Control theories in motor behavior

Intracortical brain-computer interfaces in primates: a review and outlook

Brain-computer interfaces (BCI) translate brain signals into artificial output to restore or replace natural central nervous system (CNS) functions. Multiple processes, including sensorimotor integration, decision-making, motor planning, execution, and updating, are involved in any movement. For example, a BCI may be better able to restore naturalistic motor behaviors if it uses signals from multiple brain areas and decodes natural behaviors' cognitive and motor aspects. This review provides an overview of the preliminary information necessary to plan a BCI project focusing on intracortical implants in primates. Since the brain structure and areas of non-human primates (NHP) are similar to humans, exploring the result of NHP studies will eventually benefit human BCI studies. The different types of BCI systems based on the target cortical area, types of signals, and decoding methods will be discussed. In addition, various successful state-of-the-art cases will be reviewed in more detail, focusing on the general algorithm followed in the real-time system. Finally, an outlook for improving the current BCI research studies will be debated.

The representation of egocentric space in the posterior parietal cortex

The posterior parietal cortex (PPC) is the most likely site where egocentric spatial relationships are represented in the brain. PPC cells receive visual, auditory, somaesthetic, and vestibular sensory inputs; oculomotor, head, limb, and body motor signals; and strong motivational projections from the limbic system. Their discharge increases not only when an animal moves towards a sensory target, but also when it directs its attention to it. PPC lesions have the opposite effect: sensory inattention and neglect. The PPC does not seem to contain a "map" of the location of objects in space but a distributed neural network for transforming one set of sensory vectors into other sensory reference frames or into various motor coordinate systems. Which set of transformation rules is used probably depends on attention, which selectively enhances the synapses needed for making a particular sensory comparison or aiming a particular movement.

Does the nervous system depend on kinesthetic information to control natural limb movements?

This target article draws together two groups of experimental studies on the control of human movement through peripheral feedback and centrally generated signals of motor commands. First, during natural movement, feedback from muscle, joint, and cutaneous afferents changes; in human subjects these changes have reflex and kinesthetic consequences. Recent psychophysical and microneurographic evidence suggests that joint and even cutaneous afferents may have a proprioceptive role. Second, the role of centrally generated motor commands in the control of normal movements and movements following acute and chronic deafferentation is reviewed. There is increasing evidence that subjects can perceive their motor commands under various conditions, but that this is inadequate for normal movement; deficits in motor performance arise when the reliance on proprioceptive feedback is abolished either experimentally or because of pathology. During natural movement, the CNS appears to have access to functionally useful input from a range of peripheral receptors as well as from internally generated command signals. The unanswered questions that remain suggest a number of avenues for further research.

Implications of neural networks for how we think about brain function

Engineers use neural networks to control systems too complex for conventional engineering solutions. To examine the behavior of individual hidden units would defeat the purpose of this approach because it would be largely uninterpretable. Yet neurophysiologists spend their careers doing just that! Hidden units contain bits and scraps of signals that yield only arcane hints about network function and no information about how its individual units process signals. Most literature on single-unit recordings attests to this grim fact. On the other hand, knowing a system's function and describing it with elegant mathematics tell one very little about what to expect of interneuronal behavior. Examples of simple networks based on neurophysiology are taken from the oculomotor literature to suggest how single-unit interpretability might decrease with increasing task complexity. It is argued that trying to explain how any real neural network works on a cell-by-cell, reductionist basis is futile and we may have to be content with trying to understand the brain at higher levels of organization.

What muscle variable(s) does the nervous system control in limb movements?

To control force accurately under a wide range of behavioral conditions, the central nervous system would either require a detailed, continuously updated representation of the state of each muscle (and the load against which each is acting) or else force feedback with sufficient gain to cope with variations in the properties of the muscles and loads. The evidence for force feedback with adequate gain or for an appropriate central representation is not sufficient to conclude that force is the major controlled variable in normal limb movements.

