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Claudio Galletti

Publications and source records attributed to Claudio Galletti.

11 recordsLinked to original sources
PubMed2006

Wide-field retinotopy defines human cortical visual area v6.

The retinotopic organization of a newly identified visual area near the midline in the dorsalmost part of the human parieto-occipital sulcus was mapped using high-field functional magnetic resonance imaging, cortical surface-based analysis, and wide-field retinotopic stimulation. This area was found in all 34 subjects that were mapped. It represents the contralateral visual hemifield in both hemispheres of all subjects, with upper fields located anterior and medial to areas V2/V3, and lower fields medial and slightly anterior to areas V3/V3A. It contains a representation of the center of gaze distinct from V3A, a large representation of the visual periphery, and a mirror-image representation of the visual field. Based on similarity in position, visuotopic organization, and relationship with the neighboring extrastriate visual areas, we suggest it might be the human homolog of macaque area V6, and perhaps of area M (medial) or DM (dorsomedial) of New World primates.

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PubMed2006

Somatosensory cells in area PEc of macaque posterior parietal cortex.

PEc is an area defined on cytoarchitectural grounds, located in the posterior part of the superior parietal lobule of macaque brain (Pandya and Seltzer, 1982). The aim of this work was to assess whether passive somatosensory stimulation elicited responses in PEc neurons. Extracellular recordings were performed in three awake Macaca fascicularis. Passive somatosensory stimulation was performed in darkness, and eye movements were monitored continuously. Recording sites were assigned to different areas according to the cytoarchitectonic criteria described by Pandya and Seltzer (1982) and Luppino et al. (2005). Only recording sites within the limits of the cytoarchitecturally defined area PEc were taken into account in this work. Of 147 PEc cells, 83 (56%) were modulated by passive somatosensory stimulation. The majority of them (73%) responded to joint rotations, and 24% responded to tactile skin stimulation. The majority of PEc somatosensory responses (90%) were evoked by contralateral stimulation. Joint-modulated cells were mostly activated by the upper limbs (82%). The majority of tactile receptive fields (61%) were located on the arms, and a minority was located on the legs and trunk. One-half of PEc somatosensory cells were polysensory, because they were sensitive to visual stimulation. The majority of PEc somatosensory cells were activated by active reaching movements. Somatosensory cells, somatosensory submodalities, and body part representations were not clustered in PEc subregions; in other words, PEc does not show a somatotopic organization. Although the caudal sector of the superior parietal lobule has been traditionally considered as a somatosensory area, this is the first demonstration of the presence of somatosensory cells in this cortical region.

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PubMed2005

The relationship between V6 and PO in macaque extrastriate cortex.

The cerebral cortex of three macaque monkeys, electrophysiologically studied in chronic preparations in order to recognize functionally the medial parieto-occipital area V6, was reconstructed using the software CARET. Locations of cells recorded from area V6 (n = 553) and from neighbouring cortical areas V2/V3 and V6A (n = 1341) were displayed on surface-based reconstructions of individual brains, and on a surface-based atlas of the macaque cerebral cortex. Results show that area V6 occupies the ventral part and fundus of the parieto-occipital sulcus, as well as the ventral part of the precuneate cortex. V6 borders areas V2/V3 posteriorly and laterally, and area V6A anteriorly. The visualization of individual cases on a common template (atlas), and the use of atlas datasets, allowed us to compare data coming from different individuals and different laboratories. In particular, a comparison of the location and extent of the medial parieto-occipital areas V6 and PO indicates that area PO occupies different locations according to different authors but in general includes parts of both areas V6 and V6A. We therefore suggest that the term V6 is a more appropriate designation of the visuotopically-organized area located on the anterior wall of the parieto-occipital sulcus.

Action Potentials↗
PubMed2005

Occipital (V6) and parietal (V6A) areas in the anterior wall of the parieto-occipital sulcus of the macaque: a cytoarchitectonic study.

