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

Effector general representation of movement goals in human frontal and parietal cortex

In the nonhuman primate, discrete parts of premotor frontal and parietal cortex appear to code for movements of different effectors. However, the evidence regarding homologous effector selectivity within the human brain remains inconclusive. Here, we measured neural activity in the human brain using functional magnetic resonance imaging while participants remembered a target location and planned either saccades or reaches that matched the rich kinematics used in seminal monkey studies. We compared activity patterns during the planning period and used assumption-free multivariate searchlight analysis to identify brain regions that could decode the spatial goals of planned movements. Critically, we performed two types of decoding analyses to determine if the spatial information embedded in activation patterns was effector-specific or effector-general. For effector-specific spatial coding, we compared brain regions that could decode target locations within each effector. However, we did not identify areas that coded spatial information in one effector but not the other. For effector-general spatial coding, we performed spatial decoding using trials across effectors and conducted cross-effector decoding. Both analyses identified several areas in the frontal and parietal regions that encoded spatial information for both effectors, including precentral sulcus, superior parietal lobe, and intraparietal sulcus. Our results indicate that premotor frontal and parietal cortex encode the spatial metrics of movement goals that can be read out and converted into effector-specific motor metrics for saccades and reaches.

Exploring imitation of within hand prehensile object manipulation using fMRI and graph theory analysis

This study aims to establish an imitation task of multi-finger haptics in the context of regular grasping and regrasping processes during activities of daily living. A video guided the 26 healthy, right-handed volunteers through the three phases of the task: (1) fixation of a hand holding a cuboid, (2) observation of the sensori-motor manipulation, (3) imitation of that motor action. fMRI recorded the task; graph analysis of the acquisitions revealed the associated functional cerebral connectivity patterns. Inferred from four 60 ROI weighted graphs, the functional connectivities are consistent with a motor plan for observation and manipulation in the left hemisphere and with a network in the right hemisphere involving the inferior frontal gyrus, the site of intentional control of imitation. The networks exhibit (1) rich clubs which include sensori-motor hand, dorsal attention and cingulo-opercular communities for observation and motor execution in both hemispheres and (2) diversity clubs, significant only for manipulation and observation of the left hand, which include the dorsal visual association cortex, suggesting enhanced visual perception required for guiding the movement-limited left fingers. Short pathway analyses are consistent with these findings, confirming preferential involvement of ventral premotor cortices in the mirror network.

Electrophysiological indexes of the cognitive-motor trade-off associated with motor response complexity in a cognitive task

Complex actions require more cognitive and motor control than simple ones. Literature shows that to face complexity, the brain must make a compromise between available resources usually giving priority to motor control. However, literature has minimally explored the effect of the motor response complexity on brain processing associated with cognitive tasks. Consequently, it is unknown whether carrying out a cognitive task requiring motor responses of increasing complexity could reduce cognitive processing keeping stable motor control. Therefore, this study aims to investigate possible modulations exerted by increasing motor response complexity in a cognitive task on brain processing. To this aim, we analyzed the event-related potentials and behavioral responses during a cognitive task with increasing complexity of the required motor response (keypress, reaching and stepping). Results showed the increasing motor complexity enhances early visual and attentional processing (P1 and N1 components) but reduces the late post-perceptual cognitive control (P3 component). Additionally, we found a component following the P3 which was specific for stimuli requiring a response. This component, labeled N750, increased amplitude along with the response motor complexity. Behaviorally, response accuracy was not affected by complexity. Results indicated that in cognitive tasks stimulus processing is affected by the complexity of the motor response. Complex responses require a greater investment of early perceptual and attentional resources, but at late phases of processing, cognitive resources are less available in favor of motor resources. This confirms the idea of the motor-priority cognitive-motor trade-off of the brain.

The benefit of making voluntary choices generalizes across multiple effectors

It has been shown that cognitive performance could be improved by expressing volition (e.g., making voluntary choices), which necessarily involves the execution of action through a certain effector. However, it is unclear if the benefit of expressing volition can generalize across different effectors. In the present study, participants made a choice between two pictures either voluntarily or forcibly, and subsequently completed a visual search task with the chosen picture as a task-irrelevant background. The effector for choosing a picture could be the hand (pressing a key), foot (pedaling), mouth (commanding), or eye (gazing), whereas the effector for responding to the search target was always the hand. Results showed that participants responded faster and had a more liberal response criterion in the search task after a voluntary choice (vs. a forced choice). Importantly, the improved performance was observed regardless of which effector was used in making the choice, and regardless of whether the effector for making choices was the same as or different from the effector for responding to the search target. Eye-movement data for oculomotor choice showed that the main contributor to the facilitatory effect of voluntary choice was the post-search time in the visual search task (i.e., the time spent on processes after the target was found, such as response selection and execution). These results suggest that the expression of volition may involve the motor control system in which the effector-general, high-level processing of the goal of the voluntary action plays a key role.

Functional organization of the caudal part of the human superior parietal lobule

Like in macaque, the caudal portion of the human superior parietal lobule (SPL) plays a key role in a series of perceptive, visuomotor and somatosensory processes. Here, we review the functional properties of three separate portions of the caudal SPL, i.e., the posterior parieto-occipital sulcus (POs), the anterior POs, and the anterior part of the caudal SPL. We propose that the posterior POs is mainly dedicated to the analysis of visual motion cues useful for object motion detection during self-motion and for spatial navigation, while the more anterior parts are implicated in visuomotor control of limb actions. The anterior POs is mainly involved in using the spotlight of attention to guide reach-to-grasp hand movements, especially in dynamic environments. The anterior part of the caudal SPL plays a central role in visually guided locomotion, being implicated in controlling leg-related movements as well as the four limbs interaction with the environment, and in encoding egomotion-compatible optic flow. Together, these functions reveal how the caudal SPL is strongly implicated in skilled visually-guided behaviors.

Cortical maps of somatosensory perception in human

Tactile and movement-related somatosensory perceptions are crucial for our daily lives and survival. Although the primary somatosensory cortex is thought to be the key structure of somatosensory perception, various cortical downstream areas are also involved in somatosensory perceptual processing. However, little is known about whether cortical networks of these downstream areas can be dissociated depending on each perception, especially in human. We address this issue by combining data from direct cortical stimulation (DCS) for eliciting somatosensation and data from high-gamma band (HG) elicited during tactile stimulation and movement tasks. We found that artificial somatosensory perception is elicited not only from conventional somatosensory-related areas such as the primary and secondary somatosensory cortices but also from a widespread network including superior/inferior parietal lobules and premotor cortex. Interestingly, DCS on the dorsal part of the fronto-parietal area including superior parietal lobule and dorsal premotor cortex often induces movement-related somatosensations, whereas that on the ventral one including inferior parietal lobule and ventral premotor cortex generally elicits tactile sensations. Furthermore, the HG mapping results of the movement and passive tactile stimulation tasks revealed considerable similarity in the spatial distribution between the HG and DCS functional maps. Our findings showed that macroscopic neural processing for tactile and movement-related perceptions could be segregated.

Dynamic spatial coding in parietal cortex mediates tactile-motor transformation

Movements towards touch on the body require integrating tactile location and body posture information. Tactile processing and movement planning both rely on posterior parietal cortex (PPC) but their interplay is not understood. Here, human participants received tactile stimuli on their crossed and uncrossed feet, dissociating stimulus location relative to anatomy versus external space. Participants pointed to the touch or the equivalent location on the other foot, which dissociates sensory and motor locations. Multi-voxel pattern analysis of concurrently recorded fMRI signals revealed that tactile location was coded anatomically in anterior PPC but spatially in posterior PPC during sensory processing. After movement instructions were specified, PPC exclusively represented the movement goal in space, in regions associated with visuo-motor planning and with regional overlap for sensory, rule-related, and movement coding. Thus, PPC flexibly updates its spatial codes to accommodate rule-based transformation of sensory input to generate movement to environment and own body alike.

