Spatial and effector processing in the human parietofrontal network for reaches and saccades

Planning Ahead: Object-Directed Sequential Actions Decoded from Human Frontoparietal and Occipitotemporal Networks

Object-manipulation tasks (e.g., drinking from a cup) typically involve sequencing together a series of distinct motor acts (e.g., reaching toward, grasping, lifting, and transporting the cup) in order to accomplish some overarching goal (e.g., quenching thirst). Although several studies in humans have investigated the neural mechanisms supporting the planning of visually guided movements directed toward objects (such as reaching or pointing), only a handful have examined how manipulatory sequences of actions-those that occur after an object has been grasped-are planned and represented in the brain. Here, using event-related functional MRI and pattern decoding methods, we investigated the neural basis of real-object manipulation using a delayed-movement task in which participants first prepared and then executed different object-directed action sequences that varied either in their complexity or final spatial goals. Consistent with previous reports of preparatory brain activity in non-human primates, we found that activity patterns in several frontoparietal areas reliably predicted entire action sequences in advance of movement. Notably, we found that similar sequence-related information could also be decoded from pre-movement signals in object- and body-selective occipitotemporal cortex (OTC). These findings suggest that both frontoparietal and occipitotemporal circuits are engaged in transforming object-related information into complex, goal-directed movements.

Allocentric versus egocentric representation of remembered reach targets in human cortex

The location of a remembered reach target can be encoded in egocentric and/or allocentric reference frames. Cortical mechanisms for egocentric reach are relatively well described, but the corresponding allocentric representations are essentially unknown. Here, we used an event-related fMRI design to distinguish human brain areas involved in these two types of representation. Our paradigm consisted of three tasks with identical stimulus display but different instructions: egocentric reach (remember absolute target location), allocentric reach (remember target location relative to a visual landmark), and a nonspatial control, color report (report color of target). During the delay phase (when only target location was specified), the egocentric and allocentric tasks elicited widely overlapping regions of cortical activity (relative to the control), but with higher activation in parietofrontal cortex for egocentric task and higher activation in early visual cortex for allocentric tasks. In addition, egocentric directional selectivity (target relative to gaze) was observed in the superior occipital gyrus and the inferior occipital gyrus, whereas allocentric directional selectivity (target relative to a visual landmark) was observed in the inferior temporal gyrus and inferior occipital gyrus. During the response phase (after movement direction had been specified either by reappearance of the visual landmark or a pro-/anti-reach instruction), the parietofrontal network resumed egocentric directional selectivity, showing higher activation for contralateral than ipsilateral reaches. These results show that allocentric and egocentric reach mechanisms use partially overlapping but different cortical substrates and that directional specification is different for target memory versus reach response.

Brain activity modulation during the production of imperative and declarative pointing

Pointing is a communicative gesture, commonly used for expressing two main intentions: imperative, to obtain a desired object/action from the other, or declarative, to share attention/interest about a referent with the other. Previous neuroimaging research on adults examined pointing almost exclusively as a reaching-like motor act rather than as a communicative gesture. Here, we used fMRI to record brain activity while 16 participants produced either imperative or declarative pointing gestures within a communicative context. A network of regions (the bilateral ventral premotor cortex, anterior midcingulate cortex, middle insula and the right preSMA) showed a preference for the production of declarative pointing as opposed to imperative pointing. The right preSMA also preferred declarative intention during pointing observation. Instead, independently from the intention, the right pMTG was more active during pointing observation than production. In the bilateral posterior parietal reach region we also observed a side (contra>ipsi) effect when the intention was imperative, regardless of the subject's role in the communication. Based on these results, we propose that pointing with declarative intention recruits a network of regions associated with will, motivation, emotional/affective expression and intersubjectivity, whereas pointing with imperative intention recruits regions associated with reaching. The proposal is consistent with the developmental hypothesis that declarative pointing reflects social cognitive abilities more than imperative pointing and establishes a stimulating link for future interdisciplinary research.

Resting-state connectivity and functional specialization in human medial parieto-occipital cortex

According to recent models of visuo-spatial processing, the medial parieto-occipital cortex is a crucial node of the dorsal visual stream. Evidence from neurophysiological studies in monkeys has indicated that the parieto-occipital sulcus (POS) contains three functionally and cytoarchitectonically distinct areas: the visual area V6 in the fundus of the POS, and the visuo-motor areas V6Av and V6Ad in a progressively dorsal and anterior location with respect to V6. Besides different topographical organization, cytoarchitectonics, and functional properties, these three monkey areas can also be distinguished based on their patterns of cortico-cortical connections. Thanks to wide-field retinotopic mapping, areas V6 and V6Av have been also mapped in the human brain. Here, using a combined approach of resting-state functional connectivity and task-evoked activity by fMRI, we identified a new region in the anterior POS showing a pattern of functional properties and cortical connections that suggests a homology with the monkey area V6Ad. In addition, we observed distinct patterns of cortical connections associated with the human V6 and V6Av which are remarkably consistent with those showed by the anatomical tracing studies in the corresponding monkey areas. Consistent with recent models on visuo-spatial processing, our findings demonstrate a gradient of functional specialization and cortical connections within the human POS, with more posterior regions primarily dedicated to the analysis of visual attributes useful for spatial navigation and more anterior regions primarily dedicated to analyses of spatial information relevant for goal-directed action.

