Optic ataxia as a model to investigate the role of the posterior parietal cortex in visually guided action: evidence from studies of patient M.H.

Abstract

Optic ataxia is a neuropsychological disorder that affects the ability to interact with objects presented in the visual modality following either unilateral or bilateral lesions of the posterior parietal cortex (PPC). Patients with optic ataxia fail to reach accurately for objects, particularly when they are presented in peripheral vision. The present review will focus on a series of experiments performed on patient M.H. Following a lesion restricted largely to the left PPC, he developed mis-reaching behavior when using his contralesional right arm for movements directed toward the contralesional (right) visual half-field. Given the clear-cut specificity of this patient's deficit, whereby reaching actions are essentially spared when executed toward his ipsilateral space or when using his left arm, M.H. provides a valuable “experiment of nature” for investigating the role of the PPC in performing different visually guided actions. In order to address this, we used kinematic measurement techniques to investigate M.H.'s reaching and grasping behavior in various tasks. Our experiments support the idea that optic ataxia is highly function-specific: it affects a specific sub-category of visually guided actions (reaching but not grasping), regardless of their specific end goal (both reaching toward an object and reaching to avoid an obstacle); and finally, is independent of the limb used to perform the action (whether the arm or the leg). Critically, these results are congruent with recent functional MRI experiments in neurologically intact subjects which suggest that the PPC is organized in a function-specific, rather than effector-specific, manner with different sub-portions of its mantle devoted to guiding actions according to their specific end-goal (reaching, grasping, or looking), rather than according to the effector used to perform them (leg, arm, hand, or eyes).

Optic Ataxia: From Balint’s Syndrome to the Parietal Reach Region

“…Balint’s syndrome can be caused by a variety of disease processes including vascular compromise, neurodegenerative diseases, infections, traumatic brain injury, inflammatory diseases and iatrogenic causes. Vascular infarcts involving the posterior parietal cortex are often caused by watershed infarcts at the location of the terminal branches of the middle cerebral artery and posterior cerebral artery, a location which is particularly vulnerable in cases of hypotension and cardiac arrest (Montero et al, 1982; Rafal, 2001; Caplan and van Gijn, 2012; Cavina-Pratesi et al, 2013). In addition, occlusions of the posterior circulation, such as those caused by atherosclerotic vertebrobasilar disease, can also lead to bilateral hypoperfusion of the posterior parietal cortex at the watershed area between the middle cerebral artery and posterior cerebral artery (Montero et al, 1982).…”

“…Finally, if there is a general spatial deficit it would easily explain the often simultaneous occurrence of reach and grasp inaccuracies. However, patient studies show differential effects on proximal/grasp and distal/reach behaviors (Himmelbach et al, 2006; Cavina-Pratesi et al, 2013; see also Online Control ).…”

“…They hypothesized that PPC is organized with respect to function (look versus reach) rather than effector. Support for this idea comes from the study of an OA patient who showed a hand effect but also a foot effect when reaching with either the hand or foot into the contralesional field (Cavina-Pratesi et al, 2013) and functional imaging of observed motor acts which found clustering according to the type of motor act and but not effector in AIP (Jastorff et al 2010). …”

Different Evolutionary Origins for the Reach and the Grasp: An Explanation for Dual Visuomotor Channels in Primate Parietofrontal Cortex

“…Electrical stimulation of this pathway elicits bilateral movements of the forelimbs and hindlimbs that resemble spontaneous running or leaping in monkeys (28–30, 101). In humans, regions of dorsomedial posterior parietal cortex (PPC) mediate reaching to visible targets with the arms and hands, but also subserve pointing and stepping movements to visible targets with the foot (102–106). Although the stepping and Reach pathways overlap in dorsomedial PPC, they appear to diverge in frontal cortex.…”

The enigma of Bálint's syndrome: neural substrates and cognitive deficits

“…Cavina-Pratesi et al (2013) here present a thorough examination of deficits in visually guided action in a patient with unilateral extensive lesion within left posterior parietal cortex (PPC). Based on their single case analysis these authors draw a model of the functional organization of the PPC (subdivided based on specific end-goals such as reaching, grasping, or looking).…”