Biomedical subjects

R S Johansson

Publications and source records attributed to R S Johansson.

At least 19 recordsLinked to original sources

PubMed1996

Forces applied by the incisors and roles of periodontal afferents during food-holding and -biting tasks.

The force exerted by the central incisors while holding and splitting a food morsel was analyzed to characterize human biting behavior. The force was continuously sampled by a transducer-equipped plate upon which a small piece of dry biscuit or half a peanut rested. Subjects were instructed to position the plate between the incisor teeth and to split the morsel either immediately ("split task") or after holding it for a brief period ("hold-and-split task"). While holding either food substance between the incisors, subjects automatically exerted light contact forces of less than 1 N (0.36-0.76N range among subjects). Considering that the subjects had no instructions about what force levels to employ, the hold force was remarkably stable during individual trials and highly similar among trials. Even during the split task, subjects opted to "hold" the morsel momentarily on ca. 50% of the trials with a similar, low contact force. For both tasks, subjects split the morsel by exerting a distinct, rapidly executed ramp increase in force. The split occurred at 7.8-10.3 N (range among subjects) bite force for the biscuit and 16.0-19.0 N for the peanut. The magnitude of the forces used during the hold phase were within the range over which most periodontal afferents are optimally sensitive to changes in force, i.e., forces below about 1 N. This observation suggested that the subjects automatically adjusted the force to maximize the availability of information from periodontal afferents and avoided higher forces at which the sensitivity of most afferents was not optimal. We further confirmed that the periodontal receptors serve a role in controlling the hold force by anesthetizing the periodontal tissues: subjects employed considerably higher and more variable hold forces, but there was no effect on the split phase. In addition, the morsel frequently escaped from the incisal edges of the teeth while the subject attempted to maintain it in position. It was concluded that subjects rely on signals from periodontal afferents to regulate the jaw muscles, particularly when they first contact, manipulate, and hold food substances between the teeth.

Adolescent↗

PubMed1996

Nondigital afferent input in reactive control of fingertip forces during precision grip.

Sensory inputs from the digits are important in initiating and scaling automatic reactive grip responses that help prevent frictional slips when grasped objects are subjected to destabilizing load forces. In the present study we analyzed the contribution to grip-force control from mechanoreceptors located proximal to the digits when subjects held a small manipulandum between the tips of the thumb and index finger. Loads of various controlled amplitudes and rates were delivered tangential to the grip surfaces at unpredictable times. Grip forces (normal to the grip surfaces) and the position of the manipulandum were recorded. In addition, movements of hand and arm segments were assessed by recording the position of markers placed at critical points. Subjects performed test series during normal digital sensibility and during local anesthesia of the index finger and thumb. To grade the size of movements of tissues proximal to the digits caused by the loadings, three different conditions of arm and hand support were used; (1) in the hand-support condition the subjects used the three ulnar fingers to grasp a vertical dowel support and the forearm was supported in a vacuum cast; (2) in the forearm-support condition only the forearm was supported; finally, (3) in the no-support condition the arm was free. With normal digital sensibility the size of the movements proximal to the digits had small effects on the grip-force control. In contrast, the grip control was markedly influenced by the extent of such movements during digital anesthesia. The poorest control was observed in the hand-support condition, allowing essentially only digital movements. The grip responses were either absent or attenuated, with greatly prolonged onset latencies. In the forearm and no-support conditions, when marked wrist movements took place, both the frequency and the strength of grip-force responses were higher, and the grip response latencies were shorter. However, the performance never approached normal. It is concluded that sensory inputs from the digits are dominant in reactive grip control. However, nondigital sensory input may be used for some grip control during impaired digital sensibility. Furthermore, the quality of the control during impaired sensibility depends on the extent of movements evoked by the load in the distal, unanesthetized parts of the arm. The origin of these useful sensory signals is discussed.

Adult↗

PubMed1996

Grip-force responses to unanticipated object loading: load direction reveals body- and gravity-referenced intrinsic task variables.

