Foveal crowding for large and small Landolt Cs: Similarity and Attention.

We compare the recognition of foveal crowded Landolt Cs of two sizes: brief (40 ms), large, low-contrast Cs and high-contrast (1 sec) tests at the resolution limit of the visual system. In different series, the test Landolt C was surrounded by two identical distractors located symmetrically along the horizontal or by a single distractor. The distractors were Landolt Cs or rings. At the resolution limit, the critical spacing was similar in the two series and did not depend on the type of distractor. The result supports the hypothesis that crowding at the resolution limit occurs when both the test and the distractors fall into the same smallest receptive field responsible for the target recognition. For large stimuli, at almost all separations distractors of the same shape caused greater impairment than did rings, and recognition errors were non-random. The critical spacing was equal to 0.5 test diameters only in the presence of one distracting Landolt C. This result suggests that attention is involved: When one distractor is added, involuntary attention, which is directed to the centre of gravity of the stimulus, can lead to confusion of features that are present in both tests and distractors and thus to non-random errors.

Metabotropic glutamate receptors as targets of neuromodulatory influence of nitric oxide.

A possible effect of nitric oxide (NO) on metabotropic glutamate receptor (mGluR) function in the amino acid afferent synapse was investigated in the isolated labyrinth of the frog Rana temporaria. The modification of the amplitude of responses of metabotropic glutamate receptor agonist trans-ACPD was analyzed during bath applied NO donor S-nitroso-N-acetyl-DL-penicillamine SNAP (0.1-100 µM) or nitric oxide synthase inhibitor L-NAME. It was shown that NO donor SNAP (1 µM) inhibited mGluR induced responses, and the inhibitor of NO-synthase L-NAME (100 µM) increased the amplitude of trans-ACPD evoked answers. The results suggest that NO can depress mGluR function due to modulation of functions of the endoplasmic reticulum channels.

[Noise affect to the fragmented contour image recognition].

We performed a comparison of the results of recognition of fragmented outline images in the presence of noise and without it. Contour images and visual noise were synthesized form Gabor elements. Distances between the Gabor elements in contour and in noise were changed and image sizes varied too. The percentage of correct responses was not dependent on the size of the stimuli, but differed for the same objects in the noise and without noise. The percentage of recognition was higher for images with lots of turns in case without noise. The recognition in noise was better for contour images having lines with slightly varying curvature. Identification in noise depended on the ration of the distances between the elements in noise and in contour.

[Estimates of the size of inhibitory areas in crowding effects in periphery].

In psychophysical experiments we studied how surround influences recognition of test objects. The tests were low-contrast Landolt rings of the size 1.1, 1.5 and 2.3 deg. Their centers were located at 13.2 deg from the fixation point. The additional objects were similar Landolt rings or rings without gaps. The distance between the centers of the test and the additional objects varied from 2.2 to 13.2 deg. Inone experiment, the task of the observer was to identify both the test objects and the surrounding objects. In the second experiment the stimulus layout was the same, but'identification of only the test stimulus was required. In both experiments, deterioration of performance was found at all distances between the test objects and the surround, but the deterioration was more significant when the observer carried out the dual task. The data showed that the size of the inhibitory areas in our case does not comply with the Bouma low which states that the size of the interaction areas are equal to half of the eccentricity where the test is presented. Further deterioration of performance in the dual task reveals the contribution of attention into peripheral crowding effects.

[Interdependent changes of the axon and Schwann cell in the process of reactive remodeling of a myelinated nerve fiber].

