Littérature scientifique sur le sujet « Somatotopic map »

The human neocerebellum has been hypothesized to contribute to many high-level cognitive processes including attention, language, and working memory. Support for these nonmotor hypotheses comes from evidence demonstrating structural and functional connectivity between the lateral cerebellum and cortical association areas as well as a lack of somatotopy in lobules VI and VII, a hallmark of motor representations in other areas of the cerebellum and cerebral cortex. We set out to test whether somatotopy exists in these lobules by using functional magnetic resonance imaging to measure cerebellar activity while participants produced simple or complex movements, using either fingers or toes. We observed a previously undiscovered somatotopic organization in neocerebellar lobules VI and VIIA that was most prominent when participants executed complex movements. In contrast, activation in the anterior lobe showed a similar somatotopic organization for both simple and complex movements. While the anterior somatotopic representation responded selectively during ipsilateral movements, the new cerebellar map responded during both ipsi- and contralateral movements. The presence of a bilateral, task-dependent somatotopic map in the neocerebellum emphasizes an important role for this region in the control of skilled actions.

The inverse magnification rule in cortical somatotopy is the experimentally derived inverse relationship between cortical magnification (area of somatotopic map representing a unit area of skin surface) and receptive field size (area of restricted skin surface driving a cortical neuron). We show by computer simulation of a simple, multilayer model that Hebb-type synaptic modification subject to competitive constraints is sufficient to account for the inverse magnification rule.

A traditional view of the human motor cortex is that it contains an overlapping sequence of body part representations from the tongue in a ventral location to the foot in a dorsal location. In this study, high-resolution functional MRI (1.5 × 1.5 × 2 mm) was used to examine the somatotopic map in the lateral motor cortex of humans, to determine whether it followed the traditional somatotopic order or whether it contained any violations of that somatotopic order. The arm and hand representation had a complex organization in which the arm was relatively emphasized in two areas: one dorsal and the other ventral to a region that emphasized the fingers. This violation of a traditional somatotopic order suggests that the motor cortex is not merely a map of the body but is topographically shaped by other influences, perhaps including correlations in the use of body parts in the motor repertoire.

Computational models of the somatosensory and auditory systems have been constructed with the neurosimulator GENESIS. The somatosensory model consists of a cortical layer with 1024 pyramidal cells and 512 basket cells connected to a hand surface with 512 tactile receptors. The auditory model consists of a cortical layer with 2256 pyramidal cells and 1128 basket cells connected to a cochlea with 47 receptors. The models reproduce processes related to the formation and maintenance of somatotopic and tonotopic maps and exhibit several features observed in experiments with animals such as variability in the shapes and sizes of areas of cortical representation and, in the case of somatotopy, cortical magnification values in agreement with experimental findings and linear decay of receptive field overlap as a function of cortical distance between recording Bites in normal conditions.

The ability to form an accurate map of sensory input to the brain is an essential aspect of interpreting functional brain signals. Here, we consider the somatotopic map of vibrissa-based touch in the primary somatosensory (vS1) cortex of mice. The vibrissae are represented by a Manhattan-like grid of columnar structures that are separated by inter-digitating septa. The development, dynamics and plasticity of this organization is widely used as a model system. Yet, the exact anatomical position of this organization within the vS1 cortex varies between individual mice. Targeting of a particular column in vivo therefore requires prior mapping of the activated cortical region, for instance by imaging the evoked intrinsic optical signal (eIOS) during vibrissa stimulation. Here, we describe a procedure for constructing a complete somatotopic map of the vibrissa representation in the vS1 cortex using eIOS. This enables precise targeting of individual cortical columns . We found, using C57BL/6 mice, that although the precise location of the columnar field varies between animals, the relative spatial arrangement of the columns is highly preserved. This finding enables us to construct a canonical somatotopic map of the vibrissae in the vS1 cortex. In particular, the position of any column, in absolute anatomical coordinates, can be established with near certainty when the functional representations in the vS1 cortex for as few as two vibrissae have been mapped with eIOS. This article is part of the themed issue ‘Interpreting BOLD: a dialogue between cognitive and cellular neuroscience’.

