Comparison of Effector-Specific Signals in Frontal and Parietal Cortices

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).

Localization of movable electrodes in a multi-electrode microdrive in nonhuman primates

BACKGROUND

Recently, large-scale semi-chronic recording systems have been developed, unique in their capability to record simultaneously from multiple individually moveable electrodes. As these recording systems can cover a large area, knowledge of the exact location of each individual electrode is crucial. Currently, the only method of keeping track of electrode depth and thus location is through detailed notebook keeping on neural activity.

NEW METHOD

We have improved the electrode localization by combining pre- and postoperative anatomical magnetic resonance imaging (MRI) scans with high resolution computed tomography (CT) scans throughout the experiment, and validated our method by comparing the resulting location estimates with traditional notebook-keeping. Finally, the actual location of a selection of electrodes was marked at the end of the experiment by creating small metallic depositions using electrical stimulation, and thereby made visible on MRI.

RESULTS

Combining CT scans with a high resolution, artefact reducing sequence during the experiment with a preoperative MRI scan provides crucial information about the exact electrode location of multielectrode arrays with individually moveable electrodes.

COMPARISON WITH EXISTING METHODS

The information obtained from the hybrid CT-MR image and the notes on spiking activity showed a similar pattern, with the clear advantage of the visualization of the exact position of the electrodes using our method.

CONCLUSIONS

The described technique allows for a precise anatomical identification of the recorded brain areas and thus to draw strong conclusions about the role of each targeted cortical area in the behavior under study.

Neural substrates for allocentric-to-egocentric conversion of remembered reach targets in humans

Targets for goal-directed action can be encoded in allocentric coordinates (relative to another visual landmark), but it is not known how these are converted into egocentric commands for action. Here, we investigated this using a slow event-related fMRI paradigm, based on our previous behavioural finding that the allocentric-to-egocentric (Allo-Ego) conversion for reach is performed at the first possible opportunity. Participants were asked to remember (and eventually reach towards) the location of a briefly presented target relative to another visual landmark. After a first memory delay, participants were forewarned by a verbal instruction if the landmark would reappear at the same location (potentially allowing them to plan a reach following the auditory cue before the second delay), or at a different location where they had to wait for the final landmark to be presented before response, and then reach towards the remembered target location. As predicted, participants showed landmark-centred directional selectivity in occipital-temporal cortex during the first memory delay, and only developed egocentric directional selectivity in occipital-parietal cortex during the second delay for the 'Same cue' task, and during response for the 'Different cue' task. We then compared cortical activation between these two tasks at the times when the Allo-Ego conversion occurred, and found common activation in right precuneus, right presupplementary area and bilateral dorsal premotor cortex. These results confirm that the brain converts allocentric codes to egocentric plans at the first possible opportunity, and identify the four most likely candidate sites specific to the Allo-Ego transformation for reaches.

Response Properties of Motor Equivalence Neurons of the Primate Premotor Cortex

To study the response properties of cells that could participate in eye-hand coordination we trained two macaque monkeys to perform center-out saccades and pointing movements with their right or left forelimb toward visual targets presented on a video display. We analyzed the phasic movement related discharges of neurons of the periarcuate cortex that fire before and during saccades and movements of the hand whether accompanied by movements of the other effector or not. Because such cells could encode an abstract form of the desired displacement vector without regard to the effector that would execute the movement we refer to such cells as motor equivalence neurons (Meq). Most of them (75%) were found in or near the smooth pursuit region and the grasp related region in the caudal bank of the arcuate sulcus. The onset of their phasic discharges preceded saccades by about 70 ms and hand movements by about 150 ms and was often correlated to both the onset of saccades and the onset of hand movements. The on-direction of Meq cells was uniformly distributed without preference for ipsiversive or contraversive movements. In about half of the Meq cells the preferred direction for saccades was the preferred direction for hand movements as well. In the remaining cells the difference was considerable (>90 deg), and the on-direction for eye-hand movements resembled that for isolated saccades in some cells and for isolated hand movements in others. A three layer neural network model that used Meq cells as its input layer showed that the combination of effector invariant discharges with non-invariant discharges could help reduce the number of decoding errors when the network attempts to compute the correct movement metrics of the right effector.