Humans preserve grasp stability by automatically regulating the grip forces when loads are applied tangentially to the grip surfaces of a manipulandum held in a precision grip. The effects of the direction of the load force in relation to the palm, trunk, and gravity were investigated in blindfolded subjects. Controlled, tangential load-forces were delivered in an unpredictable manner to the grip surface in contact with the index finger either in the distal and proximal directions (away from and toward the palm) or in the ulnar and radial directions (transverse to the palm). The hand was oriented in: (1) a standard position, with the forearm extended horizontally and anteriorly in intermediate pronosupination; (2) an inverted position, reversing the direction of radial and ulnar loads in relation to gravity; and (3) a horizontally rotated position, in which distal loads were directed toward the trunk. The amplitude of the grip-force responses (perpendicular to the grip surface) varied with the direction of load in a manner reflecting frictional anisotropies at the digit-object interface; that is, the subjects automatically scaled the grip responses to provide similar safety margins against frictional slips. For all hand positions, the time from onset of load increase to start of the grip-force increase was shorter for distal loads, which tended to pull the object out of the hand, than for proximal loads. Furthermore, this latency was shorter for loads in the direction of gravity, regardless of hand position. Thus, shorter latencies were observed when frictional forces alone opposed the load, while longer latencies occurred when gravity also opposed the load or when the more proximal parts of the digits and palm were positioned in the path of the load. These latency effects were due to different processing delays in the central nervous system and may reflect the preparation of a default response in certain critical directions. The response to loads in other directions would incur delays required to implement a new frictional scaling and a different muscle activation pattern to counteract the load forces. We conclude that load direction, referenced to gravity and to the hand's geometry, represents intrinsic task variables in the automatic processes that maintain a stable grasp on objects subjected to unpredictable load forces. In contrast, the grip-force safety margin against frictional slips did not vary systematically with respect to these task variables. Instead, the magnitude of the grip-force responses varied across load direction and hand orientation according to frictional differences providing similar safety margins supporting grasp stability.

Adult↗

PubMed1996

Control of grip force during restraint of an object held between finger and thumb: responses of cutaneous afferents from the digits.

Unexpected pulling and pushing loads exerted by an object held with a precision grip evoke automatic and graded increases in the grip force (normal to the grip surfaces) that prevent escape of the object; unloading elicits a decrease in grip force. Anesthesia of the digital nerves has shown that these grip reactions depend on sensory signals from the digits. In the present study we assessed the capacity of tactile afferents from the digits to trigger and scale the evoked grip responses. Using tungsten microelectrodes inserted percutaneously into the median nerve of awake human subjects, unitary recordings were made from ten FA I and 13 FA II rapidly adapting afferents, and 12 SA I and 18 SA II slowly adapting afferents. While the subject held a manipulandum between a finger and the thumb, tangential load forces were applied to the receptor-bearing digit (index, middle, or ring finger or thumb) as trapezoidal load-force profiles with a plateau amplitude of 0.5-2.0 N and rates of loading and unloading at 2-8 N/s, or as "step-loads" of 0.5 N delivered at 32 N/s. Such load trials were delivered in both the distal (pulling) and proximal (pushing) direction. FA I afferents responded consistently to the load forces, being recruited during the loading and unloading phases. During the loading ramp the ensemble discharge of the FA I afferents reflected the first time-derivative of the load force (i.e., the load-force rate). These afferents were relatively insensitive to the subject's grip force responses. However, high static finger forces appeared to suppress excitation of these afferents during the unloading phase. The FA II afferents were largely insensitive to the load trials: only with the step-loads did some afferents respond. Both classes of SA afferents were sensitive to load force and grip force, and discharge rates were graded by the rate of loading. The firing of the SA I afferents appeared to be relatively more influenced by the subject's grip-force response than the discharge of the SA II afferents, which were more influenced by the load-force stimulus. The direction in which the tangential load force was applied to the skin influenced the firing of most afferents and in particular the SA II afferents. Individual afferents within each class (except for the FA IIs) responded to the loading ramp before the onset of the subject's grip response and may thus be responsible for initiating the automatic increase in grip force. However, nearly half of the FA I afferents recruited by the load trials responded to the loading phase early enough to trigger the subject's grip-force response, whereas only ca. one-fifth of the SA Is and SA IIs did so. These observations, together with the high density of FA I receptors in the digits, might place the FA I afferents in a unique position to convey the information required to initiate and scale the reactive grip-force responses to the imposed load forces.

Adolescent↗

PubMed1996

Control of grip force during restraint of an object held between finger and thumb: responses of muscle and joint afferents from the digits.