Neurosurgery is predicated on the knowledge of the structure-function relationship of the brain. When the topic is broached in its historiography, it begins with Fritch and Hitzig's report on the localization of motor function in the cortex of the dog and skips rapidly to Wilder Penfield's homunculus. In that gap are found the origins of modern neurosurgery in 3 papers published by Jean-Martin Charcot and Albert Pitres between 1877 and 1879 in which they describe the somatotopic organization of the human motor cortex and draw the first human brain map. Their findings, obtained through the clinicopathological method, gave relevance to David Ferrier's observations in animals. Their work was extensively cited, and their illustrations reproduced by Ferrier in his landmark lecture to the Royal College of Physicians in 1878. It was known to William Macewen, who used localization to guide him in resecting intracranial mass lesions, and to William Osler and John Hughlings Jackson, who were early advocates of intracranial surgery. This paper describes Charcot and Pitres' discovery of the cortical origin of human voluntary movement and its somatotopic organization, and their influence on 19th-century intracranial surgery. It fills a gap in the historiography of cerebral localization and neurosurgery.

The somatotopic organization of spinocervical tract cells and unidentified dorsal horn neurons that lie in the same depth range as the spinocervical tract cells has been examined in detail in the lumbosacral enlargement of cats anesthetized with alpha-chloralose. Only gentle hair movement or light touch of glabrous skin were used as stimuli. Within the region of the dorsal horn containing these neurons there is a precise somatotopic organization. However, there is considerable variation between animals in the relationship between the somatotopic map and the lumbosacral segmental boundaries. The somatotopic map described here is considered to be restricted to a 300- to 500-micron thick lamina. In the medial half to two-thirds of this lamina in the L6 and L7 segments the toes are represented in a rostrocaudal sequence from toe 2 to toe 5. Over the most medial 200-500 micron of this part of the dorsal horn are found cells that respond to stimulation of the glabrous skin of the toe pads and the central pad. The toe representation is surrounded by a strip of foot representation, which is in turn surrounded by a strip of leg representation. The most lateral part of the lamina curves ventrally in the L6 and L7 segments and contains a continuous rostrocaudal representation of the skin of the thigh. In this region are found both spinocervical tract cells and unidentified dorsal horn neurons with receptive fields on the thigh. The skin of the hindlimb is represented such that a line of discontinuity runs down the posteromedial thigh, leg, and foot. Skin lateral to this line is represented caudally, while skin medial to it is represented rostrally.

Les neurones sensorials projecten al sistema nerviós central seguint una distribució espacial ordenada, formant mapes neuronals que representen propietats dels estímuls sensorials i que són considerats essencials per a la interpretació del món extern. He utilitzat la línia lateral de la larva del peix zebra com a model per a l’estudi de l’organització i el desenvolupament dels mapes neuronals sensorials. Les neurones sensorials de la línia lateral formen un mapa neuronal topogràfic, anomenat somatotopia, que representa la posició de l’estímul sensorial. He demostrat que l’ordre de neurogènesi defineix la somatotopia. A més, he identificat dues subclasses de neurones sensorials de la línia lateral que presenten diferències en els seus patrons de projecció central i en els contactes amb una neurona central: la cèl·lula de Mauthner. Proposo que aquest dimorfisme és important per a donar lloc a reaccions comportamentals adients al context sensorial. També he demostrat una contribució per part de l’ordre de neurogènesi a la formació del mapa neuronal dimòrfic de la línia lateral. Finalment, he obtingut resultats que mostren que la diversitat neuronal i la topologia del mapa observades ocorren amb normalitat en l’absència d’activitat sensorial. Page

