Chantal Bard

Attentional demands for static and dynamic equilibrium

Upright standing and walking tasks require the integration of several sources of sensory information. In a normal and highly predictable environment, locomotor synergies involving several muscles may take place at lower spinal levels with neural circuitry tuned by local loops of assistance or self-organizing processes generated in coordinative networks. When ongoing regulation of gait is necessary (obstacles, changes in direction) supraspinal involvement is necessary to perform movements adapted to the environment. Using a classical information processing framework and a dual-task methodology, it is possible to evaluate the attentional demands for performing static and dynamic equilibrium tasks. The present experiment evaluates whether the attentional requirements for a control sitting condition and for standing and walking conditions vary with the intrinsic balance demands of the tasks. The results show that standing and walking conditions required more attention than sitting in a chair. The attentional cost for walking was also significantly greater than for standing. For the walking task, reaction times when subjects were in singlesupport phase (small base of support) were significantly longer than those in double-support phase, suggesting that the attentional demands increased with an increase in the balance requirements of the task. Balance control requires a continuous regulation and integration of sensory inputs; increasing balance demands loads the higher level cognitive system.

On the cognitive penetrability of posture control

Postural sway increases with age. The decreased stability associated with postural sway often has been related to the reduced peripheral sensibility in the visual, vestibular, and proprioceptive systems. We examined whether the micropostural adjustments necessary for maintaining balance also require some cognitive processing. Young and older subjects were submitted to an auditory reaction time task while maintaining an upright posture on a force platform. The auditory stimuli were presented randomly when subjects were in a central or in an eccentric less stable postural position in four conditions of vision/surface. If the postural adjustments require some cognitive processing, a more eccentric position of the center of foot pressure (COP) would require more attention than a stable position of the COP because when an eccentric position is identified, a corrective response subsequently needs to be selected, programmed, and executed. The visual and surface conditions were altered to determine if additional attentional resources need to be allocated to the postural task when there is a reduction of the sensory information available. Results showed that as the sensory information decreased, the postural task became increasingly difficult for the elderly and required more of their attentional capacity (as indexed by increases in reaction time).

Upright Standing and Gait: Are There Changes in Attentional Requirements Related to Normal Aging?

This study evaluates attentional requirements for maintaining an upright posture and for walking among young and elderly persons to determine if, with normal aging, there is a deficit and/or a modification in the allocation of the attentional resources necessary for balance control. Eight young adults and 8 elderly persons were asked to respond to an auditory reaction time (RT) task (secondary task) while in a seated position, while in a broad-support or narrow-support upright standing position, and while walking (primary tasks). Reducing the base of support yielded slower RTs for the elderly than for the young persons. When walking, the elderly persons adopted a slower speed than young persons. They also had a shorter stride length. These adaptations have been reported to produce a more secure gait. Even so, they responded to the probe RT task with greater delays than young adults. Together, the results suggest that normal aging requires that a greater proportion of attentional resources be allocated to the balance demands of postural tasks.

Determining Movement Onsets from Temporal Series

With the advent of recent measurement techniques, kinematic and kinetic measures commonly are used to describe events over time. Often, the central and peripheral nature of the control processes involved are derived from these temporal series. For example, movement onset often arbitrarily defines the end of the central and the beginning of the peripheral processes. Because of its critical temporal location, we examined whether response dynamics (average movement velocity) affects the determination of movement onset. Interactive graphics and numerical methods of determining movement onsets from temporal series were evaluated on various kinematic signals. Variations in the initial rate of change in a given signal significantly affected the determination of movement onset. Consequently, measurements of component latency must be regarded with caution. A cursory description of related problems elucidated in previous research is discussed, and procedures that can minimize these artifacts are suggested.

Reference systems for coding spatial information in normal subjects and a deafferented patient.

To produce accurate goal-directed arm movements, subjects must determine the precise location of target object. Position of extracorporeal objects can be determined using: (a) an egocentric frame of reference, in which the target is localized in relation to the position of the body; and/or (b) an allocentric system, in which target position is determined in relation to stable visual landmarks surrounding the target (Bridgeman 1989; Paillard 1991). The present experiment was based on the premise that (a) the presence of a structured visual environment enables the use of an allocentric frame of reference, and (b) the sole presence of a visual target within a homogeneous background forces the registration of the target location by an egocentric system. Normal subjects and a deafferented patient (i.e., with an impaired egocentric system) pointed to visual targets presented in both visual environments to evaluate the efficiency of the two reference systems. For normals, the visual environment conditions did not affect pointing accuracy. However, kinematic parameters were affected by the presence or absence of a structured visual surrounding. For the deafferented patient, the presence of a structured visual environment permitted a decrease in spatial errors when compared with the unstructured surrounding condition (for movements with or without visual feedback of the trajectory). Overall, results support the existence of an egocentric and an allocentric reference system capable of organizing extracorporeal space during arm movements directed toward visual targets.

