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Parieto-Occipital Sulcus (POS)
Full text linkAmong the areas of the monkey parietal cortex,
those lying inside the parieto-occipital sulcus
(POs), especially in its anterior bank and fundus,
have been of particular interest for neuroscientists
in the last decades. In fact, while the areas in the
posterior bank show simple low-level visual
responses, areas of the anterior bank and on the
fundus have complex high-level visual and
visuomotor functions. In this chapter, we will
describe the main properties of neurons of the
areas located in the POs
Neural activity in the medial parietal area V6A while grasping with or without visual feedback
Full text linkRecent works have reported that grasping movements are controlled not only by the dorsolateral visual stream, as generally thought, but also by the dorsomedial visual stream, and in particular by the medial posterior parietal area V6A. To date, the grasping activity of V6A neurons has been studied only in darkness. Here we studied the effect of visual feedback on grasp-related discharges of V6A neurons while the monkey was preparing and executing the grasping of a handle. We found that V6A grasping activity could be excited or inhibited by visual information. The neural population was divided into Visual, Motor, and Visuomotor cells. The majority of Visual and Visuomotor neurons did not respond to passive observation of the handle, suggesting that vision of action, rather than object vision, is the most effective factor. The present findings highlight the role of the dorsomedial visual stream in integrating visual and motor signals to monitor and correct grasping
Temporal stability of reference frames in monkey area V6A during a reaching task in 3D space
No full textNeurons in the posterior parietal cortex of macaques show spatial tuning during several phases of an instructed delay reaching task, but their reference frames have been studied mostly during fixed periods without addressing how they evolve across task phases. In parietal area V6A, we reported recently that during the late delay and hand movement periods, most neurons represent target location either in body-centered frame of reference, or in mixed body/hand-centered coordinates, with no evidence of hand-centered representations. Here, we characterized the spatial representations of V6A neurons in earlier task epochs, i.e., immediately after target fixation and in the subsequent main part of the delay and examined whether the reference frames of individual neurons are stable across the task. We report no evidence of hand-centered coding also in the earlier phases of the task. Shortly, after target fixation and throughout the main part of the delay period, V6A neurons used either body-centered or mixed body/hand-centered reference frames. Most of the cells showed consistent reference frames across epochs. Interestingly, a population trend of shifting from mixed body/hand-centered frames to ‘pure’ body-centered coordinates was found as the task progressed. These findings suggest that, similar to other parietal areas, in V6A, the reference frames show a limited degree of temporal evolution. The stronger presence of mixed coding at the early task stages could reflect the early involvement of V6A in eye-hand coordination, whereas the increase in spatiotopic representations towards movement execution could be related to its role in online movement control
Vision for Prehension in the Medial Parietal Cortex
No full textIn the last 2 decades, the medial posterior parietal area V6A has been extensively studied in awake macaque monkeys for visual and somatosensory properties and for its involvement in encoding of spatial parameters for reaching, including arm movement direction and amplitude. This area also contains populations of neurons sensitive to grasping movements, such as wrist orientation and grip formation. Recent work has shown that V6A neurons also encode the shape of graspable objects and their affordance. In other words, V6A seems to encode object visual properties specifically for the purpose of action, in a dynamic sequence of visuomotor transformations that evolve in the course of reach-to-grasp action.We propose a model of cortical circuitry controlling reach-to-grasp actions, in which V6A acts as a comparator that monitors differences between current and desired hand positions and configurations. This error signal could be used to continuously update the motor output, and to correct reach direction, hand orientation, and/or grip aperture as required during the act of prehension.In contrast to the generally accepted view that the dorsomedial component of the dorsal visual stream encodes reaching, but not grasping, the functional properties of V6A neurons strongly suggest the view that this area is involved in encoding all phases of prehension, including grasping
Evidence for both reaching and grasping activity in the medial parieto-occipital cortex of the macaque.
No full textPreparatory activity for purposeful arm movements in the dorsomedial parietal area V6A: Beyond the online guidance of movement
Full text linkOver the years, electrophysiological recordings in macaque monkeys performing visuomotor tasks brought about accumulating evidence for the expression of neuronal properties (e.g., selectivity in the visuospatial and somatosensory domains, encoding of visual affordances and motor cues) in the posterior parietal area V6A that characterize it as an ideal neural substrate for online control of prehension. Interestingly, neuroimaging studies suggested a role of putative human V6A also in action preparation; moreover, pre-movement population activity in monkey V6A has been recently shown to convey grip-related information for upcoming grasping. Here we directly test whether macaque V6A neurons encode preparatory signals that effectively differentiate between dissimilar actions before movement. We recorded the activity of single V6A neurons during execution of two visuomotor tasks requiring either reach-to-press or reach-to-grasp movements in different background conditions, and described the nature and temporal dynamics of V6A activity preceding movement execution. We found striking consistency in neural discharges measured during pre-movement and movement epochs, suggesting that the former is a preparatory activity exquisitely linked to the subsequent execution of particular motor actions. These findings strongly support a role of V6A beyond the online guidance of movement, with preparatory activity implementing suitable motor programs that subsequently support action execution
Prediction of Reach Goals in Depth and Direction from the Parietal Cortex
Full text linkSummary: The posterior parietal cortex is well known to mediate sensorimotor transformations during the generation of movement plans, but its ability to control prosthetic limbs in 3D environments has not yet been fully demonstrated. With this aim, we trained monkeys to perform reaches to targets located at various depths and directions and tested whether the reach goal position can be extracted from parietal signals. The reach goal location was reliably decoded with accuracy close to optimal (>90%), and this occurred also well before movement onset. These results, together with recent work showing a reliable decoding of hand grip in the same area, suggest that this is a suitable site to decode the entire prehension action, to be considered in the development of brain-computer interfaces. : Filippini et al. show that it is possible to use parietal cortex activity to predict in which direction the arm will move and how far it will reach. This opens up the possibility of neural prostheses that can accurately guide reach and grasp using signals from this part of the brain. Keywords: neuroprosthetics, offline neural decoding, reaching in depth, monkey, V6A, machine learning, visuomotor transformations, hand guidance, prehension, robotic
Functional specialization of the human posterior parietal cortex in visually and proprioceptively driven reaching corrections
Full text linkWhile online adjustments during reaching are essential for interacting with our dynamic environment,the specialized contributions of subregions of the posterior parietal cortex (PPC) remain unclear. In thisstudy, we investigate the distinct roles of human medial PPC areas V6A (hV6A) and PEc (hPEc) inmediating online reaching corrections elicited by visual and proprioceptive perturbations. Here wedeliver online repetitive transcranial magnetic stimulation (rTMS) during the early stages of reachingcorrections triggered by an unexpected shift of the visual target or by the application of an externalforce to the wrist. Our findings reveal that rTMS over hV6A selectively impairs the ability to correctreach trajectories for visual perturbations, whereas stimulation of hPEc interferes only withproprioceptively driven corrections. These findings confirm the critical role of hV6A in processingvisual feedback, demonstrate the causal involvement of hPEc in integrating proprioceptiveinformation to guide motor adjustments, and show how the PPC selectively engages specializedneural circuits to adapt motor control strategies according to the sensory nature of thereaching perturbation
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