Motor cortical activity in a context-recall task

Viewer and object mental rotation in young adults with psychotic disorders

Schizophrenia patients have difficulty with processing visuo-spatial information, which may explain their deficits with considering other people's point-of-view. Processing visuo-spatial information operates on egocentric and allocentric frames of reference. Here, we tested the ability of individuals at different stages of psychotic disorders, specifically ultra-high-risk for psychosis individuals, as well as first-episode psychosis, and chronic schizophrenia patients, to perform a viewer mental rotation task and an object mental rotation task. The two tasks were differentiated only by the instruction given. Healthy individuals and patients with a diagnosis of anxiety/depressive mood disorder served as non-patient and patient controls, respectively. The results show that first-episode psychosis and chronic schizophrenia patients, but not ultra-high-risk individuals, had more errors and longer response times with both mental rotation tasks than the two control groups. In addition, chronic schizophrenia patients had additional difficulty with the object rotation task. The difference in performance between groups and tasks remained significant even after controlling for age, IQ, and antipsychotic medication dose. The results indicate that patients with psychotic disorders have a deficit of mental spatial imagery that include both egocentric and allocentric representations. This deficit may explain the difficulty of these patients with perspective-taking, and inferring other people's point of view, thoughts or intentions which is at the core of the pathogenesis of schizophrenia.

Comparative Ultrastructural Analysis of Thalamocortical Innervation of the Primary Motor Cortex and Supplementary Motor Area in Control and MPTP-Treated Parkinsonian Monkeys

The synaptic organization of thalamic inputs to motor cortices remains poorly understood in primates. Thus, we compared the regional and synaptic connections of vGluT2-positive thalamocortical glutamatergic terminals in the supplementary motor area (SMA) and the primary motor cortex (M1) between control and MPTP-treated parkinsonian monkeys. In controls, vGluT2-containing fibers and terminal-like profiles invaded layer II-III and Vb of M1 and SMA. A significant reduction of vGluT2 labeling was found in layer Vb, but not in layer II-III, of parkinsonian animals, suggesting a potential thalamic denervation of deep cortical layers in parkinsonism. There was a significant difference in the pattern of synaptic connectivity in layers II-III, but not in layer Vb, between M1 and SMA of control monkeys. However, this difference was abolished in parkinsonian animals. No major difference was found in the proportion of perforated versus macular post-synaptic densities at thalamocortical synapses between control and parkinsonian monkeys in both cortical regions, except for a slight increase in the prevalence of perforated axo-dendritic synapses in the SMA of parkinsonian monkeys. Our findings suggest that disruption of the thalamic innervation of M1 and SMA may underlie pathophysiological changes of the motor thalamocortical loop in the state of parkinsonism.

Activity in Primary Motor Cortex Related to Visual Feedback

Neural modulation in primate motor cortex exhibits complex patterns. We found that modulation during reaching could be separated into discrete temporal epochs. To determine if these epochs are driven by behavioral events, monkeys performed variations of a center-out reaching task. Monkeys viewed a computer cursor matched to hand position and a radial target at 1 of 16 locations. In some trials, they performed a visuomotor rotation (the cursor moved at an angle to the hand). After adaptation, encoding changes for single units are temporally structured: adaptation could affect one temporal component of a unit's response but not another. In half the normal and perturbed trials, we removed visual feedback before movement. Adaptation-sensitive firing components toward the end of movement are often weak or absent during reaches without feedback. These results show that temporal structure in motor cortical activity is driven by behavior, with a discrete component related to visual feedback.

Inhibition of protein synthesis in M1 of monkeys disrupts performance of sequential movements guided by memory

The production of action sequences is a fundamental aspect of motor skills. To examine whether primary motor cortex (M1) is involved in maintenance of sequential movements, we trained two monkeys (Cebus apella) to perform two sequential reaching tasks. In one task, sequential movements were instructed by visual cues, whereas in the other task, movements were generated from memory after extended practice. After the monkey became proficient with performing the tasks, we injected an inhibitor of protein synthesis, anisomycin, into M1 to disrupt information storage in this area. Injection of anisomycin in M1 had a marked effect on the performance of sequential movements that were guided by memory. In contrast, the anisomycin injection did not have a significant effect on the performance of movements guided by vision. These results suggest that M1 of non-human primates is involved in the maintenance of skilled sequential movements.

Corticocortical Systems Underlying High-Order Motor Control

Cortical networks are characterized by the origin, destination, and reciprocity of their connections, as well as by the diameter, conduction velocity, and synaptic efficacy of their axons. The network formed by parietal and frontal areas lies at the core of cognitive-motor control because the outflow of parietofrontal signaling is conveyed to the subcortical centers and spinal cord through different parallel pathways, whose orchestration determines, not only when and how movements will be generated, but also the nature of forthcoming actions. Despite intensive studies over the last 50 years, the role of corticocortical connections in motor control and the principles whereby selected cortical networks are recruited by different task demands remain elusive. Furthermore, the synaptic integration of different cortical signals, their modulation by transthalamic loops, and the effects of conduction delays remain challenging questions that must be tackled to understand the dynamical aspects of parietofrontal operations. In this article, we evaluate results from nonhuman primate and selected rodent experiments to offer a viewpoint on how corticocortical systems contribute to learning and producing skilled actions. Addressing this subject is not only of scientific interest but also essential for interpreting the devastating consequences for motor control of lesions at different nodes of this integrated circuit. In humans, the study of corticocortical motor networks is currently based on MRI-related methods, such as resting-state connectivity and diffusion tract-tracing, which both need to be contrasted with histological studies in nonhuman primates.