Effector-selective modulation of the effective connectivity within frontoparietal circuits during visuomotor tasks

Despite extensive research, the functional architecture of the subregions of the dorsal posterior parietal cortex (PPC) involved in sensorimotor processing is far from clear. Here, we draw a thorough picture of the large-scale functional organization of the PPC to disentangle the fronto-parietal networks mediating visuomotor functions. To this aim, we reanalyzed available human functional magnetic resonance imaging data collected during the execution of saccades, hand, and foot pointing, and we combined individual surface-based activation, resting-state functional connectivity, and effective connectivity analyses. We described a functional distinction between a more lateral region in the posterior intraparietal sulcus (lpIPS), preferring saccades over pointing and coupled with the frontal eye fields (FEF) at rest, and a more medial portion (mpIPS) intrinsically correlated to the dorsal premotor cortex (PMd). Dynamic causal modeling revealed feedforward-feedback loops linking lpIPS with FEF during saccades and mpIPS with PMd during pointing, with substantial differences between hand and foot. Despite an intrinsic specialization of the action-specific fronto-parietal networks, our study reveals that their functioning is finely regulated according to the effector to be used, being the dynamic interactions within those networks differently modulated when carrying out a similar movement (i.e. pointing) but with distinct effectors (i.e. hand and foot).

Allocation of Visuospatial Attention Indexes Evidence Accumulation for Reach Decisions

Visuospatial attention is a prerequisite for the performance of visually guided movements: perceptual discrimination is regularly enhanced at target locations before movement initiation. It is known that this attentional prioritization evolves over the time of movement preparation; however, it is not clear whether this build-up simply reflects a time requirement of attention formation or whether, instead, attention build-up reflects the emergence of the movement decision. To address this question, we combined behavioral experiments, psychophysics, and computational decision-making models to characterize the time course of attention build-up during motor preparation. Participants (n = 46, 29 female) executed center-out reaches to one of two potential target locations and reported the identity of a visual discrimination target (DT) that occurred concurrently at one of various time-points during movement preparation and execution. Visual discrimination increased simultaneously at the two potential target locations but was modulated by the experiment-wide probability that a given location would become the final goal. Attention increased further for the location that was then designated as the final goal location, with a time course closely related to movement initiation. A sequential sampling model of decision-making faithfully predicted key temporal characteristics of attentional allocation. Together, these findings provide evidence that visuospatial attentional prioritization during motor preparation does not simply reflect that a spatial location has been selected as movement goal, but rather indexes the time-extended, cumulative decision that leads to the selection, hence constituting a link between perceptual and motor aspects of sensorimotor decisions.

The human middle temporal cortex responds to both active leg movements and egomotion-compatible visual motion

The human middle-temporal region MT+ is highly specialized in processing visual motion. However, recent studies have shown that this region is modulated by extraretinal signals, suggesting a possible involvement in processing motion information also from non-visual modalities. Here, we used functional MRI data to investigate the influence of retinal and extraretinal signals on MT+ in a large sample of subjects. Moreover, we used resting-state functional MRI to assess how the subdivisions of MT+ (i.e., MST, FST, MT, and V4t) are functionally connected. We first compared responses in MST, FST, MT, and V4t to coherent vs. random visual motion. We found that only MST and FST were positively activated by coherent motion. Furthermore, regional analyses revealed that MST and FST were positively activated by leg, but not arm, movements, while MT and V4t were deactivated by arm, but not leg, movements. Taken together, regional analyses revealed a visuomotor role for the anterior areas MST and FST and a pure visual role for the anterior areas MT and V4t. These findings were mirrored by the pattern of functional connections between these areas and the rest of the brain. Visual and visuomotor regions showed distinct patterns of functional connectivity, with the latter preferentially connected with the somatosensory and motor areas representing leg and foot. Overall, these findings reveal a functional sensitivity for coherent visual motion and lower-limb movements in MST and FST, suggesting their possible involvement in integrating sensory and motor information to perform locomotion.

Egomotion-related visual areas respond to goal-directed movements

Integration of proprioceptive signals from the various effectors with visual feedback of self-motion from the retina is necessary for whole-body movement and locomotion. Here, we tested whether the human visual motion areas involved in processing optic flow signals simulating self-motion are also activated by goal-directed movements (as saccades or pointing) performed with different effectors (eye, hand, and foot), suggesting a role in visually guiding movements through the external environment. To achieve this aim, we used a combined approach of task-evoked activity and effective connectivity (PsychoPhysiological Interaction, PPI) by fMRI. We localized a set of six egomotion-responsive visual areas through the flow field stimulus and distinguished them into visual (pIPS/V3A, V6+ , IPSmot/VIP) and visuomotor (pCi, CSv, PIC) areas according to recent literature. We tested their response to a visuomotor task implying spatially directed delayed eye, hand, and foot movements. We observed a posterior-to-anterior gradient of preference for eye-to-foot movements, with posterior (visual) regions showing a preference for saccades, and anterior (visuomotor) regions showing a preference for foot pointing. No region showed a clear preference for hand pointing. Effective connectivity analysis showed that visual areas were more connected to each other with respect to the visuomotor areas, particularly during saccades. We suggest that visual and visuomotor egomotion regions can play different roles within a network that integrates sensory-motor signals with the aim of guiding movements in the external environment.

Are reaching and grasping effector-independent? Similarities and differences in reaching and grasping kinematics between the hand and foot

While reaching and grasping are highly prevalent manual actions, neuroimaging studies provide evidence that their neural representations may be shared between different body parts, i.e., effectors. If these actions are guided by effector-independent mechanisms, similar kinematics should be observed when the action is performed by the hand or by a cortically remote and less experienced effector, such as the foot. We tested this hypothesis with two characteristic components of action: the initial ballistic stage of reaching, and the preshaping of the digits during grasping based on object size. We examined if these kinematic features reflect effector-independent mechanisms by asking participants to reach toward and to grasp objects of different widths with their hand and foot. First, during both reaching and grasping, the velocity profile up to peak velocity matched between the hand and the foot, indicating a shared ballistic acceleration phase. Second, maximum grip aperture and time of maximum grip aperture of grasping increased with object size for both effectors, indicating encoding of object size during transport. Differences between the hand and foot were found in the deceleration phase and time of maximum grip aperture, likely due to biomechanical differences and the participants’ inexperience with foot actions. These findings provide evidence for effector-independent visuomotor mechanisms of reaching and grasping that generalize across body parts.

Exploring Cognition with Brain-Machine Interfaces

Traditional brain-machine interfaces decode cortical motor commands to control external devices. These commands are the product of higher-level cognitive processes, occurring across a network of brain areas, that integrate sensory information, plan upcoming motor actions, and monitor ongoing movements. We review cognitive signals recently discovered in the human posterior parietal cortex during neuroprosthetic clinical trials. These signals are consistent with small regions of cortex having a diverse role in cognitive aspects of movement control and body monitoring, including sensorimotor integration, planning, trajectory representation, somatosensation, action semantics, learning, and decision making. These variables are encoded within the same population of cells using structured representations that bind related sensory and motor variables, an architecture termed partially mixed selectivity. Diverse cognitive signals provide complementary information to traditional motor commands to enable more natural and intuitive control of external devices.

