The Effects of Objectives and Constraints on Motor Control Strategy in Reciprocal Aiming Movements

The goal of this study was to examine how the kinematics of reciprocal aiming movements were affected by both the objective of the movement and the constraints operating on that movement. In Experiment 1, the objective of the movement was indirectly manipulated by capitalizing on the fact that subjects determine their own accuracy and speed limits, despite uniform task instructions to move as quickly and accurately as possible. A Fitts' type reciprocal aiming paradigm was employed, in which 69 subjects were asked to move a stylus repetitively between two spatially separated targets. Four target widths were orthogonally combined with four movement amplitudes, resulting in 16 conditions. Movements were made on an X-Y digitizing tablet. Based on the mean variable error produced on both targets, subjects were differentiated post hoc into three movement objective groups: speed, accuracy, and speed-plus-accuracy. Kinematic analyses revealed that the programming and execution of movements were systematically influenced by both the movement objective and the movement constraints. That is, movement time, peak velocity, dwell time, acceleration and deceleration time, normalized acceleration and normalized deceleration varied systematically as a function of both the speed-accuracy movement objective and the movement constraints of target size and movement distance. Moreover, the consequences of changing the constraints of the movement were affected by an interaction with the objective of the movement. In Experiment 2, the objective of the movement was directly manipulated by varying speed and/or accuracy instructions to subjects. The basic results of Experiment 1 were substantiated. Overall, the results were consistent with the view that motor control is dependent upon sensory consequences.     

Pre-Motor and Motor Reaction Time Differences Associated with Stretching of the Hamstring Muscles

Pre-motor and motor RT scores were obtained from 24 male Ss using a knee flexion task. It was predicted that an increased arousal state due to proprioceptive feedback from stretched hamstring muscles would shorten pre-motor RT while motor RT would shorten because of changes in muscle tension development due to changes in the series elastic and/or contractile components of the muscle tissue. A finger RT task was also included in order to determine whether other factors not related to changes in the stretch of the hamstring muscles were operative. Motor RT decreased with increased muscle stretch and constituted 46% of leg RT. Pre-motor RT as well as finger RT increased rather than decreased with increased muscle stretch. No relationship was found between pre-motor and motor RT indicating that lags in CNS processing are independent of lags associated with the rate of muscular tension development.     

Cortical Cell Assemblies: A Possible Mechanism for Motor Programs

The concept of a motor program has been used to interpret a diverse range of empirical findings related to preparation and initiation of voluntary movement. In the absence of an underlying mechanism, its explanatory power has been limited to that of an analogy with running a stored computer program. We argue that the theory of cortical cell assemblies suggests a possible neural mechanism for motor programming. According to this view, a motor program may be conceptualized as a cell assembly, which is stored in the form of strengthened synaptic connections between cortical pyramidal neurons. These connections determine which combinations of corticospinal neurons are activated when the cell assembly is ignited. The dynamics of cell assembly ignition are considered in relation to the problem of serial order. These considerations lead to a plausible neural mechanism for the programming of movements and movement sequences that is compatible with the effects of precue information and sequence length on reaction times. Anatomical and physiological guidelines for future quantitative models of cortical cell assemblies are suggested. By taking into account the parallel, re-entrant loops between the cerebral cortex and basal ganglia, the theory of cortical cell assemblies suggests a mechanism for motor plans that involve longer sequences. The suggested model is compared with other existing neural network models for motor programming.     

Interference Effects in Learning Similar Sequences of Discrete Movements

Three experiments were conducted to examine proactive and retroactive interference effects in learning 2 similar sequences of discrete movements. In each experiment, the participants in the experimental group practiced 2 movement sequences on consecutive days (1 on each day, order counterbalanced across participants) followed by retention tests on the third day. In all, 2 out of 8 target locations differed between the 2 sequences. Experiment 1 established the nature of the interference effects in the present setup. Clear evidence was found for button-specific proactive and retroactive interference effects. Experiments 2 and 3 further probed the mechanisms underlying those effects, by varying the numbers of repetitions (50 or 250) of the 1st and 2nd sequence (Experiment 2) and the hand, dominant or nondominant, with which the sequences were practiced (Experiment 3). Experiment 2 showed that after a mere 50 repetitions, the representation of the movement structure was strong enough to evoke the effects observed in Experiment 1. Experiment 3 revealed that learning with the dominant hand did not result in more pronounced interference effects compared with learning with the nondominant hand. In combination, these results suggest that changes in the representation of the movement structure are primarily responsible for the observed interference effects.     

