Posterior parietal cortex encodes autonomously selected motor plans

The posterior parietal cortex (PPC) of rhesus monkeys has been found to encode the behavioral meaning of categories of sensory stimuli. When animals are instructed with sensory cues to make either eye or hand movements to a target, PPC cells also show specificity depending on which effector (eye or hand) is instructed for the movement. To determine whether this selectivity retrospectively reflects the behavioral meaning of the cue or prospectively encodes the movement plan, we trained monkeys to autonomously choose to acquire a target in the absence of direct instructions specifying which effector to use. Activity in PPC showed strong specificity for effector choice, with cells in the lateral intraparietal area selective for saccades and cells in the parietal reach region selective for reaches. Such differential activity associated with effector choice under identical stimulus conditions provides definitive evidence that the PPC is prospectively involved in action selection and movement preparation.     

Posterior parietal cortex neurons encode target motion in world-centered coordinates

The motion areas of posterior parietal cortex extract information on visual motion for perception as well as for the guidance of movement. It is usually assumed that neurons in posterior parietal cortex represent visual motion relative to the retina. Current models describing action guided by moving objects work successfully based on this assumption. However, here we show that the pursuit-related responses of a distinct group of neurons in area MST of monkeys are at odds with this view. Rather than signaling object image motion on the retina, they represent object motion in world-centered coordinates. This representation may simplify the coordination of object-directed action and ego motion-invariant visual perception.     

Human parietal cortex in action

Experiments using functional neuroimaging and transcranial magnetic stimulation in humans have revealed regions of the parietal lobes that are specialized for particular visuomotor actions, such as reaching, grasping and eye movements. In addition, the human parietal cortex is recruited by processing and perception of action-related information, even when no overt action occurs. Such information can include object shape and orientation, knowledge about how tools are employed and the understanding of actions made by other individuals. We review the known subregions of the human posterior parietal cortex and the principles behind their organization.     

Gaze control: role of the parietal cortex

Eye movements constitute one of the most basic means of interacting with our environment, allowing to orient to, localize and scrutinize the variety of potentially interesting objects that surround us. In this review we discuss the role of the parietal cortex in the control of saccadic and smooth pursuit eye movements, whose purpose is to rapidly displace the line of gaze and to maintain a moving object on the central retina, respectively. From single cell recording studies in monkey we know that distinct sub-regions of the parietal lobe are implicated in these two kinds of movement. The middle temporal (MT) and medial superior temporal (MST) areas show neuronal activities related to moving visual stimuli and to ocular pursuit. The lateral intraparietal (LIP) area exhibits visual and saccadic neuronal responses. Electrophysiology, which in essence is a correlation method, cannot entirely solve the question of the functional implication of these areas: are they primarily involved in sensory processing, in motor processing, or in some intermediate function? Lesion approaches (reversible or permanent) in the monkey can provide important information in this respect. Lesions of MT or MST produce deficits in the perception of visual motion, which would argue for their possible role in sensory guidance of ocular pursuit rather than in directing motor commands to the eye muscle. Lesions of LIP do not produce specific visual impairments and cause only subtle saccadic deficits. However, recent results have shown the presence of severe deficits in spatial attention tasks. LIP could thus be implicated in the selection of relevant objects in the visual scene and provide a signal for directing the eyes toward these objects. Functional imaging studies in humans confirm the role of the parietal cortex in pursuit, saccadic, and attentional networks, and show a high degree of overlap with monkey data. Parietal lobe lesions in humans also result in behavioral deficits very similar to those that are observed in the monkey. Altogether, these different sources of data consistently point to the involvement of the parietal cortex in the representation of space, at an intermediate stage between vision and action.     

Spatial updating in human parietal cortex

Single neurons in monkey parietal cortex update visual information in conjunction with eye movements. This remapping of stimulus representations is thought to contribute to spatial constancy. We hypothesized that a similar process occurs in human parietal cortex and that we could visualize it with functional MRI. We scanned subjects during a task that involved remapping of visual signals across hemifields. We observed an initial response in the hemisphere contralateral to the visual stimulus, followed by a remapped response in the hemisphere ipsilateral to the stimulus. We ruled out the possibility that this remapped response resulted from either eye movements or visual stimuli alone. Our results demonstrate that updating of visual information occurs in human parietal cortex.     