Encoding of Serial Order in Working Memory: Neuronal Activity in Motor, Premotor, and Prefrontal Cortex during a Memory Scanning Task

We have adapted Sternberg's context-recall task to investigate the neural mechanisms of encoding serial order information in working memory, in 2 male rhesus monkeys. We recorded from primary motor, premotor, and dorsolateral prefrontal cortex while the monkeys performed the task. In each cortical area, most neurons displayed marked modulation of activity during the list presentation period of the task, whereas the serial order of the stimuli needed to be encoded in working memory. The activity of many neurons changed in a consistent manner over the course of the list presentation period, without regard to the location of the stimuli presented. Remarkably, these neurons encoded serial position information in a relative (rather than absolute) manner across different list lengths. In addition, many neurons showed activity related to both location and serial position, in the form of an interaction effect. Surprisingly, the activity of these neurons was often modulated by the location of stimuli presented before the epoch in which the activity changes occurred. In motor and premotor areas, a large proportion of neurons with list presentation activity also showed direction-related activity during the response phase, whereas in prefrontal cortex most cells showed only list presentation effects. These results show that many neurons had a heterogeneous functionality by representing distinct task variables at different periods of the task. Finally, potential confounds could not account for the effects observed. For these reasons, we conclude that these neurons were indeed participating in sequence encoding in working memory.SIGNIFICANCE STATEMENT Traditionally, primary motor, premotor, and prefrontal areas have been considered to be mainly engaged in motor output, visuomotor transformation, and higher cognitive functions, respectively. Here we show that neurons in all three cortical regions participate in the encoding of a sequence of spatial stimuli in working memory. Furthermore, a central question in cognitive neuroscience has been the manner in which the position of an item within a sequence is encoded in the brain. Our findings provide direct neurophysiological support for a specific hypothesis from cognitive psychology: that of relative coding of serial order.

Object-based processes in the planning of goal-directed hand movements

Theories in motor control suggest that the parameters specified during the planning of goal-directed hand movements to a visual target are defined in spatial parameters like direction and amplitude. Recent findings in the visual attention literature, however, argue widely for early object-based selection processes. The present experiments were designed to examine the contributions of object-based and space-based selection processes to the preparation time of goal-directed pointing movements. Therefore, a cue was presented at a specific location. The question addressed was whether the initiation of responses to uncued target stimuli could benefit from being either within the same object (object based) or presented at the same direction (space based). Experiment 1 replicated earlier findings of object-based benefits for non-goal-directed responses. Experiment 2 confirmed earlier findings of space-based benefits for goal-directed hand pointing movements. In Experiments 3 and 4, space-based and object-based manipulations were combined while requiring goal-directed hand pointing movements. The results clearly favour the notion that the selection processes for goal-directed pointing movements are primarily object based. Implications for theories on selective attention and action planning are discussed.

Computational Architecture of the Parieto-Frontal Network Underlying Cognitive-Motor Control in Monkeys

The statistical structure of intrinsic parietal and parieto-frontal connectivity in monkeys was studied through hierarchical cluster analysis. Based on their inputs, parietal and frontal areas were grouped into different clusters, including a variable number of areas that in most instances occupied contiguous architectonic fields. Connectivity tended to be stronger locally: that is, within areas of the same cluster. Distant frontal and parietal areas were targeted through connections that in most instances were reciprocal and often of different strength. These connections linked parietal and frontal clusters formed by areas sharing basic functional properties. This led to five different medio-laterally oriented pillar domains spanning the entire extent of the parieto-frontal system, in the posterior parietal, anterior parietal, cingulate, frontal, and prefrontal cortex. Different information processing streams could be identified thanks to inter-domain connectivity. These streams encode fast hand reaching and its control, complex visuomotor action spaces, hand grasping, action/intention recognition, oculomotor intention and visual attention, behavioral goals and strategies, and reward and decision value outcome. Most of these streams converge on the cingulate domain, the main hub of the system. All of them are embedded within a larger eye–hand coordination network, from which they can be selectively set in motion by task demands.

Insights into Seeing and Grasping: Distinguishing the Neural Correlates of Perception and Action

Vision contributes to both perception and visuomotor control, and it has been suggested that many higher brain structures specialize in one or the other function. An alternative view, presented here, is that most higher brain areas participate in both visuomotor and perceptual functions. In the anterior frontal cortex, for example, the activity of one population of neurons reflects perceptual reports about a visual stimulus, whereas the activity of an intermingled population reflects movements aimed at the same stimulus. Similarly, posterior parietal and inferior temporal areas appear to function in both visual perception and visuomotor control. Visuomotor signals in higher order cortical areas could contribute to the perception of one’s own action. They also might reflect the existence of two systems for visual information processing: one stressing accuracy for the control of movement and the other generating hypotheses about the world.

The primary motor cortex is involved in the control of a non-motor cognitive action

OBJECTIVE

Adaptive interactions with the outer world necessitate effective connections between cognitive and executive functions. The primary motor cortex (M1) with its control of the spinal cord motor apparatus and its involvement in the processing of cognitive information related to motor functions is one of the best suited structures of this cognition-action connection. The question arose whether M1 might be involved also in situations where no overt or covered motor action is present.

