Biomedical subjects

Siegrid Löwel

Publications and source records attributed to Siegrid Löwel.

11 recordsLinked to original sources

PubMed2005

Optical imaging in cat area 18: strabismus does not enhance the segregation of ocular dominance domains.

While early-onset strabismus leads to clearly segregated domains of the left and the right eye in cat primary visual cortex (area 17), far less is known about experience-dependent plasticity of ocular dominance in area 18. We therefore used optical imaging of intrinsic signals to analyze the influence of strabismus on cortical maps in cat area 18. Monocular visual stimulation of the left and right eye with moving square wave gratings of four different orientations induced patchy activity maps. Unlike our previous observations in cat area 17, the monocular activity maps in area 18 of strabismic cats were rather similar so that functional ocular dominance domains were not clearly segregated. Imaging of the 17/18 border region confirmed this observation and revealed a sudden change in the segregation of the left and right eye domains across the border. Our results demonstrate that modified visual input can have different consequences for different visual areas: while the decorrelation of activity between the two eyes (as induced by strabismus) clearly enhances the segregation of ocular dominance domains in cat area 17, area 18 does not show this effect although electrophysiological studies have confirmed that the percentage of binocularly driven neurons is as reduced as in area 17.

Animals↗

PubMed2003

Postnatal growth and column spacing in cat primary visual cortex.

The primary visual cortex (area 17) of cats continues to grow substantially during early postnatal development. To assess the implications of this growth for the organization of visual cortical maps, we analysed both postnatal growth of area 17 and the spacing of ocular dominance columns in the same animals using 2-deoxyglucose autoradiography to label cortical activation patterns. Quantitative analyses of area size and column spacing were performed on flat-mount sections of the cortical hemispheres. Our analysis of the surface area revealed an average increase of the size of area 17 between the third and tenth postnatal weeks by about 51%. About 75% of this increase occurred during the third and sixth postnatal weeks (size increase of 37%). However, the distance between adjacent columns did not exhibit a similar increase but rather remained constant during the same postnatal period. Since cortical growth is not accompanied by an increased spacing of ocular dominance columns, new functional modules must somehow be added during the first postnatal weeks to occupy the enlarging cortical sheet. Possible mechanisms underlying the formation of new modules are discussed.

Age Factors↗

PubMed2003

The growth of cat cerebral cortex in postnatal life: a magnetic resonance imaging study.

To follow up the development of an individual brain over time and to measure its growth we have analysed the brains of individual cats from postnatal day 12 to adulthood using magnetic resonance imaging. From the anatomical images, four parameters were calculated: anteroposterior extent of the telencephalon, brain volume, neocortical surface area and neocortical volume. The development of the anteroposterior extent was similar in all cats. It increased between the 3rd and 6th postnatal week from 33 to 37.5 mm ending up approximately 40 mm in adulthood. The brain volume showed greater variability. On average, the volume increased from 11.5 to 16.5 cm3 in the same period. Adult values were approximately 19 cm3. Considerable interindividual variability was observed in neocortical surface area. In one cat, it expanded from 12.5 to 26 cm2 between days 14 and 41. In another cat, this area expanded from 16 to 24.5 cm2 between days 12 and 40. On average, the surface area expanded by 34% between the 3rd and 6th week. Adult values ranged from 27 to 30 cm2. Neocortical volume increased from 2.9 to 4.1 cm3 between the 3rd and 6th postnatal week and to 4.5-5.2 cm3 in adulthood. The asymmetry between the hemispheres in both neocortical surface area and volume was < 3% in all animals for most of the observation period. Comparison of the neocortical surface measurements with data on postnatal growth of cat primary visual cortex obtained by 2-deoxyglucose autoradiography indicates that the primary visual cortex grows at the same speed and amounts to approximately 15% of the entire neocortical surface area throughout development.

Aging↗

PubMed2003

The pattern of ocular dominance columns in cat primary visual cortex: intra- and interindividual variability of column spacing and its dependence on genetic background.

We present a comprehensive analysis of the intrinsic variability of the periodicity of ocular dominance columns in cat primary visual cortex (area 17) and its relationship to genetic background and visual experience. We characterized the intra-areal and interindividual variability of column spacing in a large set (n = 49) of ocular dominance patterns adapting a recently developed technique for the two-dimensional analysis of orientation column patterns. Patterns were obtained from three different cat colonies (termed F, M and D), the cats having either normal visual experience or experimentally induced strabismus. Two-dimensional maps of local column spacing were calculated for every pattern. In individual cortices, local column spacings varied by > 50% with the majority of column spacings ranging between 0.6 and 1.5 mm in different animals. In animals from colonies F and M (n = 29), the mean column spacing ranged between 1.03 and 1.27 mm and exhibited no significant differences, either between the two breeds or between strabismic and normal animals. The mean spacing was moderately clustered in the left and right brain hemisphere of individual animals but not in littermates. In animals from colony D (n = 2), average column spacing ranged between 0.73 and 0.95 mm, and was thus significantly different from the distribution of spacings in animals from breeds F and M, suggesting an influence of genetic factors on the layout of ocular dominance columns. Local column spacing exhibited a considerable systematic intra-areal variation, with largest spacings along the representation of the horizontal meridian and smallest spacings along the peripheral representation of the vertical meridian. The total variability of ocular dominance column spacing comprised 24% systematic intra-areal variation, 18% interindividual differences of mean column spacing and 58% nonsystematic intra-areal variability.