Repetition effects in action planning reflect effector- but not hemisphere-specific coding

Action choices are influenced by future and recent past action states. For example, when performing two actions in succession, response times (RTs) to initiate the second action are reduced when the same hand is used. These findings suggest the existence of effector-specific processing for action planning. However, given that each hand is primarily controlled by the contralateral hemisphere, the RT benefit might actually reflect effector-independent, hemisphere-specific rather than effector-specific repetition effects. Here, participants performed two consecutive movements, each with a hand or a foot, in one of two directions. Direction was specified in an egocentric reference frame (inward, outward) or in an allocentric reference frame (left, right). Successive actions were initiated faster when the same limb (e.g., left hand-left hand), but not the other limb of the same body side (e.g., left foot-left hand), executed the second action. The same-limb advantage was evident even when the two movements involved different directions, whether specified egocentrically or allocentrically. Corroborating evidence from computational modeling lends support to the claim that repetition effects in action planning reflect persistent changes in baseline activity within neural populations that encode effector-specific action plans.

NEW & NOTEWORTHY Repeated hand use facilitates the initiation of successive actions (repetition effect). This finding has been interpreted as evidence for effector-specific action plans. However, given that each hand is primarily controlled by the contralateral hemisphere, any differences might reflect effector-independent, hemisphere-specific rather than effector-specific processing. We dissociated these alternatives by asking participants to perform successive actions with hands and feet and provide novel evidence that repetition effects in limb use truly reflect effector-specific coding.

Multisensory integration in cortical regions responding to locomotion-related visual and somatomotor signals

During real-world locomotion, in order to be able to move along a path or avoid an obstacle, continuous changes in self-motion direction (i.e. heading) are needed. Control of heading changes during locomotion requires the integration of multiple signals (i.e., visual, somatomotor, vestibular). Recent fMRI studies have shown that both somatomotor areas (human PEc [hPEc], human PE [hPE], primary somatosensory cortex [S-I]) and egomotion visual regions (cingulate sulcus visual area [CSv], posterior cingulate area [pCi], posterior insular cortex [PIC]) respond to either leg movements and egomotion-compatible visual stimulations, suggesting a role in the analysis of both visual attributes of egomotion and somatomotor signals with the aim of guiding locomotion. However, whether these regions are able to integrate egomotion-related visual signals with somatomotor inputs coming from leg movements during heading changes remains an open question. Here we used a combined approach of individual functional localizers and task-evoked activity by fMRI. In thirty subjects we first localized three egomotion areas (CSv, pCi, PIC) and three somatomotor regions (S-I, hPE, hPEc). Then, we tested their responses in a multisensory integration experiment combining visual and somatomotor signals relevant to locomotion in congruent or incongruent trials. We used an fMR-adaptation paradigm to explore the sensitivity to the repeated presentation of these bimodal stimuli in the six regions of interest. Results revealed that hPE, S-I and CSv showed an adaptation effect regardless of congruency, while PIC, pCi and hPEc showed sensitivity to congruency. PIC exhibited a preference for congruent trials compared to incongruent trials. Areas pCi and hPEc exhibited an adaptation effect only for congruent and incongruent trials, respectively. PIC, pCi and hPEc sensitivity to the congruency relationship between visual (locomotion-compatible) cues and (leg-related) somatomotor inputs suggests that these regions are involved in multisensory integration processes, likely in order to guide/adjust leg movements during heading changes.

Cortical beta-band power modulates with uncertainty in effector selection during motor planning

Although beta-band activity during motor planning is known to be modulated by uncertainty about where to act, less is known about its modulations to uncertainty about how to act. To investigate this issue, we recorded oscillatory brain activity with EEG while human participants (n = 17) performed a hand choice reaching task. The reaching hand was either predetermined or of participants' choice, and the target was close to one of the two hands or at about equal distance from both. To measure neural activity in a motion artifact-free time window, the location of the upcoming target was cued 1,000-1,500 ms before the presentation of the target, whereby the cue was valid in 50% of trials. As evidence for motor planning during the cuing phase, behavioral observations showed that the cue affected later hand choice. Furthermore, reaction times were longer in the choice trials than in the predetermined trials, supporting the notion of a competitive process for hand selection. Modulations of beta-band power over central cortical regions, but not alpha-band or theta-band power, were in line with these observations. During the cuing period, reaches in predetermined trials were preceded by larger decreases in beta-band power than reaches in choice trials. Cue direction did not affect reaction times or beta-band power, which may be due to the cue being invalid in 50% of trials, retaining effector uncertainty during motor planning. Our findings suggest that effector uncertainty modulates beta-band power during motor planning.NEW & NOTEWORTHY Although reach-related beta-band power in central cortical areas is known to modulate with the number of potential targets, here we show, using a cuing paradigm, that the power in this frequency band, but not in the alpha or theta band, is also modulated by the uncertainty of which hand to use. This finding supports the notion that multiple possible effector-specific actions can be specified in parallel up to the level of motor preparation.

Sensory-Motor Modulations of EEG Event-Related Potentials Reflect Walking-Related Macro-Affordances

One fundamental principle of the brain functional organization is the elaboration of sensory information for the specification of action plans that are most appropriate for interaction with the environment. Using an incidental go/no-go priming paradigm, we have previously shown a facilitation effect for the execution of a walking-related action in response to far vs. near objects/locations in the extrapersonal space, and this effect has been called “macro-affordance” to reflect the role of locomotion in the coverage of extrapersonal distance. Here, we investigated the neurophysiological underpinnings of such an effect by recording scalp electroencephalography (EEG) from 30 human participants during the same paradigm. The results of a whole-brain analysis indicated a significant modulation of the event-related potentials (ERPs) both during prime and target stimulus presentation. Specifically, consistent with a mechanism of action anticipation and automatic activation of affordances, a stronger ERP was observed in response to prime images framing the environment from a far vs. near distance, and this modulation was localized in dorso-medial motor regions. In addition, an inversion of polarity for far vs. near conditions was observed during the subsequent target period in dorso-medial parietal regions associated with spatially directed foot-related actions. These findings were interpreted within the framework of embodied models of brain functioning as arising from a mechanism of motor-anticipation and subsequent prediction error which was guided by the preferential affordance relationship between the distant large-scale environment and locomotion. More in general, our findings reveal a sensory-motor mechanism for the processing of walking-related environmental affordances.

The macaque ventral intraparietal area has expanded into three homologue human parietal areas

The macaque ventral intraparietal area (VIP) in the fundus of the intraparietal sulcus has been implicated in a diverse range of sensorimotor and cognitive functions such as motion processing, multisensory integration, processing of head peripersonal space, defensive behavior, and numerosity coding. Here, we exhaustively review macaque VIP function, cytoarchitectonics, and anatomical connectivity and integrate it with human studies that have attempted to identify a potential human VIP homologue. We show that human VIP research has consistently identified three, rather than one, bilateral parietal areas that each appear to subsume some, but not all, of the macaque area's functionality. Available evidence suggests that this human "VIP complex" has evolved as an expansion of the macaque area, but that some precursory specialization within macaque VIP has been previously overlooked. The three human areas are dominated, roughly, by coding the head or self in the environment, visual heading direction, and the peripersonal environment around the head, respectively. A unifying functional principle may be best described as prediction in space and time, linking VIP to state estimation as a key parietal sensorimotor function. VIP's expansive differentiation of head and self-related processing may have been key in the emergence of human bodily self-consciousness.