Morton's hypothesis, that length is controlled by a follow-up servo, has a number of difficulties related to the delays, gains, variability, and specificity in feedback pathways comprising potential servo loops. However, experimental evidence is consistent with these pathways providing servo assistance for some movements produced by coactivation of α- and static γ-motoneurons. Dynamic γ-motoneurons may provide an additional input for adaptive control of different types of movements.

The idea that feedback is used to compensate for changes in muscle stiffness has received experimental support under static postural conditions. However, reflexes tend to increase rather than decrease the range of variation in muscle stiffness during some cyclic movements. Theoretical problems associated with the regulation of stiffness are also discussed. The possibilities of separate control systems for velocity or viscosity are considered, but the evidence is either negative or lacking. I conclude that different physical variables can be controlled depending on the type of limb movement required. The concept of stiffness regulation is also useful under some conditions, but should probably be extended to the regulation of the visco-elastic properties (i.e., the mechanical impedance) of a muscle or joint.

Thinking as the control of imagination: a conceptual framework for goal-directed systems

This paper offers a conceptual framework which (re)integrates goal-directed control, motivational processes, and executive functions, and suggests a developmental pathway from situated action to higher level cognition. We first illustrate a basic computational (control-theoretic) model of goal-directed action that makes use of internal modeling. We then show that by adding the problem of selection among multiple action alternatives motivation enters the scene, and that the basic mechanisms of executive functions such as inhibition, the monitoring of progresses, and working memory, are required for this system to work. Further, we elaborate on the idea that the off-line re-enactment of anticipatory mechanisms used for action control gives rise to (embodied) mental simulations, and propose that thinking consists essentially in controlling mental simulations rather than directly controlling behavior and perceptions. We conclude by sketching an evolutionary perspective of this process, proposing that anticipation leveraged cognition, and by highlighting specific predictions of our model.

Automatic behaviour: efficient not mindless

Automaticity is a core construct underpinning theoretical accounts of human performance and cognition. In spite of this, its current conceptualisation is plagued by circularity - automaticity is typically defined in terms of the very behaviour it seeks to explain - and a lack of internal consistency-defining features of automaticity do not reliably co-occur. Furthermore, invoking automaticity tends to be post hoc as it is used to explain violations of dominant theories of attention. Prevailing models of automaticity explain automatic processing as merely faster processing than controlled processing. We present an alternative conceptualisation of automaticity as efficient, elegant and economical but not fast. This is supported by functional imaging studies, which reveal a pattern of reduced global activation as well as a shift in activation from cortical to subcortical areas once automaticity has been achieved. Were automaticity to be faster processing, functional imaging would indicate greater activation when an automatic task is performed. We propose possible circuitry of automaticity incorporating the direct pathways of the basal ganglia.

The micro-structure of tapping movements in children

In order to investigate the development of movement speed in relation to movement organization, children of 5, 6, 7, 8 and 9 years of age and adults carried out a reciprocal tapping task, in which time pressure and distance were manipulated. The duration, velocity, acceleration and accuracy of the movements were compared between age groups. Age differences appeared mainly in the homing time, not in the duration of the distance covering movement phase. Accuracy and velocity of the distance covering movement phase differed with age. Time pressure affected the homing time, but not the duration of the distance covering phase. Distance manipulation affected mainly the velocity and duration of the distance covering movement phase and the homing time. In the discussion it is contended that age differences in homing time may be related to both the accuracy of the distance covering movement phase and the rate of information processing of the subject.

Characteristics of cursor trajectories controlled by the computer mouse

An analysis of computer screen cursor trajectories can provide insights into the factors limiting efficient cursor positioning and can assist in the design of human-computer interfaces. Cursor locations as controlled by a Microsoft computer mouse with standard settings were therefore sampled at 5 ms intervals and kinematic analyses addressed the proportions of time spent in the initiation, accelerative and terminal guidance phases of cursor positioning. Twelve participants used a computer mouse to move a cursor over different distances (7.5 cm, 15 cm) from a home location in the lower centre of the screen to targets of different diameters (8 mm, 16 mm), situated to the left, middle or right of the computer screen. Cursor trajectories were irregular, and participants regularly overshot their targets, spending 70% of movement duration in terminal guidance. Participants appeared to use the initial part of their movement to establish mappings between controller and display. Interventions should seek to reduce the terminal guidance phase of cursor positioning.