Effector specificity in macaque frontal and parietal cortex

Single neurons in the frontal eye fields (FEFs) and lateral intraparietal area (LIP) of macaques are preferentially activated by saccade- versus reach-related processes. fMRI studies focusing on saccade- and reach-specific activity in human cortex, however, provided conflicting evidence for effector specificity. To gain further insights into effector preferences throughout monkey cortex using the same technique as in humans, we performed a mixed block/event-related fMRI experiment in macaques. Within single fMRI runs, monkeys alternated between a visually guided saccade task, a visually guided arm movement task, and a fixation-only task requiring no saccades or arm movements. The detection of a peripheral pop-out go cue initiating the required operant behavior and the identification of a target among distractors was identical in the arm and saccade tasks. We found saccade-related activity in parietal areas V6, V6A, LIP, and caudal intraparietal area and frontal areas FEF, 45a, 45b, and 46. Areas 45 and FEF even showed markedly decreased fMRI activity during arm movements relative to fixation only. Conversely, medial and anterior intraparietal areas (MIP and AIP), and parietal area PEip; somatosensory areas S1 and S2; and (pre)motor areas F1, F3, F5, and F6 showed increased arm movement-related activity. F1, F5, PEip, and somatosensory cortex also showed deactivations during saccades relative to fixation only. Control experiments showed that such deactivations in both operant-specific functional networks did not depend on training history or rapid task switching requiring active suppression of the unpreferred operant behavior. Therefore, although both tasks required divided attention to detect a pop-out go cue and target, two largely segregated and mainly effector-driven cortical networks were activated.

Region-Specific Summation Patterns Inform the Role of Cortical Areas in Selecting Motor Plans

Given an instruction regarding which effector to move and what location to move to, simply adding the effector and spatial signals together will not lead to movement selection. For this, a nonlinearity is required. Thresholds, for example, can be used to select a particular response and reject others. Here we consider another useful nonlinearity, a supralinear multiplicative interaction. To help select a motor plan, spatial and effector signals could multiply and thereby amplify each other. Such an amplification could constitute one step within a distributed network involved in response selection, effectively boosting one response while suppressing others. We therefore asked whether effector and spatial signals sum supralinearly for planning eye versus arm movements from the parietal reach region (PRR), the lateral intraparietal area (LIP), the frontal eye field (FEF), and a portion of area 5 (A5) lying just anterior to PRR. Unlike LIP neurons, PRR, FEF, and, to a lesser extent, A5 neurons show a supralinear interaction. Our results suggest that selecting visually guided eye versus arm movements is likely to be mediated by PRR and FEF but not LIP.

Spectral Signatures of Saccade Target Selection

Action generation relies on a widely distributed network of brain areas. However, little is known about the spatiotemporal dynamics of neuronal activity in the network that gives rise to voluntary action in humans. Here, we used magnetoencephalography (MEG) and source analysis (n = 15, 7 female subjects) to investigate the spectral signatures of human cortical networks engaged in active and intrinsically motivated viewing behavior. We compared neuronal activity of externally cued saccades with saccades to freely chosen targets. For planning and execution of both saccade types, we found an increase in gamma band (~64-128 Hz) activity and a concurrent decrease in beta band (~12-32 Hz) activity in saccadic control areas, including the intraparietal sulcus and the frontal eye fields. Guided compared to voluntary actions were accompanied by stronger transient increases in the gamma and low frequency (<16 Hz) range immediately following the instructional cue. In contrast, action selection between competing alternatives was reflected by stronger sustained fronto-parietal gamma increases that occurred later in time and persisted until movement execution. This sustained enhancement for free target selection was accompanied by a spatially widespread reduction of lower frequency power (~8-45 Hz) in parietal and extrastriate areas. Our results suggest that neuronal population activity in the gamma frequency band in a distributed network of fronto-parietal areas reflects the intrinsically driven process of selection among competing behavioral alternatives.

