Effects of distractors on the perception of right angles.

Psychophysiological experiments were performed in which imaginary right angles formed by three spots of light were supplemented by distracting line segments. The subjects' task was to restore the orthogonality disturbed by the distractors. Subjects' errors were assessed in relation to the distance between the basal spots and the distractors. The results led to the suggestion that there are specific lateral interaction processes involving local averaging of information on the positions of stimulus components. Quantitative evaluation of the parameters of this interaction was addressed using a simplified mathematical model constructed on the basis of the concept of displacements of the centers of gravity resulting from summation of the corresponding excitation patterns in the neural networks of the visual system. Good consistency between theoretical and experimental data was obtained.

[Stimulus size and visual illusion of extent].

In psychophysical experiments, the dependence of magnitude of the illusion of extent on the length of the referential part of a modified Brentano figure with different size of the imaginary wings was studied. The experimental data obtained were interpreted in terms of a model based on the concept ofcentroid biases of excitatory patterns, evoked by stimulus terminators. A good resemblance between the expetimental and computational data confirms the modeling assumption on increase of the area of perceptual influence in dependence on the eccentricity in the visual field. The estimated model parameters support a suggestion that distortions of perceived position of the centers-of-masses of stimulus terminators may be considered as one of the main reasons of the illusion of extent. The model parameters also have showed favor to a possible link of the phenomenon of the illusion investigated with perception of three-dimensional coordinates of visual objects.

[Distracter effect on the right angle perception].

On the monitor three light spots form imaginary sides of the right angle. The perceived angle distortions arose when distracting stripes were flanking each stimulus spot and subjects were asked to readjust the orthogonal orientation of the sides. The dependence of the subject's error value on the spot-to-stripe distance was measured. The data obtained was referred to a certain lateral interaction processes which manifested themselves as a local information averaging concerned with the positions of the stimulus parts. The quantitative parameters of the lateral interaction were established by a simplified mathematical model based on the conception of the centroid biases which are due to summation of the corresponding patterns of excitations in the visual pathways. A good agreement between the computational and experimental data was obtained.

Distortions in length perception: visual field anisotropy and geometrical illusions.

Psychometric experiments were performed to study the effects of visual field anisotropy and geometrical illusions on the accuracy of comparison of objects in terms of length. Stimuli consisting of V-shaped symbols were used, made up of three light spots or one spot plus components of illusory figures, which were pairs of Müller-Layer wings or an interval of the Oppel-Kundt figure filled with spots of light. Relationships between the length comparison errors and the orientations of the reference and test parts of the stimulus were obtained. The experimental data showed that the simultaneous appearance of illusions and visual field anisotropy can be summed algebraically.

[Distortions of length perception: anisotropy of the visual field and geometric illusions]. / Iskazheniia vospriiatiia dliny: anizotropiia zritel'nogo polia i geometricheskie illiuzii.

A combined influence of stimulus orientation and structure on the judgement of length was tested in psychophysical experiments. The subjects adjusted the test part of a stimulus to be equal in length to the reference part. The V-shaped stimuli (three dots, or the Oppel-Kundt figure, or one dot and two Müller-Layer wings) were generated on the monitor. In the Oppel-Kundt and Müller-Layer figures, the filled part was considered as a reference and the empty part as a test. In session of the experiments, values of errors measured as functions of orientation of the parts of the stimuli. We assume that experiments with the three-dot stimuli yielded pure characteristics of visual field anisotropy, while those with the Oppel-Kundt and Müller-Layer figures showed a combined effect of both anisotropy and illusions. The data demonstrated that illusions and anisotropy are to be interpreted as independent factors, which converge to an algebraic summation in a simultaneous manifestation.

Distortions of length perception.

A combined influence of stimulus orientation and structure on the judgment of length was tested in psychophysiological experiments. The subjects adjusted the test part of a stimulus to be equal in length to the reference part. The orientation of the parts of the stimulus varied in the experiments. The stimuli (three dots or the Oppel-Kundt figure, which had ten dots within the filled part) were generated on the monitor. In the Oppel-Kundt figure, the filled part was considered as a reference and the empty part as a test. In sessions of the experiments, values of errors were measured as functions of the size and orientation of the stimulus. The reference part length varied within 14-150 min are range, and the orientation was fixed in 0 degree, 90 degrees, 180 degrees or 270 degrees positions. The orientation of the test part varied from 0 degree to 360 degrees in 7 degrees steps. We assume, that the experiments with the three-dot stimuli yielded pure characteristics of visual field anisotropy, while those with the Oppel-Kundt figure showed the combined effect of both the components (anisotropy and spatial filtering). The data demonstrated independence of the two factors from each other in a simultaneous manifestation. The characteristics of a pure Oppel-Kundt illusion have been found to be in close correspondence with the predictions of the model of spatial filtering.

Geometrical illusions: study and modelling.

The phenomena of geometrical illusions of extent suggest that the metric of a perceived field is different from the metric of a physical stimulus. The present study investigated the Müller-Lyer and Oppel-Kundt illusions as functions of spatial parameters of the figures, and constructed a neurophysiological model. The main idea of the modelling is based on the uncertainty principle, according to which distortions of size relations of certain parts of the stimulus, so-called geometrical illusions, are determined by processes of spatial filtering in the visual system. Qualitative and quantitative agreement was obtained between psychophysical measurement of the strength value of the illusions and the predictions of our model.

Colour-spatial vision.

A critical survey is made of neurophysiological and psychophysiological investigations of colour vision. A neuronal model of colour-spatial vision is suggested. The model allows a unified explanation of the whole range of psychophysiological phenomena: the mixing of colours of high-frequency image components, the McCollough type colour after-effects, the simultaneous and successive colour contrast, the hue constancy perception, the appearance of non-spectral colours by mixing of monochromatic lights. A suggestion is made as to the existence of two main mechanisms of colour vision. The first of these, by means of Fourier transforms, gives a set of coefficients which describes the spatial distribution of light (quantity of energy) and hue (quality of energy) in the visual field. The second mechanism establishes colour names in each chromatically homogenous area of the field described by the first mechanism. Both mechanisms cooperate on the basis of their common spatial organization.

Spectral and orientation specificity of single cells in foveal striate cortex of the vervet monkey, Cercopithecus aethiops.

1. The spectral sensitivity, orientation specificity and inhibitory surround of seventy-three cells were studied in the vervet monkey. The eye was in the dark or illuminated with steady white or spectral light. The cells were in the striate cortex corresponding to the foveal representation. Nearly all the cells gave on- or on/off-responses.2. More than a third of the cells (41%) responded over a narrow spectral band, in the blue, green, yellow or red section. Three quarters of them were orientation specific with flanking inhibitory surround and half of these were of the ;stopped-end' variety as well. The effect of the wave-length of the background indicated that only a half were activated by such excitatory-inhibitory colour pairs as have been described in the geniculate nucleus.3. A third of the cells (36%) responded to most colours but with the greatest response to green, yellow or red. Less than half were orientation specific. Unlike the narrowband cells, the response decreased with the intensity of the light. Two thirds were activated by the excitatory-inhibitory colour pairs that have been described in the geniculate nucleus.4. In both groups of cells the wave-length of the spectral peak could be different when the stimuli were weak compared with when they were strong.5. One quarter of the cells (23%) gave on/off-responses to all spectral flashes; half were not orientation specific. The difference in orientation specificity between narrow- and broadband cells is significant (P < 0.01).