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Biomedical subjects

W Singer

Publications and source records attributed to W Singer.

At least 19 recordsLinked to original sources
PubMed1998

Two segmentation mechanisms with differential sensitivity for colour and luminance contrast.

In a texture-segregation paradigm, subjects were asked to detect figures whose elements were segregated from background either because of temporal offset or because of differing orientations. Texture elements were either isoluminant or had high or low luminance contrast. At high luminance contrast, figures could be segregated both on the basis of orientation and temporal cues whereby temporal offsets as short as 10 msec supported detection. At isoluminance, orientation defined figures were as readily distinguishable as in the high contrast condition but temporally defined figures were perceived only for offset intervals > 50 msec. With low luminance contrast, performance for orientation defined figures was impaired relative to the high contrast condition, but for temporally defined figures, it was superior to the isoluminant condition; detection was possible for offset intervals as short as 22 msec. These results suggest that the temporal and orientation cues which support scene segmentation are transmitted by both the luminance and colour sensitive pathways. However, if temporal offsets are < 50 msec, segmentation of temporally defined figures is supported only by the luminance sensitive system.

Adult↗
PubMed1998

Modification of discharge patterns of neocortical neurons by induced oscillations of the membrane potential.

We investigated, with whole-cell recordings from rat visual cortex slices, how sinusoidal modulation of the membrane potential affects signal transmission. Subthreshold oscillations activate tetrodotoxin sensitive, transient inward currents whose threshold, phase lag and duration change with modulation frequency. These periodically recurring phases of enhanced excitability affect synaptic transmission in two ways. Weak and short lasting excitatory postsynaptic potentials evoke discharges only if they are coincident within a few milliseconds with these active membrane responses. Long-lasting, N-methyl-D-aspartate-mediated or polysynaptic excitatory postsynaptic potentials, by contrast, evoke trains of spikes, that are precisely time-locked to the oscillations and may last for more than 100 ms. Thus, oscillations impose a precise temporal window for the integration of synaptic inputs, favouring coincidence detection and they generate temporally-structured responses whose timing and amplitude are largely independent of the input. These properties are ideally suited for the synchronization of neuronal activity and the encoding of information in the precise timing of discharges. A preliminary account of these data has appeared in an abstract form [Volgushev M. et al. (1995) Eur. J Neurosci. 8, 77].

Action Potentials↗
PubMed1997

Synchronization of oscillatory responses in visual cortex correlates with perception in interocular rivalry.

In subjects suffering from early onset strabismus, signals conveyed by the two eyes are not perceived simultaneously but in alternation. We exploited this phenomenon of interocular suppression to investigate the neuronal correlate of binocular rivalry in primary visual cortex of awake strabismic cats. Monocularly presented stimuli that were readily perceived by the animal evoked synchronized discharges with an oscillatory patterning in the gamma-frequency range. Upon dichoptic stimulation, neurons responding to the stimulus that continued to be perceived increased the synchronicity and the regularity of their oscillatory patterning while the reverse was true for neurons responding to the stimulus that was no longer perceived. These differential changes were not associated with modifications of discharge rate, suggesting that at early stages of visual processing the degree of synchronicity rather than the amplitude of responses determines which signals are perceived and control behavioral responses.

Animals↗
PubMed1997

NCAM 180 in the postnatal development of cat visual cortex: an immunohistochemical study.

The neural cell adhesion molecule (NCAM) consists of three major isoforms with differing extents of intracellular domains. The largest, NCAM 180, may play an important role in the development and stabilization of cell contacts because of its interaction with the cytoskeleton, its accumulation at contact sites between cells, and age-dependent modifications of the immunoreactivity of its intracellular domain. The developmental expression pattern of NCAM 180 was examined in the cat visual cortex to explore its relation with the age-dependent decline of synaptic malleability. Western blot and immunohistochemical analyses were carried out by using a monoclonal antibody (D3) directed against an epitope of the intracellular domain specific to NCAM 180 and antibodies against all NCAM isoforms. The latter revealed a similar time course of increased expression during development by both Western blot and immunohistochemical analysis. In contrast, the D3 antibody showed an age-dependent increase of immunoreactivity by Western blot analysis and a decrease of reactivity by immunohistochemistry. In addition, the D3 antibody revealed characteristic developmental changes of immunoreactivity in the neuropil and distribution of immunopositive neuronal cell bodies in the different cortical laminae. The observations from this and another study (Kramer et al., 1997) suggest that the D3 antibody-specific NCAM 180 epitope is masked during the stabilization of connectivity.