Pulling or pushing forces applied to an object gripped between finger and thumb excite tactile afferents in the digits in a manner awarding these afferents probable roles in triggering the reactive increases in grip force and in scaling the changes in grip force to the changes in applied load-force. In the present study we assessed the possible contributions from slowly adapting afferents supplying muscles involved in the generation of grip forces and from digital joint afferents. Impulses were recorded from single afferents via tungsten microelectrodes inserted percutaneously into the median or ulnar nerves of awake human subjects. The subject held a manipulandum with a precision grip between the receptor-related digit (index finger, middle finger, ring finger or thumb) and an opposing digit (thumb or index finger). Ramp-and-hold load forces of various amplitudes (0.5-2.0 N) and ramp rates (2-32 N/s) were delivered tangential to the parallel grip surfaces in both the distal (pulling) and the proximal (pushing) directions. Afferents from the long flexors of the digits (n = 19), regardless of their muscle-spindle or tendon-organ origin, did not respond to the load forces before the onset of the automatic grip response, even with the fastest ramp rates. Their peak discharge closely followed the peak rate of increase in grip force. During the hold phase of the load stimulus, the afferents sustained a tonic discharge. The discharge rates were significantly lower with proximally directed loads despite the mean grip-force being similar in the two directions. This disparity could be explained by the differing contributions of these muscles to the finger-tip forces necessary to restrain the manipulandum in the two directions. Most afferents from the short flexors of the digits (n = 17), including the lumbricals, dorsal interossei, opponens pollicis, and flexor pollicis brevis, did not respond at all, even with the fastest ramps. Furthermore, the ensemble pattern from the joint afferents (n = 6) revealed no significant encoding of changes in finger-tip forces before the onset of the increase in grip force. We conclude that mechanoreceptors in the flexors of the digits and in the interphalangeal joints cannot be awarded a significant role in triggering the automatic changes in grip force. Rather, their responses appeared to reflect the reactive forces generated by the muscles to restrain the object. Hence, it appears that tactile afferents of the skin in contact with the object are the only species of receptor in the hand capable of triggering and initially scaling an appropriate change in grip force in response to an imposed change in load force, but that muscle and joint afferents may provide information related to the reactive forces produced by the subject.

Adolescent↗

PubMed1996

Encoding of tooth loads by human periodontal afferents and their role in jaw motor control.

Microneurography has been used to analyze the functional properties of human periodontal mechanoreceptors. Signals were recorded from single afferents in the inferior alveolar nerve while controlled forces were applied to the teeth. We have found that all periodontal afferents adapt slowly to maintained loads. Most afferents are tuned broadly to direction of force application, and about half respond to forces applied to teeth adjacent to the one to which the afferent distributes. Populations of periodontal afferents, nevertheless, reliably encode information about both the teeth stimulated and the direction of forces applied to the individual teeth. Information about the magnitude of steady forces is made available in the mean firing-rate response of periodontal afferents. Most afferents exhibit a marked "hyperbolic" relationship between the static discharge rate and the force amplitude; the highest sensitivity to changes in static force is observed at forces below 1 N. Similarly, the dynamic sensitivity is highest at low forces. These afferents efficiently encode food contact during biting and continuously discharge while food is held between the incisors. Subjects spontaneously exert low contact forces matched to the sensitivity characteristics of these periodontal afferents when holding food substances between the incisors. If periodontal afferent information is not available, the control of the hold forces is severely impaired. Moreover, since only a few afferents encode information about the rapid and strong force increase employed to bite through food, we conclude that subjects rely on signals from periodontal afferents to regulate the jaw muscles primarily when they first contact, manipulate and hold food substances between the teeth. A potential role for periodontal afferents in the spatio-intensive control of jaw actions is discussed.

Humans↗

PubMed1996

Modulation of corticospinal influence over hand muscles during gripping tasks in man and monkey.

Transcranial magnetic brain stimulation (TMS) was used to investigate corticospinal influences during a task in which human subjects had to reach out and grasp and lift an object. TMS applied to the hand area of the motor cortex was delivered during eight different phases of the task. There was a striking phase-related modulation in the amplitude of the short-latency EMG responses elicited by TMS in six arm and hand muscles. Although several mechanisms probably contribute to this modulation, one result of their operation is a potentially greater influence of the cortex during particular phases of the task. Evidence is produced that one factor contributing to this modulation is a phase-related change in corticospinal excitability. The results are consistent with a strong excitatory corticospinal drive throughout the reach to brachioradialis and anterior deltoid, which contribute to hand transport, and to the extrinsic hand muscles, which orientate the hand and fingertips. In contrast, the intrinsic hand muscles appear to receive their strongest cortical input as the digits close around and first touch the object. TMS just before contact delayed the isometric parallel increase in load and grip forces necessary to lift the object. The particularly strong EMG and behavioral effects seen at touch may reflect a powerful interaction, at the cortical level, between cutaneous inputs signalling contact with the object and the effects of TMS. Central interactions between tactile afferent input and TMS were tested by delivering TMS at different times relative to the application of an unexpected load to an object held between the fingertips. The largest responses occurred when TMS was applied 60-80 ms after load onset. THe enhanced corticospinal influence that this represents probably contributes to the powerful, short-latency boosting in grip force observed when the object was suddenly subjected to an external load. Recording of corticospinal cells in the primary motor cortex of the awake monkey suggests that the phasic modulation observed with TMS may reflect the phasic-tonic pattern of corticomotoneuronal cell discharge during the task. Since the activation of corticospinal cells by low-intensity TMS is dependent upon their level of excitability, EMG responses evoked by TMS during the performance of skilled tasks in man may, in part, reflect changes in the excitability of these cells.