METHODS

The EEG data analyzed were recorded during an oddball task in one epileptic patient (19 years) with depth multilead electrodes implanted for diagnostic reasons into the M1 and several prefrontal areas.

RESULTS

The main result was the finding of an evoked response to non-target stimuli with a pronounced late component in all frontal areas explored, including three loci of the M1. The late component was implicated in the evaluation of predicted and actual action and was synchronized in all three precentral loci and in the majority of prefrontal loci.

CONCLUSION

The finding is considered as direct evidence of functional involvement of the M1 in cognitive activity not related to motor function.

SIGNIFICANCE

Our results contribute to better understanding of neural mechanisms underlying cognition.

Dynamic reorganization of neural activity in motor cortex during new sequence production

Although previous studies have shown that primary motor cortex (M1) neurons are modulated during the performance of a sequence of movements, it is not known how this neural activity in the M1 reorganizes during new learning of sequence-dependent motor skills. Here we trained monkeys to move to each of four spatial targets to produce multiple distinct sequences of movements in which the spatial organization of the targets determined uniquely the serial order of the movements. After the monkeys memorized the sequences, we changed one element of these over-practised sequences and the subjects were required to learn the new sequence through trial and error. When one element in an over-learned four-element sequence was changed, the sequence-specific neural activity was totally disrupted, but relatively minor changes in the direction-specific activity were observed. The data suggest that sequential motor skills are represented within M1 in the context of the complete sequential behavior rather than as a series of single consecutive movements; and sequence-specific neurons in the M1 are involved in new learning of sequence by using memorized knowledge to acquire complex motor skill efficiently.

Organization and evolution of parieto-frontal processing streams in macaque monkeys and humans

The functional organization of the parieto-frontal system is crucial for understanding cognitive-motor behavior and provides the basis for interpreting the consequences of parietal lesions in humans from a neurobiological perspective. The parieto-frontal connectivity defines some main information streams that, rather than being devoted to restricted functions, underlie a rich behavioral repertoire. Surprisingly, from macaque to humans, evolution has added only a few, new functional streams, increasing however their complexity and encoding power. In fact, the characterization of the conduction times of parietal and frontal areas to different target structures has recently opened a new window on cortical dynamics, suggesting that evolution has amplified the probability of dynamic interactions between the nodes of the network, thanks to communication patterns based on temporally-dispersed conduction delays. This might allow the representation of sensory-motor signals within multiple neural assemblies and reference frames, as to optimize sensory-motor remapping within an action space characterized by different and more complex demands across evolution.

Motor system evolution and the emergence of high cognitive functions

In human and nonhuman primates, the cortical motor system comprises a collection of brain areas primarily related to motor control. Existing evidence suggests that no other mammalian group has the number, extension, and complexity of motor-related areas observed in the frontal lobe of primates. Such diversity is probably related to the wide behavioral flexibility that primates display. Indeed, recent comparative anatomical, psychophysical, and neurophysiological studies suggest that the evolution of the motor cortical areas closely correlates with the emergence of high cognitive abilities. Advances in understanding the cortical motor system have shown that these areas are also related to functions previously linked to higher-order associative areas. In addition, experimental observations have shown that the classical distinction between perceptual and motor functions is not strictly followed across cortical areas. In this paper, we review evidence suggesting that evolution of the motor system had a role in the shaping of different cognitive functions in primates. We argue that the increase in the complexity of the motor system has contributed to the emergence of new abilities observed in human and nonhuman primates, including the recognition and imitation of the actions of others, speech perception and production, and the execution and appreciation of the rhythmic structure of music.

Cell directional spread determines accuracy, precision, and length of the neuronal population vector

The neuronal population vector (NPV) for movement direction is the sum of weighted neuronal directional contributions. Based on theoretical considerations, we proposed recently that the sharpness of tuning will impact the directional precision, accuracy, and length of the NPV, such that sharper tuning will yield NPV with higher precision, higher accuracy, and shorter length (Mahan and Georgopoulos in Front Neural Circuits 7:92, 2013). Furthermore, we proposed that controlling the inhibitory drive in a local network could be the mechanism by which the sharpness of directional tuning would be varied, resulting in a continuous specification and control of movement's directional precision, accuracy, and speed (Mahan and Georgopoulos in Front Neural Circuits 7:92, 2013, Fig. 5). As a first step in testing this idea, here we analyzed data from 899 cells recorded in the motor cortex during performance of a center → out task. There were two major findings. First, directional selectivity varied with cell activity, such that it was higher in cells with lower mean discharge rates. And second, NPVs calculated from subsets of cells with higher directional selectivity (and, correspondingly, lower mean discharge rates) were more accurate (i.e., closer to the movement), precise (i.e., less variable), and shorter (i.e., slower; Schwartz in Science 265:540-542, 1994). These findings confirm our predictions above made from modeling (Mahan and Georgopoulos in Front Neural Circuits 7:92, 2013) and provide a simple mechanism by which desired attributes of the directional motor command can be implemented. We hypothesize that the inhibitory drive in a local network is controlled directly and independently of recurrent collaterals or common excitatory inputs to other cells. This could be achieved by a private excitation/inhibition of key inhibitory interneurons in a way similar to that in operation for Renshaw cells in the spinal cord. The presence of such a private line of inhibitory control remains to be investigated.

