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C Habas

Publications and source records attributed to C Habas.

7 recordsLinked to original sources
PubMed2004

The cerebellar second homunculus remains silent during passive bimanual movements.

In a previous study, we showed that the second homunculus in lobule VIII of the cerebellum is activated during bilateral out-of-phase index finger-thumb opposition, implying a role in motor coordination. However, several recent studies indicate that the cerebellum could be more actively involved in sensory information processing during movement. Therefore, as lobule VIII activation could involve either a motor or a proprioceptive component, these two components must be distinguished and their relative contribution must be determined. Using functional imaging, we studied cerebellar activation of the same region during passively induced index finger-thumb opposition of both hands in in-phase and out-of-phase modes, thereby excluding the voluntary movement component. No significant activation was detected in lobule VIII. Intense activation of lobule VIII, obtained during active, out-of-phase bimanual movements, therefore does not involve a significant sensory component related to direct proprioceptive feedback. This result is strongly in favour of the specific recruitment of lobule VIII during out-of-phase movements related more to complex motor timing than to sensory function.

Adult↗
PubMed2004

Specific neocerebellar activation during out-of-phase bimanual movements.

We used fMRI to study cerebellar activation during index finger-thumb opposition of the right hand and index finger-thumb opposition of both hands in in-phase and out-of-phase modes. The right hand movement activates the contralateral anterior lobe of the cerebellum. During bimanual in-phase movements, this activity pattern becomes bilateral. More interestingly, bilateral out-of-phase movements recruit the cerebellar posterior lobe VIII, which likely corresponds to the second homunculus. As out-of-phase movements differ from the in-phase movements only by their temporal complexity and their attentional awareness, this study demonstrates the preferential involvement of the cerebellar second homunculus in the control of complex movements.

Adult↗
PubMed2004

[Basic principles of diffusion tensor MR tractography].

Diffusion tensor MR tractography allows in vivo depiction of anatomical bundles composing the white matter of the brain and of spinal cord. Diffusion MRI uses the effects of heterogeneous water molecule movement to determine for each pixel the main axis and magnitude of local anisotropy. Tractography exploits these data to reconstruct the tridimensional geometry of the bundles providing neurologists with precise information about white matter tract architecture involvement by various pathologies. In this paper, the basic principles of molecular diffusion and the subsequent diffusion tensor that describes its geometrical and quantitative characteristics will be reviewed, in particular within bundles of white matter. Then main principles of diffusion tensor MR imaging and tractography will be presented.

Biophysical Phenomena↗
PubMed2003

[Coordination of rhythmic movements: a dynamic approach].

The dynamic theoretic approach regards motor coordination not as a supervised sequential planning but as a spontaneous self-organized process. Owing only to transitory local interactions between sensorimotor areas (without any pre-existing motor program) a coherent collective neural behavior should emerge and underlie voluntary rhythmic synergies (in our example, bimanual on-phase and out-of-phase movements). Therefore this collective behavior should govern and reflect functional constraints slaving independent movement of effectors in a global synergy. In order to understand such a process, it proves necessary to study movement from the effector level to the cerebral level, by experimentally ascertaining: (1) the set of all possible synergies for a given movement, (2) the order parameters and the control parameters, (3) the task-dependent laws for the movement of each effector, and (4) the general and slaving laws responsible for the synergy itself which must be qualitatively similar to those applying to cerebral collective behavior. This paper tries to introduce experimental results for bimanual rhythmic movements as well as the main theoretic tools issued from dynamics of dissipative systems located far from equilibrium (order parameter, multistability, bifurcation, fluctuation...) used to grasp the self-organized nature of motor synergies.

Humans↗
PubMed2002

[Physiological basis of functional MRI].

Afferent neurotransmission of a given neuronal group and its subsequent dendritic activity are followed by a transient hemodynamic response such as an increase of the local blood flow and of the intravascular level of oxygen. This neurovascular regulation involves neurons, astrocytes, interneurons and various transmitter substances. The resulting changes in the magnetic susceptibility is then detected by the MRI machine which can specifically localize regional brain activity. Blood oxygenation level dependent contrast reflects afferent inputs and their post-synaptic (dendritic) processing, rather than the output from the concerned cerebral zone (spiking). In other words, this contrast depends on local informational treatment of an active zone without implying any effective influence upon its targets.

Afferent Pathways↗
PubMed2001

[The cerebellum: from motor coordination to cognitive function].

Clinical data in man, as well as experimental results in animals, classically involve the cerebellum in the coordination of ballistic movements and in their accompanying postural adjustment. The cerebellum intervenes in the coding of the order and duration of contraction of the different protagonist muscular groups contributing to the same movement. In normal life, this is an automatic, non conscious procedure. Recent studies seem to indicate that the human neocerebellum (lateral hemispheres and dentate nuclei) plays a role in the regulation of some neocortical cognitive functions. This new functional aspect of cerebellar activity has been inferred from the results obtained by three quite different domains: neuroanatomical data showing the existence of, sometimes reciprocal, pathways between the neocerebellum and associative and limbic areas in primates, neuropsychological data assessing the presence, in some cerebellar patients, of purely cognitive impairments, and data from functional imagery pointing out cerebellar activation in healthy subjects during non motor tasks. II would ensue that, thanks to new cortical targets. The cerebellum could regulate sensorial, procedural, linguistic and emotional activities, so that a cerebellar lesion could be followed by a cognitive and affective syndrome, depending on the importance and on the location of the lesion.

Animals↗