Galina I. Rozhkova

Visual acuity in 5–7‐year‐old children: individual variability and dependence on observation distance

Accurate measurements of uncorrected binocular and monocular visual acuity were performed in 65 children aged 5–7 years at five viewing distances in the range 0.5–5.0 m by means of the test charts containing widely spaced E stimuli in four orientations. It was found that, in most children of this age, visual acuity (V) changed with test distance, as had been reported previously with older subjects. Visual acuity could be considered as practically independent of observation distance (Vmax−Vmin ≤ 0.2 decimal units) in <50% of children. The remaining children demonstrated evident distance dependence of visual acuity but the shape of the acuity–distance curve varied. The acuity–distance curve of most children peaked at an intermediate distance (1–2 m), the typical difference Vmax−Vmin being 0.4 decimal units. To explain the existence of an optimal observation distance in the majority of children, a number of developmental and environmental factors could be proposed that adjust the parameters of the visual system to the parameters of operational visual space and prevailing activity.

Yarbus's Conceptions on the General Mechanisms of Color Perception.

In the last series of papers published during 1975 to 1980, Alfred Yarbus tried to formulate general conceptions concerning the basic principles of retinal image processing in the human visual system. The original ideas of Yarbus were based on the results of his numerous and various experiments carried out with extraordinary inventiveness and great skill. Being concentrated primarily on the problems of color vision, Alfred Yarbus dreamed of elaborating a comprehensive model that would simulate visual information processing at the monocular precognitive level in the visual system of humans with normal trichromatic color perception. In this article, the most important of Yarbus’ experimental paradigms, findings, statements, and conclusions are systematized and considered in relation to the classical theories of color perception and, in particular, fundamental theses of the Nyberg school. The perceptual model developed by Alfred Yarbus remained incomplete. Nevertheless, it is already evident that some intrinsic contradictions make it inadequate in terms of comprehensive modeling. However, certain partial advantages deserve more thorough appreciation and further investigation.

Visual Percepts in the Cases of Binocular and Monocular Viewing Stabilized Test Objects, Ganzfeld Stimuli, and Prolonged Afterimages.

A thorough analysis of the literature on retinal image stabilization, as well as our own experimental data, present evidence that Yarbus’s concept, implying inevitable and irreversible fading of a visible image evoked by stabilized retinal stimulus of any size, color, and luminance in 1 to 3 s after its onset, is not valid in a general case. It has been demonstrated that, even with Yarbus’s stabilization techniques, the lifetime of visible images varies from fractions of a second to the whole stimulus duration—up to 30 min in our experiments—depending on many factors: monocular or binocular viewing, stimulus parameters, characteristics of subjects, and so forth. The dynamics of perceived images is determined mainly by the processes at the higher levels of the visual system. In the cases of such unusual visual stimuli as stabilized retinal images, it is problematic for the visual brain to find their proper interpretations in terms of everyday natural experience. Usually, the responses of retinal units are determined by three types of coexisting images: (a) the optical projections of external objects, (b) shadows of the blood vessels and other internal eye structures, (c) virtual patterns caused by the traces of previous stimuli. A task of the visual system is to recognize and visualize only external objects separating their projections from all the entoptic images of the two remaining types. To implement separation, visual brain employs a number of approaches—in particular, the eye movements that cause sliding over the retina but only the projection of the external objects. This means that the peculiar phenomena observed in the cases of stabilized retinal images can be determined not by invariability of such stimuli per se but rather by the fact that stabilization eliminates a powerful cue helping to identify the retinal images belonging to the external objects, thereby increasing the probability to treat them as the entoptic ones which should be ignored or canceled rather than perceived. However, the probability of canceling—image fading—can be essentially reduced in conditions of concordant, large, bright, and sharp binocular stimuli.

Scientific Activity of Alfred Yarbus: The Stages of Research Work, Senior and Younger Colleagues, Conditions of Investigations.

1941: graduated from the Moscow State University (Faculty of Physics) 1941–1942: factory engineer (Moscow) 1942–1946: Military service (Karelia, then Far East Japanese front) 1946–1947: researcher at the Institute of Crystallography 1947–1950: doctoral student at the Psychology Department of the Institute of Philosophy, (Academy of Sciences of the USSR), supervisor—S. V. Kravkov 1950–1963: researcher, senior scientist (since 1957) at the Institute of Biophysics (Academy of Sciences of the USSR), the Laboratory for biophysics of vision 1963–1986: senior scientist at the Institute for Information Transmission Problems (Academy of Sciences of the USSR)

Optimal Optotype Structure for Monitoring Visual Acuity

Abstract To date, there are no generally accepted optotypes for monitoring visual acuity. All common optotypes are not completely suitable for some reasons. The tasks requiring visual monitoring - investigation of visual development, early diagnostics, assessment of treatment - impose heavy demands on the test stimuli. They must be: (1) suitable for patients of any age; (2) convenient for repeatable examinations; and (3) accurate enough for revealing the smallest physiologically significant changes of visual acuity. From theoretical consideration, one could conclude that the optotypes for monitoring visual acuity should be designed for measuring visual resolution but not recognition, unlike most popular optotypes. The best optotypes for visual resolution are gratinglike stimuli whose recognition could only be based on the high frequency part of the Fourier spectrum around the characteristic frequency (not on the low-frequency components). On the basis of theoretical analysis we elaborated modified 3-bar optotypes, which minimise the possibility of using low-frequency cues for stimulus recognition. In this paper we present the results of theoretical and experimental comparison of these optotypes with the two widely used ones: tumbling-E and standard 3-bar targets. According to the data obtained, our modified optotypes seem to be better than other investigated ones for monitoring visual acuity.

The Influence of Monocular Spatial Cues on Vergence Eye Movements in Monocular and Binocular Viewing of 3-D and 2-D Stimuli.

The influence of monocular spatial cues on the vergence eye movements was studied in two series of experiments: (I) the subjects were viewing a 3-D video and also its 2-D version—binocularly and monocularly; and (II) in binocular and monocular viewing conditions, the subjects were presented with stationary 2-D stimuli containing or not containing some monocular indications of spatial arrangement. The results of the series (I) showed that, in binocular viewing conditions, the vergence eye movements were only present in the case of 3-D but not 2-D video, while in the course of monocular viewing of 2-D video, some regular vergence eye movements could be revealed, suggesting that the occluded eye position could be influenced by the spatial organization of the scene reconstructed on the basis of the monocular depth information provided by the viewing eye. The data obtained in series (II), in general, seem to support this hypothesis.