Missing mass approximations for the partition function of stimulus driven Ising models

Ising models are routinely used to quantify the second order, functional structure of neural populations. With some recent exceptions, they generally do not include the influence of time varying stimulus drive. Yet if the dynamics of network function are to be understood, time varying stimuli must be taken into account. Inclusion of stimulus drive carries a heavy computational burden because the partition function becomes stimulus dependent and must be separately calculated for all unique stimuli observed. This potentially increases computation time by the length of the data set. Here we present an extremely fast, yet simply implemented, method for approximating the stimulus dependent partition function in minutes or seconds. Noting that the most probable spike patterns (which are few) occur in the training data, we sum partition function terms corresponding to those patterns explicitly. We then approximate the sum over the remaining patterns (which are improbable, but many) by casting it in terms of the stimulus modulated missing mass (total stimulus dependent probability of all patterns not observed in the training data). We use a product of conditioned logistic regression models to approximate the stimulus modulated missing mass. This method has complexity of roughly O(LNNpat) where is L the data length, N the number of neurons and N pat the number of unique patterns in the data, contrasting with the O(L2 (N) ) complexity of alternate methods. Using multiple unit recordings from rat hippocampus, macaque DLPFC and cat Area 18 we demonstrate our method requires orders of magnitude less computation time than Monte Carlo methods and can approximate the stimulus driven partition function more accurately than either Monte Carlo methods or deterministic approximations. This advance allows stimuli to be easily included in Ising models making them suitable for studying population based stimulus encoding.

Hypothesis regarding the transformation of the intended direction of movement during the production of graphic trajectories: a study of drawing movements in 8- to 12-year-old children

Children from 8 to 12 years of age drew figure-eights and ellipses at a self-chosen tempo on a digitizing tablet. Global aspects (perimeter and average speed) and local aspects (relation between instantaneous speed and curvature) of performance were analyzed across age groups and types of figures. We tested the predictions of the transformation model, which is based on the hypothesis that changing the intended direction of movement is a time-consuming process that affects the evolution in time of the movement trajectory, and compared how well it fitted the data relative to the power law. We found that the relation between speed and curvature was typically better described by the transformation model than by the power law. However, the power law provided a better description when ellipses were drawn at a fast speed. The analyses of the parameters of the transformation model indicate that processing speed increased linearly with age. In addition, the results suggest that the effects of the spring-like properties of the arm were noticeable when ellipses were drawn at a fast speed. This study indicates that both biomechanical properties and central processes have an effect on the kinematics of continuous movements and particularly on the relation between speed and curvature. However, their relative importance varies with the type of figure and average movement speed. In conclusion, the results support the hypothesis that a time-consuming process of transformation of the intended direction of movement is operating during the production of continuous movements and that this process increases in speed between 8 to 12 years of age.

Motor Imagery

Background and Purpose— Understanding brain plasticity after stroke is important in developing rehabilitation strategies. Active movement therapies show considerable promise but depend on motor performance, excluding many otherwise eligible patients. Motor imagery is widely used in sport to improve performance, which raises the possibility of applying it both as a rehabilitation method and to access the motor network independently of recovery. Specifically, whether the primary motor cortex (M1), considered a prime target of poststroke rehabilitation, is involved in motor imagery is unresolved.

Summary of Review— We review methodological considerations when applying motor imagery to healthy subjects and in patients with stroke, which may disrupt the motor imagery network. We then review firstly the motor imagery training literature focusing on upper-limb recovery, and secondly the functional imaging literature in healthy subjects and in patients with stroke.

Conclusions— The review highlights the difficulty in addressing cognitive screening and compliance in motor imagery studies, particularly with regards to patients with stroke. Despite this, the literature suggests the encouraging effect of motor imagery training on motor recovery after stroke. Based on the available literature in healthy volunteers, robust activation of the nonprimary motor structures, but only weak and inconsistent activation of M1, occurs during motor imagery. In patients with stroke, the cortical activation patterns are essentially unexplored as is the underlying mechanism of motor imagery training. Provided appropriate methodology is implemented, motor imagery may provide a valuable tool to access the motor network and improve outcome after stroke.

Premotor neuronal plasticity in monkeys adapting to a new dynamic environment

Recent evidence indicates that premotor cortex (PM) in addition to their well-established motor functions, also play a role in nonmotor processes such as spatial attention and working memory. In the present study, neuronal activities in dorsal PM (PMd) and ventral PM (PMv) were recorded in a force field adaptation task. This study found that PM neurons show learning-related plasticity and that a neuron demonstrates either one type or multiple types of properties (i.e. kinematic, dynamic, and memory). The current study reveals that memory properties could be displayed by one or a combination of the cell activity parameters [i.e. average firing rate (AFR), dynamic range (DR), and preferred direction (PD)]. A predominant percentage of cells displayed memory properties with AFR or AFR plus other parameters. This study investigated the memory properties vs. the time sequence of the task trial [i.e. delay time (DT), movement time (MT), and target holding time (THT)] and found that: (i) most neurons display memory properties only in one time window; (ii) few neurons display memory properties in three time windows, and (iii) there are significantly more cells showing memory properties during MT than during any other time windows. There are cells that show memory I (changing their tuning curves in the force field and retaining those changes after the force field was removed), memory II (changing their tuning curves after the force field was removed), or both properties. Significantly more cells display one type of memory property (memory I or memory II) rather than both types of memory properties (memory I and memory II).