Animals↗
PubMed1997

Functional specificity of long-range intrinsic and interhemispheric connections in the visual cortex of strabismic cats.

The development of both long-range intracortical and interhemispheric connections depends on visual experience. Previous experiments showed that in strabismic but not in normal cats, clustered horizontal axon projections preferentially connect cell groups activated by the same eye. This indicates that there is selective stabilization of fibers between neurons exhibiting correlated activity. Extending these experiments, we investigated in strabismic cats: (1) whether tangential connections remain confined to columns of similar orientation preference within the subsystems of left and right eye domains; and (2) whether callosal connections also extend predominantly between neurons activated by the same eye and preferring similar orientations. To this end, we analyzed in strabismic cats the topographic relationships between orientation preference domains and both intrinsic and callosal connections of area 17. Red and green latex microspheres were injected into monocular iso-orientation domains identified by optical imaging of intrinsic signals. Additionally, domains sharing the ocular dominance and orientation preference of the neurons at the injection sites were visualized by 2-deoxyglucose (2-DG) autoradiography. Quantitative analysis revealed that 56% of the retrogradely labeled cells within the injected area 17 and 60% of the transcallosally labeled neurons were located in the 2-DG-labeled iso-orientation domains. This indicates: (1) that strabismus does not interfere with the tendency of long-range horizontal fibers to link predominantly neurons of similar orientation preference; and (2) that the selection mechanisms for the stabilization of callosal connections are similar to those that are responsible for the specification of the tangential intrinsic connections.

Animals↗
PubMed1997

Visuomotor integration is associated with zero time-lag synchronization among cortical areas.

Information processing in the cerebral cortex invariably involves the activation of millions of neurons that are widely distributed over its various areas. These distributed activity patterns need to be integrated into coherent representational states. A candidate mechanism for the integration and coordination of neuronal activity between different brain regions is synchronization on a fine temporal scale. In the visual cortex, synchronization occurs selectively between the responses of neurons that represent related features and that need to be integrated for the generation of coherent percepts; neurons in other areas of the cerebral cortex also synchronize their discharges. However, little is known about the patterns and the behavioural correlates of synchrony among widely separated cortical regions. Here we report that synchronization occurs between areas of the visual and parietal cortex, and between areas of the parietal and motor cortex, in the awake cat. When cats responded to a sudden change of a visual pattern, neuronal activity in cortical areas exhibited synchrony without time lags; this synchrony was particularly strong between areas subserving related functions. During reward and inter-trial episodes, zero-time-lag synchrony was lost and replaced by interactions exhibiting large and unsystematic time lags.

Action Potentials↗
PubMed1997

The effect of exercise on lymphocyte redistribution and leucocyte function in asymptomatic HIV-infected subjects.

This study was undertaken to examine the responsiveness of circulating leucocyte and lymphocyte populations to the physiological demands of exercise in asymptomatic HIV-infected subjects with CD4+ counts greater than 500/microliter. Thirteen subjects infected with HIV and 14 control subjects underwent 20 min of defined moderate exercise at estimated 65% of their maximal ventilatory capacity on a bicycle ergometer. Blood samples were obtained for serum cortisol, norepinephrine, lymphocyte subsets (CD4, CD8, CD19, CD16-CD56), and phagocytic function at rest immediately after exercise and 20 min following the cessation of the exercise. The HIV-infected subjects had increased circulating concentrations of CD8 cells (p = .007) and CD16-CD56+ NK cells (p = .02) in response to the exercise, whereas the control group did not. There was a greater increase in monocyte respiratory burst activity following recovery from exercise in the control subjects (p = .016) but not in the HIV-infected subjects. The control subjects experienced an increase in serum cortisol in response to the exercise (p = .006), but the HIV-infected subjects did not. Our results show that the changes in the distribution and function of circulating leucocytes and adrenal neuroendocrine responses to moderate exercise differ in asymptomatic HIV-infected and control subjects.

Adult↗
PubMed1997

Selective temporal interactions between processing streams with differential sensitivity for colour and luminance contrast.