Animals↗

PubMed2002

Genetic influence on quantitative features of neocortical architecture.

The layout of functional cortical maps exhibits a high degree of interindividual variability that may account for individual differences in sensory and cognitive abilities. By quantitatively assessing the interindividual variability of orientation preference columns in the primary visual cortex, we demonstrate that column sizes and shapes as well as a measure of the homogeneity of column sizes across the visual cortex are significantly clustered in genetically related animals and in the two hemispheres of individual brains. Taking the developmental timetable of column formation into account, our data indicate a substantial genetic influence on the developmental specification of visual cortical architecture and suggest ways in which genetic information may influence an individual's visual abilities.

Animals↗

PubMed2002

Two-dimensional analysis of the spacing of ocular dominance columns in normally raised and strabismic kittens.

In the primary visual cortex of cats, ferrets and macaque monkeys, the thalamocortical afferents conveying signals from the two eyes terminate in alternating regions of layer IV known as ocular dominance columns. Previous experiments have indicated that the periodicity of these columns can be influenced by visual experience: compared to normally raised animals both strabismic cats and cats raised with alternating monocular exposure displayed an increased spacing of adjacent ocular dominance columns in the primary visual cortex (area 17). However, recently it was shown that the formation of ocular dominance columns begins much earlier than previously supposed, indicating that early visual experience might only have a limited influence on the development of the spatial pattern of ocular dominance columns. We therefore visualized the complete pattern of ocular dominance columns in area 17 of normally raised and strabismic kittens during early postnatal development (age 3-6 weeks), particularly focussing on littermates. In addition, we used a previously developed spatial analysis (period statistics) to quantify columnar spacing two-dimensionally. We observed a pronounced interindividual variability in both normally raised and strabismic animals, with column spacings ranging from 783 to 1362 microm. In contrast to previous reports, there were no significant differences in columnar periodicity between normally raised and strabismic cats. These data indicate that rearing has less influence on column spacing while the interindividual variability is much greater than previously supposed, suggesting that genetic differences have an influence on column spacing.

Animals↗

PubMed2002

GABA-inactivation attenuates colinear facilitation in cat primary visual cortex.

Neurons in primary visual cortex (V1) respond preferentially to stimuli of a particular orientation falling within a circumscribed region of visual space known as their receptive field (RF). However, the response to an optimally oriented stimulus presented within the RF can be enhanced by the simultaneous presentation of co-oriented, co-linearly aligned flank stimuli falling outside the RF which, when presented alone, fail to activate the cell. This type of contextual effect, termed colinear facilitation, presumably forms the physiological substrate for the integration of the line elements of a contour and the perceptual saliency of a contour in a complex environment. Here we show that colinear facilitation in single cells of cat area V1 can be substantially reduced or abolished by focal inactivation of laterally remote cells in the same area which respond strongly to the co-oriented, colinear flank stimulus inducing the facilitatory effect. The results provide evidence that horizontal intrinsic connections between cells with co-oriented and co-linearly aligned RFs make a major contribution to colinear facilitation in V1. They imply that the neuronal circuitry underlying contour integration and saliency is already present at the earliest stage of visual cortical information processing.

Action Potentials↗

PubMed2002

Neuroanatomical and neurophysiological consequences of strabismus: changes in the structural and functional organization of the primary visual cortex in cats with alternating fixation and strabismic amblyopia.

In recent years, evidence has accumulated indicating that long-ranging neuronal connections within the primary visual cortex (area 17) mediate the influences of context and experience, possibly also those of expectation. After early onset strabismus, the layout of these connections is massively modified: in strabismic but not in normally raised cats, horizontal connections extend primarily between neurons activated by the same eye. As a possible consequence of the modified circuitry, neuronal synchronization between different ocular dominance domains is also massively reduced. Thus, the inability of strabismics to combine the signals arriving from the two eyes into a single percept may be caused by these structural and functional changes. Strabismic amblyopia is also accompanied by significant modifications of intracortical associational interactions: corresponding to the psychophysical deficits, neurons driven by the normal eye displayed stronger synchronization of their responses than neurons dominated by the amblyopic eye.(1) These data demonstrated for the first time a clear neurophysiological correlate of strabismic amblyopia in area 17. They suggest that - similar to our observations in divergent squinters - at least some of the perceptual deficits of amblyopic patients are due to experience-dependent changes in intracortical circuitry. We analyze this question by combining optical imaging of intrinsic signals with 3-D reconstructions of neuronal circuitry.

