Search PubMed⌕ Search

Biomedical subjects

M Imbert

Publications and source records attributed to M Imbert.

At least 145 records · Page 8Linked to original sources
PubMedDate not supplied

Dynamics of orientation coding in area V1 of the awake primate.

To investigate the importance of feedback loops in visual information processing, we have analyzed the dynamic aspects of neuronal responses to oriented gratings in cortical area V1 of the awake primate. If recurrent feedback is important in generating orientation selectivity, the initial part of the neuronal response should be relatively poorly selective, and full orientation selectivity should only appear after a delay. Thus, by examining the dynamics of the neuronal responses it should be possible to assess the importance of feedback processes in the development of orientation selectivity. The results were base on a sample of 259 cells recorded in two monkeys, of which 89% were visually responsive. Of these, approximately two-thirds were orientation selective. Response latency varied considerably between neurons, ranging from a minimum of 41 ms to over 150 ms, although most had latencies of 50-70 ms. Orientation tuning (defined as the bandwidth at half-height) ranged from 16 deg to over 90 deg, with a mean value of around 55 deg. By examining the selectivity of these different neurons by 10-ms time slices, starting at the onset of the neuronal response, we found that the orientation selectivity of virtually every neuron was fully developed at the very start of the neuronal response. Indeed, many neurons showed a marked tendency to respond at somewhat longer latencies to stimuli that were nonoptimally oriented, with the result that orientation selectivity was highest at the very start of the neuronal response. Furthermore, there was no evidence that the neurons with the shortest onset latencies were less selective. Such evidence is inconsistent with the hypothesis that recurrent intracortical feedback plays an important role in the generation of orientation selectivity. Instead, we suggest that orientation selectivity is primarily generated using feedforward mechanisms, including feedforward inhibition. Such a strategy has the advantage of allowing orientation to be computed rapidly, and avoids the initially poorly selective neuronal responses that characterize processing involving recurrent loops.

Animals↗
PubMedDate not supplied

Effect of monocular deprivation on NMDAR1 immunostaining in ocular dominance columns of the marmoset Callithrix jacchus.

We previously showed that immunoreactivity to N-Methyl-D-aspartate (NMDA) receptors in primary visual cortex of Callithrix jacchus is regulated by visual activity during the second and third postnatal months (Fonta et al., 1997). The purpose of the present study was to show that the columnar pattern of high and low NMDAR1 immunoreactivity observed in monocularly deprived animals corresponds to ocular dominance columns linked to the nondeprived and deprived eye, respectively. We compared cortical distribution of NMDAR1 receptors and the projection zones of thalamic afferents, revealed by transneuronal transport of tritiated proline, in 2-month-old, either monocularly deprived or control, marmosets. The data show that ocular dominance columns exist in 2-month-old marmosets and that a 2-week monocular deprivation by means of eyelid suture leads to a modification of the thalamo-cortical afferents organization. Experiments of neuronal tracing and immunohistochemistry performed on the same animals demonstrated that cortical domains with decreased NMDAR1 level correspond to the deprived eye columns. These investigations, coupled to the previous results, strongly suggest that the NMDA receptors, regulated by visual activity, are involved in the refining of ocular dominance columns in the primary visual cortex of juvenile marmoset.

Animals↗
PubMedDate not supplied

Ocular dominance columns in the adult New World Monkey Callithrix jacchus.

In the marmoset Callithrix jacchus, ocular dominance columns (ODC) have been reported to be present in young animals, but absent in adults (Spatz, 1989). We have studied in juvenile and adult animals the postnatal organization of the retino-geniculo-cortical afferents by means of transneuronal labeling. We show in the present work that ODC are present in the primary visual cortex of Callithrix jacchus, both in the adult and in the juvenile animal. The present work confirms the presence of ODC in the visual cortex of juvenile marmoset before the end of the first postnatal month. In 2-month-old animals, ODC are well demarcated in IVcalpha and IVcbeta. In the adult marmosets, the present data clearly show that the primary visual cortex is also organized with ODC. In horizontal sections, they form a mosaic through the ventral and dorsal calcarine cortex and through the dorso-lateral occipital part of the striate cortex. In frontal sections, their presence is manifest in IVcbeta within the calcarine cortex and they only faintly appear in IVcalpha. These new findings are important since they underline the usefulness of the adult New World Monkeys as a model in visual research.

Aging↗
PubMedDate not supplied

A mathematical approach to the connectivity between the cortical visual areas of the macaque monkey.

The visual cortex of the macaque monkey is divided into many distinct visual information processing areas. In many cases, anatomical and physiological results allow one to determine the presence or the absence of neuronal connections from one area to another. We have approached the topology of this neuronal network within the mathematical framework of graph theory. At first, we studied the unknown part of the network, i.e. the part where anatomical and physiological results are lacking. Relying on a specific topological property of the network established on the known part, we developed an interpolation algorithm for reducing the level of uncertainty concerning the unknown part. From these results, we then constructed a connectional model of the neuronal network for the entire cortical visual system. Subsequently, a topological analysis of this model, with the help of factorial analysis and clustering technics, shows its structural properties and singular vertices. This analysis suggests the existence of two distinct classes of areas, one in the parietal part of the cortex and the other in the temporal part, which are connected to each other via relay areas, especially involving the frontal eye field. These results may help to understand the functional role of particular cortical areas in vision and, more generally, to explore how visual information flows within the visual cortex.

Algorithms↗
PubMedDate not supplied

[Cardiopulmonary arrest].

Cardiopulmonary arrest is an acute emergency characterized by the absence of central pulse and ventilatory movements. Only the fast intervention of a resuscitation unit allows in some cases to preserve the vital prognostic. However its action can be efficient only if resuscitation gestures have been undertaken as soon as the cardiopulmonary arrest is established by the practitioner. These gestures are chest compressions and artificial ventilation. Their efficiency can be increased by the use of a simple and inexpensive oxygen therapy equipment.

Cardiopulmonary Resuscitation↗
PubMedDate not supplied

[The place of the dentist in the chain of care. The call].

After a short summary of the circumstances that can arise a call for help, this article describes the different organisms able to intervene in case of an emergency situation: medical emergency units, firemen, other organisms and the means to contact them. The modalities of this call and its minimal and indispensable content are then repeated. The modalities are on one side description of the emergency situation and on the other side the geographical informations. When the message received, the organism called can adapt the means that need to be send to the emergency situation and can manage to send them on location as soon as possible.

Dentists↗
PubMedDate not supplied

[Acquired idiopathic sideroblastic anemia (author's transl)].

The authors consider the clinical and biological data of Acquired Idiopathic Sideroblastic Anemia (AISA). The physiopathology of the syndrome is discussed; the relationships between pathologic sideroblastosis, dyserythropoiesis and ferrokinetic modifications are pointed out. The associated abnormalities of granulocytic and megacaryocytic series linked AISA to other myelodysplasia.

Anemia, Sideroblastic↗