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.

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.

Intention, action planning, and decision making in parietal-frontal circuits

The posterior parietal cortex and frontal cortical areas to which it connects are responsible for sensorimotor transformations. This review covers new research on four components of this transformation process: planning, decision making, forward state estimation, and relative-coordinate representations. These sensorimotor functions can be harnessed for neural prosthetic operations by decoding intended goals (planning) and trajectories (forward state estimation) of movements as well as higher cortical functions related to decision making and potentially the coordination of multiple body parts (relative-coordinate representations).

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

It is generally accepted that interactions between parietal and frontal cortices subserve the visuomotor processing for eye and hand movements. Here, we used a sequential-instruction paradigm in 3-T functional MRI to test the processing of effector and spatial signals, as well as their interaction, as a movement is composed and executed in different stages. Subjects prepared either a saccade or a reach following two successive visual instruction cues, presented in either order. One cue instructed which effector to use (eyes, right hand); the other signaled the spatial goal (leftward vs. rightward target location) of the movement. During the first phase of the prepared movement, after cueing of either goal or effector information, we found significant spatial goal selectivity but no effector specificity along the parietofrontal network. During the second phase of the prepared movement, when both goal and effector information were available, we found a large overlap in the neural circuitry involved in the planning of eye and hand movements. Gradually distributed along this network, we observed clear spatial goal selectivity and limited, but significant, effector specificity. Regions in the intraparietal sulcus and the dorsal premotor cortex were selective to both goal location and motor effector. Taken together, our results suggest that the relative weight of spatial goal and effector selectivity changes along the parietofrontal network, depending on the status of the movement plan.

Attention and active vision

Visual perception results from the interaction of incoming sensory signals and top down cognitive and motor signals. Here we focus on the representation of attended locations in parietal cortex and in earlier visual cortical areas. We review evidence that these spatial representations are modulated not only by selective attention but also by the intention to move the eyes. We describe recent experiments in monkey and human that elucidate the mechanisms and circuitry involved in updating, or remapping, the representations of salient stimuli. Two central ideas emerge. First, selective attention and remapping are closely intertwined, and together contribute to the percept of spatial stability. Second, remapping is accomplished not by a single area but by the participation of parietal, frontal and extrastriate cortex as well as subcortical structures. This neural circuitry is distinguished by significant redundancy and plasticity, suggesting that the updating of salient stimuli is fundamental for spatial stability and visuospatial behavior. We conclude that multiple processes and pathways contribute to active vision in the primate brain.

Saccade preparation signals in the human frontal and parietal cortices

Our ability to prepare an action in advance allows us to respond to our environment quickly, accurately, and flexibly. Here, we used event-related functional MRI to measure human brain activity while subjects maintained an active state of preparedness. At the beginning of each trial, subjects were instructed to prepare a pro- or antisaccade to a visual cue that was continually present during a long and variable preparation interval, but to defer the saccade's execution until a go signal. The deferred saccade task eliminated the mnemonic component inherent in memory-guided saccade tasks and placed the emphasis entirely on advance motor preparation. During the delay while subjects were in an active state of motor preparedness, the blood oxygen level-dependent signal in the frontal cortex showed 1) a sustained elevation throughout the preparation interval; 2) a linear increase with increasing delay length; 3) a bias for contra- rather than ipsiversive movements; 4) greater activity when the specific metrics of the planned saccade were known compared with when they were not; and 5) increased activity when the saccade was directed toward an internal versus an external representation (i.e., anticue location). These findings support the hypothesis that both the human frontal and parietal cortices are involved in the spatial selection and preparation of saccades.

Hand position modulates saccadic activity in the frontal eye field

Recent neurophysiological studies have begun to uncover the neuronal correlates of eye hand coordination. This study was designed to test whether the frontal eye field (FEF) saccadic activity is modulated by hand position. Single neurons were recorded in two macaque monkeys performing visually guided saccades while holding their hand at given locations on a touchscreen. To determine the relative contributions of hand vision and its proprioception, monkeys executed the task with or without vision of the hand. We found that saccadic activity of more than half of the neuronal sample (54%; n=130) was dependent on hand position relative to the saccade end point. Both visual and proprioceptive signals contributed to this modulation. These data demonstrate that the oculomotor function of the FEF takes into account hand position in space.