Understanding effector selectivity in human posterior parietal cortex by combining information patterns and activation measures

The posterior parietal cortex (PPC) has traditionally been viewed as containing separate regions for the planning of eye and limb movements, but recent neurophysiological and neuroimaging observations show that the degree of effector specificity is limited. This has led to the hypothesis that effector specificity in PPC is part of a more efficient than strictly modular organization, characterized by both distinct and common activations for different effectors. It is unclear, however, what differentiates the distinctions and commonalities in effector representations. Here, we used fMRI in humans to study the cortical representations involved in the planning of eye, hand, and foot movements. We used a novel combination of fMRI measures to assess the effector-related representational content of the PPC: a multivariate information measure, reflecting whether representations were distinct or common across effectors and a univariate activation measure, indicating which representations were actively involved in movement preparation. Active distinct representations were evident in areas previously reported to be effector specific: eye specificity in the posterior intraparietal sulcus (IPS), hand tuning in anterior IPS, and a foot bias in the anterior precuneus. Crucially, PPC regions responding to a particular effector also contained an active representation common across the other two effectors. We infer that rostral PPC areas do not code single effectors, but rather dichotomies of effectors. Such combinations of representations could be well suited for active effector selection, efficiently coding both a selected effector and its alternatives.

Optic ataxia: from Balint's syndrome to the parietal reach region

Optic ataxia is a high-order deficit in reaching to visual goals that occurs with posterior parietal cortex (PPC) lesions. It is a component of Balint's syndrome that also includes attentional and gaze disorders. Aspects of optic ataxia are misreaching in the contralesional visual field, difficulty preshaping the hand for grasping, and an inability to correct reaches online. Recent research in nonhuman primates (NHPs) suggests that many aspects of Balint's syndrome and optic ataxia are a result of damage to specific functional modules for reaching, saccades, grasp, attention, and state estimation. The deficits from large lesions in humans are probably composite effects from damage to combinations of these functional modules. Interactions between these modules, either within posterior parietal cortex or downstream within frontal cortex, may account for more complex behaviors such as hand-eye coordination and reach-to-grasp.

Object size modulates fronto-parietal activity during reaching movements

In both monkeys and humans, reaching-related sensorimotor transformations involve the activation of a wide fronto-parietal network. Recent neurophysiological evidence suggests that some components of this network host not only neurons encoding the direction of arm reaching movements, but also neurons whose involvement is modulated by the intrinsic features of an object (e.g. size and shape). To date, it has yet to be investigated whether a similar modulation is evident in the human reaching-related areas. To fill this gap, we asked participants to reach towards either a small or a large object while kinematic and electroencephalographic signals were recorded. Behavioral results showed that the precision requirements were taken into account and the kinematics of reaching was modulated depending on the object size. Similarly, reaching-related neural activity at the level of the posterior parietal and premotor cortices was modulated by the level of accuracy determined by object size. We therefore conclude that object size is engaged in the neural computations for reach planning and execution, consistent with the results from physiological studies in non-human primates.

Contribution of the posterior parietal cortex in reaching, grasping, and using objects and tools

Neuropsychological and neuroimaging data suggest a differential contribution of posterior parietal regions during the different components of a transitive gesture. Reaching requires the integration of object location and body position coordinates and reaching tasks elicit bilateral activation in different foci along the intraparietal sulcus. Grasping requires a visuomotor match between the object's shape and the hand's posture. Lesion studies and neuroimaging confirm the importance of the anterior part of the intraparietal sulcus for human grasping. Reaching and grasping reveal bilateral activation that is generally more prominent on the side contralateral to the hand used or the hemifield stimulated. Purposeful behavior with objects and tools can be assessed in a variety of ways, including actual use, pantomimed use, and pure imagery of manipulation. All tasks have been shown to elicit robust activation over the left parietal cortex in neuroimaging, but lesion studies have not always confirmed these findings. Compared to pantomimed or imagined gestures, actual object and tool use typically produces activation over the left primary somatosensory region. Neuroimaging studies on pantomiming or imagery of tool use in healthy volunteers revealed neural responses in possibly separate foci in the left supramarginal gyrus. In sum, the parietal contribution of reaching and grasping of objects seems to depend on a bilateral network of intraparietal foci that appear organized along gradients of sensory and effector preferences. Dorsal and medial parietal cortex appears to contribute to the online monitoring/adjusting of the ongoing prehensile action, whereas the functional use of objects and tools seems to involve the inferior lateral parietal cortex. This functional input reveals a clear left lateralized activation pattern that may be tuned to the integration of acquired knowledge in the planning and guidance of the transitive movement.

Multiple aspects of neural activity during reaching preparation in the medial posterior parietal area V6A

The posterior parietal cortex is involved in the visuomotor transformations occurring during arm-reaching movements. The medial posterior parietal area V6A has been shown to be implicated in reaching execution, but its role in reaching preparation has not been sufficiently investigated. Here, we addressed this issue exploring the neural correlates of reaching preparation in V6A. Neural activity of single cells during the instructed delay period of a foveated Reaching task was compared with the activity in the same delay period during a Detection task. In this latter task, animals fixated the target but, instead of performing an arm reaching movement, they responded with a button release to the go signal. Targets were allocated in different positions in 3-D space. We found three types of neurons: cells where delay activity was equally spatially tuned in the two tasks (Gaze cells), cells spatially tuned only during reaching preparation (Set cells), and cells influenced by both gaze and reaching preparation signals (Gaze/Set cells). In cells influenced by reaching preparation, the delay activity in the Reaching task could be higher or lower compared with the Detection task. All the Set cells and a minority of Gaze/Set cells were more active during reaching preparation. Most cells modulated by movement preparation were also modulated with a congruent spatial tuning during movement execution. Present results highlight the convergence of visuospatial information, reach planning and reach execution signals on V6A, and indicate that visuospatial processing and movement execution have a larger influence on V6A activity than the encoding of reach plans.