Lower visual field preference for the visuomotor control of limb movements in the human dorsomedial parietal cortex

Visual cues coming from the lower visual field (VF) play an important role in the visual guidance of upper and lower limb movements. A recently described region situated in the dorsomedial parietal cortex, area hPEc (Pitzalis et al. in NeuroImage 202:116092, 2019), might have a role in integrating visually derived information with somatomotor signals to guide limb interaction with the environment. In macaque, it has been demonstrated that PEc receives visual information mostly from the lower visual field but, to date, there has been no systematic investigation of VF preference in the newly defined human homologue of macaque area PEc (hPEc). Here we examined the VF preferences of hPEc while participants performed a visuomotor task implying spatially directed delayed eye-, hand- and foot-movements towards different spatial locations within the VF. By analyzing data as a function of the different target locations towards which upcoming movements were planned (and then executed), we observed the presence of asymmetry in the vertical dimension of VF in area hPEc, being this area more strongly activated by limb movements directed towards visual targets located in the lower compared to the upper VF. This result confirms the view, first advanced in macaque monkey, that PEc is involved in processing visual information to guide body interaction with the external environment, including locomotion. We also observed a contralateral dominance for the lower VF preference in the foot selective somatomotor cortex anterior to hPEc. This result might reflect the role of this cortex (which includes areas PE and S-I) in providing highly topographically organized signals, likely useful to achieve an appropriate foot posture during locomotion.

Automatic coding of environmental distance for walking-related locomotion in the foot-related sensory-motor system: A TMS study on macro-affordances

We have recently described a facilitation effect for the execution of a walking-related action in response to distant objects/locations in the extrapersonal space. Based on the parallelism with the well-known effect of "micro-affordance", observed during the execution of functionally appropriate hand-related actions towards manipulable objects, we have referred to this effect in terms of "macro-affordance". Here we used transcranical magnetic stimulation (TMS) to investigate whether a foot-related region located in the human dorsal precuneate cortex plays a causal role in the generation and maintenance of such behavioral effect. This question was addressed by comparing the magnitude of the facilitation effect during an incidental go/no-go task, i.e. advantage for walking-related actions to pictures framing an environment from a far vs. near distance, during three different TMS conditions. The three TMS conditions were collected in all subjects in a randomized order and included stimulation of: i. a foot-related region in the anterior precuneus, ii. a control region in the middle intraparietal sulcus (mIPS), and iii. a sham condition. Enrollment in the TMS protocol was based on analysis of individual performance during a preliminary session conducted using a sham stimulation. TMS was administered at a low frequency range before the beginning of each condition. The results showed that stimulation of the foot-related region in the anterior precuneus produced a significant reduction of the walking-related facilitation effect as compared to both stimulation of the active-control region and the non-active sham stimulation. These findings suggest that the foot-related sensory-motor system directly participates in the process of extraction of the spatial features (i.e. distance) from an environmental scene that are useful for locomotion. More in general, these findings support an automatic coding of environmental affordance or "macro-affordances" in the walking-related sensory-motor system.

Evidence for an effector-independent action system from people born without hands

What are the principles of brain organization? In the motor domain, separate pathways were found for reaching and grasping actions performed by the hand. To what extent is this organization specific to the hand or based on abstract action types, regardless of which body part performs them? We tested people born without hands who perform actions with their feet. Activity in frontoparietal association motor areas showed preference for an action type (reaching or grasping), regardless of whether it was performed by the foot in people born without hands or by the hand in typically-developed controls. These findings provide evidence that some association areas are organized based on abstract functions of action types, independent of specific sensorimotor experience and parameters of specific body parts.

Many parts of the visuomotor system guide daily hand actions, like reaching for and grasping objects. Do these regions depend exclusively on the hand as a specific body part whose movement they guide, or are they organized for the reaching task per se, for any body part used as an effector? To address this question, we conducted a neuroimaging study with people born without upper limbs—individuals with dysplasia—who use the feet to act, as they and typically developed controls performed reaching and grasping actions with their dominant effector. Individuals with dysplasia have no prior experience acting with hands, allowing us to control for hand motor imagery when acting with another effector (i.e., foot). Primary sensorimotor cortices showed selectivity for the hand in controls and foot in individuals with dysplasia. Importantly, we found a preference based on action type (reaching/grasping) regardless of the effector used in the association sensorimotor cortex, in the left intraparietal sulcus and dorsal premotor cortex, as well as in the basal ganglia and anterior cerebellum. These areas also showed differential response patterns between action types for both groups. Intermediate areas along a posterior–anterior gradient in the left dorsal premotor cortex gradually transitioned from selectivity based on the body part to selectivity based on the action type. These findings indicate that some visuomotor association areas are organized based on abstract action functions independent of specific sensorimotor parameters, paralleling sensory feature-independence in visual and auditory cortices in people born blind and deaf. Together, they suggest association cortices across action and perception may support specific computations, abstracted from low-level sensorimotor elements.

Properties and temporal dynamics of choice- and action-predictive signals during item recognition decisions

Decision-making is in the service of action regardless of whether the decision concerns perceptual information, goods or memories. Compared to recent advances in the neurobiology of perceptual or value-based decisions, however, the neural bases supporting the sampling of evidence in long-term memory, and the transformation of memory-based decisions into appropriate actions, are still poorly understood. In the present fMRI study, we used multivariate pattern analysis to investigate the temporal dynamics of choice- and action-predictive signals during an item recognition task that manipulated the association between memory choices (old/new) and motor responses (eye/hand) across subjects. Choice-predictive activity was mainly observed in striatal, lateral prefrontal and lateral parietal regions, was sensitive to the amount of decision evidence and showed a rapid increase after stimulus onset, followed by a fast decay. Action-predictive signals were found in primary sensory motor, premotor and occipito-parietal regions, were generally observed at the end of the decision phase and were not modulated by decision evidence. These findings suggest that a memory decision variable, potentially represented in a fronto-striato-parietal network, is not directly transformed into an action plan as often observed in perceptual decisions. Regions exhibiting choice predictive activity, and especially the striatum, however, also showed a second peak of decision-related activity that, unlike pure choice- or action-predictive signals, depended on the particular choice-response association. This second peak of activity in the striatum might represent the neural signature of the transformation of a memory decision into an appropriate motor response based on the specific choice-response association.

Hierarchical Action Encoding Within the Human Brain

Humans are able to interact with objects with extreme flexibility. To achieve this ability, the brain does not only control specific muscular patterns, but it also needs to represent the abstract goal of an action, irrespective of its implementation. It is debated, however, how abstract action goals are implemented in the brain. To address this question, we used multivariate pattern analysis of functional magnetic resonance imaging data. Human participants performed grasping actions (precision grip, whole hand grip) with two different wrist orientations (canonical, rotated), using either the left or right hand. This design permitted to investigate a hierarchical organization consisting of three levels of abstraction: 1) “concrete action” encoding; 2) “effector-dependent goal” encoding (invariant to wrist orientation); and 3) “effector-independent goal” encoding (invariant to effector and wrist orientation). We found that motor cortices hosted joint encoding of concrete actions and of effector-dependent goals, while the parietal lobe housed a convergence of all three representations, comprising action goals within and across effectors. The left lateral occipito-temporal cortex showed effector-independent goal encoding, but no convergence across the three levels of representation. Our results support a hierarchical organization of action encoding, shedding light on the neural substrates supporting the extraordinary flexibility of human hand behavior.

