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W Singer

Publications and source records attributed to W Singer.

At least 37 records · Page 2Linked to original sources
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↗
PubMed1996

Stimulus-dependent synchronization of neuronal responses in the visual cortex of the awake macaque monkey.

In visual areas of the cerebral cortex, most neurons exhibit preferences for particular features of visual stimuli, but in general, the tuning is broad. Thus, even simple stimuli evoke responses in numerous neurons with differing but overlapping feature preferences, and it is commonly held that a particular feature is encoded in the pattern of graded responses of the activated population rather than in the optimal responses of individual cells. To decipher this population code, responses evoked by a particular stimulus need to be identified and bound together for further joint processing and must not be confounded with responses to other, nearby stimuli. Such selection of related responses could be achieved by synchronizing the respective discharges at a time scale of milliseconds, as this would selectively and jointly enhance their saliency. This hypothesis predicts that a given set of neurons should exhibit synchronized discharges more often when responding to a single stimulus than when activated by different but simultaneously presented stimuli. To test this prediction, recordings were performed with two electrodes from spatially segregated cells in the middle temporal area (MT) of the awake behaving macaque monkey. It was found that cells with overlapping receptive fields, but different preferences for directions of motion, can engage in synchronous activity if they are stimulated with a single moving bar. In contrast, if the same cells are activated with two different bars, each moving in the direction preferred by the cells at the two respective sites, responses show no or much fewer synchronous epochs. Control experiments exclude that this effect is attributable to changes in response amplitude, the mere presence of two stimuli, or the specific orientation of the bars. The critical variable determining the strength of correlation is the extent to which both sites are activated by a common stimulus or by two different stimuli with different directions of motion.

Action Potentials↗
PubMed1996

Long-range synchronization of oscillatory light responses in the cat retina and lateral geniculate nucleus.

Visual responses in the retina and the lateral geniculate nucleus (LGN) exhibit oscillatory patterning within a broad range of frequencies. Oscillatory activity is often associated with the synchronization of spatially distributed responses. Here we demonstrate, with simultaneous multi-electrode recordings from the retina and the LGN, that stationary and moving light stimuli evoke in retinal ganglion cells oscillatory responses in the frequency range of 61 to 114 Hz that become synchronized over distances larger than 20 degrees of visual angle across the nasal and temporal halves of the retina. This temporal patterning of retinal responses is transmitted reliably by LGN neurons, such that stimuli crossing the vertical meridian evoke synchronous responses in the LGNs of both hemispheres. The oscillatory responses are not phase-locked to the stimulus onset, indicating that synchronization results from horizontal interactions in the retina. The occurrence of synchronization depends on global stimulus properties such as size and continuity, suggesting that temporal correlation among responses of spatially segregated ganglion cells can be exploited to convey information relevant for perceptual grouping.

Animals↗
PubMed1996

Lack of association between thyroid and pineal responses to antidepressant treatment.

With antidepressant treatment for major depression there are decreases in thyroid hormone levels and increases in pineal function. We have conducted a study to examine whether there is a relationship between pineal and thyroid hormone measures as well as between hormone measures and response to treatment in patients treated with desipramine. Measures of thyroid activity included thyroxine, triiodothyronine, T3 resin uptake, and TSH. Pineal function was determined by measurement of 6-suphatoxymelatonin in three consecutive 8 hour pools. Hormone measures as well as Hamilton depression scores were obtained prior to treatment and at the end of 5 weeks of treatment. As in previous studies, thyroid measures decreased, pineal activity increased, and Hamilton scores decreased significantly with treatment. No correlations were found between these measures, suggesting that if there is a relationship between them it is not a direct one.

Adult↗
PubMed1996

The influence of temporal phase differences on texture segmentation.

Scene segmentation and perceptual grouping are important operations in visual processing. Pattern elements constituting individual perceptual objects need to be segregated from those of other objects and the background and have to be bound together for further joint evaluation. Both textural (spatial) and temporal cues are exploited for this grouping operation. Thus, pattern elements might get bound that share certain textural features and/or appear in close spatial or temporal contiguity. However, results on the involvement of temporal cues in perceptual grouping are contradictory [Kiper et al. (1991). Society for Neuroscience Abstracts, 17, 1209; Fahle (1993). Proceedings of the Royal Society of London B, 254, 199-203]. We therefore reinvestigated the relative contributions of temporal and spatial cues and their interactions in a texture-segmentation paradigm. Our data show that the visual system can segregate figures solely on the basis of temporal cues if the temporal offset between figure and ground elements exceeds 10 msec. Moreover, segregation of figures defined by orientation differences among pattern elements is facilitated by additional temporal cues if these define the same figure. If temporal and textural cues define different figures, the two cues compete and only the more salient pattern is perceived. By contrast, the detection of a figure defined by orientation is not impaired by conflicting temporal cues if these do not define a figure themselves and do not exceed offset intervals of 100 msec. These results indicate the existence of a flexible binding mechanism that exploits both temporal and textural cues either alone or in combination if they serve perceptual grouping but can exclude either of the two cues if they are in conflict or do not define a figure. It is proposed that this flexibility is achieved by the implementation of two segmentation mechanisms which differ in their sensitivity for spatial and temporal cues and interact in a facultative way.

Adult↗
PubMed1996

Involvement of silent synapses in the induction of long-term potentiation and long-term depression in neocortical and hippocampal neurons.

Changes in the latency of small excitatory postsynaptic potentials were observed in association with induction of long-term modifications of synaptic transmission in slices of rat neocortex and guinea-pig hippocampus. After potentiation response latency decreased in 3/10 cases in the neocortex and in 6/24 cases in the hippocampus, and increased after depression in 4/8 cases in the neocortex. These latency changes could not be attributed to changes in presynaptic fibre excitability, monosynaptic inhibition, release kinetics or activation kinetics of postsynaptic ion channels. We conclude therefore that potentiation led to the activation of previously silent synapses of fast-conducting afferents and depression to the inactivation of previously functional synapses. Thus, neocortical and hippocampal synapses can be in a non-functional state, and regimes that induce long-term potentiation and depression not only change the efficacy of synapses but also alter their functional state.

Animals↗