Covert oculo-manual coupling induced by visually guided saccades

Hand pointing to objects under visual guidance is one of the most common motor behaviors in everyday life. In natural conditions, gaze and arm movements are commonly aimed at the same target and the accuracy of both systems is considerably enhanced if eye and hand move together. Evidence supports the viewpoint that gaze and limb control systems are not independent but at least partially share a common neural controller. The aim of the present study was to verify whether a saccade execution induces excitability changes in the upper-limb corticospinal system (CSS), even in the absence of a manual response. This effect would provide evidence for the existence of a common drive for ocular and arm motor systems during fast aiming movements. Single-pulse TMS was applied to the left motor cortex of 19 subjects during a task involving visually guided saccades, and motor evoked potentials (MEPs) induced in hand and wrist muscles of the contralateral relaxed arm were recorded. Subjects had to make visually guided saccades to one of 6 positions along the horizontal meridian (±5°, ±10°, or ±15°). During each trial, TMS was randomly delivered at one of 3 different time delays: shortly after the end of the saccade or 300 or 540 ms after saccade onset. Fast eye movements toward a peripheral target were accompanied by changes in upper-limb CSS excitability. MEP amplitude was highest immediately after the end of the saccade and gradually decreased at longer TMS delays. In addition to the change in overall CSS excitability, MEPs were specifically modulated in different muscles, depending on the target position and the TMS delay. By applying a simple model of a manual pointing movement, we demonstrated that the observed changes in CSS excitability are compatible with the facilitation of an arm motor program for a movement aimed at the same target of the gaze. These results provide evidence in favor of the existence of a common drive for both eye and arm motor systems.

Broad domain generality in focal regions of frontal and parietal cortex

Unlike brain regions that respond selectively to specific kinds of information content, a number of frontal and parietal regions are thought to be domain- and process-general: that is, active during a wide variety of demanding cognitive tasks. However, most previous evidence for this functional generality in humans comes from methods that overestimate activation overlap across tasks. Here we present functional MRI evidence from single-subject analyses for broad functional generality of a specific set of brain regions: the same sets of voxels are engaged across tasks ranging from arithmetic to storing information in working memory, to inhibiting irrelevant information. These regions have a specific topography, often lying directly adjacent to domain-specific regions. Thus, in addition to domain-specific brain regions tailored to solve particular problems of longstanding importance to our species, the human brain also contains a set of functionally general regions that plausibly endow us with the cognitive flexibility necessary to solve novel problems.

The human homologue of macaque area V6A

In macaque monkeys, V6A is a visuomotor area located in the anterior bank of the POs, dorsal and anterior to retinotopically-organized extrastriate area V6 (Galletti et al., 1996). Unlike V6, V6A represents both contra- and ipsilateral visual fields and is broadly retinotopically organized (Galletti et al., 1999b). The contralateral lower visual field is over-represented in V6A. The central 20°-30° of the visual field is mainly represented dorsally (V6Ad) and the periphery ventrally (V6Av), at the border with V6. Both sectors of area V6A contain arm movement-related cells, active during spatially-directed reaching movements (Gamberini et al., 2011). In humans, we previously mapped the retinotopic organization of area V6 (Pitzalis et al., 2006). Here, using phase-encoded fMRI, cortical surface-based analysis and wide-field retinotopic mapping, we define a new cortical region that borders V6 anteriorly and shows a clear over-representation of the contralateral lower visual field and the periphery. As with macaque V6A, the eccentricity increases moving ventrally within the area. The new region contains a non-mirror-image representation of the visual field. Functional mapping reveals that, as in macaque V6A, the new region, but not the nearby area V6, responds during finger pointing and reaching movements. Based on similarity in position, retinotopic properties, functional organization and relationship with the neighboring extrastriate visual areas, we propose that the new cortical region is the human homologue of macaque area V6A.

Deciding where to look based on visual, auditory, and semantic information

Neurons in the dorsal frontal and parietal cortex are thought to transform incoming visual signals into the spatial goals of saccades, a process known as target selection. Here, we used functional magnetic resonance imaging (fMRI) to test how target selection may generalize beyond visual transformations when auditory and semantic information is used for selection. We compared activity in the frontal and parietal cortex when subjects made visually, aurally, and semantically guided saccades to one of four differently colored dots. Selection was based on a visual cue (i.e., one of the dots blinked), an auditory cue (i.e., a white noise burst was emitted at one of the dots' location), or a semantic cue (i.e., the color of one of the dots was spoken). Although neural responses in frontal and parietal cortex were robust, they were non-specific with regard to the type of information used for target selection. Decoders, however, trained on the patterns of activity in the intraparietal sulcus could classify both the type of cue used for target selection and the direction of the saccade. Therefore, we find evidence that the posterior parietal cortex is involved in transforming multimodal inputs into general spatial representations that can be used to guide saccades.

Decoding the neural mechanisms of human tool use

Sophisticated tool use is a defining characteristic of the primate species but how is it supported by the brain, particularly the human brain? Here we show, using functional MRI and pattern classification methods, that tool use is subserved by multiple distributed action-centred neural representations that are both shared with and distinct from those of the hand. In areas of frontoparietal cortex we found a common representation for planned hand- and tool-related actions. In contrast, in parietal and occipitotemporal regions implicated in hand actions and body perception we found that coding remained selectively linked to upcoming actions of the hand whereas in parietal and occipitotemporal regions implicated in tool-related processing the coding remained selectively linked to upcoming actions of the tool. The highly specialized and hierarchical nature of this coding suggests that hand- and tool-related actions are represented separately at earlier levels of sensorimotor processing before becoming integrated in frontoparietal cortex. DOI:http://dx.doi.org/10.7554/eLife.00425.001.