Using the inverted phase-contrast microscope, the living undamaged frog sciatic nerve fibers and the fibers mechanically injured to varying degrees, were studied. It was found that the swelling of myelin incisures (MI) (of Schmidt-Lanterman) occured according to the principles similar to those controlling the changes of the myelin gap (node of Ranvier) and depended on the swelling of a Schwann cell (SC) perikaryon. It was detected that this was a single process, which which could be united in a complex of nonspecific changes of a myelinated nerve fiber. It was also demonstrated that under the action of mechanical injury and hypotonic solution, swelling of MI, nodes of Ranvier and SC perikaryon occurred without modifications of outer fiber diameter, due to the pronounced local axon thinning. Electron microscopic study of the cytoskeletal axonal structures showed that there was not a simple local contraction of an axon, but a significant local increase in the density of cytoskeletal components of the axoplasm (by 200-275%). Reactive reversible remodeling of a myelinated fiber suggests a new type of interaction between the axon and SC, the mechanism of reversible translocation of liquid axoplasmic fraction to the glial cell cytoplasm.

[Crowding-effect and attention].

We studied the influence of additional objects on recognition of the test visual objects. The test objects were stylized low-contrast letters having size 1.1, 2.1 or 4.3 ang. deg. The additional objects after 30 ms were followed by the test objects which were presented in the middle of the screen. The additional objects were digits 1-9 having size 1.3 ang. deg. The digits were presented at various distances from the centre of the screen. The observers' task was to identify both the test objects and the digits. Recognition of the test objects deteriorated when the digits were at small distances to the tests (crowding-effect). Recognition of digits deteriorated with the increasing distances from the centre of the screen; the effect was more pronounced when the tests were large. The contribution of laterals masking and attention into crowding-effect is discussed.

[Effects of histamine on contractile activity of lymphatic node capsules. The NO role].

Effects of histamine (10(-9)--5 x 10(-5) M) on the phase and tonic contractile activity of capsular smooth muscles of isolated bovine mesentery lymph node were investigated. Dual dose-depended effect of histamine was found. Low concentrations of histamine less than 10(-7) M caused a decrease of contractile activity, whereas higher concentrations of histamine (more than 5 x 10(-7) M) resulted in increase of the phase and tonic contractions. Both H1- and H2-receptors of smooth muscle cells are involved in the response. Much of the relaxing histamine-induced response is produced by the stimulation of the endothelial cells. We believe that activating effect of histamine is due to the excitation of H1-receptors located on the membrane of myocytes, whereas its inhibitory effect occurs in two ways: 1) via excitation of H2-receptors located on the membrane of myocytes; 2) via stimulation of the NO production by the endothelial cells of lymph node sinus.

Perception of fragmented images of three-dimensional objects as the observation angle changes.

Recognition thresholds for incomplete two-dimensional images of three-dimensional objects were measured as the observation angle was changed. A new experimental psychophysical method was developed and programmed for this purpose, this being a modification of the Gollin test, which measures perception thresholds of incomplete outline images. After training to a stimulus alphabet, observers' responses were found to be invariant to changes in the observation angles of three-dimensional objects from 15 degrees to 60 degrees. It is suggested that possible algorithms for the formation of models of three-dimensional images in the human visual system do not operate on the basis of simple section, but involve invariance mechanisms.

[Age related size invariance in perception of illusory and fragmented contours].

We investigated invariant perception to sizes of images. Observers were schoolmates of 7-17 years and adults. Two types of stimuli were used: fragmented line drawing of common objects and discs with deleted sectors, which represented illusory Kanizsa contours when discs were in particular positions. In experiments with fragmented images, we found an improvement in image recognition with observers' age, increasing up to 13-14 years. The probability of recognition of fragmented line drawings increased significantly with decreasing stimulus size for children of 7-12 years, indicating that size invariance at recognition for fragmented line drawings was absent in these children. However, size invariance was found for observers of 13-17 years and for adults in this task. At Kanizsa illusion appearance, the ratio of the separation between discs and disc diameter was smaller when we used larger disc diameters. This ratio increased with increasing age of observers. Obtained results provide evidence for the absence of size invariance when perceiving the Kanizsa illusion in our experimental conditions.

Analysis of the studies of the perception of fragmented images: global description and perception using local features.

