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Biomedical subjects

R Sireteanu

Publications and source records attributed to R Sireteanu.

At least 19 recordsLinked to original sources
PubMed2000

Perceptual learning in visual search generalizes over tasks, locations, and eyes.

In a visual search task, targets containing elementary features are detected in parallel, while a serial search is necessary for the detection of a target without a feature, or for targets containing conjunctions of features. In this study, we re-investigated the role of practice in visual search tasks, using an uncued visual search paradigm. Under some circumstances, initially serial tasks can become parallel with practice. Perceptual learning of feature search tasks is rapid (a few hundreds of trials are sufficient to transform serial into parallel search), long-lasting (a learned task is retained over several months), but far less specific than learning of other visual tasks (see also Sireteanu & Rettenbach, 1995a [Vision Research, 35, 2037-2043]). Learning transfers from one task to another, from one location in the visual field to another, and between the two eyes of a given subject, even if the subject has reduced stereopsis. Search for a conjunction of orientation and colour becomes more efficient, suggesting that a different search strategy emerges after prolonged practice. These results suggest that learning of visual search tasks modifies neural structures located at a high level in the visual pathway, involving different, presumably more central neural circuits, than the learning of visual discriminations and hyperacuity.

Analysis of Variance↗
PubMed2000

Preliminary report: monocular spatial localization in children with strabismic amblyopia.

Defective spatial localization is an important feature of strabismic amblyopia. Based on our experience from testing adult strabismics under various test conditions, we developed a test for assessing vertical alignment in strabismic children. Patients had to align a vertical test line with the apices of two vertically arranged reference triangles, under the control of both the dominant eye and the amblyopic eye. Means and standard deviations of several judgements represent systematic errors and uncertainty of alignment. We tested 27 strabismic and 34 age-matched control children aged 4.5-10 years. Control children showed a scatter of mean systematic alignment around the correct position of up to 7 minarc. In the amblyopic eyes of strabismic children, uncertainty was consistently higher than in the eyes of the control children. Systematic errors outside the normal range frequently occurred. In children tested repeatedly during occlusion therapy, uncertainty decreased as visual acuity improved. In several cases we observed changes of systematic vertical alignment during therapy, sometimes unexpectedly in the sense of a change in the direction of mislocalization or an initial increase and later decrease of errors. Thus, children with strabismic amblyopia show spatial localization deficits which are similar to those of adult strabismic amblyopes. Both spatial uncertainty and systematic distortions are susceptible to change due to enforced use of the amblyopic eye during occlusion therapy.

Amblyopia↗
PubMed1999

Do deaf people see better? Texture segmentation and visual search compensate in adult but not in juvenile subjects.

The research concerning the visual perception in deaf subjects has led to contradictory results: Deaf subjects have been reported to show enhanced visual perceptual skills compared to hearing subjects (Neville & Lawson, 1987). On the other hand, there are indications that acoustic deprivation may produce an inferiority in all sensory modalities (Myklebust, 1964). These contradictions may be due to methodological differences: The investigators selected different conditions (e.g. attentive/nonattentive) and various samples of deaf subjects (e.g., different age, language, and aetiology groups). In our study, we tested a large sample of deaf subjects with texture segmentation and visual search conditions, which allowed us to differentiate between visual processing with and without attentional load. All deaf subjects had profound hearing loss within the first year of life. Our results suggest that the visual processing capacity of deaf children and adolescents does not exceed that of age- and gender-matched hearing subjects. Rather, deaf school children show deficits in visual processing in conditions with and without attentional load. Age (6 to 20 years), language used (oral, sign, oral + sign), and aetiology for deafness (genetic, maternal rubella, perinatal, infection in the first year of life, unknown) did not consistently influence the results. The deficits in visual processing were partially compensated for in adult deaf subjects. The performances of deaf and hearing adults in trials that could be solved preattentively did not differ statistically significantly, but in attention-dependent trials the deaf subjects were more efficient than the hearing controls. We conclude that visual compensation for deafness is limited to attention-dependent tasks and does not develop until adulthood.

Adolescent↗
PubMed1998

Parallel visual search is not always effortless.

Serial visual search can become parallel with practice. We tested whether this parallelisation is accompanied by a transition from an attentive to a pre-attentive and thus effortless mode of visual perception. While psychophysiological indicators of attentional effort (galvanic skin conductance, muscle tonus) reflected the distinction between serial and parallel search modes, they did not follow the perceptual changes during learning. Despite the perceptual parallelisation with practice, the attentional load remained high for initially serial tasks.

Attention↗
PubMed1996

[Texture discrimination and visual search: development, learning and plasticity].