Where one hand meets the other: limb-specific and action-dependent movement plans decoded from preparatory signals in single human frontoparietal brain areas

Planning object-directed hand actions requires successful integration of the movement goal with the acting limb. Exactly where and how this sensorimotor integration occurs in the brain has been studied extensively with neurophysiological recordings in nonhuman primates, yet to date, because of limitations of non-invasive methodologies, the ability to examine the same types of planning-related signals in humans has been challenging. Here we show, using a multivoxel pattern analysis of functional MRI (fMRI) data, that the preparatory activity patterns in several frontoparietal brain regions can be used to predict both the limb used and hand action performed in an upcoming movement. Participants performed an event-related delayed movement task whereby they planned and executed grasp or reach actions with either their left or right hand toward a single target object. We found that, although the majority of frontoparietal areas represented hand actions (grasping vs reaching) for the contralateral limb, several areas additionally coded hand actions for the ipsilateral limb. Notable among these were subregions within the posterior parietal cortex (PPC), dorsal premotor cortex (PMd), ventral premotor cortex, dorsolateral prefrontal cortex, presupplementary motor area, and motor cortex, a region more traditionally implicated in contralateral movement generation. Additional analyses suggest that hand actions are represented independently of the intended limb in PPC and PMd. In addition to providing a unique mapping of limb-specific and action-dependent intention-related signals across the human cortical motor system, these findings uncover a much stronger representation of the ipsilateral limb than expected from previous fMRI findings.

Cross-modal plasticity preserves functional specialization in posterior parietal cortex

In congenitally blind individuals, many regions of the brain that are typically heavily involved in visual processing are recruited for a variety of nonvisual sensory and cognitive tasks (Rauschecker 1995; Pascual-Leone et al. 2005). This phenomenon-cross-modal plasticity-has been widely documented, but the principles that determine where and how cross-modal changes occur remain poorly understood (Bavelier and Neville 2002). Here, we evaluate the hypothesis that cross-modal plasticity respects the type of computations performed by a region, even as it changes the modality of the inputs over which they are carried out (Pascual-Leone and Hamilton 2001). We compared the fMRI signal in sighted and congenitally blind participants during proprioceptively guided reaching. We show that parietooccipital reach-related regions retain their functional role-encoding of the spatial position of the reach target-even as the dominant modality in this region changes from visual to nonvisual inputs. This suggests that the computational role of a region, independently of the processing modality, codetermines its potential cross-modal recruitment. Our findings demonstrate that preservation of functional properties can serve as a guiding principle for cross-modal plasticity even in visuomotor cortical regions, i.e. beyond the early visual cortex and other traditional visual areas.

Neural activity in superior parietal cortex during rule-based visual-motor transformations

Cognition allows for the use of different rule-based sensorimotor strategies, but the neural underpinnings of such strategies are poorly understood. The purpose of this study was to compare neural activity in the superior parietal lobule during a standard (direct interaction) reaching task, with two nonstandard (gaze and reach spatially incongruent) reaching tasks requiring the integration of rule-based information. Specifically, these nonstandard tasks involved dissociating the planes of reach and vision or rotating visual feedback by 180°. Single unit activity, gaze, and reach trajectories were recorded from two female Macaca mulattas. In all three conditions, we observed a temporal discharge pattern at the population level reflecting early reach planning and on-line reach monitoring. In the plane-dissociated task, we found a significant overall attenuation in the discharge rate of cells from deep recording sites, relative to standard reaching. We also found that cells modulated by reach direction tended to be significantly tuned either during the standard or the plane-dissociated task but rarely during both. In the standard versus feedback reversal comparison, we observed some cells that shifted their preferred direction by 180° between conditions, reflecting maintenance of directional tuning with respect to the reach goal. Our findings suggest that the superior parietal lobule plays an important role in processing information about the nonstandard nature of a task, which, through reciprocal connections with precentral motor areas, contributes to the accurate transformation of incongruent sensory inputs into an appropriate motor output. Such processing is crucial for the integration of rule-based information into a motor act.

Distributed sensitivity for movement amplitude in directionally tuned neuronal populations

Neurons in macaque primary motor cortex and dorsal premotor cortex are tuned to movement direction. In humans, neuronal populations tuned to movement direction have recently been described using multivoxel pattern analysis and functional magnetic resonance imaging adaptation. It is unclear, however, to what extent directionally tuned neuronal populations are sensitive to movement amplitude. Here we used functional magnetic resonance imaging adaptation to determine whether directionally tuned neuronal populations are modulated by movement amplitude. In different blocks, participants were adapted to small- or large-amplitude hand-reaching movements. On occasional test trials, we parametrically varied the angular difference between adaptation and test direction and the congruency between adapted and tested amplitude (same or different). We predicted that the blood oxygen level-dependent signal in directionally tuned regions should be adapted in proportion to the angular difference between adaptation and test direction. Directionally tuned regions insensitive to movement amplitude should show a transfer of adaptation from the adapted to the nonadapted amplitude. In contrast, regions sensitive to the specific combination of movement direction and amplitude should show directional tuning only for the adapted amplitude. We identified a network of parietal and frontal regions tuned to movement direction. We found that parietal areas contain neuronal populations sensitive to specific combinations of movement direction and amplitude, while frontal areas show transfer from the adapted to the nonadapted amplitude during small-amplitude movements after adaptation to large amplitude, but not vice versa. Our results thus imply different processing of movement amplitude in directionally tuned frontal and parietal areas.