Anticipatory activity in primary motor cortex codes memorized movement sequences

Movement sequences, defined both by the component movements and by the serial order in which they are produced, are fundamental building blocks of motor behavior. The serial order of sequence production is strongly encoded in medial motor areas. It is not known to what extent sequences are further elaborated or encoded in primary motor cortex. Here, we describe cells in the primary motor cortex of the monkey that show anticipatory activity exclusively related to a specific memorized sequence of upcoming movements. In addition, the injection of muscimol, a GABA agonist, into motor cortex resulted in an increase in the error rate during sequence production, without concomitant effects on nonsequenced motor performance. Our results challenge the role of medial motor areas in the control of well-practiced movement sequences and suggest that motor cortex contains a complete apparatus for the planning and production of this complex behavior.

Participation of primary motor cortical neurons in a distributed network during maze solution: representation of spatial parameters and time-course comparison with parietal area 7a

Traditionally, primary motor cortex (M1) has been thought to be involved solely in planning and generating movements. Recent evidence suggests that the arm area of M1 plays a role in other functions, such as the representation of serial order (Pellizzer et al. 1995, Science 269:702-705; Carpenter et al. 1999, Science 283:1752-1757) and spatial processing (Georgopoulos et al. 1989, Science 243:234-236). Previous studies of such cognitive processes have used tasks in which a directed arm movement was required, raising a question as to whether this brain area is involved in cognitive processing per se, or whether such cognitive signals may be gated into the arm area of M1 only when arm movements are required. To study this question, we developed a task that required a spatial analysis of a complex visual stimulus, but required no arm movement as a response. In this task, monkeys were shown an octagonal maze. After an imposed delay of 2 to 2.5 s, they indicated whether a path that emanated from the center of the maze exited at the perimeter (exit maze) or terminated within the maze (no-exit maze) by pressing a pedal with their left or right foot, respectively. We recorded from 785 cells from the arm area of M1 from two monkeys during the delay period of the maze task. We found that cell activity was influenced by both the exit status and the direction of the path, beginning soon after the maze was displayed. This activity was not related to the activation of arm muscles, suggesting that the directional signals observed represented abstract spatial aspects of maze processing. Finally, we compared maze-related activity of M1 neurons with those recorded from posterior parietal area 7a, reported previously (Crowe et al. 2004). Interestingly, cells from each area exhibited similar properties. Both the exit status and path direction were encoded by cells in M1 and 7a, although to different extents. An analysis of the time-course of the neural representation of these factors revealed that area 7a and M1 begin to encode these factors at the same time, suggesting these brain areas are part of a distributed system performing the spatial computations involved in maze solution.

Motor planning: effect of directional uncertainty with discrete spatial cues

We investigated the effect of spatial uncertainty on motor planning by using the cueing method in a reaching task (experiment 1). Discrete spatial cues indicated the different locations in which the target could be presented. The number of cues as well as their direction changed from trial to trial. We tested the adequacy of two models of motor planning to account for the data. The switching model assumes that only one motor response can be planned at a time, whereas the capacity-sharing model assumes that multiple motor responses can be planned in parallel. Both models predict the same relation between average reaction time (RT) and number of cues, but they differ in their prediction of the shape of the distribution of the reaction time. The results showed that RT increased with the number of cues independently from their spatial dispersion. This relation was well described by the function predicted by both models, whereas it was poorly described by the Hick-Hyman law. In addition, the distribution of RT conformed to the prediction of the capacity-sharing model and not to that of the switching model. We investigated the role that the requirement of a spatially directed motor response might have had on this pattern of results by testing subjects in a simple RT task (experiment 2) with the same cueing presentation as in experiment 1. The results contrasted with those in experiment 1 and showed that RT was dependent on the spatial dispersion of the cues and not on their number. The results of the two experiments suggest that the mode of processing of potential targets is dependent on the spatial constraints of the task. The processing resources can be either divided relative to the spatial distribution of possible targets or across multiple independent discrete representations of these targets.

Neural activity in human primary motor cortex areas 4a and 4p is modulated differentially by attention to action

The mechanisms underlying attention to action are poorly understood. Although distracted by something else, we often maintain the accuracy of a movement, which suggests that differential neural mechanisms for the control of attended and nonattended action exist. Using functional magnetic resonance imaging (fMRI) in normal volunteers and probabilistic cytoarchitectonic maps, we observed that neural activity in subarea 4p (posterior) within the primary motor cortex was modulated by attention to action, while neural activity in subarea 4a (anterior) was not. The data provide the direct evidence for differential neural mechanisms during attended and unattended action in human primary motor cortex.

Plasticity and primary motor cortex

One fundamental function of primary motor cortex (MI) is to control voluntary movements. Recent evidence suggests that this role emerges from distributed networks rather than discrete representations and that in adult mammals these networks are capable of modification. Neuronal recordings and activation patterns revealed with neuroimaging methods have shown considerable plasticity of MI representations and cell properties following pathological or traumatic changes and in relation to everyday experience, including motor-skill learning and cognitive motor actions. The intrinsic horizontal neuronal connections in MI are a strong candidate substrate for map reorganization: They interconnect large regions of MI, they show activity-dependent plasticity, and they modify in association with skill learning. These findings suggest that MI cortex is not simply a static motor control structure. It also contains a dynamic substrate that participates in motor learning and possibly in cognitive events as well.

