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.     

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.     

Contrast transfer function of the visual system]

Visually evoked potentials were used to determine the spatial contrast response function of the visual system and the visual acuity of the pigeon. The spatial contrast response describes the relationship between the contrast in a pattern of vertical stripes, whose luminance is a function of position, and the amplitude of the visually evoked response at various spatial frequencies for a given temporal frequency (pattern reversal frequency); it indicates how particular spatial frequencies are attenuated in the visual system. The visually evoked responses were recorded using monopolar stainless steel electrodes inserted into the stratum griseum superficiale of the optic tectum; the depth of penetration was determined on the basis of a stereotactic atlas. The stimulus patterns were generated on a video monitor placed 75 cm in front of the animal's eye perpendicular to the optic axis. The spatial contrast response function measured at 10% contrast and 0.5 Hz reversal frequency shows a peak at a spatial frequency of 0.5 c/deg, corresponding to 1 degree of visual angle, and decreases progressively at higher spatial frequencies. The high-frequency limit (cut-off frequency) for resolution of sinusoidal gratings, estimated from the contrast response function, is 15.5 c/deg, corresponding to a visual acuity of 1.9 min of arc.     

Modeling of the vertebrate visual system. II. Application to the turtle cone retina

The model of the vertebrate cone retina was adapted to the turtle retina with its red cone- and L-channel-dominances. The model consists of an ordering of four spatial organizations of unit hexagons, weighted inputs for all cones in the receptive fields, and linear polarization factors based on data from literature on turtle retina. Data generated by the model for spatial and chromatic patterns of receptive fields, intensity-response curves, dynamic ranges for cones, horizontal and bipolar cells proved remarkably consistent with literature. The model also generates observed phenomena such as near-field enhancement of cones due to stray light effects and electrical coupling of like-cones and far-field decrease in responses due to negative feedback from L-type horizontal cells to cones. Annular stimuli were shown to be more effective than spot stimuli for horizontal cells. The formal approach of the model demonstrates factors which play roles in various observed phenomena and all aspects of model can be displayed and tested both qualitatively and quantitatively.     

Photochromism of visual pigments. II. Kinetics of photoconversion of frog rhodopsin]

A kinetic scheme is suggested of reversible processes of rhodopsin phototransformation at --22 degrees C under light effect with lambda 579 and 435 mm. On the basis of this scheme quantum yields of certain stages are calculated from initial rates of transformations. Dependence of the quantum yield of photolysis of rhodopsin transformation on the wavelength of light and temperature is studied. The scheme of frog rhodopsin transformation is compared with the similar scheme of bovine rhodopsin phototransformation.     

Intra-analyzer temporary connections in the visual system]

The possibility of setting up intraanalyser temporary connections integrating the activity of various elements within the central part of the visual analyser at a neuronal level was studied in acute experiments on unanaesthetized immobilized cats. In the given model of temporary connection the unit activity in the lateral geniculate bodies (LGB) was investigated. Electrical stimulation of the superior colliculi was used as a conditioned stimulus, and illumination of the receptive fields of the studied LGB neurones, as an unconditioned one. In the process of conditioning, 10 to 11% of learning elements were revealed in LGB. The possibility is discussed of integration of learning elements into a definite microsystem achieving the process of temporary connection closing in the visual analyser in the course of conditioning.     

Three-dimensional human pattern visual evoked potentials. II. Multiple sclerosis patients

Pattern visual evoked potentials were obtained from 46 patients with definite relapsing/remitting multiple sclerosis, using both a conventional 5-channel occipital array and a 3-D recording technique consisting of three bipolar derivations approximating the three dimensions of space. These three orthogonal wave forms were displayed as a 3-D Lissajous trajectory for each subject. Two of the 15 patients with completely normal conventional pattern VEPs had abnormalities of the orientation of the B-C curvilinear segment of the 3-D pattern VEPs. Delays in the first major occipital positive component (P100) were evident using both techniques; the correlation between P100 latency and the latency of the corresponding trajectory apex was r = 0.99 (P < 0.01). Post-chiasmal MRI abnormalities were associated with 3-D VEP orientation abnormalities. Three-dimensional pattern VEPs are moderately more sensitive than conventional pattern VEPs at detecting dysfunction posterior to the optic chiasm in demyelinating disease and do not require the use of eccentric fixation to do so.     

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.     

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     

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     

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.     

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.     

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.     

Event-related neuronal responses in the human strio-pallido-thalamic system. II. Cognitive functions

Multiunit activity was recorded from the strio-pallido-thalamic system in the same parkinsonian patients (as described in the previous paper) who bore gold electrodes for diagnosis and therapy. The patients voluntarily participated in various tasks designed to study neuronal correlates of cognitive functions: “odd-omissions,” “short-term memory,” “cued bimodal,” “assessment,” and “dual-stage delayed response” tasks. Preparatory-related activities were found in multiunits reacting to sensory cues. In a few multiunits these activities depended upon the specific features of the stimuli that were anticipated for evaluation. The most striking characteristic of stimulus-related activity was the suppression of the multiunit responses when the stimuli become behaviourally meaningless. The hypothesis of action programming is discussed: the loop, including the cortex, neostriatum, pallidum and appropriate parts of the thalamus, is involved in the selection of actions, thus providing the organization of sequential behaviour.     

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.     

Effect of visual deprivation on the composition of structural proteins of the visual system of rabbit brain]

Composition of membrane proteins of the visual system structures of normal and dark-reared rabbits (for 2 1/2 months from birth) was studied by using polyacrylamide gel electrophoresis. Solubilization of membrane proteins was carried out using a 1% solution of Triton X-100 and a 0.1% solution of sodium dodecylsulfate, in succession. One fraction representing the fraction of high-molecular proteins was absent in membrane proteins of the visual analyser in the dark-reared animals; there was also a change in the percentage content of proteins in different fractions in comparison with the normal. The data obtained led to the conclusion that visual deprivation caused marked quantitative and qualitative reconstruction in the composition of membrane proteins of the brain with some specific characteristics for the formation of the visual analyser.