Decoding effector-dependent and effector-independent movement intentions from human parieto-frontal brain activity

Our present understanding of the neural mechanisms and sensorimotor transformations that govern the planning of arm and eye movements predominantly come from invasive parieto-frontal neural recordings in nonhuman primates. While functional MRI (fMRI) has motivated investigations on much of these same issues in humans, the highly distributed and multiplexed organization of parieto-frontal neurons necessarily constrain the types of intention-related signals that can be detected with traditional fMRI analysis techniques. Here we employed multivoxel pattern analysis (MVPA), a multivariate technique sensitive to spatially distributed fMRI patterns, to provide a more detailed understanding of how hand and eye movement plans are coded in human parieto-frontal cortex. Subjects performed an event-related delayed movement task requiring that a reach or saccade be planned and executed toward one of two spatial target positions. We show with MVPA that, even in the absence of signal amplitude differences, the fMRI spatial activity patterns preceding movement onset are predictive of upcoming reaches and saccades and their intended directions. Within certain parieto-frontal regions we show that these predictive activity patterns reflect a similar spatial target representation for the hand and eye. Within some of the same regions, we further demonstrate that these preparatory spatial signals can be discriminated from nonspatial, effector-specific signals. In contrast to the largely graded effector- and direction-related planning responses found with fMRI subtraction methods, these results reveal considerable consensus with the parieto-frontal network organization suggested from primate neurophysiology and specifically show how predictive spatial and nonspatial movement information coexists within single human parieto-frontal areas.

A functional and structural investigation of the human fronto-basal volitional saccade network

Almost all cortical areas are connected to the subcortical basal ganglia (BG) through parallel recurrent inhibitory and excitatory loops, exerting volitional control over automatic behavior. As this model is largely based on non-human primate research, we used high resolution functional MRI and diffusion tensor imaging (DTI) to investigate the functional and structural organization of the human (pre)frontal cortico-basal network controlling eye movements. Participants performed saccades in darkness, pro- and antisaccades and observed stimuli during fixation. We observed several bilateral functional subdivisions along the precentral sulcus around the human frontal eye fields (FEF): a medial and lateral zone activating for saccades in darkness, a more fronto-medial zone preferentially active for ipsilateral antisaccades, and a large anterior strip along the precentral sulcus activating for visual stimulus presentation during fixation. The supplementary eye fields (SEF) were identified along the medial wall containing all aforementioned functions. In the striatum, the BG area receiving almost all cortical input, all saccade related activation was observed in the putamen, previously considered a skeletomotor striatal subdivision. Activation elicited by the cue instructing pro or antisaccade trials was clearest in the medial FEF and right putamen. DTI fiber tracking revealed that the subdivisions of the human FEF complex are mainly connected to the putamen, in agreement with the fMRI findings. The present findings demonstrate that the human FEF has functional subdivisions somewhat comparable to non-human primates. However, the connections to and activation in the human striatum preferentially involve the putamen, not the caudate nucleus as is reported for monkeys. This could imply that fronto-striatal projections for the oculomotor system are fundamentally different between humans and monkeys. Alternatively, there could be a bias in published reports of monkey studies favoring the caudate nucleus over the putamen in the search for oculomotor functions.

Three-dimensional transformations for goal-directed action

Much of the central nervous system is involved in visuomotor transformations for goal-directed gaze and reach movements. These transformations are often described in terms of stimulus location, gaze fixation, and reach endpoints, as viewed through the lens of translational geometry. Here, we argue that the intrinsic (primarily rotational) 3-D geometry of the eye-head-reach systems determines the spatial relationship between extrinsic goals and effector commands, and therefore the required transformations. This approach provides a common theoretical framework for understanding both gaze and reach control. Combined with an assessment of the behavioral, neurophysiological, imaging, and neuropsychological literature, this framework leads us to conclude that (a) the internal representation and updating of visual goals are dominated by gaze-centered mechanisms, but (b) these representations must then be transformed as a function of eye and head orientation signals into effector-specific 3-D movement commands.

Intentional signals during saccadic and reaching delays in the human posterior parietal cortex

In the monkey posterior parietal cortex (PPC), there is clear evidence of anatomically segregated neuronal populations specialized for planning saccades and arm-reaching movements. However, functional neuroimaging studies in humans have yielded controversial results. Here we show that the human PPC contains distinct subregions responsive to salient visual cues, some of which combine spatial and action-related signals into 'intentional' signals. Participants underwent event-related functional magnetic resonance imaging while performing delayed saccades and long-range arm reaches instructed by visual cues. We focused on activity in the time period following the cue and preceding the actual movement. The use of individual cortical surface reconstructions with detailed sulcal labeling allowed the definition of six responsive regions with distinctive anatomical locations in the PPC. Each region exhibited a distinctive combination of transient and sustained signals during the delay, modulated by either the cue spatial location (contralateral vs. ipsilateral), the instructed action (saccades vs. reaching) or both. Importantly, a lateral and a medial dorsal parietal region showed sustained responses during the delay preferentially for contralateral saccadic and reaching trials, respectively. In the lateral region, preference for saccades was evident only as a more sustained response during saccadic vs. reaching delays, whereas the medial region also showed a higher transient response to cues signaling reaching vs. saccadic actions. These response profiles closely match the behavior of neurons in the macaque lateral and medial intraparietal area, respectively, and suggest that these corresponding human regions are encoding spatially directed action plans or 'intentions'.

Parietofrontal circuits in goal-oriented behaviour

Parietal and frontal cortical areas play important roles in the control of goal-oriented behaviour. This review examines how signal processing in the parietal and frontal eye fields is involved in coding and storing space, directing attention and processing the sensorimotor transformation for saccades. After a survey of the functional specialization of these areas in monkeys, we discuss homologous regions in the human brain in terms of topographic organization, storage capacity, target selection, spatial remapping, reference frame transformations and effector specificity. The overall picture suggests that bottom-up sensory, top-down cognitive signals and efferent motor signals are integrated in dynamic sensorimotor maps as part of a functionally flexible parietofrontal network. Neuronal synchronization in these maps may be instrumental in amplifying behaviourally relevant representations and setting up a functional pathway to route information in this parietofrontal circuit.