The anterior wall of the parieto-occipital sulcus (POs) of the macaque monkey, classically considered as part of Brodmann's area 19, contains two functionally distinct areas: a ventral, purely visual area, V6, and a dorsal area, V6A, containing visual neurons and neurons related to the control of arm movements. The aim of this study was to establish whether areas V6 and V6A, so far identified only on a functional basis, have a cytoarchitectonic counterpart. The cytoarchitectonic analysis of 13 hemispheres from ten macaque brains, cut along different planes of section, showed that the anterior wall of the POs contains three distinct areas. One is located in the ventralmost part of the wall, another in the dorsalmost part of the wall, and the third occupies an intermediate position. The ventralmost region displays architectonic features typical of the occipital cytoarchitectonic domain, whereas the two dorsal areas display architectonic features typical of the posterior parietal cortex. Analysis of myeloarchitecture and of the distribution of SMI-32 immunoreactivity confirmed the cytoarchitectonic parcellation. Correlation of cytoarchitectonic maps with functional and hodological data strongly suggests that the ventral region corresponds to area V6, whereas the other two regions correspond to different subsectors of V6A, here named V6Av and V6Ad, respectively. The present data are in line with electrophysiological and hodological data, which suggest that area V6 is a classic extrastriate area, whereas V6A is an area of the posterior parietal cortex. They also suggest that V6A includes two separate cortical subdivisions, a view supported by preliminary functional and hodological data that needs further confirmation.

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PubMed2005

Spatial tuning of reaching activity in the medial parieto-occipital cortex (area V6A) of macaque monkey.

We recorded neural activity from the medial parieto-occipital area V6A while three monkeys performed an instructed-delay reaching task in the dark. Targets to be reached were in different spatial positions. Neural discharges were recorded during reaching movements directed outward from the body (towards visual objects), during the holding phase (when the hand was on the target) and during inward movements of the hand towards the home button (which was near the body and outside the field of view). Reach-related activity was observed in the majority of 207 V6A cells, during outward (78%) and inward (65%) movements as well as during the holding phase (62%). Most V6A reaching neurons (84%) were modulated in more than one phase of the task. The reach-related activity in V6A could depend on somatosensory inputs and/or on corollary discharges from the dorsal premotor cortex. Although visual and oculomotor inputs are known to have a strong influence on V6A activity, we excluded the possibility that the reach-related activity which we observed was due to visual stimulation and/or oculomotor activity. Reach-related activity for movements towards different locations was spatially modulated during outward (40%) and inward (47%) reaching movements. The position of the hand/arm in space modulated about 40% of V6A cells. Preferred reach directions and spatial locations were represented uniformly across the workspace. These data suggest that V6A reach-related neurons are able to code the direction of movement of the arm and the position of the hand/arm in space. We suggest that the V6A reach-related neurons are involved in the guidance of goal-directed arm movements, whether these actions are visually guided or not.

Action Potentials↗
PubMed2004

Evidence for both reaching and grasping activity in the medial parieto-occipital cortex of the macaque.

In humans, the caudal pole of the superior parietal lobule is involved in the control of both reaching and grasping movements, whereas in monkey it is reported to be involved only in the control of reaching. Using single-unit recordings from trained macaque monkeys, we investigated whether area V6A, a visuomotor area located in the caudal part of the posterior parietal cortex, is involved in both components of prehension, the hand transport towards the visual target and the grip formation to secure the grasp. In Experiment 1, neural activity was recorded in V6A while two monkeys performed two instructed-delay reaching tasks (reach-to-point and reach-to-grasp) under controlled conditions in darkness. Fourty-five of 93 tested neurons (48%) were modulated during reach-to-point and 62% (52/84) during reach-to-grasp. In 63% of cells (51/81) neural activity was significantly different between reach-to-point and reach-to-grasp tasks, suggesting that grip formation could influence neural activity. In Experiment 2, two monkeys performed natural reach-to-grasp movements in fully lit environment; V6A neural activity and arm-hand movements were recorded by a digital camcorder and analysed frame-by-frame using a digital video technique. Thirty of the 58 tested neurons (52%) were modulated during natural prehension; about 30% of these neurons (8/30) were modulated only during the last phase of prehension, i.e. during finger flexion around the object to be grasped. This is the first direct demonstration that both reaching and grasping modulate neural activity in the caudal part of the posterior parietal cortex of the macaque. Our work suggests a strict functional homology between human and monkey superior parietal lobule.