Animals↗

PubMed1995

Development of human precision grip. IV. Tactile adaptation of isometric finger forces to the frictional condition.

The adaptation of the grip forces to the frictional condition between the digits and an object relies on feedforward sensorimotor mechanisms that use tactile afferent input to intermittently update a sensorimotor memory that controls the force coordination, i.e., the ratio between grip force (normal to the grip surface) and load force (tangential to the grip surface). The present study addressed the development of these mechanisms. Eighty-nine children and 15 adults lifted an instrumented object with exchangeable grip surfaces measuring the grip and load forces. Particularly in trials with high friction (sandpaper), the youngest children used a high grip force to load force ratio. Although this large safety margin against slips indicated an immature capacity to adapt to the frictional condition, higher grip forces were produced for more slippery material (silk versus sandpaper). The safety margin decreased during the first 5 years of age, in parallel with a lower variability in the grip force and a better adaptation to the current frictional condition. The youngest children (18 months) could adapt the grip force to load force ratio to the frictional condition in a series of lifts when the same surface structure was presented in blocks of trials, but failed when the surface structure was unpredictably changed between subsequent lifts. The need for repetitive presentation suggests a poor capacity to form a sensorimotor memory representation of the friction or an immature capacity to control the employed ratio from this representation. The memory effects, reflected by the influences of the frictional condition in the previous trial, gradually increased with age. Older children required a few lifts and adults only one lift to update their force coordination to a new friction. Hence, the present finding suggests that young children use excessive grip force, a strategy to avoid frictional slips, to compensate for an immature tactile control of the precision grip.

Adolescent↗

PubMed1995

Corticospinal control during reach, grasp, and precision lift in man.

Transcranial magnetic brain stimulation (TMS) was used to assess the influence of the corticospinal system on motor output in seven human subjects during a task in which they had to reach out, grasp, and lift an object. Stimuli, directed at the hand area of the motor cortex, were delivered at eight defined points during the task: during reach, at grip closure, during object manipulation, during the parallel isometric increase in grip and load forces, during the lifting movement, and while the object was held in air. The amplitudes of short-latency EMG responses evoked by TMS in six arm and hand muscles showed a striking modulation across the different phases of the task. This modulation may well reflect phasic changes in corticospinal excitability because: (1) it did not simply reflect phasic changes in muscular activity associated with task performance, (2) it could vary inversely with the amplitude of the background EMG, and (3) it was only obtained with weak TMS intensities, below threshold for evoking responses in hand muscles of the relaxed subject. Our results suggest that the cortical representations of extrinsic hand muscles, which act to orientate the hand and finger tips, were subjected to a strong excitatory drive throughout the reach. This drive was also observed for brachioradialis and anterior deltoid, which contribute to transport of the hand. In contrast, the intrinsic hand muscles appear to receive their strongest cortical input as the digits closed around the object, and just after the subject first touched the object at the onset of manipulation. The isometric parallel increase in load and grip forces necessary to lift the object, which is normally triggered by tactile contact, was delayed by TMS delivered late during the reach. TMS at this time may disrupt processing necessary to control this critical phase of the task.

Adult↗

PubMed1994

Time-varying enhancement of human cortical excitability mediated by cutaneous inputs during precision grip.