Decoding action intentions from preparatory brain activity in human parieto-frontal networks

How and where in the human brain high-level sensorimotor processes such as intentions and decisions are coded remain important yet essentially unanswered questions. This is in part because, to date, decoding intended actions from brain signals has been primarily constrained to invasive neural recordings in nonhuman primates. Here we demonstrate using functional MRI (fMRI) pattern recognition techniques that we can also decode movement intentions from human brain signals, specifically object-directed grasp and reach movements, moments before their initiation. Subjects performed an event-related delayed movement task toward a single centrally located object (consisting of a small cube attached atop a larger cube). For each trial, after visual presentation of the object, one of three hand movements was instructed: grasp the top cube, grasp the bottom cube, or reach to touch the side of the object (without preshaping the hand). We found that, despite an absence of fMRI signal amplitude differences between the planned movements, the spatial activity patterns in multiple parietal and premotor brain areas accurately predicted upcoming grasp and reach movements. Furthermore, the patterns of activity in a subset of these areas additionally predicted which of the two cubes were to be grasped. These findings offer new insights into the detailed movement information contained in human preparatory brain activity and advance our present understanding of sensorimotor planning processes through a unique description of parieto-frontal regions according to the specific types of hand movements they can predict.

Role of the medial part of the intraparietal sulcus in implementing movement direction

The contribution of the posterior parietal cortex (PPC) to visually guided movements has been originally inferred from observations made in patients suffering from optic ataxia. Subsequent electrophysiological studies in monkeys and functional imaging data in humans have corroborated the key role played by the PPC in sensorimotor transformations underlying goal-directed movements, although the exact contribution of this structure remains debated. Here, we used transcranial magnetic stimulation (TMS) to interfere transiently with the function of the left or right medial part of the intraparietal sulcus (mIPS) in healthy volunteers performing visually guided movements with the right hand. We found that a "virtual lesion" of either mIPS increased the scattering in initial movement direction (DIR), leading to longer trajectory and prolonged movement time, but only when TMS was delivered 100-160 ms before movement onset and for movements directed toward contralateral targets. Control experiments showed that deficits in DIR consequent to mIPS virtual lesions resulted from an inappropriate implementation of the motor command underlying the forthcoming movement and not from an inaccurate computation of the target localization. The present study indicates that mIPS plays a causal role in implementing specifically the direction vector of visually guided movements toward objects situated in the contralateral hemifield.

Functional rather than effector-specific organization of human posterior parietal cortex

Neurophysiological and neuroimaging studies have shown that the posterior parietal cortex (PPC) distinguishes between the planning of eye and hand movements. This distinction has usually been interpreted as evidence for a modular, effector-specific organization of this cerebral region. However, the eyes differ markedly from other body parts both in terms of their functional purpose and with regard to the spatial transformations required to plan goal-directed movements. PPC may therefore provide specialized subregions for eye movements, but distinguish less for other effectors. Using functional magnetic resonance imaging, we compared activity during memory-guided eye, hand, and foot movements in human participants. The results did not reveal any significant activation differences during the planning of hand and foot movements, except in the most anterior part of PPC [Brodmann's area (BA) 5], marginally extending into anterior BA 7/40. This region showed a lateral-to-medial gradient for hand versus foot movement planning. The limb-unspecific PPC regions were functionally connected with hand and foot motor regions. In contrast, a gradient-like organization was found for all of PPC for the planning of eye versus hand and foot movements. Although planning-related activity across the three effectors considerably overlapped, saccade planning activated occipitoparietal regions more than limb movements, whereas limb movements activated anterior regions of the superior parietal lobule more than saccades. We infer that PPC does not follow a strict effector-specific organization. Rather, the large-scale organization of this region might reflect the different computational constraints that need to be satisfied when planning eye and limb movements.

Effect of reinforcement history on hand choice in an unconstrained reaching task

Choosing which hand to use for an action is one of the most frequent decisions people make in everyday behavior. We developed a simple reaching task in which we vary the lateral position of a target and the participant is free to reach to it with either the right or left hand. While people exhibit a strong preference to use the hand ipsilateral to the target, there is a region of uncertainty within which hand choice varies across trials. We manipulated the reinforcement rates for the two hands, either by increasing the likelihood that a reach with the non-dominant hand would successfully intersect the target or decreasing the likelihood that a reach with the dominant hand would be successful. While participants had minimal awareness of these manipulations, we observed an increase in the use of the non-dominant hand for targets presented in the region of uncertainty. We modeled the shift in hand use using a Q-learning model of reinforcement learning. The results provided a good fit of the data and indicate that the effects of increasing and decreasing the rate of positive reinforcement are additive. These experiments emphasize the role of decision processes for effector selection, and may point to a novel approach for physical rehabilitation based on intrinsic reinforcement.

Mapping reflexive shifts of attention in eye-centered and hand-centered coordinate systems

Behavioral studies indicate that directional gaze and hand pointing are fundamental social signals that may capture spatial attention more powerfully than directional arrows. By using fMRI, we explored whether reflexive shifts of attention triggered by different distracters were influenced by the motor effector used for performing an overt response. In separate blocks, healthy participants performed a directional saccadic or a hand pointing movement. Color changes of a central black fixation point constituted the imperative instruction signal to make a leftward (red color) or a rightward (blue color) movement while ignoring distracting leftward or rightward oriented gaze, hand pointing, or arrow. Distracters that were directionally incongruent with the instruction cue impaired the saccadic and pointing-release RTs. The comparison of incongruent vs. congruent conditions showed an increase of BOLD signal in the frontal eye field (FEF), the intraparietal sulcus (IPS), and the posterior parietal cortex (PPC) bilaterally. Importantly, a specific relationship between distracter and effector used for the response was found in these frontal and parietal regions. In particular, higher activity in the FEF, for distracting gaze was found mainly during the saccadic response task. In the same vein, higher activity in the left and right IPS regions was found for the distracting hand mainly in the hand pointing task. The results suggest that reflexive shifts of attention triggered by social signals are coded in the fronto-parietal cortex according to effector-specific mapping rules.