Action Observation and Effector Independency

The finding of reasonably consistent spatial and temporal productions of actions across different body parts has been used to argue in favor of the existence of a high-order representation of motor programs. In these terms, a generalized motor program consists of an abstract memory structure apt to specify a class of non-specific instructions used to guide a broad range of movements (e.g., “grasp,” “bite”). Although a number of studies, using a variety of tasks, have assessed the issue of effector independence in terms of action execution, little is known regarding the issue of effector independence within an action observation context. Here corticospinal excitability (CSE) of the right hand’s first dorsal interosseous (FDI) and abductor digiti minimi (ADM) muscles was assessed by means of single-pulse transcranial magnetic stimulation (spTMS) during observation of a grasping action performed by the hand, the foot, the mouth, the elbow, or the knee. The results indicate that observing a grasping action performed with different body parts activates the effector typically adopted to execute that action, i.e., the hand. We contend that, as far as grasping is concerned, motor activations by action observation are evident in the muscles typically used to perform the observed action, even when the action is executed with another effector. Nevertheless, some exceptions call for a deeper analysis of motor coding.

No effect of triple-pulse TMS medial to intraparietal sulcus on online correction for target perturbations during goal-directed hand and foot reaches

Posterior parietal cortex (PPC) is central to sensorimotor processing for goal-directed hand and foot movements. Yet, the specific role of PPC subregions in these functions is not clear. Previous human neuroimaging and transcranial magnetic stimulation (TMS) work has suggested that PPC lateral to the intraparietal sulcus (IPS) is involved in directing the arm, shaping the hand, and correcting both finger-shaping and hand trajectory during movement. The lateral localization of these functions agrees with the comparably lateral position of the hand and fingers within the motor and somatosensory homunculi along the central sulcus; this might suggest that, in analogy, (goal-directed) foot movements would be mediated by medial portions of PPC. However, foot movement planning activates similar regions for both hand and foot movement along the caudal-to-rostral axis of PPC, with some effector-specificity evident only rostrally, near the central regions of sensorimotor cortex. Here, we attempted to test the causal involvement of PPC regions medial to IPS in hand and foot reaching as well as online correction evoked by target displacement. Participants made hand and foot reaches towards identical visual targets. Sometimes, the target changed position 100–117 ms into the movement. We disturbed cortical processing over four positions medial to IPS with three pulses of TMS separated by 40 ms, both during trials with and without target displacement. We timed TMS to disrupt reach execution and online correction. TMS did not affect endpoint error, endpoint variability, or reach trajectories for hand or foot. While these negative results await replication with different TMS timing and parameters, we conclude that regions medial to IPS are involved in planning, rather than execution and online control, of goal-directed limb movements.

A putative human homologue of the macaque area PEc

The cortical area PEc is anatomically and functionally well-defined in macaque, but it is unknown whether it has a counterpart in human. Since we know that macaque PEc, but not the nearby posterior regions, hosts a lower limb representation, in an attempt to recognize a possible human PEc we looked for the existence of leg representations in the human parietal cortex using individual cortical surface-based analysis, task-evoked paradigms and resting-state functional connectivity. fMRI images were acquired while thirty-one participants performed long-range leg movements through an in-house MRI-compatible set-up. We revealed the existence of multiple leg representations in the human dorsomedial parietal cortex, here defined as S-I (somatosensory-I), hPE (human PE, in the postcentral sulcus), and hPEc (human PEc, in the anterior precuneus). Among the three "leg" regions, hPEc had a unique functional profile, in that it was the only one responding to both arm and leg movements, to both hand-pointing and foot pointing movements, and to flow field visual stimulation, very similar to macaque area PEc. In addition, hPEc showed functional connections with the somatomotor regions hosting a lower limb representation, again as in macaque area PEc. Therefore, based on similarity in brain position, functional organization, cortical connections, and relationship with the neighboring areas, we propose that this cortical region is the human homologue of macaque area PEc.

Hand forces and placement are modulated and covary during anticipatory control of bimanual manipulation

Dexterous object manipulation relies on the feedforward and feedback control of kinetics (forces) and kinematics (hand shaping and digit placement). Lifting objects with an uneven mass distribution involves the generation of compensatory moments at object lift-off to counter object torques. This is accomplished through the modulation and covariation of digit forces and placement, which has been shown to be a general feature of unimanual manipulation. These feedforward anticipatory processes occur before performance-specific feedback. Whether this adaptation is a feature unique to unimanual dexterous manipulation or general across unimanual and bimanual manipulation is not known. We investigated the generation of compensatory moments through hand placement and force modulation during bimanual manipulation of an object with variable center of mass. Participants were instructed to prevent object roll during the lift. Similar to unimanual grasping, we found modulation and covariation of hand forces and placement for successful performance. Thus this motor adaptation of the anticipatory control of compensatory moment is a general feature across unimanual and bimanual effectors. Our results highlight the involvement of high-level representation of manipulation goals and underscore a sensorimotor circuitry for anticipatory control through a continuum of force and placement modulation of object manipulation across a range of effectors.

NEW & NOTEWORTHY This is the first study, to our knowledge, to show that successful bimanual manipulation of objects with asymmetrical centers of mass is performed through the modulation and covariation of hand forces and placements to generate compensatory moments. Digit force-to-placement modulation is thus a general phenomenon across multiple effectors, such as the fingers of one hand, and both hands. This adds to our understanding of integrating low-level internal representations of object properties into high-level task representations.

Subjective vividness of motor imagery has a neural signature in human premotor and parietal cortex

Motor imagery (MI) is the process in which subjects imagine executing a body movement with a strong kinesthetic component from a first-person perspective. The individual capacity to elicit such mental images is not universal but varies within and between subjects. Neuroimaging studies have shown that these inter-as well as intra-individual differences in imagery quality mediate the amplitude of neural activity during MI on a group level. However, these analyses were not sensitive to forms of representation that may not map onto a simple modulation of overall amplitude. Therefore, the present study asked how far the subjective impression of motor imagery vividness is reflected by a spatial neural code, and how patterns of neural activation in different motor regions relate to specific imagery impressions. During fMRI scanning, 20 volunteers imagined three different types of right-hand actions. After each imagery trial, subjects were asked to evaluate the perceived vividness of their imagery. A correlation analysis compared the rating differences and neural dissimilarity values of the rating groups separately for each region of interest. Results showed a significant positive correlation in the left vPMC and right IPL, indicating that these regions particularly reflect perceived imagery vividness in that similar rated trials evoke more similar neural patterns. A decoding analysis revealed that the vividness of the motor image related systematically to the action specificity of neural activation patterns in left vPMC and right SPL. Imagined actions accompanied by higher vividness ratings were significantly more distinguishable within these areas. Altogether, results showed that spatial patterns of neural activity within the human motor cortices reflect the individual vividness of imagined actions. Hence, the findings reveal a link between the subjective impression of motor imagery vividness and objective physiological markers.

Egomotion-related visual areas respond to active leg movements

Monkey neurophysiology and human neuroimaging studies have demonstrated that passive viewing of optic flow stimuli activates a cortical network of temporal, parietal, insular, and cingulate visual motion regions. Here, we tested whether the human visual motion areas involved in processing optic flow signals simulating self-motion are also activated by active lower limb movements, and hence are likely involved in guiding human locomotion. To this aim, we used a combined approach of task-evoked activity and resting-state functional connectivity by fMRI. We localized a set of six egomotion-responsive visual areas (V6+, V3A, intraparietal motion/ventral intraparietal [IPSmot/VIP], cingulate sulcus visual area [CSv], posterior cingulate sulcus area [pCi], posterior insular cortex [PIC]) by using optic flow. We tested their response to a motor task implying long-range active leg movements. Results revealed that, among these visually defined areas, CSv, pCi, and PIC responded to leg movements (visuomotor areas), while V6+, V3A, and IPSmot/VIP did not (visual areas). Functional connectivity analysis showed that visuomotor areas are connected to the cingulate motor areas, the supplementary motor area, and notably to the medial portion of the somatosensory cortex, which represents legs and feet. We suggest that CSv, pCi, and PIC perform the visual analysis of egomotion-like signals to provide sensory information to the motor system with the aim of guiding locomotion.

