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.

[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.

[Acuity and hyperacuity for pupils of 11-17 years old].

Estimations of hyperacuity and visual acuity (VA) have been compared in schoolchildren aged 11-17 years with normal vision. VA was measured using Landolt Cs and Tumbling Es. Hyperacuity was measured by vernier stimuli. Acuity estimations depended on the test stimuli. They were in 1.1 times over for Landolt Cs than for Tumbling Es. Hyperacuity estimations exceeded VA in 1.25-4.1 times. They were almost twice as high among pupils of 16 years compared to 13-year-olds, in contrast to estimates of VA, which practically did not change with age. Binocular VA was significantly higher monocular VA in 1.05 times regardless of age. The ratio between the binocular and monocular hyperacuity estimates for thirteen years pupils in average was equal to 1.9, while for sixteen years pupils--1.2. We discuss the contribution of binocular vision in the development of mechanisms of VA and hyperacuity in ontogenesis and the difference between these mechanisms.

Detection of narrow bars when presented in the right and left visual hemifields.

Three series of psychophysical experiments were performed to study the effects of additional images on the contrast thresholds for detection of narrow bars of width 1 arc min and length 1 arc degree presented randomly to the left and right sides of the fixation point with an eccentricity of 4 degrees. The additional images were bars of the same size but with lower contrast, presented to the right and left of the test bar at varying distances, i.e., 1-2 degrees. Different series used different contrast ratios for the test and additional bars. The first series revealed significant predominance of one visual hemifield over the other in performing the bar detection task, though the predominant side varied in different subjects. This predominance disappeared or changed to the opposite side in the second or third series. There was a tendency for the additional images to have asymmetrical influences on the detection threshold for the test bar: additional bars had more extensive inhibitory influences when they were on the peripheral side of the test bar. These data provide evidence for the absence of specificity in hemisphere operation in the detection task. The mechanism of description of peripheral images is discussed.

Resolving ability and image discretization in the visual system.

Psychophysiological studies were performed to measure the spatial threshold for resolution of two "points" and the thresholds for discriminating their orientations depending on the distance between the two points. Data were compared with the scattering of the "point" by the eye's optics, the packing density of cones in the fovea, and the characteristics of the receptive fields of ganglion cells in the foveal area of the retina and neurons in the corresponding projection zones of the primary visual cortex. The effective zone was shown to have to contain a scattering function for several receptors, as this allowed preliminary blurring of the image by the eye's optics to decrease the subsequent (at the level of receptors) discretization noise created by a matrix of receptors. The concordance of these parameters supports the optical operation of the spatial elements of the neural network determining the resolving ability of the visual system at different levels of visual information processing. It is suggested that the special geometry of the receptive fields of neurons in the striate cortex, which are concordant with the statistics of natural scenes, results in a further increase in the signal:noise ratio.

[Contrast detection thresholds during their presentation in right and left hemifields of vision]. / Obnaruzhenie uzkikh polos pri pred'iavlenii ikh v pravom i levom polupoliakh zreniia.

We measured contrast detection thresholds for thin black vertical bars (1 min of arc wide, 1 deg long) against a bright background, which were presented eccentrically at 4 deg to the left or right of fixation and flanked on one side by a subthreshold bar having the same dimensions. On each presentation, the stimulus randomly appeared in the left or right visual hemifield. The separation between the test and the flanks varied from 1 to 60-120 min of arc. Different subjects showed better performance in different hemifields. This preference was lost or changed to opposite in the course of training. The central excitatory areas are more or less symmetrical, but inhibitory areas reveal asymmetrical tendency: when flanked bars were more peripheral than the test line, the inhibitory areas were larger. Our results show absence of hemispheric specificity in the line detection task. We discuss the ways of stimulus description in fovea and periphery.

[The resolution and discrimination of images in the visual system]. / Razreshaiushchaia sposobnost' i diskreditatsiia izobrazhenii v zritel'noi sisteme.

We analysed the conformity of optic and retinal cones anatomy factors by the two-point test. Obtained by F. Campbell and R. W. Gubish, the point spread function has a width of about 1 arc min. Cones sizes are equal to 0.5 arc min in the fovea. Functional pixel consists of 3-5 cones under the point spread function. Such an organisation in very useful in decreasing the samping noise of receptors. We carried out psychophysical investigations to show a consensus among the optic, receptors', and neuronal levels. In experiments we studied changes of the two-point pattern perception in respect to the points separation, measured the orientation threshold of small size stimuli. Data were compared with optical point-spread function, the hexagonal mosaic of cones, and line spread function of spatial elements, which form spatial frequency channels at the cortical level.

Spatial interval discrimination in the presence of flanking lines.

Spatial interval discrimination was studied in the absence or presence of distractors. In the latter case, two flanking lines surrounded two vertical lines delimiting the spatial interval. Using a temporal 2AFC technique with a method of constant stimuli we measured the accuracy of performance (discrimination thresholds) and biases (points of subjective equality) depending on the separations between the target and the flanking lines. For separations less than or comparable to the size of the spatial interval we found both a reduction of precision and the increase of perceived sizes of the spatial intervals: the discrimination thresholds were increased, the size of the spatial interval was overestimated. For larger separations, the size of the spatial interval was underestimated, but the precision of performance was not affected by the presence of flanking lines. We discuss the possible mechanisms underlying spatial interval discrimination in the presence of flanking lines.

What spatial frequency do we use to detect the orientation of a Landolt C?

We calculated the two-dimensional Fourier spectrum of a Landolt C. For Landolt Cs of orthogonal orientation, the main differences in the amplitude spectrum were found at a low frequency such that 1.3 periods were equal to the size fo the Landolt C, rather than at the high frequency corresponding to the size of the gap, i.e., such that 2.5 periods were equal to the size of the Landolt C. We compared visual acuity assessments obtained with Landolt C optotypes and the cut-off of the contrast sensitivity function measured with sinusoidal gratings. We found that the frequency corresponding to the size of the gap is twice the latter frequency. We suggest that, in fact, this lower frequency can be used to determine the position of the gap.

Similarity between Fourier transforms of objects predicts their experimental confusions.

Two sets of stimuli were presented tachistoscopically to 4 subjects. On each trial, a single stimulus was presented, and the subject was required to identify the stimulus by verbal response. An exposure duration was chosen such that the subject's identification performance fell within a range from faultless identification to chance guessing. The object-identification data of each subject obtained for all stimulus exposures were pooled to form an object confusion matrix. A model of visual processing based on two-dimensional spatial frequency content (Fourier transforms) was used to predict confusions among stimulus pairs. The model properties that appear to be the most essential are those that allow it (1) to account for the obvious dependence of the Fourier transform on the choice of an origin point; and (2) choose the point of origin for each object separately, irrespective of other objects of the set. The point of origin of the reference frame, in which Fourier transforms are performed, is chosen so as to minimize the low-frequency phase component for each object. A high correlation (up to .96) between confusion matrices and model interobject distances was attained. The results demonstrate that such a distance measure gives a good prediction of object confusability.