The parietal cortex and episodic memory: an attentional account

The contribution of the parietal cortex to episodic memory is a fascinating scientific puzzle. On the one hand, parietal lesions do not normally yield severe episodic-memory deficits; on the other hand, parietal activations are seen frequently in functional-neuroimaging studies of episodic memory. A review of these two categories of evidence suggests that the answer to the puzzle requires us to distinguish between the contributions of dorsal and ventral parietal regions and between the influence of top-down and bottom-up attention on memory.     

Association connections of the cat's parietal cortex

Intercortical connections of primary sensory (visual, auditory, somatosensory) areas with the parietal association cortex were studied in cats by the retrograde axonal transport of horseradish peroxidase and the Fink-Heimer silver impregnation of degenerated fibers techniques. This combined study revealed the shape, size, and intracortical location of cells connecting the primary sensory areas monosynaptically with the parietal cortex and also the distribution of preterminals and terminals of the fibers of these cells in the parietal association cortex. The greatest number of cells forming connections with area 7 of the parietal association cortex was shown to occur in visual area V1, and with area 5 in somatosensory area S1. Besides pyramidal neurons tagged with horseradish peroxidase, which were located mainly in layers II–IV, a few tagged stellate and fusiform cells also were found. The results supplement and confirm data on afferent connections of the parietal association cortex in cats.M. Gor'kii Donetsk Medical Institute. Translated from Neirofiziologiya, Vol. 13, No. 1, pp. 3–6, January, 1981.     

Associative connections of the cat parietal cortex

Interneuronal connections of area 7 of the cat parietal cortex with projection areas of the visual, auditory, and somatosensory cortex were analyzed by orthograde degeneration and retrograde transport of horseradish peroxidase methods. By combined investigation the cortico-cortical sources of afferentation of parietal area 7 could be precisely identified and concentration sites of neurons sending their axons into this area identified, and the morphological characteristics of these neurons could also be determined.A. A. Ukhtomskii Physiological Institute, A. A. Zhdanov Leningrad State University. Donetsk Medical Institute. Translated from Neirofiziologiya, Vol. 12, No. 1, pp. 13–17, January–February, 1980.     

Spatio-temporal updating in the left posterior parietal cortex

Adopting an unusual posture can sometimes give rise to paradoxical experiences. For example, the subjective ordering of successive unseen tactile stimuli delivered to the two arms can be affected when people cross them. A growing body of evidence now highlights the role played by the parietal cortex in spatio-temporal information processing when sensory stimuli are delivered to the body or when actions are executed; however, little is known about the neural basis of such paradoxical feelings resulting from such unusual limb positions. Here, we demonstrate increased fMRI activation in the left posterior parietal cortex when human participants adopted a crossed hands posture with their eyes closed. Furthermore, by assessing tactile temporal order judgments (TOJs) in the same individuals, we observed a positive association between activity in this area and the degree of reversal in TOJs resulting from crossing arms. The strongest positive association was observed in the left intraparietal sulcus. This result implies that the left posterior parietal cortex may be critically involved in monitoring limb position and in spatio-temporal binding when serial events are delivered to the limbs.     

Spatial and non-spatial functions of the parietal cortex

Although the parietal cortex is traditionally associated with spatial attention and sensorimotor integration, recent evidence also implicates it in higher order cognitive functions. We review relevant results from neuron recording studies showing that inferior parietal neurons integrate information regarding target location with a variety of non-spatial signals. Some of these signals are modulatory and alter a stimulus-evoked response according to the action, category, or reward associated with the stimulus. Other non-spatial inputs act independently, encoding the context or rules of a task even before the presentation of a specific target. Despite the ubiquity of non-spatial information in individual neurons, reversible inactivation of the parietal lobe affects only spatial orienting of attention and gaze, but not non-spatial aspects of performance. This suggests that non-spatial signals contribute to an underlying spatial computation, possibly allowing the brain to determine which targets are worthy of attention or action in a given task context.     