This report presents an analysis of studies of the visual perception of fragmented images. Two approaches to the description of brain functioning during the solution of this task are considered: studies of the perception of the statistical properties of global whole images and studies of the perception of images in terms of local higher-order informative features. These approaches describe two different mechanisms which the human brain may use for recognizing incomplete images. Studies performed using the Gollin test (measurement of recognition thresholds for fragmented outline images during gradual construction of the outline) are given most attention. In solving this task, the visual system extracts the statistical properties of the whole image. Local higher-order informative features are used by the brain as additional sources of information about the image. The role of this source increases on learning a given alphabet of stimuli. In accordance with a matched filtration model, the fragmented images used in the Gollin test are perceived as a whole structure, compared with a reference which is stored in memory and selected using the selective attention mechanism. At the primary filtration step and the matched filtration step, the recognition thresholds of images in the Gollin test reflect the processes of extracting the signal from noise. The Gollin test can be used as a differential tool for the diagnosis of different types of cognitive impairments.

[Incomplete 2-D images perception at viewpoint variation of 3-D objects].

Thresholds of recognition of 3-D objects when viewed as incomplete 2-D images were measured in psychophysical experiments varying perspective of 3-D objects during subject's learning and testing. The new method of measurement of recognition thresholds of the 2-D incomplete images of 3-D objects in Gollin-test has been created. The visual system appears to be capable of extrapolating results that were obtained at the first presentation of object, at one viewing angle to other, subsequent viewing angles. It appears that a template forms immediately after the first presentation of a 2-D projection of that object. This template has "invariance" properties since its use for object identification does not depend on the orientation at which the object is subsequently presented, at least up to an angle of 60 degrees. The algorithm for template formation of 3-D objects in the human visual system needs to be modeled, taking account of the invariance mechanism.

[Analysis of fragmented images perception: local features and global description].

Analysis of experimental investigations of the perception of incomplete images is presented. It illustrates two different approaches to work of the brain mechanisms involved: one approach is based on the perception of whole images and another on local informative features. These approaches describe two different mechanisms, which are possibly used by brain systems for incomplete image recognition. Performance on the Gollin test (measuring recognition thresholds for fragmented line drawings of everyday objects and animals) depends upon recognition based on image informational-statistical characteristics. We suggest that recognition thresholds for Gollin stimuli in part reflect the extraction of signal from noise. The brain uses local informative features as an additional source of information about them. We have suggested that fragmented images in the Gollin-test are perceived as whole structures. This structure is compared with a template in memory which is extracted with the help of selective attention mechanism in accordance with a matched filtration model. The Gollin-test is a tool for differential diagnosis of a various forms of cognitive disorders.

The threshold signal:noise ratio in the perception of fragmented figures.

Perception thresholds were measured for fragmented outline figures (the Gollin test). A new approach to the question of the perception of incomplete images was developed. In this approach, figure fragmentation consisted of masking with multiplicative texture-like noise--this interference was termed "invisible" masking. The first series of studies established that the "similarity" between the amplitude-frequency spectra of test figures and "invisible" masks, expressed as a linear correlation coefficient, had significant effects on the recognition thresholds of these figures. The second series of experiments showed that progressing formation of the figures was accompanied by increases in the correlation between their spatial-frequency characteristics and the corresponding characteristics of the incomplete figure, while the correlation with the "invisible" mask decreased. It is suggested that the ratio of the correlation coefficients, characterizing the "similarity" of the fragmented figure with the intact figure and the "invisible" mask, corresponds to the signal:noise ratio. The psychophysical recognition threshold for figures for naive subjects not familiar with the test image alphabet was reached after the particular level of fragmentation at which this ratio was unity.

The optical-geometrical characteristics and thresholds of perception of fragmented outline figures.

Measurements were made of the threshold of recognition of cumulatively forming line figures. The threshold value of the outline, expressed in pixels, depended on the length of the outline of the whole unfragmented figure. Relative threshold values were constant, and for the measures of figure fragments used in the present study, averaged 12.5%. A spatial frequency analysis of the test images was performed. Variation of the amplitude-frequency parameters of the spectra of the images of various figures with threshold fragmentation was minimal as compared with the variation of these parameters in figures with subthreshold or suprathreshold levels of fragmentation.

