[Spatial misperceptions in amblyopic vision: abnormal activation of the primary visual cortex?]. / Basiert der verzerrte Seheindruck bei Amblyopie auf einer erhöhten Aktivierung in der primären Sehrinde?

BACKGROUND: The aim of this study was to provide a description of the spatial distortions and temporal instability in amblyopic vision, and to attempt to define a cortical substrate of the spatial distortions in strabismic amblyopia. MATERIAL AND METHODS: The perceptual distortions and instabilities occurring in amblyopic vision were investigated psychophysically, by asking 17 subjects to describe and sketch their percepts. This was then visualised with an animated computer programme and validated by the subjects. In a second experiment, the cortical responses of normal observers to patterns corresponding to the spatial distortions reported by amblyopic subjects were investigated using functional magnetic resonance imaging. RESULTS: Spatial distortions were more marked in strabismic than in anisometropic amblyopes or in strabismic subjects with alternating fixation. Temporal instability occurred mainly in strabismic amblyopes and affected mainly patterns with higher spatial frequencies. Experiments with functional magnetic resonance imaging showed that the patterns with the highest spatial distortions yield increased activation in the primary visual cortex of normally-sighted observers. CONCLUSIONS: The results of the imaging experiment suggest that the occurrence of spatial distortions might explain the higher activation in the primary visual cortex of some amblyopic subjects. The occurrence of temporal instability in strabismic amblyopia suggests an involvement of higher-order, extrastriate visual areas of the dorsal, "where" visual pathway in amblyopia, in addition to the known deficits in the ventral, "what" visual pathway.

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.

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.

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.

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.

Monocular acuity in preschool children: Assessment with the Teller and Keeler acuity cards in comparison to the C-test.

The development of monocular grating acuity was studied in 210 children between 1 and 6 years using Teller Acuity Cards (TAC). 95 of these children were also tested with the Keeler Acuity Cards (KAC). Monocular visual acuity measured with the TAC showed a gradual increase from 8.4 to 32.4 c/deg for 7 to 79 month-olds and with the KAC from 14.5 to 31.4 c/deg for 19 to 78 month-olds. Mean visual acuity at 6 years was still 0.5-1.0 octave lower than grating acuity in adults. The results of 95 children tested with TAC and KAC did not indicate any significant differences between the acuity scores obtained with the two tests. Stimulus configuration of the TAC did not result in overestimation of acuity in children 1-6 years of age. In 141 older children (39-79 months), the TAC test was compared with a recognition test (C-test). Children between 3 and 5 years of age reached significantly lower acuities with the C-test than with the TAC. The C-test showed a higher sensitivity for unconnected refractive errors than the acuity card tests.

[Texture discrimination and visual search: development, learning and plasticity]. / Textursegmentierung und visuelle Suche: Entwicklung, Lernen und Plastizität.

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.

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.

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.

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.

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.

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.

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.

Eye alignment and cortical binocularity in strabismic kittens: a comparison between tenotomy and recession.

Interocular alignment was assessed by corneal light reflex photography in 15 normal and 26 strabismic kittens. Strabismus was induced at 3-4 weeks of age by severing one extraocular muscle (tenotomy), by cutting and reinserting the muscle at another position on the ocular globe (recession), or by combining recession of the medial rectus muscle with resection of the lateral rectus muscle of the same eye. Nineteen strabismic and five normal kittens were followed longitudinally from 12 days to about 6 months of age. Three out of six longitudinally followed tenotomized cats and six out of the 13 recessed cats conserved their postoperative ocular deviation throughout the testing period ("large-angle strabismics"). Three tenotomized and seven recessed cats showed a transient deviation for 1-2 weeks after surgery, after which the interocular deviation diminished to values found in normal cats ("microstrabismic" cats). Both recessed-resected cats showed a transient interocular deviation. In spite of their different developmental histories, all cats showed a clear breakdown of binocularity in area 17. Large-angle strabismics showed a dominance of the non-operated eye, while in microstrabismic cats, both eyes were equally effective in driving cortical cells. It thus appears that a transient strabismus is sufficient to produce a reduction of binocularity in area 17.

Distortions in two-dimensional visual space perception in strabismic observers.

Amblyopic subjects were asked to memorize circles of different radii (2, 4 and 6 degrees), and then to reconstruct them monocularly with each eye, point by point (12 points per circle). The resulting two-dimensional maps of visual space showed considerable distortions, including expansion, shrinkage and torsion of specific regions of the visual field of the amblyopic eye, but not the normal eye. Based on the differences between the two eyes, we computed complex two-dimensional patterns (gratings, checkerboards, optotypes, written text, natural scenes) as "seen" monocularly with the amblyopic eye. These reconstructed patterns were then compared with drawings of the same patterns observed through the amblyopic eyes of the same subjects. The reconstructed patterns only partially reflected the actual perception for the amblyopic eyes. The compensation of complex, globally-extended scenes in comparison to the distortions obtained by local, punctate settings probably reflects cooperative interactions occurring at higher brain levels.

[Errors of monocular localization in strabismic amblyopia. Two-dimensional distortion]. / Fehler der monokularen Lokalisation bei Schielamblyopie. Zweidimensionale Verzerrungen.

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.