Spinal position sense is independent of the magnitude of movement

STUDY DESIGN

Position sense in the spine was recorded at T1, T7, L1, and S2 in three incremental angular ranges of flexion and on return to upright standing from these movements.

OBJECTIVES

To determine the effect of angular range of movement on position sense. The main purpose was to establish a protocol for whole spine assessment of position sense in healthy and pathologic spines.

SUMMARY OF BACKGROUND DATA

Position sense is one dimension of proprioception, classically assessed by the ability to reproduce preselected target positions. This approach was used in the current study to determine whether spinal position sense is affected by the magnitude of movement traversed in repositioning tasks.

METHODS

Spinal position sense was assessed in 20 healthy subjects during repeated flexion movements carried out in one-third, half, and two thirds of the full range of movement in the sagittal and coronal planes. During each movement, the 3-Space Fastrak (Polhemus Inc., Colchester, VT) was used to record angular movement of the spine at four sensor locations (T1, T7, L1, and S2). The absolute difference in the sensor angles between repeated trials was calculated for each flexed position and on return to upright standing from these. These absolute differences were used as a measure of position sense.

RESULTS

Absolute position sense after one-third angular movements was accurate to within 4.30 degrees +/- 2.84 degrees in flexed positions and 2.70 degrees +/- 2.20 degrees in upright postures. Corresponding results for two-thirds movements were 4.75 degrees +/- 2.63 degrees and 3.33 degrees +/- 2.60 degrees, respectively. Range of movement had no significant influence on the accuracy of position sense.

CONCLUSIONS

1) Healthy individuals are able to reposition their spine accurately under conditions of incremental increases in angular range. 2) Range-related variations in position sense are small and unlikely to be of clinical significance.

Knee position error detection in closed and open kinetic chain tasks during concurrent cognitive distraction

It is important to establish whether presumed differences among varieties of motor responses are manifested in related differences in performance. In order to determine possible functional distinctions between closed and open kinetic chain tasks, participants' performance in the presence or absence of cognitive distraction on an error-detection task was assessed. Individual testing of participants consisted of knee extension and flexion movements to experimenter-defined positions, approximating the 25th, 50th, and 75th percentile of the participants' range of motion on the apparatus. Performance under conditions of distraction was significantly worse than in the absence of distraction. Performance using longer movements was significantly more accurate. No substantial differences were found between closed and open kinetic chain movements. Limitations of this research for the distinction between open and closed chain tasks are addressed, and clinical implications of the effects of distraction are presented.

Movement dysfunction in patients with Parkinson's disease: A literature review

Experimental evidence has shown that people with Parkinson's disease have deficits in the initiation and execution of movements. The delay in response initiation may be due to impairment in the organisation or translation of motor programs into muscle actions. The slowness in the execution of simple movements may result from inappropriate scaling of muscle activity, defective predictive function or defective memory for the computed forces. Extra slowness in the execution of complicated concurrent movements appears to be a result of deficits in switching from one program to another within a motor plan in sequential movements, or in superimposition of motor programs to form a motor plan in simultaneous movements.

Stratification in perception and action

This paper discusses the concept of stratification, in the sense of levels of analysis and levels of control, in relation to human perception and performance. It is contended that functional analysis is the proper level of analysis for the domain of perception and action. This is illustrated by means of models of cognitive energetics and motor control. The functional level of analysis can be situated in between the level of symbolic representations and the level of neurophysiological mechanisms. It is also argued that the concept of levels of control provides a way of integrating or relating ecological and information-processing approaches to psychology, in the sense that ecological psychology is concerned with lower and more peripheral levels of control (coordinative structures). Finally, some remarks are made on the relation between cognition and perception and action, drawing on Piaget, and it is proposed that the concept of schema is more appropriate for the domain of perception and action than is the notion of symbolic representations.