Feasibility of identifying the ideal locations for motor intention decoding using unimodal and multimodal classification at 7T-fMRI

Invasive Brain-Computer Interfaces (BCIs) require surgeries with high health-risks. The risk-to-benefit ratio of the procedure could potentially be improved by pre-surgically identifying the ideal locations for mental strategy classification. We recorded high-spatiotemporal resolution blood-oxygenation-level-dependent (BOLD) signals using functional MRI at 7 Tesla in eleven healthy participants during two motor imagery tasks. BCI diagnostic task isolated the intent to imagine movements, while BCI simulation task simulated the neural states that may be yielded in a real-life BCI-operation scenario. Imagination of movements were classified from the BOLD signals in sub-regions of activation within a single or multiple dorsal motor network regions. Then, the participant's decoding performance during the BCI simulation task was predicted from the BCI diagnostic task. The results revealed that drawing information from multiple regions compared to a single region increased the classification accuracy of imagined movements. Importantly, systematic unimodal and multimodal classification revealed the ideal combination of regions that yielded the best classification accuracy at the individual-level. Lastly, a given participant's decoding performance achieved during the BCI simulation task could be predicted from the BCI diagnostic task. These results show the feasibility of 7T-fMRI with unimodal and multimodal classification being utilized for identifying ideal sites for mental strategy classification.

Human posterior parietal and dorsal premotor cortex encode the visual properties of an upcoming action

Behavioral studies show that motor actions are planned by adapting motor programs to produce desired visual consequences. Does this mean that the brain plans these visual consequences independent of the motor actions required to obtain them? Here we addressed this question by investigating planning-related fMRI activity in human posterior parietal (PPC) and dorsal premotor (PMd) cortex. By manipulating visual movement of a virtual end-effector controlled via button presses we could dissociate motor actions from their sensory outcome. A clear representation of the visual consequences was visible in both PPC and PMd activity during early planning stages. Our findings suggest that in both PPC and PMd action plans are initially represented on the basis of the desired sensory outcomes while later activity shifts towards representing motor programs.

Right Hemisphere Lateralization in Neural Connectivity Within Fronto-Parietal Networks in Non-human Primates During a Visual Reaching Task

A fronto-parietal network, comprised of the posterior parietal cortex (PPC) and the dorsal premotor cortex (PMd) has been proposed to be involved in planning and guiding movement. However, the issue of how the network is expressed across the bilateral cortical area according to the effector's side remains unclear. In this study, we tested these questions using electrocorticographic (ECoG) recordings in non-human primates and using a simple visual guided reaching task that induced a left or right hand response based on relevant cues provided for the task. The findings indicate that right hemisphere lateralized network patterns in which the right PMd was strongly coordinated with bilateral PPC immediately after presentation of the movement cue occurred, while the coherence with the left PMd was not enhanced. No difference was found in the coherence pattern between the effector's side (left hand or right hand), but the strength of coherence was different, in that animals showed a higher coherence in the right hand response compared to the left. Our data support that right lateralization in long-range phase synchrony in the 10–20 Hz low beta band is involved in motor preparation stage, irrespective of the upcoming effector's side.

Imagined and Executed Actions in the Human Motor System: Testing Neural Similarity Between Execution and Imagery of Actions with a Multivariate Approach

Simulation theory proposes motor imagery (MI) to be a simulation based on representations also used for motor execution (ME). Nonetheless, it is unclear how far they use the same neural code. We use multivariate pattern analysis (MVPA) and representational similarity analysis (RSA) to describe the neural representations associated with MI and ME within the frontoparietal motor network. During functional magnetic resonance imaging scanning, 20 volunteers imagined or executed 3 different types of right-hand actions. Results of MVPA showed that these actions as well as their modality (MI or ME) could be decoded significantly above chance from the spatial patterns of BOLD signals in premotor and posterior parietal cortices. This was also true for cross-modal decoding. Furthermore, representational dissimilarity matrices of frontal and parietal areas showed that MI and ME representations formed separate clusters, but that the representational organization of action types within these clusters was identical. For most ROIs, this pattern of results best fits with a model that assumes a low-to-moderate degree of similarity between the neural patterns associated with MI and ME. Thus, neural representations of MI and ME are neither the same nor totally distinct but exhibit a similar structural geometry with respect to different types of action.

The dorsal "action" pathway

In 1992, Goodale and Milner proposed a division of labor in the visual pathways of the primate cerebral cortex. According to their account, the ventral pathway, which projects to occipitotemporal cortex, constructs our visual percepts, while the dorsal pathway, which projects to posterior parietal cortex, mediates the visual control of action. Although the framing of the two-visual-system hypothesis has not been without controversy, it is clear that vision for action and vision for perception have distinct computational requirements, and significant support for the proposed neuroanatomic division has continued to emerge over the last two decades from human neuropsychology, neuroimaging, behavioral psychophysics, and monkey neurophysiology. In this chapter, we review much of this evidence, with a particular focus on recent findings from human neuroimaging and monkey neurophysiology, demonstrating a specialized role for parietal cortex in visually guided behavior. But even though the available evidence suggests that dedicated circuits mediate action and perception, in order to produce adaptive goal-directed behavior there must be a close coupling and seamless integration of information processing across these two systems. We discuss such ventral-dorsal-stream interactions and argue that the two pathways play different, yet complementary, roles in the production of skilled behavior.

Time-resolved decoding of planned delayed and immediate prehension movements

Different contexts require us either to react immediately, or to delay (or suppress) a planned movement. Previous studies that aimed at decoding movement plans typically dissociated movement preparation and execution by means of delayed-movement paradigms. Here we asked whether these results can be generalized to the planning and execution of immediate movements. To directly compare delayed, non-delayed, and suppressed reaching and grasping movements, we used a slow event-related functional magnetic resonance imaging (fMRI) design. To examine how neural representations evolved throughout movement planning, execution, and suppression, we performed time-resolved multivariate pattern analysis (MVPA). During the planning phase, we were able to decode upcoming reaching and grasping movements in contralateral parietal and premotor areas. During the execution phase, we were able to decode movements in a widespread bilateral network of motor, premotor, and somatosensory areas. Moreover, we obtained significant decoding across delayed and non-delayed movement plans in contralateral primary motor cortex. Our results demonstrate the feasibility of time-resolved MVPA and provide new insights into the dynamics of the prehension network, suggesting early neural representations of movement plans in the primary motor cortex that are shared between delayed and non-delayed contexts.

Partially Mixed Selectivity in Human Posterior Parietal Association Cortex

To clarify the organization of motor representations in posterior parietal cortex, we test how three motor variables (body side, body part, cognitive strategy) are coded in the human anterior intraparietal cortex. All tested movements were encoded, arguing against strict anatomical segregation of effectors. Single units coded for diverse conjunctions of variables, with different dimensions anatomically overlapping. Consistent with recent studies, neurons encoding body parts exhibited mixed selectivity. This mixed selectivity resulted in largely orthogonal coding of body parts, which "functionally segregate" the effector responses despite the high degree of anatomical overlap. Body side and strategy were not coded in a mixed manner as effector determined their organization. Mixed coding of some variables over others, what we term "partially mixed coding," argues that the type of functional encoding depends on the compared dimensions. This structure is advantageous for neuroprosthetics, allowing a single array to decode movements of a large extent of the body.