BACKGROUND: The abilities to segment coherent surfaces out of a discrepant background (texture segmentation) and to localize single objects out of a complex scene (visual search) are believed to occur at a peripheral level in the cortical visual pathway. AIM OF THE STUDY: Are texture segmentation and visual search innate or acquired? Can these abilities be improved in adulthood by learning? Does a disturbed early visual experience (strabismus, amblyopia) lead to a reduction of these abilities? RESULTS: Texture segmentation and visual search undergo a protracted development which continues well into adulthood. Even in adult observers, these abilities can be improved with practice. Learning is enduring, but not specific: It can be transferred from one task to another and between the two eyes of a given subject (including the two eyes of stereoblind subjects). DISCUSSION: We conclude that learning of texture segmentation and visual search does not involve the specific features of a visual scene, but rather reflects an improved strategy of localizing visual objects in a cluttered environment. This type of learning probably occurs at a very high level of cortical processing.

Adolescent↗
PubMed1995

Perceptual learning in visual search: fast, enduring, but non-specific.

Visual search has been suggested as a tool for isolating visual primitives. Elementary "features" were proposed to involve parallel search, while serial search is necessary for items without a "feature" status, or, in some cases, for conjunctions of "features". In this study, we investigated the role of practice in visual search tasks. We found that, under some circumstances, initially serial tasks can become parallel after a few hundred trials. Learning in visual search is far less specific than learning of visual discriminations and hyperacuity, suggesting that it takes place at another level in the central visual pathway, involving different neural circuits.

Discrimination, Psychological↗
PubMed1994

Reduced synchronization in the visual cortex of cats with strabismic amblyopia.

Synchronous firing of spatially separate neurons was studied with multi-electrode recordings in area 17 of the visual cortex of strabismic cats which had developed behaviourally verified amblyopia of the deviated eye. Responses of neurons were evoked with moving light bars or gratings of different spatial frequency. Neurons driven by the normal eye displayed stronger synchronization of their responses than neurons dominated by the amblyopic eye. These interocular differences were highly significant and particularly pronounced for grating stimuli of high spatial frequency. No interocular differences were noted with respect to the amplitudes of responses to the light bars and gratings. These results suggest reduced synchronization of population responses as a neurophysiological correlate of strabismic amblyopia and underline the importance of correlated firing of spatially separate cortical neurons for normal processing of visual information.

Amblyopia↗
PubMed1994

Pointing errors in strabismics: complex patterns of distorted visuomotor coordination.

Monocular eye-hand coordination was tested in a pointing experiment in the central and peripheral visual field of each eye of strabismic and anisometropic amblyopes, strabismic alternators and normal controls. In the normal controls, pointing was accurate in the central visual field. Towards the periphery, normal observers showed a varying amount of undershoot as well as increasing uncertainty. The pointing pattern was similar in the two eyes of these subjects. Increased uncertainty of pointing responses was found in most amblyopic eyes as compared to the dominant eyes. Three strabismic amblyopes and one strabismic alternator also had systematic pointing errors (i.e. systematic differences between pointing under the control of the dominant as compared to the nondominant eye). The pointing pattern was not uniform across the visual field in some of the strabismics, but was composed of areas of larger and smaller pointing errors. Larger pointing errors (i.e. larger differences between the dominant and nondominant eyes) were usually found in the central visual field. These errors were not closely related to the angle of squint. The clinical history of the strabismics seems to be an important factor for the occurrence of pointing errors.

Adolescent↗
PubMed1994

The development of visual acuity in the peripheral visual field of human infants: binocular and monocular measurements.

We measured binocular and monocular grating acuity in the peripheral visual field of infants, using a modified preferential looking procedure. Both binocular and monocular peripheral acuity increased between 2 and 11 months of age, but had not reached adult levels at the end of the first year of life. Binocular acuity was always higher than monocular acuity. At all ages tested, acuity was higher in the temporal than in the nasal visual field. We conclude that, in spite of the relative morphological maturity of the peripheral retina, visual acuity develops in the peripheral visual field.

Adult↗
PubMed1994

Texture segmentation and 'pop-out' in infants and children: the effect of test field size.

The ability of infants and children to segment textures based on differences in line orientation and blob size was investigated, using a forced-choice preferential looking method. In the first experiment, a stimulus pair (a homogeneous texture and a texture containing either a group of sixteen elements or a single element of an orthogonal orientation or a larger blob size) was presented on two separate test fields. Preference for the figure defined by differences in blob size was seen already in 2-month-old infants. In contrast, preference for a figure defined by differences in orientation emerged at 9-12 months of age and became adult-like around school age (see also Sireteanu and Rieth, Behavioural Brain Res., 49, 133-139, 1992). Preference for the single discrepant element was always lower than preference for the discrepant group. In the second experiment, segmentation of oriented textures presented on a single, rather than two separate surfaces was tested. A significant preference for the embedded discrepant group, but not for the single discrepant element, was seen already at 3 months of age. These results show that infants as young as 3 months of age are able to detect a boundary defined by differences in line orientation (see also Atkinson and Braddick, Behavioural Brain Res., 49, 123-131, 1992). However, this ability does not appear to lead to the 'pop-out' phenomenon, as seen in adult observers, until much later.