Investigation of Perceptual-Motor Behavior Across the Expert Athlete to Disabled Patient Skill Continuum can Advance Theory and Practical Application

A framework is presented of how theoretical predictions can be tested across the expert athlete to disabled patient skill continuum. Common-coding theory is used as the exemplar to discuss sensory and motor system contributions to perceptual-motor behavior. Behavioral and neural studies investigating expert athletes and patients recovering from cerebral stroke are reviewed. They provide evidence of bi-directional contributions of visual and motor systems to perceptual-motor behavior. Majority of this research is focused on perceptual-motor performance or learning, with less on transfer. The field is ripe for research designed to test theoretical predictions across the expert athlete to disabled patient skill continuum. Our view has implications for theory and practice in sports science, physical education, and rehabilitation.     

How Can You Best Measure Reaction Times?

Comparing many ways of measuring and analyzing reaction times reveals that the chosen method influences both the judged reaction time and, more importantly, conclusions about how the reaction time depends on the circumstances under study. The task was to lift one's finger in response to a tone. The response amplitude was either constrained or not. Constraining the amplitude made the response less vigorous. When the response was less vigorous it took longer to move far enough to release a switch or exceed the elasticity of the finger pulp. Although using a micro-switch would have made the reaction time appear to be longer for the constrained movement, reaction times determined in the most reliable ways were not systematically longer for the constrained movement. The most reliable method is to use extrapolation of the change in the average force that the finger exerts on the surface to estimate the reaction time.     

The Motor System Computes Well but Remembers Poorly

The psychological approach to human voluntary movements advocated by F. Mechsner (2004) can be aligned with the view that motor commands are generated afresh but are not retrieved from memory. This movement-generation hypothesis gains indirect support in the findings Mechsner reviewed and obtains more direct support from the results of studies of memory for movement and computational modeling of motor planning.     

What the Eye Tells the Hand

The information flow necessary to coordinate hand movements with vision is summarized. A series of experiments was performed which quantitatively examined and confirmed the summary. In general, an information-theoretic approach was used. It was confirmed that the speed and accuracy of hand movements are in a reciprocal relationship. The accuracy of a movement is more sensitive to a loss of visual information than the pacing mechanism. Both speed and accuracy are sensitive to withdrawal of information to maintain posture. It is suggested that the retrieval of a movement pre-program is disturbed by afferent “noise.” The contribution of visual information to controlling standing posture, arm and shoulder movement, and wrist movement were investigated.     

Sensorimotor Impairment of Speech and Hand Movement Timing Processing in Parkinson’s Disease

Models of motor control have highlighted the role of temporal predictive mechanisms in sensorimotor processing of speech and limb movement timing. We investigated how these mechanisms are affected in Parkinson’s disease (PD) while patients performed speech and hand motor reaction time tasks in response to sensory stimuli with predictable and unpredictable temporal dynamics. Results showed slower motor reaction times in PD vs. control in response to temporally predictable, but not unpredictable stimuli. This effect was driven by faster motor responses to predictable stimuli in control subjects; however, no such effect was observed in the PD group. These findings indicated the relationship between PD pathology and sensorimotor deficits in temporal predictive mechanisms of timing processing during speech production and hand movement.     

Anomalous global effects induced by ‘blind’ distractors in visual hemifield defects

Previous research has revealed that a stimulus presented in the blind visual field of participants with visual hemifield defects can evoke oculomotor competition, in the absence of awareness. Here we studied three cases to determine whether a distractor in a blind hemifield would be capable of inducing a global effect, a shift of saccade endpoint when target and distractor are close to each other, in participants with lesions of the optic radiations or striate cortex. We found that blind field distractors significantly shifted saccadic endpoints in two of three participants with lesions of either the striate cortex or distal optic radiations. The direction of the effect was paradoxical, however, in that saccadic endpoints shifted away from blind field distractors, whereas endpoints shifted towards distractors in the visible hemifields, which is the normal global effect. These results provide further evidence that elements presented in the blind visual field can generate modulatory interactions in the oculomotor system, which may differ from interactions in normal vision.