of pattern movement in order to obtain clear perception of moving patterns. The pattern integration mechanism is likely to be direction-selective filtering by V1 simple cells, but the integration of the local pattern information into a global figure should be guided by a higher-order motion mechanism such as MT pattern cells.