Specificity of human cortical areas for reaches and saccades

Electrophysiological studies in monkeys have identified effector-related regions in the posterior parietal cortex (PPC). The lateral intraparietal area, for example, responds preferentially for saccades, whereas the parietal reach region responds preferentially for arm movements. However, the degree of effector selectivity actually observed is limited; each area contains neurons selective for the nonpreferred effector, and many neurons in both areas respond for both effectors. We used functional magnetic resonance imaging to assess the degree of effector preference at the population level, focusing on topographically organized regions in the human PPC [visual area V7, intraparietal sulcus 1 (IPS1), and IPS2]. An event-related design adapted from monkey experiments was used. In each trial, an effector cue preceded the appearance of a spatial target, after which a Go signal instructed subjects to produce the specified movement with the specified effector. Our results show that the degree of effector specificity is limited in many cortical areas and transitions gradually from saccade to reach preference as one moves through the hierarchy of areas in the occipital, parietal, and frontal cortices. Saccade preference was observed in visual cortex, including early areas and V7. IPS1 exhibited balanced activation to saccades and reaches, whereas IPS2 showed a weak but significant preference for reaches. In frontal cortex, areas near the central sulcus showed a clear and absolute preference for reaches, whereas the frontal eye field showed little or no effector selectivity. Although these results contradict many theoretical conclusions about effector specificity, they are compatible with the complex picture arising from electrophysiological studies and also with previous imaging studies that reported mostly overlapping saccade- and arm-related activation. The results are also compatible with theories of efficient coding in cortex.

Preparatory delay activity in the monkey parietal reach region predicts reach reaction times

To acquire something that we see, visual spatial information must ultimately result in the activation of the appropriate set of muscles. This sensory to motor transformation requires an interaction between information coding target location and information coding which effector will be moved. Activity in the monkey parietal reach region (PRR) reflects both spatial information and the effector (arm or eye) that will be used in an upcoming reach or saccade task. To further elucidate the functional role of PRR in visually guided movement tasks and to obtain evidence that PRR signals are used to drive arm movements, we tested the hypothesis that increased neuronal activity during a preparatory delay period would lead to faster reach reaction times but would not be correlated with saccade reaction times. This proved to be the case only when the type of movement and not the spatial goal of that movement was known in advance. The correlation was strongest in cells that showed significantly more activity on arm reach compared with saccade trials. No significant correlations were found during delay periods in which spatial information was provided in advance. These data support the idea that PRR constitutes a bottleneck in the processing of spatial information for an upcoming arm reach. The lack of a correlation with saccadic reaction time also supports the idea that PRR processing is effector specific, that is, it is involved in specifying targets for arm movements but not targets for eye movements.

Integration of target and effector information in the human brain during reach planning

To plan a reaching movement, the brain must integrate information about the location of the target with information about the limb selected for the reach. Here, we applied rapid event-related 3-T fMRI to investigate this process in human subjects (n = 16) preparing a reach following two successive visual instruction cues. One cue instructed which arm to use; the other cue instructed the location of the reach target. We hypothesized that regions involved in the integration of target and effector information should not only respond to each of the two instruction cues, but should respond more strongly to the second cue due to the added integrative processing to establish the reach plan. We found bilateral regions in the posterior parietal cortex, the premotor cortex, the medial frontal cortex, and the insular cortex to be involved in target-arm integration, as well as the left dorsolateral prefrontal cortex and an area in the right lateral occipital sulcus to respond in this manner. We further determined the functional properties of these regions in terms of spatial and effector specificity. This showed that the posterior parietal cortex and the dorsal premotor cortex specify both the spatial location of a target and the effector selected for the response. We therefore conclude that these regions are selectively engaged in the neural computations for reach planning, consistent with the results from physiological studies in nonhuman primates.