Observing Others Speak or Sing Activates Spt and Neighboring Parietal Cortex

To obtain further evidence that action observation can serve as a proxy for action execution and planning in posterior parietal cortex, we scanned participants while they were (1) observing two classes of action: vocal communication and oral manipulation, which share the same effector but differ in nature, and (2) rehearsing and listening to nonsense sentences to localize area Spt, thought to be involved in audio-motor transformation during speech. Using this localizer, we found that Spt is specifically activated by vocal communication, indicating that Spt is not only involved in planning speech but also in observing vocal communication actions. In addition, we observed that Spt is distinct from the parietal region most specialized for observing vocal communication, revealed by an interaction contrast and located in PFm. The latter region, unlike Spt, processes the visual and auditory signals related to other's vocal communication independently. Our findings are consistent with the view that several small regions in the temporoparietal cortex near the ventral part of the supramarginal/angular gyrus border are involved in the planning of vocal communication actions and are also concerned with observation of these actions, though involvements in those two aspects are unequal.

Differential Recruitment of Parietal Cortex during Spatial and Non-spatial Reach Planning

The planning of goal-directed arm reaching movements is associated with activity in the dorsal parieto-frontal cortex, within which multiple regions subserve the integration of arm- and target-related sensory signals to encode a motor goal. Surprisingly, many of these regions show sustained activity during reach preparation even when target location is not specified, i.e., when a motor goal cannot be unambiguously formed. The functional role of these non-spatial preparatory signals remains unresolved. Here this process was investigated in humans by comparing reach preparatory activity in the presence or absence of information regarding upcoming target location. In order to isolate the processes specific to reaching and to control for visuospatial attentional factors, the reaching task was contrasted to a finger movement task. Functional MRI and electroencephalography (EEG) were used to characterize the spatio-temporal pattern of reach-related activity in the parieto-frontal cortex. Reach planning with advance knowledge of target location induced robust blood oxygenated level dependent and EEG responses across parietal and premotor regions contralateral to the reaching arm. In contrast, reach preparation without knowledge of target location was associated with a significant BOLD response bilaterally in the parietal cortex. Furthermore, EEG alpha- and beta-band activity was restricted to parietal scalp sites, the magnitude of the latter being correlated with reach reaction times. These results suggest an intermediate stage of sensorimotor transformations in bilateral parietal cortex when target location is not specified.

Mixed Body/Hand Reference Frame for Reaching in 3D Space in Macaque Parietal Area PEc

The neural correlates of coordinate transformations from vision to action are expressed in the activity of posterior parietal cortex (PPC). It has been demonstrated that among the medial-most areas of the PPC, reaching targets are represented mainly in hand-centered coordinates in area PE, and in eye-centered, body-centered, and mixed body/hand-centered coordinates in area V6A. Here, we assessed whether neurons of area PEc, located between V6A and PE in the medial PPC, encode targets in body-centered, hand-centered, or mixed frame of reference during planning and execution of reaching. We studied 104 PEc cells in 3 Macaca fascicularis. The animals performed a reaching task toward foveated targets located at different depths and directions in darkness, starting with the hand from 2 positions located at different depths, one next to the trunk and the other far from it. We show that most PEc neurons encoded targets in a mixed body/hand-centered frame of reference. Although the effect of hand position was often rather strong, it was not as strong as reported previously in area PE. Our results suggest that area PEc represents an intermediate node in the gradual transformation from vision to action that takes place in the reaching network of the dorsomedial PPC.

Decoding Movement Goals from the Fronto-Parietal Reach Network

During reach planning, fronto-parietal brain areas need to transform sensory information into a motor code. It is debated whether these areas maintain a sensory representation of the visual cue or a motor representation of the upcoming movement goal. Here, we present results from a delayed pro-/anti-reach task which allowed for dissociating the position of the visual cue from the reach goal. In this task, the visual cue was combined with a context rule (pro vs. anti) to infer the movement goal. Different levels of movement goal specification during the delay were obtained by presenting the context rule either before the delay together with the visual cue (specified movement goal) or after the delay (underspecified movement goal). By applying functional magnetic resonance imaging (fMRI) multivoxel pattern analysis (MVPA), we demonstrate movement goal encoding in the left dorsal premotor cortex (PMd) and bilateral superior parietal lobule (SPL) when the reach goal is specified. This suggests that fronto-parietal reach regions (PRRs) maintain a prospective motor code during reach planning. When the reach goal is underspecified, only area PMd but not SPL represents the visual cue position indicating an incomplete state of sensorimotor integration. Moreover, this result suggests a potential role of PMd in movement goal selection.

Neuronal correlates of continuous manual tracking under varying visual movement feedback in a virtual reality environment

To accurately guide one's actions online, the brain predicts sensory action feedback ahead of time based on internal models, which can be updated by sensory prediction errors. The underlying operations can be experimentally investigated in sensorimotor adaptation tasks, in which moving under perturbed sensory action feedback requires internal model updates. Here we altered healthy participants' visual hand movement feedback in a virtual reality setup, while assessing brain activity with functional magnetic resonance imaging (fMRI). Participants tracked a continually moving virtual target object with a photorealistic, three-dimensional (3D) virtual hand controlled online via a data glove. During the continuous tracking task, the virtual hand's movements (i.e., visual movement feedback) were repeatedly periodically delayed, which participants had to compensate for to maintain accurate tracking. This realistic task design allowed us to simultaneously investigate processes likely operating at several levels of the brain's motor control hierarchy. FMRI revealed that the length of visual feedback delay was parametrically reflected by activity in the inferior parietal cortex and posterior temporal cortex. Unpredicted changes in visuomotor mapping (at transitions from synchronous to delayed visual feedback periods or vice versa) activated biological motion-sensitive regions in the lateral occipitotemporal cortex (LOTC). Activity in the posterior parietal cortex (PPC), focused on the contralateral anterior intraparietal sulcus (aIPS), correlated with tracking error, whereby this correlation was stronger in participants with higher tracking performance. Our results are in line with recent proposals of a wide-spread cortical motor control hierarchy, where temporoparietal regions seem to evaluate visuomotor congruence and thus possibly ground a self-attribution of movements, the LOTC likely processes early visual prediction errors, and the aIPS computes action goal errors and possibly corresponding motor corrections.

Foot placement relies on state estimation during visually guided walking

As we walk, we must accurately place our feet to stabilize our motion and to navigate our environment. We must also achieve this accuracy despite imperfect sensory feedback and unexpected disturbances. In this study we tested whether the nervous system uses state estimation to beneficially combine sensory feedback with forward model predictions to compensate for these challenges. Specifically, subjects wore prism lenses during a visually guided walking task, and we used trial-by-trial variation in prism lenses to add uncertainty to visual feedback and induce a reweighting of this input. To expose altered weighting, we added a consistent prism shift that required subjects to adapt their estimate of the visuomotor mapping relationship between a perceived target location and the motor command necessary to step to that position. With added prism noise, subjects responded to the consistent prism shift with smaller initial foot placement error but took longer to adapt, compatible with our mathematical model of the walking task that leverages state estimation to compensate for noise. Much like when we perform voluntary and discrete movements with our arms, it appears our nervous systems uses state estimation during walking to accurately reach our foot to the ground.