Neuroimaging of cognitive functions in human parietal cortex

Functional neuroimaging has proven highly valuable in mapping human sensory regions, particularly visual areas in occipital cortex. Recent evidence suggests that human parietal cortex may also consist of numerous specialized subregions similar to those reported in neurophysiological studies of non-human primates. However, parietal activation generalizes across a wide variety of cognitive tasks and the extension of human brain mapping into higher-order "association cortex" may prove to be a challenge.     

Interactions between number and space in parietal cortex

Since the time of Pythagoras, numerical and spatial representations have been inextricably linked. We suggest that the relationship between the two is deeply rooted in the brain's organization for these capacities. Many behavioural and patient studies have shown that numerical-spatial interactions run far deeper than simply cultural constructions, and, instead, influence behaviour at several levels. By combining two previously independent lines of research, neuroimaging studies of numerical cognition in humans, and physiological studies of spatial cognition in monkeys, we propose that these numerical-spatial interactions arise from common parietal circuits for attention to external space and internal representations of numbers.     

Tracking route progression in the posterior parietal cortex

Quick and efficient traversal of learned routes is critical to the survival of many animals. Routes can be defined by both the ordering of navigational epochs, such as continued forward motion or execution of a turn, and the distances separating them. The neural substrates conferring the ability to fluidly traverse complex routes are not well understood, but likely entail interactions between frontal, parietal, and rhinal cortices and the hippocampus. This paper demonstrates that posterior parietal cortical neurons map both individual and multiple navigational epochs with respect to their order in a route. In direct contrast to spatial firing patterns of hippocampal neurons, parietal neurons discharged in a place- and direction-independent fashion. Parietal route maps were scalable and versatile in that they were independent of the size and spatial configuration of navigational epochs. The results provide a framework in which to consider parietal function in spatial cognition.     

Neural coding of behavioral relevance in parietal cortex

Flexible control of behavior requires the selective processing of task-relevant sensory information and the appropriate linkage of sensory input to action. A great deal of evidence suggests a central role for the parietal cortex in these functions. Recent results from neurophysiological studies in non-human primates and neuroimaging experiments in humans illuminate the importance of parietal cortex for attention, and suggest how parietal neurons might allow the dynamic representation of behaviorally relevant information.     

Control over conflict during movement preparation: role of posterior parietal cortex

Flexible behavior in humans often requires that rapid choices be made between conflicting action plans. Although much attention has focused on prefrontal regions, little is understood about the contribution of parietal cortex under situations of response conflict. Here we show that right parietal damage associated with spatial neglect leads to paradoxical facilitation (speeding) of rightward movements in the presence of conflicting leftward response plans. These findings indicate a critical role for parietal regions in action planning when there is response competition. In contrast, patients with prefrontal damage have an augmented cost of conflict for both leftward and rightward movements. The results suggest involvement of two independent systems in situations of response conflict, with right parietal cortex being a crucial site for automatic activation of competing motor plans and prefrontal regions acting independently to inhibit action plans irrelevant to current task goals.     

The associative parietal cortex and spatial processing in rodents

It is widely acknowledged that the hippocampal formation has a central function in rodents' spatial memory and navigation. However, recent work has shown that other structures participate in specific spatial processing. That is so for the associative parietal cortex (APC). Although this neocortical region is far less developed in rodents than in humans and non-human primates, APC damage in rodents induces deficits which affect both egocentrically and allocentrically organized spatial behaviours. On the basis of behavioural (following parietal lesions) and neuroanatomical data, we propose that the APC could be at the interface between the level of perception of the physical world (egocentrically organized) and that of representations or maps (allocentrically organized) of this world. Reciprocally, the APC could also be involved in the transformation, in the opposite direction, of computations made on the basis of representations into motor actions necessary for the efficient execution of oriented behaviours within the physical world.