Temporal interactions between spatially separated visual stimuli were investigated in human observers. Subjects had to judge whether briefly presented targets consisted of a single or a double flash. Simultaneous presentation of unattended single or double flash distractors impaired performance if target and distractor followed different time courses, confirming previous findings. This interference occurred only when targets had high luminance contrast or were isoluminant and when distractors had high or low luminance contrast, but not when targets had low luminance contrast or when distractors were isoluminant. Low luminance contrast distractors strongly influenced high luminance contrast targets but not vice versa. These results suggest that (i) information about the precise temporal structure of stimuli is conveyed preferentially by the luminance-sensitive magnocellular system; and (ii) that this information influences judgements on the temporal patterning of signals transmitted by the colour-sensitive parvocellular system.

Adolescent↗
PubMed1997

Towards a European forum for the neurosciences.

In 1994, TINS published a brief analysis of the problems faced by the European neuroscience community in its attempt to meet the challenges associated with the proclamation of the European 'Decade of the Brain'. Since then numerous initiatives have been taken by national and European neuroscience societies with the common goals of improving communication among European neuroscientists, increasing the visibility of the neurosciences as an autonomous and increasingly important domain of research in the appreciation of national and European granting bodies, and facilitating integration of colleagues from Eastern Europe into the international scientific community. These initiatives have led to major changes in the organization and self-apprehension of national and European neuroscience societies.

Europe↗
PubMed1997

Role of the temporal domain for response selection and perceptual binding.

Most cognitive functions are based on highly parallel and distributed information processing by the brain. A paradigmatic example is provided by the vertebrate visual system where numerous cortical areas have been described which analyse different types of visual information. At present, it is unclear how information can be integrated and how coherent representational states can be established in such distributed systems. We suggest that this so-called 'binding problem' may be solved in the temporal domain. The hypothesis is that synchronization of neuronal discharges can serve for the integration of distributed neurons into cell assemblies and that this process may underlie the selection of perceptually and behaviourally relevant information. We review experimental results, mainly obtained in the visual system, which support this temporal binding hypothesis.

Animals↗
PubMed1997

The perceptual grouping criterion of colinearity is reflected by anisotropies of connections in the primary visual cortex.

An important step in the processing of visual patterns is the segmentation of the retinal image. Neuronal responses evoked by the contours of individual objects need to be identified and associated for further joint processing. These grouping operations are based on a number of Gestalt criteria. Here we report that connections in the visual cortex of the cat exhibit a highly significant anisotropy, preferentially linking neurons activated by contours that have similar orientation and are aligned colinearly. These anatomical data suggest a close relation between the perceptual grouping criterion of colinearity and the topology of tangential intracortical connections. We propose that tangential intracortical connections support perceptual grouping by modulating the saliency of distributed cortical responses in a context-dependent way. The present data are compatible with the hypothesis that the criteria for this grouping operation are determined by the architecture of the tangential connections.

Animals↗
PubMed1997

A metabolic mapping study of orientation discrimination and detection tasks in the cat.

Increasing evidence suggests that a large number of distinct cortical areas and associated subcortical structures participate in the processing of visual information and that different aspects of visual scenes are evaluated in different areas. This necessitates identification of cortical and subcortical regions cooperating in particular visual tasks. Using the 2-deoxyglucose technique, we monitored the differential activation of areas in the cat visual cortex participating in an orientation discrimination and a detection task. Concordant with previous lesion studies, we found increased activity levels in area 17 in the discrimination condition relative to the detection condition. In addition, the 2-deoxyglucose technique revealed discrimination-related increased activations in the claustrum, the putamen and in parts of the anteromedial, anterolateral and posterolateral lateral suprasylvian visual areas. Regions activated differentially with the detection task comprised subdivisions of areas 17, 18, 19 and 21, posterior area 7 (7p), several areas of the posterior part of the middle and posterior suprasylvian sulcus, the pulvinar complex and the superior colliculus. These results show that the 2-deoxyglucose technique is useful to investigate cognitive brain functions, and that different sets of cortical and subcortical regions are activated during two visual tasks with similar visual stimulation.

Animals↗
PubMed1997

Relations between long-term synaptic modifications and paired-pulse interactions in the rat neocortex.