Nella vita di tutti i giorni, capita numerose volte di toccare oggetti di diverso tipo con le mani. Il nostro cervello non è un ricevitore passivo di tali informazioni. Infatti, l'elaborazione degli input sensoriali coinvolge diverse rappresentazioni corporee che influiscono sul modo in cui percepiamo gli oggetti. Numerosi studi hanno dimostrato che l’elaborazione degli stimoli tattili avviene a diversi livelli. Un primo livello coinvolge le mappe somatotopiche, rappresentazioni stabili del corpo che non tengono conto dei cambiamenti posturali. Un secondo livello, coinvolge lo schema corporeo, una rappresentazione dinamica che si riadatta in base ai cambiamenti posturali. Questa tesi indaga il ruolo delle diverse rappresentazioni corporee nel modulare la percezione di stimoli innocui e dolorosi applicati sulle dita. Tali aspetti sono stati indagati attraverso due illusioni somatosensoriali: l’illusione di Aristotele e l’illusione della griglia termica. Nello specifico, il primo studio indaga i correlati percettivi delle alterazioni a carico della rappresentazioni somatotopiche delle dita e il modo in cui tali alterazioni possono influenzare la relazione funzionale tra più dita nella percezione tattile. Per tale scopo, abbiamo utilizzato l' illusione di Aristotele quale paradigma sperimentale e la distonia focale della mano come modello di rappresentazioni somatotopiche delle dita alterate. Tali alterazioni potrebbero riflettersi sulla relazione funzionale tra più dita nella percezione tattile. La distonia focale della mano, tuttavia, presenta disordini somatosensoriali comuni ad altre forme di distonia focale e alla malattia di Parkinson. Questo potrebbe essere dovuto ad una comune alterazione a carico dei gangli della base. Al fine di escludere ogni possibile ruolo dei gangli della base nel modulare l’illusione di Aristotele in pazienti con distonia focale della mano abbiamo condotto due esperimenti in cui i tali pazienti e quelli con altre forme di distonia focale e malattia di Parkinson sono stati confrontati rispetto a quanto percepissero o meno l’illusione di Aristotele. I risultati dei due esperimenti 1) confermano che la rappresentazione somatotopica delle dita è specificamente alterata nella distonia focale della mano, e 2) dimostrano che tale alterazione si associa ad un cambiamento nella relazione funzionale tra le dita nella percezione tattile specifica per la distonia focale della mano. Il secondo studio ha analizzato il ruolo dello schema corporeo nell’elaborazione di stimoli termo-tattili. L’obiettivo in questo caso era quello di studiare come l’organizzazione spaziale di diverse parti del corpo oggetto di stimolazione moduli l’illusione della griglia termica. Tale illusione è frequentemente utilizzata per studiare i fattori che influenzano la percezione del dolore. L’illusione della griglia termica deriva dall’applicare temperature calde e fredde innocue secondo uno specifico pattern caldo-freddo-caldo. Tale pattern di stimolazione determina che la temperatura fredda sia percepita come bollente. Nel nostro studio abbiamo utilizzato lo stesso pattern di stimolazione ma applicando ciascuna temperatura sull’indice, il medio e l’anulare. Incrociando o meno l’indice e il medio dei partecipanti, è emerso che la sensazione di caldo bruciante si riduceva quando il pattern spaziale delle temperature era freddo-caldo-caldo, indipendentemente che le dita fossero o meno incrociate. Tali risultati dimostrano come il cervello tenga conto della posizione relativa che ciascuno stimolo occupa rispetto agli altri nel definire la sensazione di calore doloroso evocato dall’illusione della griglia termica.
In everyday life, we use our hands to explore or manipulate objects. The processing of somatosensory inputs involves different representations of the body, and the resulting subjective experience of touch is strongly influenced by the type of body representation used for sensory processing. In the first instance tactile stimuli are processed within somatotopic maps that preserve the topographical organization of the physical body. Successively, the somatosensory inputs are further processed within the body schema which takes track of the postural changes of the body. In this framework, the current thesis is aimed at investigating the specific role of somatotopic maps and body schema in modulating the multi-digit tactile and thermo-tactile perception. This purpose has been achieved by “fooling” the sense of touch through two somatosensory illusions: Aristotle’s illusion and the Thermal Grill Illusion. In the first study we investigated the tactile processing occurring within the somatotopic maps. We approached to this issue by studying the multi-digit tactile perception as influenced by alterations in somatosensory maps. To address this specific aim, focal hand dystonia was chosen as a model of altered somatotopic maps. We used the Aristotle’s illusion to specifically investigate the functional meaning of the interplay between different fingers in tactile perception. Since, behavioural and neurophysiological studies indicate that the mechanisms underlying this illusion involve primary somatosensory cortex, and since a number of FHD neuroimaging studies have widely demonstrated anatomical alterations in the primary somatosensory cortex of FHD patients, we would expect that the Aristotle’s illusion is compromised in focal hand dystonia. This result would demonstrate a case of interdigit functional somatosensory alterations as specific counterpart of the widely reported anatomical alteration of somatosensory maps. However, FHD shares common somatosensory alterations features with other forms of focal dystonia and with PD. This is probably due to a common underlying factor: the abnormal basal ganglia activity. In order to exclude any possible role of basal ganglia in potentially modulation of Aristotle’s illusion in FHD we performed two experiments in which we compared the Aristotle’s illusion in FHD with other forms of focal dystonia and PD. Altogether, the two experiments: 1) confirm that fingers representation is specifically altered in FHD, and 2) demonstrate that abnormal alterations in fingers representation determine distorted tactile perception of objects simultaneously touched by different digits. The second study investigated the role the of body schema in processing thermotactile stimuli. More precisely, we studied how multiple thermotactile innocuous stimuli are combined in an unusual paradoxical painful sensation, termed Thermal Grill Illusion (TGI). In the classical TGI, innocuous warm and cold stimuli are arranged in a warm-cold-warm fashion. Intriguingly, this type of stimulation evokes a feeling of burning heat. In the second study, we applied the warm-cold-warm pattern on the first three digits (index, middle, and ring) and we observed whether the TGI was modulated by changing the relative spatial position of the fingers, that is by crossing or uncrossing the middle over the index. Using this simple method we found that the perceptual experience of paradoxical pain might arise from the integration of multiple thermal inputs based on the relative position of the stimulated fingers according to external frame of reference. Indeed, in the crossed position, the paradoxical heat sensation was reduced when the cooled finger was the middle, and increased when the cooled finger was the index. This interesting result indicates that the brain takes into account the relative spatial position of each stimulus to produce the paradoxical heat sensation termed TGI.