Action Potentials↗
PubMed2003

Role of the medial parieto-occipital cortex in the control of reaching and grasping movements.

The medial parieto-occipital cortex is a central node in the dorsomedial visual stream. Recent physiological studies in the macaque monkey have demonstrated that the medial parieto-occipital cortex contains two areas, the visual area V6 and the visuomotor area V6A. Area V6 is a retinotopically organized visual area that receives form and motion information directly from V1 and is heavily connected with the other areas of the dorsal visual stream, including V6A. Area V6A is a bimodal visual/somatosensory area that elaborates visual information such as form, motion and space suitable for the control of both reaching and grasping movements. Somatosensory and skeletomotor activities in V6A affect the upper limbs and involve both the transport phase of reaching and grasping movements. Finally, V6A is strongly and reciprocally connected with the dorsal premotor cortex controlling arm movements. The picture emerging from these data is that the medial parieto-occipital cortex is well equipped to control both proximal and distal movements in the online visuomotor guidance of prehension. In agreement with this view, selective V6A lesions in monkey produce misreaching and misgrasping with the arm contralateral to the lesion in visually guided movements. These deficits are similar to those observed in optic ataxia patients and suggest that human and monkey superior parietal lobules are homologous structures, and that optic ataxia syndrome is the result of the lesion of a 'human' area V6A.

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PubMed2003

Neuronal mechanisms for detection of motion in the field of view.

The visual system cannot rely only upon information from the retina to perceive object motion because identical retinal stimulations can be evoked by the movement of objects in the field of view as well as by the movements of retinal images self-evoked by eye movements. We clearly distinguish the two situations, perceiving object motion in the first case and stationarity in the second. The present work deals with the neuronal mechanisms that are likely involved in the detection of real motion. In monkeys, cells that are able to distinguish real from self-induced motion (real-motion cells) are distributed in several cortical areas of the dorsal visual stream. We suggest that the activity of these cells is responsible for motion perception, and hypothesize that these cells are the elements of a cortical network representing an internal map of a stable visual world. Supporting this view are the facts that: (i) the same cortical regions in humans are activated in brain imaging studies during perception of object motion; and (ii) lesions of these same regions produce selective impairments in motion detection, so that patients interpret any retinal image motion as object motion, even when they result from her/his eye movements. Among the areas of the dorsal visual stream rich in real-motion cells, V3A and V6, likely involved in the fast form and motion analyses needed for visual guidance of action, could use real-motion signals to orient the animal's attention towards moving objects, and/or to help grasping them. Areas MT/V5, MST and 7a, known to be involved in the control of pursuit eye movements and in the analysis of visual signals evoked by slow ocular movements, could use real-motion signals to give a proper evaluation of motion during pursuits.

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PubMed2002

Somatosensory cells in the parieto-occipital area V6A of the macaque.

The aim of this study was to assess whether neurones of area V6A, a part of Brodmann's area 19, are modulated by passive somatosensory stimulations. Extracellular activity was recorded in four awake while passive tactile stimulations of the skin and passive rotations of the joints were performed in complete darkness and under eye movement control. Out of 240 V6A units, 78 (32%) were modulated by somatosensory stimulations. The majority of somatic receptive fields were located on both proximal and distal parts of the contralateral arm. V6A somatosensory cells may play a role in the feedback control of the actual state of the arm while reaching its target in peripersonal space.

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PubMed2002

Effects of lesions to area V6A in monkeys.

In order to assess the role played by area V6A in visuomotor control, two adult green monkeys ( Cercopithecus aethiops) were subjected to small, bilateral lesions in the anterior bank of the parieto-occipital sulcus. Before and after the lesions, monkeys were tested for naturally designed reaching, grasping and picking-up pieces of food from various positions on a plate and from a differently oriented narrow slit. All movements were recorded with closed circuit TV and analysed offline on a single-photogram basis for defective reaching and wrist orientation. V6A lesions provoked parietal weakness, reluctance to move, and specific deficits in reaching, wrist orientation and grasping. Recovery from the observed deficits was rapid, even after a second, contralateral lesion was given, creating a bilateral lesion. Thus, together with previous anatomical and electrophysiological data, these results directly support the hypothesis that area V6A is part of the network involved in the control of reaching movements and wrist orientation.

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