The phenomenon of paired-pulse facilitation (PPF) was exploited to investigate the role of presynaptic mechanisms in the induction and maintenance of long-term synaptic plasticity in the neocortex. Long-term potentiation (LTP) and depression (LTD) were induced without afferent activation by applying tetani of intracellular pulses. Our results show that synaptic modifications closely resembling LTP and LTD can be induced by postsynaptic activation alone. The polarity of these synaptic modifications depends on initial properties of the input, as indicated by a correlation between initial PPF ratio and post-tetanic amplitude changes: inputs exhibiting strong PPF, which might be associated with low release probability tend to be potentiated, while inputs with small PPF are more likely to show depression. Maintenance of both LTP and LTD involve presynaptic mechanisms, as indicated by changes in PPF ratios and in failure rate after LTP or LTD induction. Presynaptic mechanisms could include changes in release probability and/or in the number of active release sites. Because induction was postsynaptic, this supports the notion of a retrograde signal. The relative contribution of pre- and postsynaptic mechanisms in the maintenance of long-term synaptic modifications depends on the initial state of the synaptic input and on LTP magnitude. PPF changes were especially pronounced in inputs which had initially high PPF and underwent strong potentiation. Since LTP and LTD are associated with changes of PPF ratios these synaptic modifications do not only alter the gain but also the temporal properties of synaptic transmission. Because of the LTP associated reduction of PPF, potentiated inputs profit less from temporal summation, favouring transmission of synchronized, low frequency activity.

Animals↗
PubMed1997

Development of orientation preference maps in area 18 of kitten visual cortex.

We investigated the development of orientation preference maps in the visual cortex of kittens by repeated optical imaging from the same animal. Orientation maps became detectable for the first time around postnatal day (P) 17 and improved continuously in strength unitl P30, the time at which their appearance became adultlike. During this developmental period the overall geometry of the maps remained unchanged, suggesting that the layout of the orientation map is specified prior to P17. Hence, before the visual cortex becomes susceptible to experience-dependent modifications its functional architecture is largely specified. This suggests that the initial development and layout of orientation preference maps are determined by intrinsic processes that are independent of visual experience. This conclusion is further supported by the result that orientation maps were well expressed at P24 in binocularly deprived kittens. Because the appearance of the first orientation-selective neurons and the subsequent development of orientation preference maps correlated well with the time course of the expression and refinement of clustered horizontal connections, we propose that these connections might contribute to the specification of orientation preference maps.

Animals↗
PubMed1997

Relation between dendritic Ca2+ levels and the polarity of synaptic long-term modifications in rat visual cortex neurons.

Long-term changes of synaptic efficacy, in particular when they are use-dependent, are candidate mechanisms for the storage of information in the nervous system. In a variety of brain structures, including the neocortex and hippocampus, synapses are susceptible to long-term potentiation (LTP) and long-term depression (LTD). It has been hypothesized that the polarity of the synaptic gain change depends on the amplitude of the postsynaptic [Ca2+]i rise, the threshold for the induction of LTD being lower than that for the induction of LTP. To test this assumption, we characterized Ca2+ signals in layer II/III pyramidal cells of rat visual cortex slices, using the fluorescent Ca2+ indicator fura-2, during application of stimulation protocols that had been adjusted to reliably induce either LTP or LTD in cells not loaded with fura-2. At dendritic sites activated by the stimulated afferents the intracellular [Ca2+] concentration ([Ca2+]i) reached higher amplitudes and decayed more slowly with stimuli inducing LTP than with those inducing LTD. To directly analyse the functional significance of the observed difference in the Ca2+ signal amplitude, we examined whether a tetanization protocol suitable for the induction of LTP can be converted into a protocol inducing LTD by injecting the postsynaptic cells with Ca2+ chelators that reduce the concentration of effective free Ca2+. In the presence of fura-2 or BAPTA [bis(2-aminophenoxy) ethane-N,N,N',N'-tetraacetate], the stimulation protocol that would normally produce LTP induced either LTD or failed to induce synaptic modifications altogether. These results support the hypothesis that the amplitude of the postsynaptic rise in [Ca2+]i is a key factor in the determination of the polarity of synaptic gain change.

Animals↗
PubMed1996

Role of reticular activation in the modulation of intracortical synchronization.

During aroused states of the brain, electroencephalographic activity is characterized by fast, irregular fluctuations of low amplitude, which are thought to reflect desynchronization of neuronal activity. This phenomenon seems at odds with the proposal that synchronization of cortical responses may play an important role in the processing of sensory signals. Here, activation of the mesencephalic reticular formation (MRF), an effective way to "desynchronize the electroencephalogram," was shown to facilitate oscillatory activity in the gamma frequency range and to enhance the stimulus-specific synchronization of neuronal spike responses in the visual cortex of cats.

Action Potentials↗
PubMed1996

Neurophysiology: the changing face of inhibition.

Inhibition is commonly thought to suppress neuronal responses, but new discoveries suggest that it may also gate transmission by coordinating the temporal patterning of neuronal responses and so play an important part in information processing in the brain.

Neurons↗