Adolescent↗
PubMed1993

Interocular suppression in normal and amblyopic subjects: the effect of unilateral attenuation with neutral density filters.

In normal subjects, binocular rivalry suppression takes time to build up (Wolfe, 1986a). The time courses of interocular suppression are different and heterogeneous in amblyopic subjects (de Belsunce & Sireteanu, 1991). In the present study, we found that, in normal observers, progressive reduction of one eye's stimulus luminance with neutral density filters produces time courses similar to those of amblyopic subjects. Conversely, in amblyopes, attenuation of the dominant eye's stimulus produces time courses similar to those of normal observers. Under conditions of balancing of the two eyes, amblyopes experience alternating suppression, similarly to binocular rivalry of normals.

Adult↗
PubMed1992

Texture segregation in infants and children.

Segregation of textures based on differences in line orientation and blob size was tested in adults, infants and children, with a forced-choice preferential looking technique. Preference for a figure defined by differences in blob size already occurs in 2-month-old infants. In contrast, preference for a figure differing from the background by the orientation of its elements emerges at the end of the first year of life and becomes adult-like by school age.

Adult↗
PubMed1992

[Localization disorders in squint amblyopia: horizontal line bisection and relative vertical localization].

We used two different procedures for the evaluation of spatial localization errors in strabismic and anisometropic amblyopes, strabismic alternators and normal control subjects: partitioning of horizontal lines and vertical alignment. We found defective spatial localization in the amblyopic eyes of squinters with both procedures. However, in the horizontal partitioning experiment, the control subjects also showed asymmetries between the two hemifields. Thus, there is no good baseline under these conditions for the results of the amblyopic eyes of squinters. Our results show that vertical alignment is the more appropriate procedure to quantify defective spatial localization in strabismic amblyopes.

Discrimination Learning↗
PubMed1992

[Errors of monocular localization in strabismic amblyopia. Two-dimensional distortion].

Spatial distortions, i.e. spatial localization errors, and precision of localization were measured monocularly in the central visual field of 7 normal observers, 17 strabismic amblyopes and 2 anisometropic amblyopes. The task of the subjects was to construct circles of 2 degrees, 4 degrees and 6 degrees radius around a fixation point, using the dominant and the amblyopic eye in turn. Normal observers set distances on the vertical meridian smaller than distances on the horizontal meridian. Anisometropic amblyopes showed localization errors and variances similar to those of normal observers. The amblyopic eyes of strabismic observers with a large angle strabismus and deep amblyopia showed significant, individually different localization errors correlating with the depth of amblyopia. Strabismics with microstrabismus exhibited parallels between monocular localization and dichoptic retinal correspondence.

Amblyopia↗
PubMed1991

Two-dimensional spatial distortions in human strabismic amblyopia.

Two-dimensional space perception was measured in the central visual field of amblyopic subjects under special consideration of spatial distortions and spatial uncertainty. The subjects had to construct circles with different radii around a given fixation point. Subjects with deep strabismic amblyopia were showed to exhibit considerable distortion in the amblyopic eye. Each subject exhibited an individual distortion pattern. The dominant eye showed distortions similar to those seen in normal observers. Subjects with mild strabismic amblyopia, strabismic alternators and anisometropic amblyopes did not exhibit any significant spatial distortions. Data from dichoptic localization experiments indicate that anomalous retinal correspondence might contribute to monocular spatial distortions.

Adult↗
PubMed1991

The time course of interocular suppression in normal and amblyopic subjects.

The authors measured the time course of interocular suppression of five normal subjects and eleven patients with amblyopia (strabismic and/or anisometropic). Orthogonal gratings were presented dichoptically for durations that ranged from 10-6000 msec. All normal observers reported fusion or superimposition of the orthogonal gratings for short stimuli and reported binocular rivalry for stimuli longer than 150 msec. At long presentation times, all amblyopes constantly suppressed the pattern that was presented to their amblyopic eye. Six amblyopes showed superimposition of the two patterns at short presentation times. Of these, three had time courses similar to those of normal observers; the other three had a much shorter onset of suppression (about 80 msec). The remaining five amblyopes perceived only the pattern of the dominant eye at short stimulus durations; at intermediate durations, they reported partial superimposition of the stimuli, whereas at the longest stimulus durations, again only the stimulus of the dominant eye was perceived. The results suggest that binocular rivalry in normal observers and strabismic suppression in amblyopes are mediated by different mechanisms. The heterogeneity of the time courses of suppression in amblyopes might result from differences in the disturbances of early visual development (age at onset of strabismus and/or anisometropia, origin, and therapy).

Adult↗