Prior information in motor and premotor cortex: activity during the delay period and effect on pre-movement activity

In instructed-delay (ID) tasks, instructional cues provide prior information about the nature of a movement to execute after a delay. Neuronal responses in dorsal premotor cortex (PMd) during the instructed-delay period (IDP) between the CUE and subsequent GO signals are presumed to reflect early planning stages initiated by the prior information. In contrast, in multiple-choice reaction-time (RT) tasks, all motor planning and execution processes must occur after the GO signal. These assumptions predict that neuronal planning correlates recorded during the IDP of ID trials should share common features with early post-GO activity in RT trials, and that those response components need not be recapitulated after the GO signal of ID trials. These two predictions were tested by comparing activity recorded in RT and ID tasks from 503 neurons in PMd and caudal (MIc) and rostral (MIr) primary motor cortex. The incidence and strength of directionally tuned IDP activity declined progressively from PMd to MIc. The directional tuning of activity during the IDP of ID trials was more similar to that in the reaction-time epoch (RTE) of RT trials than after movement onset, especially in PMd. A modulation of post-GO activity was often observed between RT and ID trials and was confined mainly to the RTE. This effect was also most prominent in PMd. The most common change was a reduction in intensity of short-latency phasic responses to the GO signal between RT and ID trials, especially in PMd cells with a short-latency phasic response to CUE signals. However, the largest group of cells in each area showed no large change in peak RTE activity between RT and ID trials, whether they were active in the IDP or not. Since early phasic CUE-related responses are least likely to be recapitulated after the GO signal in ID trials, they may be a neuronal correlate of an early planning stage such as response selection. Tonic IDP responses, which are not as strongly associated with a post-GO reduction in activity, may be related to other aspects of motor planning and preparation. Finally, a major component of the movement-related activity in both MI and PMd is not susceptible to modification by prior information and is indivisibly coupled temporally to movement execution.

Neural aspects of cognitive motor control

Traditionally, motor and cognitive functions were studied separately; however, the investigation of processes at the interface between cognition and action has become more and more popular recently. Typical research goals include the identification of the processes involved using experimental psychological methods, and understanding the neural mechanisms underlying these processes using neurophysiological and functional neuroimaging methods. Specifically, there has been a special emphasis during the past few years on timing mechanisms, practice effects, and the application of rules in guiding action. New information concerning the neural mechanisms involved is being acquired at a rapid pace, albeit mostly within a descriptive framework. With respect to specific brain areas, a key finding has been the clear involvement of the primary motor cortex in complex tasks engaging diverse motor and cognitive dimensions.

An alternative interpretation of population vector rotation in macaque motor cortex

Neural recordings from the primary motor cortex of monkeys performing movements at an angle to a cue stimulus have yielded two main results: (A) the population vector rotates from the direction of the cue to the direction of movement, and (B) cells with intermediate preferred directions are recruited during the middle of this rotation. These results have been interpreted as the neural correlates of a process of 'mental rotation'. Here we propose that results A and B are also consistent with an alternate hypothesis of 'response substitution', given four well known features of cortical neurophysiology.

A theory of geometric constraints on neural activity for natural three-dimensional movement

Although the orientation of an arm in space or the static view of an object may be represented by a population of neurons in complex ways, how these variables change with movement often follows simple linear rules, reflecting the underlying geometric constraints in the physical world. A theoretical analysis is presented for how such constraints affect the average firing rates of sensory and motor neurons during natural movements with low degrees of freedom, such as a limb movement and rigid object motion. When applied to nonrigid reaching arm movements, the linear theory accounts for cosine directional tuning with linear speed modulation, predicts a curl-free spatial distribution of preferred directions, and also explains why the instantaneous motion of the hand can be recovered from the neural population activity. For three-dimensional motion of a rigid object, the theory predicts that, to a first approximation, the response of a sensory neuron should have a preferred translational direction and a preferred rotation axis in space, both with cosine tuning functions modulated multiplicatively by speed and angular speed, respectively. Some known tuning properties of motion-sensitive neurons follow as special cases. Acceleration tuning and nonlinear speed modulation are considered in an extension of the linear theory. This general approach provides a principled method to derive mechanism-insensitive neuronal properties by exploiting the inherently low dimensionality of natural movements.

Motor cortical encoding of serial order in a context-recall task

The neural encoding of serial order was studied in the motor cortex of monkeys performing a context-recall memory scanning task. Up to five visual stimuli were presented successively on a circle (list presentation phase), and then one of them (test stimulus) changed color; the monkeys had to make a single motor response toward the stimulus that immediately followed the test stimulus in the list. Correct performance in this task depends on memorization of the serial order of the stimuli during their presentation. It was found that changes in neural activity during the list presentation phase reflected the serial order of the stimuli; the effect on cell activity of the serial order of stimuli during their presentation was at least as strong as the effect of motor direction on cell activity during the execution of the motor response. This establishes the serial order of stimuli in a motor task as an important determinant of motor cortical activity during stimulus presentation and in the absence of changes in peripheral motor events, in contrast to the commonly held view of the motor cortex as just an "upper motor neuron."