Mental blocks: fMRI reveals top-down modulation of early visual cortex when obstacles interfere with grasp planning

When grasping an object, the fingers, hand and arm rarely collide with other non-target objects in the workspace. Kinematic studies of neurological patients (Schindler et al., 2004) and healthy participants (Chapman and Goodale, 2010a) suggest that the location of potential obstacles and the degree of interference they pose are encoded by the dorsal visual stream during action planning. Here, we used a slow event-related paradigm in functional magnetic resonance imaging (fMRI) to examine the neural encoding of obstacles in normal participants. Fifteen right-handed participants grasped a square target object with a thumb-front or thumb-side wrist-posture with (1) no obstacle present, (2) an obstacle behind the target object (interfering with the thumb-front grasp), or (3) an obstacle beside the target object (interfering with the thumb-side grasp). Within a specified network of areas involved in planning, a group voxelwise analysis revealed that one area in the left posterior intraparietal sulcus (pIPS) and one in early visual cortex were modulated by the degree of obstacle interference, and that this modulation occurred prior to movement execution. Given previous reports of a functional link between IPS and early visual cortex, we suggest that the increasing activity in the IPS with obstacle interference provides the top-down signal to suppress the corresponding obstacle coding in early visual areas, where we observed that activity decreased with interference. This is the first concrete evidence that the planning of a grasping movement can modulate early visual cortex and provides a unifying framework for understanding the dual role played by the IPS in motor planning and attentional orienting.

Independent Allocation of Attention to Eye and Hand Targets in Coordinated Eye-Hand Movements

When reaching for objects, people frequently look where they reach. This raises the question of whether the targets for the eye and hand in concurrent eye and hand movements are selected by a unitary attentional system or by independent mechanisms. We used the deployment of visual attention as an index of the selection of movement targets and asked observers to reach and look to either the same location or separate locations. Results show that during the preparation of coordinated movements, attention is allocated in parallel to the targets of a saccade and a reaching movement. Attentional allocations for the two movements interact synergistically when both are directed to a common goal. Delaying the eye movement delays the attentional shift to the saccade target while leaving attentional deployment to the reach target unaffected. Our findings demonstrate that attentional resources are allocated independently to the targets of eye and hand movements and suggest that the goals for these effectors are selected by separate attentional mechanisms.

Brain activation related to combinations of gaze position, visual input, and goal-directed hand movements

Humans reach to and acquire objects by transforming visual targets into action commands. How the brain integrates goals specified in a visual framework to signals into a suitable framework for an action plan requires clarification whether visual input, per se, interacts with gaze position to formulate action plans. To further evaluate brain control of visual-motor integration, we assessed brain activation, using functional magnetic resonance imaging. Humans performed goal-directed movements toward visible or remembered targets while fixating gaze left or right from center. We dissociated movement planning from performance using a delayed-response task and manipulated target visibility by its availability throughout the delay or blanking it 500 ms after onset. We found strong effects of gaze orientation on brain activation during planning and interactive effects of target visibility and gaze orientation on movement-related activation during performance in parietal and premotor cortices (PM), cerebellum, and basal ganglia, with more activation for rightward gaze at a visible target and no gaze modulation for movements directed toward remembered targets. These results demonstrate effects of gaze position on PM and movement-related processes and provide new information how visual signals interact with gaze position in transforming visual inputs into motor goals.

Specificity of human parietal saccade and reach regions during transcranial magnetic stimulation

Single-unit recordings in macaque monkeys have identified effector-specific regions in posterior parietal cortex (PPC), but functional neuroimaging in the human has yielded controversial results. Here we used on-line repetitive transcranial magnetic stimulation (rTMS) to determine saccade and reach specificity in human PPC. A short train of three TMS pulses (separated by an interval of 100 ms) was delivered to superior parieto-occipital cortex (SPOC), a region over the midposterior intraparietal sulcus (mIPS), and a site close to caudal IPS situated over the angular gyrus (AG) during a brief memory interval while subjects planned either a saccade or reach with the left or right hand. Behavioral measures then were compared to controls without rTMS. Stimulation of mIPS and AG produced similar patterns: increased end-point variability for reaches and decreased saccade accuracy for contralateral targets. In contrast, stimulation of SPOC deviated reach end points toward visual fixation and had no effect on saccades. Contralateral-limb specificity was highest for AG and lowest for SPOC. Visual feedback of the hand negated rTMS-induced disruptions of the reach plan for mIPS and AG, but not SPOC. These results suggest that human SPOC is specialized for encoding retinally peripheral reach goals, whereas more anterior-lateral regions (mIPS and AG) along the IPS possess overlapping maps for saccade and reach planning and are more closely involved in motor details (i.e., planning the reach vector for a specific hand). This work provides the first causal evidence for functional specificity of these parietal regions in healthy humans.