Availability of visual and proprioceptive afferent messages and postural control in elderly adults

The ability of young and elderly adults to keep a stable upright posture while facing changes in the availability of visual and/or propriomuscular information was investigated. The two sensory sources of information were alternatively available and withdrawn, jointly and separately, during 10-s alternating sequences. Vision was modified by means of liquid-crystal goggles, and proprioception was altered by means of tendon vibration of both antagonistic ankle muscles. Elderly adults were less stable than young adults when vision was withdrawn. Both groups were greatly affected when proprio-muscular inputs were altered by vibration. Under constant visual conditions and following a propriomuscular perturbation (i.e., vibration), elderly adults were unable to take advantage of the reinsertion of propriomuscular inputs. They showed a transient, decreased stability and were unable to fully recover during a 10-s period, whereas young adults were able to rapidly integrate the information to stabilize their posture. When both propriomuscular and visual inputs were withdrawn and concurrently reinserted, the elderly adults did not show a transitory increase in the velocity of the center of foot pressure. The present results extend findings on the inability of elderly adults to reconfigure rapidly the postural set following reinsertion of sensory inputs. The results also suggest that elderly adults have difficulties in taking advantage of sensory redundancy in postural control.

Gait problems in diabetic neuropathic patients

OBJECTIVE To examine whether a reduced peripheral sensibility caused by diabetic neuropathy increases the attentional demands necessary for controlling and regulating gait. DESIGN Nonrandomized control trial. SETTING University motor performance laboratory. SUBJECTS Twelve diabetic patients with peripheral neuropathy and 7 control subjects, all volunteers. INTERVENTIONS All subjects first performed a control seated reaction time task. For the walking task, auditory stimuli were randomly presented in the third, fourth, or fifth walking cycle on left foot toe off on left foot heel contact. The subjects task was to respond verbally as fast as possible to the auditory stimulus, while maintaining progression. MAIN OUTCOME MEASURES Simple reaction times and kinematics of the gait pattern (cycle amplitude, cycle duration, cycle speed, cadence and percentage of time spent in the single support phase) were evaluated. RESULTS For the walking task, diabetic neuropathic patients had a smaller cycle amplitude, cycle speed, and percentage of time spent in the single support phase than control subjects. Also, reaction times while walking were higher for diabetic neuropathic patients than for control subjects. CONCLUSIONS Diabetic neuropathic patients show a less destabilizing and more conservative gait than control subjects. The increased attentional demands in gait for the diabetic neuropathic patients, along with their more conservative gait pattern, suggest that a lack of proprioception from the legs affects the control of gait. Diminished sensory information makes gait control more cognitively dependent in diabetic neuropathic persons than in control subjects.

Role of afferent information in the timing of motor commands: A comparative study with a deafferented patient

The accuracy of the motor system in synchronizing simultaneous movements initiations was tested in two conditions: (1) when the motor commands were triggered by an external signal (reactive condition), and (2) when subjects self-paced their movement onsets (self-paced condition). The task consisted of initiating simultaneously ipsilateral finger extension and heel raising. Eight normal subjects and a deafferented patient were tested. In the reactive condition, both normal subjects and the deafferented patient exhibited a precession of finger initiation over heel raising. This delay corresponds to the difference observed in the reaction time of the two limbs when measured independently. It reflects the difference in conduction times of the efferent pathways, as if the two motor commands were released simultaneously through a common triggering signal in the motor cortex. In contrast, in the self-paced condition normal subjects showed precession of heel over finger onsets, suggesting that synchrony is based upon the evaluation of afferent information. Unlike normal subjects, the patient showed no heel precession in the self-paced condition. These findings suggest that reactive and self-paced responses are produced through two different control modes and that afferent information contributes to the timing of motor commands in the self-paced mode.

The role of proprioceptive information for the production of isometric forces and for handwriting tasks

A patient showing a total loss of all the large sensory myelinated fibers but intact peripheral motor system produced simple isometric force pulses and more complex tasks like handwriting and drawing. Overall, the patient was able to perform the isometric force task with an accuracy that approached that of normal subjects. The writing tasks, however, proved to be more challenging. In absence of vision, the different forms and cursive trajectories forming letters (morphocinetic components) were preserved but their localization within the constraints of the graphic space (topocinetic components) were severely impaired. These results demonstrate that, in absence of visual information, proprioceptive information is necessary to calibrate the hand in space.

Deafferentation and pointing with visual double-step perturbations.

Abstract The capability of reprogramming movement responses following changes in the visual goal has been studied through the double-step paradigm. These studies have shown that: (a) continuous internal feedback-loops correct unconsciously the dynamic errors throughout the movement; (b) proprioceptive information and/or the efference copy have a privileged status among central processes, insuring on-line regulation of the initial motor commands; and (c) generation of the motor program starts after target presentation, and is continuously updated in the direction of the current internal representation of the target, at least until the onset of hand movement. This main corrective process of the initial program appears to be basically independent of visual reafference from the moving hand. However, the agreement with the possibility of a visuomotor loop, based on the comparison of the new updated representation of the target position and on the information from the moving hand, has not determined whether the correcting process is proprioceptive feedback dependent, or whether internal feedback-loops (efferent copies) are responsible for quick corrections of unfolding motor responses. To answer this question, the present experiment investigated the pointing behavior of a deafferented subject, using a double-step paradigm under various conditions of visual feedback and movement initiation. Overall, the present study (a) clearly showed the capacity of the motor system to modify and correct erroneous trajectories on the mere basis of internal feedback-loops and (b) emphasized the crucial role played by the target jump/arm triggering delay and the importance of the eye efferent copy for providing information about the spatial goal of the movement.