Computerized morphometric analysis of the dendrites of Dogiel type II neurons.

Silver nitrate impregnation by the Bil'shovskii-Gros method was used to study the dendrites of Dogiel type II cells in the cat large intestine (local receptors) by computerized morphometric analysis. The following parameters were measured: the areas of the receptive field of the dendrite, the total area of all receptor elements, the total length of all branches of the receptor. There was significant individual variation in all these measures: the ratios of maximum to minimum values were 41, 29, and 45 respectively. There were no correlations between these three measures. It is suggested that this morphological heterogeneity of dendrites may underlie functional differences in Dogiel type II cells.

Operant behavior in conditions of activation and blockade of neostriatal muscarinic receptors.

Chronic experiments were performed on four dogs using a model of an operant defensive reflex associated with maintaining a flexion posture to study the effects of bilateral intraneostriatal microinjection of the non-selective muscarinic receptor agonist carbachol, the selective D2 dopamine receptor blocker raclopride, and the selective M1 muscarinic receptor blocker pirenzipine on the performance of the operant defensive reflex and differentiation of signals. The results show that microinjection of carbachol induced increases in the tonic component and inhibition of the phasic component of the reflex, an ordering rearrangement of the posture, and increases in the amplitudes of its components. Raclopride microinjection gave similar but less marked results. The greatest effects with both substances were seen using differential stimuli. There were sharp increases in the process of differentiation of sound signals. Pirenzipine microinjections gave the opposite result. These data are assessed on the basis of concepts of the existence of two efferent outputs from the neostriatum with opposite effects on their targets and the roles of muscarinic and dopamine receptors in triggering and blocking these effects.

[Signal/noise ratio at the visual recognition threshold of incomplete figures]. / Porogovoe otnoshenie signal/shum pri vospriiatii fragmentirovannykh figur.

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.

Comparison of the postural and movement components during learning by dogs of an operant defensive reflex.

Chronic experiments on six dogs using a model of an operant defensive reflex associated with maintenance of flexion of the hindlimb of specified amplitude were performed to compare the characteristics of the postural and movement components during the learning process. Dogs were placed standing on four tension platforms. Signals were activated and data were recorded and stored using original PC programs. Original programs running on another PC were used to analyze the data. All dogs showed a series of characteristics for the appearance of a diagonal pattern of conditioned reflex posture rearrangement. During the period of complete formation of the "coordinated" program of the operant reflex (indicated by high performance criteria for execution of the operant task), the diagonal pattern of posture rearrangement was seen extremely rarely, in only occasional performances in calm dogs. It was only during the period of complete automatization of the movement habit that the diagonal pattern of postural rearrangement was seen consistently in all performances. By this time, there was a sharp increase in the tensogram amplitudes for all four limbs, which sharp increases in the correlation coefficients between individual tensogram performances. These results suggest that in the experimental conditions used here, the diagonal pattern of postural rearrangement appeared significantly later than the movement pattern needed for resolving the operant task. These data also lead to the conclusion that dogs can complete operant defensive limb movements associated with maintaining a specified flexor posture in the absence of preliminary rearrangement of the posture having the diagonal pattern.

[Optical-geometrical parameters and perception threshold of the fragmented contour figures]. / Optiko-geometricheskie kharakteristiki i porogi vospriiatiia fragmentirovannykh konturnykh figur.

The thresholds of recognition of line drawings of common objects were measured using the Gollin-test procedure, in which separate random line fragments are displayed cumulatively up to the point of recognition. It was shown that the mean percentage of contour displayed at threshold recognition for different images was always about 12.5%, despite inter-subject variability between 5% and 25%. The comparative and spatial-frequency analysis of the geometrical parameters of images was carried out for different levels of fragmentation (before threshold, at threshold, and for the complete contour). The magnitude information of the Fourier domain image of figures was characterized by maximum at low and high levels of fragmentation, but at recognition threshold fragmentation it was characterized by minimum variability.