Functionally dissociating temporal and motor components of response preparation in left intraparietal sulcus

To optimise speed and accuracy of motor behaviour, we can prepare not only the type of movement to be made but also the time at which it will be executed. Previous cued reaction-time paradigms have shown that anticipating the moment in time at which this response will be made (“temporal orienting”) or selectively preparing the motor effector with which an imminent response will be made (motor intention or “motor orienting”) recruits similar regions of left intraparietal sulcus (IPS), raising the possibility that these two preparatory processes are inextricably co-activated. We used a factorial design to independently cue motor and temporal components of response preparation within the same experimental paradigm. By differentially cueing either ocular or manual response systems, rather than spatially lateralised responses within just one of these systems, potential spatial confounds were removed. We demonstrated that temporal and motor orienting were behaviourally dissociable, each capable of improving performance alone. Crucially, fMRI data revealed that temporal orienting activated the left IPS even if the motor effector that would be used to execute the response was unpredictable. Moreover, temporal orienting activated left IPS whether the target required a saccadic or manual response, and whether this response was left- or right-sided, thus confirming the ubiquity of left IPS activation for temporal orienting. Finally, a small region of left IPS was also activated by motor orienting for manual, though not saccadic, responses. Despite their functional independence therefore, temporal orienting and manual motor orienting nevertheless engage partially overlapping regions of left IPS, possibly reflecting their shared ontogenetic roots.

Human Cortical Control of Hand Movements: Parietofrontal Networks for Reaching, Grasping, and Pointing

In primates, control of the limb depends on many cortical areas. Whereas specialized parietofrontal circuits have been proposed for different movements in macaques, functional neuroimaging in humans has revealed widespread, overlapping activations for hand and eye movements and for movements such as reaching and grasping. This review examines the involvement of frontal and parietal areas in hand and arm movements in humans as revealed with functional neuroimaging. The degree of functional specialization, possible homologies with macaque cortical regions, and differences between frontal and posterior parietal areas are discussed, as well as a possible organization of hand movements with respect to different spatial reference frames. The available evidence supports a cortical organization along gradients of sensory (visual to somatosensory) and effector (eye to hand) preferences.

Reference frames for reach planning in human parietofrontal cortex

To plan a reaching movement, the brain must integrate information about the spatial goal of the reach with positional information about the selected hand. Recent monkey neurophysiological evidence suggests that a mixture of reference frames is involved in this process. Here, using 3T functional magnetic resonance imaging (fMRI), we tested the role of gaze-centered and body-centered reference frames in reach planning in the human brain. Fourteen human subjects planned and executed arm movements to memorized visual targets, while hand starting position and gaze direction were monitored and varied on a trial-by-trial basis. We further introduced a variable delay between target presentation and movement onset to dissociate cerebral preparatory activity from stimulus- and movement-related responses. By varying the position of the target and hand relative to the gaze line, we distinguished cerebral responses that increased for those movements requiring the integration of peripheral target and hand positions in a gaze-centered frame. Posterior parietal and dorsal premotor areas showed such gaze-centered integration effects. In regions closer to the primary motor cortex, body-centered hand position effects were found. These results suggest that, in humans, spatially contiguous neuronal populations operate in different frames of reference, supporting sensorimotor transformations according to gaze-centered or body-centered coordinates. The former appears suited for calculating a difference vector between target and hand location, whereas the latter may be related to the implementation of a joint-based motor command.

Neuronal synchronization in human posterior parietal cortex during reach planning

Although single-unit studies in monkeys have identified effector-related regions in the posterior parietal cortex (PPC) during saccade and reach planning, the degree of effector specificity of corresponding human regions, as established by recordings of the blood oxygen level-dependent signal, is still under debate. Here, we addressed this issue from a different perspective, by studying the neuronal synchronization of the human PPC during both reach and saccade planning. Using magnetoencephalography (MEG), we recorded ongoing brain activity while subjects performed randomly alternating trials of memory-guided reaches or saccades. Additionally, subjects performed a dissociation task requiring them to plan both a memory-guided saccade and reach to locations in opposing visual hemifields. We examined changes in spectral power of the MEG signal during a 1.5 s memory period in relation to target location (left/right) and effector type (eye/hand). The results show direction-selective synchronization in the 70-90 Hz gamma frequency band, originating from the medial aspect of the PPC, when planning a reaching movement. In contrast, activity in a more central portion of the PPC was synchronized in a lower gamma band (50-60 Hz) when planning the direction of a saccade. Both observations were corroborated in the dissociation task. In the lower frequency bands, we observed sustained alpha-band (8-12 Hz) desynchronization in occipitoparietal regions, but in an effector-unspecific manner. These results suggest that distinct modules in the posterior parietal cortex encode movement goals of different effectors by selective gamma-band activity, compatible with the functional organization of monkey PPC.

Multifocal intraparietal activation during discrimination of action intention in observed tool grasping

The way we grasp an object varies depending on how we want to use that object, and this knowledge can be used to predict the object-related behavior of others. In this study, we assessed the neural correlates that determine the action intention of another person based on observed prehensile movements. Fourteen right-handed volunteers watched video clips of a person performing right-handed transitive grasping gestures that were either aimed at displacing or using a tool-object. Clips showing the grasping and displacement of neutral shapes served as a control condition. By discrimination of the actor's intention, three roughly symmetrical foci were activated in the anterior, middle, and caudal segments of the intraparietal sulci, and in the fusiform gyri and parts of the lateral occipital complex. Anterior intraparietal activation has been associated with the representation of object goals (object specific), and the present findings extend its involvement to functional goals (use-specific). Activation in the middle intraparietal region during intention discrimination was very similar to the activation elicited in a saccadic localizer task, suggesting a relation with spatial attention and eye movements. The caudal intraparietal region has been related with visuospatial guidance of reaching, and its activation during action intention discrimination indicates that the visuospatial properties of the observed reaching movement contribute to understanding of actions. As these parietal regions are strongly linked with motor behavior, our results appear to support the motor simulation hypothesis for action understanding with the preferential recruitment of the mirror-neuron system. This could at least be the case when no contextual information other than the visual properties of the movement is provided to discriminate the intention of an observed hand action.