Event-related magnetic fields in the auditory cortex of man during unilateral movements: a discriminant function analysis

It is often assumed that sensorimotor coordination is a feature of the sensorimotor areas of the neocortex only. The purpose of the present study was to examine how this phenomenon is reflected in the auditory cortex of man. Ten subjects were engaged in a stimulus-reaction paradigm, in which each of two acoustical tones was associated to either of two motor reactions. Magnetic fields recorded with a 122-channel magnetometer were modelled by current dipoles. The spatial coordinates as well as the amplitudes of the dipoles were analyzed from 90 to 110 ms after stimulus onset using discriminant analysis. The results suggest that the dipole trajectory in the auditory cortex of the right hemisphere and amplitudes of the dipoles in the auditory cortex of the left hemisphere already 90-110 ms after the beginning of the stimulus could be affected not only by physical features of the stimulus, but also by the motor task required as a reaction.

Conversion of sensory signals into motor commands in primary motor cortex

Movement triggered by sensory stimuli requires that the networks generating the motor commands receive an adequate driving input, which, in general, is a transformed version of the initial sensory signal. We investigated the nature of this transformation in a task in which monkeys categorize the speed of tactile stimuli as either low or high, reaching for one of two pushbuttons to indicate their choice. Extracellular recordings from primary motor cortex revealed two types of neurons selective for the speed categories: ones that fire at higher rates for low versus high speeds, and others that do the opposite. These differential responses are task-specific; no firing rate modulation was seen when identical arm movements were triggered by visual cues or when stimuli were delivered passively. Analyses using decoding and modeling techniques produced two main results. First, the neurons accurately encode the chosen category; an observer measuring their responses can exhibit a psychophysical performance during categorization identical to the monkey's. Second, by analyzing separately the trials in which hits and errors were scored, it is possible to distinguish purely sensory activity from activity exclusively related to arm motion. The recorded responses did not match either of these alternatives but were consistent with a model in which the category-tuned neurons are the link between the output of the sensory categorization process and the motor command used to indicate the animal's decision. Thus, the observed activity seems to encode a preprocessed version of the sensory stimulus and to participate in driving the arm motion.

A model of reaching dynamics in primary motor cortex

Features of virtually all voluntary movements are represented in the primary motor cortex. The movements can be ongoing, imminent, delayed, or imagined. Our goal was to investigate the dynamics of movement representation in the motor cortex. To do this we trained a fully recurrent neural network to continually output the direction and magnitude of movements required to reach randomly changing targets. Model neurons developed preferred directions and other properties similar to real motor cortical neurons. The key finding is that when the target for a reaching movement changes location, the ensemble representation of the movement changes nearly monotonically, and the individual neurons comprising the representation exhibit strong, nonmonotonic transients. These transients serve as internal recurrent signals that force the ensemble representation to change more rapidly than if it were limited by the time constants of individual neurons. These transients, if they exist, could be observed in experiments that require only slight modifications of the standard paradigm used to investigate movement representation in the motor cortex.

Reaching movements: concurrency of continuous and discrete programming

The present study was undertaken to investigate further how human subjects prepare default parameters of reaching movements. Two qualitatively different modes for setting default values of direction are possible (continuous and discrete mode) and there is a threshold visual target separation for continuous or discrete programming. The most prominent finding is that the continuous and discrete modes of programming can be used concurrently.

Cortical control of reaching movements

Recent studies provide further support for the hypothesis that spatial representations of limb position, target locations, and potential motor actions are expressed in the neuronal activity in parietal cortex. In contrast, precentral cortical activity more strongly expresses processes involved in the selection and execution of motor actions. As a general conceptual framework, these processes may be interpreted in terms of such formalisms as sensorimotor transformations and 'internal models'.

Mental Rotation Studied by Functional Magnetic Resonance Imaging at High Field (4 Tesla): Performance and Cortical Activation

We studied the performance and cortical activation patterns during a mental rotation task (Shepard & Metzler, 1971) using functional magnetic resonance imaging (fMlU) at high field (4 Tesla). Twenty-four human subjects were imaged (fMRI group), whereas six additional subjects performed the task without being imaged (control group). All subjects were shown pairs of perspective drawings of 31, objects and asked to judge whether they were the same or mirror images. The measures of performance examined included (1) the percentage of errors, (2) the speed of performance, calculated as the inverse of the average response time, and (3) the rate of rotation for those object pairs correctly identified as “same.” We found the following: (1) Subjects in the fMRI group performed well outside and inside the magnet, and, in the latter case, before and during data acquisition. Moreover, performance over time improved in the same manner as in the control group. These findings indicate that exposure to high magnetic fields does not impair performance in mental rotation. (2) Functional activation data were analyzed from 16 subjects of the fMRI goup. Several cortical areas were activated during task performance. The relations between the measures of performance above and the magnitude of activation of specific cortical areas were investigated by anatomically demarcating these areas of interest and calculating a normalized activation for each one of them. (3) We used the multivariate technique of hierarchical tree modeling to determine functional clustering among areas of interest and performance measures. Two main branches were distinguished: One comprised areas in the right hemisphere and the extrastriate and superior parietal lobules bilaterally, whereas the other comprised areas of the left hemisphere and the frontal pole bilaterally; all three performance measures above clustered with the former branch. Specifically, performance outcome (“percentage of errors”) clustered with the parieto-occipital subcluster, whereas both the speed of performance and the rate of mental rotation clustered with the right precentral gyms. We conclude that the mental rotation paradigm used involves the cooperative interaction of functional groups of cortical areas of which some are probably more specifically associated with performance, whereas others may serve a more general function within the task constraints.