This chapter addresses the various cranial neuropathies and brainstem syndromes and their respective anatomical components. Included among these disorders are pupillary reflex pathologies, oral-palatal deviations, gag reflex, facial palsy, Bell’s palsy, internuclear ophthalmoplegia, midbrain syndromes, pontine syndromes, and medullary syndromes. Instructions are presented on how to draw the elements of the neuropathies and syndromes, as well as the trigeminal nerve, central pathways, central somatotopic maps, and smooth pursuit eye movements. Finally, case histories of specific disorders are presented along with discussion of the elements involved in making the diagnosis.

The processing of sensory information, coordination of movement, and other higher brain functions are carried out by millions of neurons that form elaborate networks. Anatomical and physiological investigations of the mammalian brain have demonstrated its extraordinary complexity. How these neurons and their intricate connections endowed the brain with its remarkable performance is an important question which can greatly benefit from the development of new technologies. Recent progress in the development and application of two optical imaging techniques to the investigation of the intact mammalian brain is described. In the first methods fluorescent voltage-sensitive dyes are used to image the flow of information from one cortical site to the next in real time. This imaging method provided information about the retinoptic organization of the cortex and its functional organization into various modules. It revealed extensive long-range interactions between these cortical modules, much larger than those predicted from retinooptic or somatotopic maps, indicating a large degree of parallel processing. The combination of optical imaging with single unit recordings permitted the visualization of coherent activity in neuronal assemblies even in cases where they are spatially mixed (real time optical imaging is illustrated with a movie). A second imaging method, which does not require dyes, is based on reflection measurements of activity dependent intrinsic signals resulting from changes in optical properties of active brain tissue. This method permitted the high resolution visualization of many elements of the functional architecture of cortex in the living brain of cats and monkeys. These two complementary optical imaging techniques are particularly attractive for providing new insights to the development, organization, and function of the mammalian brain. The second technique is also likely to have clinical application in certain neurosurgical procedures on human patients.