NEW & NOTEWORTHY

Accurate foot placement is essential for safe walking. We used computational models and human walking experiments to test how our nervous system achieves this accuracy. We find that our control of foot placement beneficially combines sensory feedback with internal forward model predictions to accurately estimate the body's state. Our results match recent computational neuroscience findings for reaching movements, suggesting that state estimation is a general mechanism of human motor control.

Functional versus effector-specific organization of the human posterior parietal cortex: revisited

In the present study, we show that regions in posterior parietal regions process information independent of the currently used effector (hand, foot, or eye) during goal-directed actions. Functional MRI repetition suppression analysis suggests that generality across effectors holds also on the neuronal level and not just at the level of entire regions. More anterior parietal regions process information only for a specific effector or a subset of effectors.

It has been proposed that the posterior parietal cortex (PPC) is characterized by an effector-specific organization. However, strikingly similar functional MRI (fMRI) activation patterns have been found in the PPC for hand and foot movements. Because the fMRI signal is related to average neuronal activity, similar activation levels may result either from effector-unspecific neurons or from intermingled subsets of effector-specific neurons within a voxel. We distinguished between these possibilities using fMRI repetition suppression (RS). Participants made delayed, goal-directed eye, hand, and foot movements to visual targets. In each trial, the instructed effector was identical or different to that of the previous trial. RS effects indicated an attenuation of the fMRI signal in repeat trials. The caudal PPC was active during the delay but did not show RS, suggesting that its planning activity was effector independent. Hand and foot-specific RS effects were evident in the anterior superior parietal lobule (SPL), extending to the premotor cortex, with limb overlap in the anterior SPL. Connectivity analysis suggested information flow between the caudal PPC to limb-specific anterior SPL regions and between the limb-unspecific anterior SPL toward limb-specific motor regions. These results underline that both function and effector specificity should be integrated into a concept of PPC action representation not only on a regional but also on a fine-grained, subvoxel level.

The Pointing Errors in Optic Ataxia Reveal the Role of "Peripheral Magnification" of the PPC

Interaction with visual objects in the environment requires an accurate correspondence between visual space and its internal representation within the brain. Many clinical conditions involve some impairment in visuo-motor control and the errors created by the lesion of a specific brain region are neither random nor uninformative. Modern approaches to studying the neuropsychology of action require powerful data-driven analyses and error modeling in order to understand the function of the lesioned areas. In the present paper we carried out mixed-effect analyses of the pointing errors of seven optic ataxia patients and seven control subjects. We found that a small parameter set is sufficient to explain the pointing errors produced by unilateral optic ataxia patients. In particular, the extremely stereotypical errors made when pointing toward the contralesional visual field can be fitted by mathematical models similar to those used to model central magnification in cortical or sub-cortical structure(s). Our interpretation is that visual areas that contain this footprint of central magnification guide pointing movements when the posterior parietal cortex (PPC) is damaged and that the functional role of the PPC is to actively compensate for the under-representation of peripheral vision that accompanies central magnification. Optic ataxia misreaching reveals what would be hand movement accuracy and precision if the human motor system did not include elaborated corrective processes for reaching and grasping to non-foveated targets.

Beta band modulations underlie action representations for movement planning

To be able to interact with our environment, we need to transform incoming sensory information into goal-directed motor outputs. Whereas our ability to plan an appropriate movement based on sensory information appears effortless and simple, the underlying brain dynamics are still largely unknown. Here we used magnetoencephalography (MEG) to investigate this issue by recording brain activity during the planning of non-visually guided reaching and grasping actions, performed with either the left or right hand. Adopting a combination of univariate and multivariate analyses, we revealed specific patterns of beta power modulations underlying varying levels of neural representations during movement planning. (1) Effector-specific modulations were evident as a decrease in power in the beta band. Within both hemispheres, this decrease was stronger while planning a movement with the contralateral hand. (2) The comparison of planned grasping and reaching led to a relative increase in power in the beta band. These power changes were localized within temporal, premotor and posterior parietal cortices. Action-related modulations overlapped with effector-related beta power changes within widespread frontal and parietal regions, suggesting the possible integration of these two types of neural representations. (3) Multivariate analyses of action-specific power changes revealed that part of this broadband beta modulation also contributed to the encoding of an effector-independent neural representation of a planned action within fronto-parietal and temporal regions. Our results suggest that beta band power modulations play a central role in movement planning, within both the dorsal and ventral stream, by coding and integrating different levels of neural representations, ranging from the simple representation of the to-be-moved effector up to an abstract, effector-independent representation of the upcoming action.

Contributions of Subsurface Cortical Modulations to Discrimination of Executed and Imagined Grasp Forces through Stereoelectroencephalography

Stereoelectroencephalographic (SEEG) depth electrodes have the potential to record neural activity from deep brain structures not easily reached with other intracranial recording technologies. SEEG electrodes were placed through deep cortical structures including central sulcus and insular cortex. In order to observe changes in frequency band modulation, participants performed force matching trials at three distinct force levels using two different grasp configurations: a power grasp and a lateral pinch. Signals from these deeper structures were found to contain information useful for distinguishing force from rest trials as well as different force levels in some participants. High frequency components along with alpha and beta bands recorded from electrodes located near the primary motor cortex wall of central sulcus and electrodes passing through sensory cortex were found to be the most useful for classification of force versus rest although one participant did have significant modulation in the insular cortex. This study electrophysiologically corroborates with previous imaging studies that show force-related modulation occurs inside of central sulcus and insular cortex. The results of this work suggest that depth electrodes could be useful tools for investigating the functions of deeper brain structures as well as showing that central sulcus and insular cortex may contain neural signals that could be used for control of a grasp force BMI.

Position and Identity Information Available in fMRI Patterns of Activity in Human Visual Cortex

Parietal cortex is often implicated in visual processing of actions. Action understanding is essentially abstract, specific to the type or goal of action, but greatly independent of variations in the perceived position of the action. If certain parietal regions are involved in action understanding, then we expect them to show these generalization and selectivity properties. However, additional functions of parietal cortex, such as self-action control, may impose other demands by requiring an accurate representation of the location of graspable objects. Therefore, the dimensions along which responses are modulated may indicate the functional role of specific parietal regions. Here, we studied the degree of position invariance and hand/object specificity during viewing of tool-grasping actions. To that end, we characterize the information available about location, hand, and tool identity in the patterns of fMRI activation in various cortical areas: early visual cortex, posterior intraparietal sulcus, anterior superior parietal lobule, and the ventral object-specific lateral occipital complex. Our results suggest a gradient within the human dorsal stream: along the posterior-anterior axis, position information is gradually lost, whereas hand and tool identity information is enhanced. This may reflect a gradual transformation of visual input from an initial retinotopic representation in early visual areas to an abstract, position-invariant representation of viewed action in anterior parietal cortex.

SIGNIFICANCE STATEMENT

Since the seminal study of Goodale and Milner (1992), there is general agreement that visual processing is largely divided between a ventral and dorsal stream specializing in object recognition and vision for action, respectively. Here, we address the specific representation of viewed actions. Specifically, we study the degree of position invariance and hand/object manipulation specificity in the human visual pathways, characterizing the information available in patterns of fMRI activation during viewing of object-grasping videos, which appeared in different retinal locations. We find converging evidence for a gradient within the dorsal stream: along the posterior-anterior axis, position information is gradually lost, whereas hand and action identity information is enhanced, leading to an abstract, position-invariant representation of viewed action in the anterior parietal cortex.