Neural Networks and Motor Control

Motor control is accomplished by the cooperative interaction of many brain networks, among which the motor cortex holds a central place. This article reviews some of the structural and functional properties of neurons of the motor cortical network, some principles of connectivity with other motor networks, the handling of spatial information regarding reaching movements, and some ideas on how motor cortical commands could be translated to muscle activations by spinal motor networks. Finally, I review recent neural network modeling studies of motor cortical ensemble operations.

The significance of neural ensemble codes during behavior and cognition

The development of techniques to record from populations of neurons has made it possible to ask questions concerning the encoding of task-relevant information in awake, behaving animals. The issue of how groups of neurons within different brain structures register and retrieve representations of behaviorally significant events can now be addressed using multineuron-recording techniques. This review examines recent studies employing simultaneous recording of ten or more individual neurons in the mammalian brain. A major issue discussed is whether ensemble information content reconstructed from single-neuron recordings may be underestimated if compared to ensembles where those same neurons were recorded simultaneously. The mechanics of ensemble information encoding in the hippocampus is illustrated from population statistical analyses of ensemble activity during performance of a delay task. Detailed descriptions of methods of extracting ensemble information, as well as cross-correlational analyses, are discussed in the context of emergent issues regarding interpretation of ensemble data.

Mental transformations in the motor cortex

The behavioral and neural correlates of processing of motor directional information are described for two visuomotor tasks: mental rotation and context-recall. Psychological studies with human subjects suggested that these two tasks involve different time-consuming processes of directional information. Analyses of the activity of single cells and neuronal populations in the motor cortex of behaving monkeys performing in the same tasks provided direct insight into the neural mechanisms involved and confirmed their different nature. In the mental rotation task the patterns of neuronal activity revealed a rotation of the intended direction of movement. In contrast, in the context-recall task the patterns of neural activity identified a switching process of the intended direction of movement.

Cortical dynamic tuning: the role of intrinsic connections, as revealed by modeling

A current challenge in computational neuroscience is to elucidate the role of cortical circuitry in information processing and in generating motor output. Our understanding of the functional significance of specifically organized feedback connections is progressing rapidly as researchers establish the equivalence of theoretical models to biological neural circuits. Modeling studies of different neural structures, along with quantitative comparisons of model performance to biological data, have recently helped to identify the basic features of synaptic connectivity that may play important roles in cortical operations.

Hippocampal place fields: relationship between degree of field overlap and cross-correlations within ensembles of hippocampal neurons

The capacity to record from multiple neurons in awake freely moving animals provides a means for characterizing organizational principles of place field encoding within ensembles of hippocampal neurons. In this study, cross-correlations between pairs of hippocampal place cells and degree of overlap between their respective place fields were analyzed during behavioral performance of delayed matching (DMS) or non-matching sample (DNMS) tasks, or while the same rats chased pellets in a different environment. The relationship between field overlap and cross-correlations of neural spike activity within ensembles was shown to be a positive, exponentially increasing, function. Place fields from the same neurons were markedly "remapped" between the Delay and Pellet-chasing tasks, with respect to physical location and size of fields. However individual pairs of place cells within each ensemble retained nearly the same degree of overlap and cross-correlation even though the spatial environment and the tasks differed markedly. This suggested that place cells were organized in functional "clusters" which exhibited the same inter-relations with respect to place field overlap and cross-correlations, irrespective of actual field of location. When cross-correlations between place cells were compared to placement of the array recording electrodes within the hippocampus, the strongest correlations were found along previously defined posterior-projecting fiber gradients between CA3 and CA1 subfields (Ishizuka et al. [1990], J Comp Neurol 295:580-623; Li et al. [1994] (J Comp Neurol 339:181-208). These findings suggest that the functional organization of place fields conforms to anatomical principles suspected to operate within hippocampal ensembles.

The mental and the neural: psychological and neural studies of mental rotation and memory scanning

In this article we review studies pertaining to psychophysical measurements and neural correlates of tasks requiring the processing of directional information in spatial motor tasks. The results of psychological studies in human subjects indicate that time-consuming processes underlie mental rotation and memory scanning. Other studies have suggested that these processes may rely on different basic mechanisms. A direct insight into their neural mechanisms was obtained analyzing the activity of single cells and neuronal populations in the brain of behaving monkeys performing the same tasks. These studies revealed the nature of the neural processes underlying mental rotation and memory scanning and confirmed their different nature.

Current issues in directional motor control

Many studies during the past 15 years have shown that the direction of motor output (movement or isometric force) is an important factor for neuronal activity in the motor cortex, both at the level of single cells and at the level of neuronal populations. Recent studies have investigated several new aspects of this problem including the effect of posture, the relations to time-varying movement parameters (for example, position, velocity and acceleration) and the cortical representation of memorized simple movements and complex-movement trajectories. Furthermore, the neural correlates of directional operations, such as mental rotation and memory-scanning of visuomotor directions, have also been investigated. In addition, neural networks have been used to model dynamic, time-varying, spatial motor trajectories.