Amblyopia↗

PubMed2002

Spatial analysis of ocular dominance patterns in monocularly deprived cats.

Monocular deprivation during a critical period in postnatal development leads to a shift in functional and anatomical ocular dominance at the expense of the deprived eye. We analyzed the complete two-dimensional pattern of [(3)H]proline-labeled afferents in primary visual cortex (area 17) of cats monocularly deprived of vision at eye opening. Substantial shrinkage of deprived eye territory in favor of the normal eye extended into optic disc and monocular segment representations. However, small domains of deprived eye afferents were distributed evenly over the entire visual field representation. Interestingly, normal and deprived eye afferents overlapped extensively in the ipsilateral and in the peripheral contralateral visual field representation of the deprived eye, so that ipsi- and contralateral ocular dominance patterns are not at all complementary. We suggest that this could be the result of both an earlier maturation of crossed versus uncrossed visual pathways and of a maturational gradient within area 17 leading to a lower vulnerability of the central visual field representation to monocular deprivation. Quantitative analysis, using a triangulation algorithm which confirmed previously described larger spacing of adjacent ocular dominance columns in strabismic cats, revealed no difference in spacing of ocular dominance domains in area 17 between monocularly deprived and normals cats. In addition, column spacing was very similar in the same animal and in littermates, indicating that the genetic influence on columnar layout is stronger than previously assumed.

Age Factors↗

PubMedDate not supplied

Optical imaging of orientation and ocular dominance maps in area 17 of cats with convergent strabismus.

Strabismus (or squint) is both a well-established model for developmental plasticity of the brain and a frequent clinical symptom. While the layout and topographic relationship of functional domains in area 17 of divergently squinting cats has been analyzed extensively in recent years (e.g. Löwel et al., 1998), functional maps in convergently squinting animals have so far not been visualized with comparable detail. We have therefore investigated the functional organization of area 17 in adult cats with a surgically induced convergent squint angle. In these animals, visual acuity was determined by both behavioral tests and recordings of visual evoked potentials, and animals with comparable acuities in both eyes were selected for further experiments. The functional layout of area 17 was visualized using optical imaging of intrinsic signals. Monocular iso-orientation domains had a patchy appearance and their layout was different for left and right eye stimulation, so that segregated ocular dominance domains could be visualized. Iso-orientation domains exhibited a pinwheel-like organization, as previously described for normal and divergently squinting cats. Mean pinwheel density was the same in the experimental and control animals (3.4 pinwheel centers per mm2 cortical surface), but significantly (P < 0.00001) higher than that reported previously for normal and divergently squinting cats (2.7/mm2). A comparison of orientation with ocular dominance maps revealed that iso-orientation domains were continuous across the borders of ocular dominance domains and tended to intersect these borders at steep angles. However, in contrast to previous reports in normally raised cats, orientation pinwheel centers showed no consistent topographical relationship to the peaks of ocular dominance domains. Taken together, these observations indicate an overall similarity between the functional layout of orientation and ocular dominance maps in area 17 of convergently and divergently squinting cats. The higher pinwheel densities compared with previous reports suggest that animals from different gene pools might generally differ in this parameter and therefore also in the space constants of their cortical orientation maps.

Animals↗

PubMedDate not supplied

Optical imaging of the intrinsic signal as a measure of cortical plasticity in the mouse.

The responses of cells in the visual cortex to stimulation of the two eyes changes dramatically following a period of monocular visual deprivation (MD) during a critical period in early life. This phenomenon, referred to as ocular dominance (OD) plasticity, is a widespread model for understanding cortical plasticity. In this study, we designed stimulus patterns and quantification methods to analyze OD in the mouse visual cortex using optical imaging of intrinsic signals. Using periodically drifting bars restricted to the binocular portion of the visual field, we obtained cortical maps for both contralateral (C) and ipsilateral (I) eyes and computed OD maps as (C - I)/(C + I). We defined the OD index (ODI) for individual animals as the mean of the OD map. The ODI obtained from an imaging session of less than 30 min gives reliable measures of OD for both normal and monocularly deprived mice under Nembutal anesthesia. Surprisingly, urethane anesthesia, which yields excellent topographic maps, did not produce consistent OD findings. Normal Nembutal-anesthetized mice have positive ODI (0.22 +/- 0.01), confirming a contralateral bias in the binocular zone. For mice monocularly deprived during the critical period, the ODI of the cortex contralateral to the deprived eye shifted negatively towards the nondeprived, ipsilateral eye (ODI after 2-day MD: 0.12 +/- 0.02, 4-day: 0.03 +/- 0.03, and 6- to 7-day MD: -0.01 +/- 0.04). The ODI shift induced by 4-day MD appeared to be near maximal, consistent with previous findings using single-unit recordings. We have thus established optical imaging of intrinsic signals as a fast and reliable screening method to study OD plasticity in the mouse.

Animals↗