1. We have investigated the afferent neurogram, muscular activity and mechanical responses while subjects restrained, with a precision grip, an object subjected to pulling loads directed away from the hand. At unpredictable times 'ramp-and-hold' loads of 1 N were delivered at a rate of ca 80 N s-1. The load ramp produced a sharp increase in multiunit activity recorded from cutaneous afferents of the median nerve. The first response in the EMG of distal hand muscles commenced at 51 +/- 2.4 ms (mean +/- S.D.); a further steep increase in activity began about 20 ms later, and this was associated with a marked augmentation of the grip force increase. 2. In four subjects, transcranial magnetic stimulation (TMS) was delivered to the contralateral motor cortex in 1000 out of a total of 1500 loading trials. The time of the stimulus was randomly selected to occur either at one of nine defined points (separated by 20 ms) before and after the computer command triggering the load force increase, or during steady periods of grip. 3. In most hand and arm muscles, there was a powerful facilitation of the short-latency EMG responses evoked by TMS delivered 40-140 ms after the load force command. The amplitudes of the largest responses (TMS delivered at 80-100 ms) were 850% higher on average than those observed when subjects gripped the unloaded object or when they restrained the statically loaded object. This large modulation was only obtained with stimulus intensities that were subthreshold in the relaxed subject. 4. The modulation was not simply a reflection of the time-varying level of motoneuronal activity during the loading trial. In most muscles, changes in the amplitude of the TMS-evoked responses were disproportionately larger than the corresponding modulation of the background EMG activity. At its maximum, the modulation in the TMS-evoked response was nearly 300% larger. Furthermore, the strength of the TMS-evoked responses did not strictly co-vary with amplitude of background EMG, i.e. inverse relationships were seen. 5. Since motor responses to the loading of the object depend on cutaneous afferent input from the gripping digits, the results demonstrate an interaction between the effects of these inputs and those of TMS. A possible site of this interaction is the primary motor cortex; the strong modulation of the responses to TMS could reflect variation in the excitability of cortical neurons mediated by the cutaneous afferent input. However, such excitability changes lagged the predicted onset of cortical excitation in a manner suggesting that the earliest 20 ms of the subjects' EMG responses to the load increase are subcortical in origin. In contrast, the results are consistent with a cortical mediation of the subsequent powerful boosting of the EMG responses associated with the robust grip force response.

Adolescent↗

PubMed1994

Electrical signs of cortical involvement in the automatic control of grip force.

Evoked potentials were recorded in 10 subjects to assess cortical involvement in automatic control of grip force during restraint of a manipulandum held between finger and thumb. Brisk pulling loads elicited an increase in first dorsal interosseous EMG after 60.6 +/- 2.4 ms. Preceding this response were several time-locked scalp potentials, the most significant being a widespread negativity that appeared first over the frontal cortex and peaked 19.5 +/- 1.8 ms before the EMG peak, and a positivity that followed the EMG by 24.3 +/- 5.4 ms. The slope of the negativity and the amplitude of the positivity were greater than during passive conditions, suggesting that these potentials reflect cortical processes associated with automatic regulation.

Adolescent↗

PubMed1994

Grasp stability during manipulative actions.

The control of adequate contact forces between the skin and an object (grasp stability) is examined for two classes of prehensile actions that employ a precision grip: lifting objects that are "passive" (subject only to inertial forces and gravity) and preventing "active" objects from moving. For manipulating either passive or active objects the relevant fingertip forces are determined by at least two control processes. "Anticipatory parameter control" is a feedforward controller that specifies the values for motor command parameters on the basis of predictions of critical characteristics, such as object weight and skin-object friction, and initial condition information. Through vision, for instance, common objects can be identified in terms of the fingertip forces necessary for a successful lift according to previous experiences. After contact with the object, sensory information representing discrete mechanical events at the fingertips can (i) automatically modify the motor commands, (ii) update sensorimotor memories supporting the anticipatory parameter control policy, (iii) inform the central nervous system about completion of the goal for each action phase, and (iv) trigger commands for the task's sequential phases. Hence, the central nervous system monitors specific, more or less expected peripheral sensory events to produce control signals that are appropriate for the task at its current phase. The control is based on neural modelling of the entire dynamics of the control process that predicts the appropriate output for several steps ahead. This "discrete-event, sensor-driven control" is distinguished from feedback or other continuous regulation. Using these two control processes, slips are avoided at each digit by independent control mechanisms that specify commands and process sensory information on a local, digit-specific basis. This scheme obviates explicit coordination of the digits and is employed when independent nervous systems lift objects. The force coordination across digits is an emergent property of the local control mechanisms operating over the same time span.

Hand↗

PubMed1994

Encoding of amplitude and rate of forces applied to the teeth by human periodontal mechanoreceptive afferents.

1. The encoding of force amplitude and force rate by human periodontal mechanoreceptive afferents was studied. Recordings were obtained from 19 single periodontal afferents in the inferior alveolar nerve with the use of tungsten microelectrodes. Loads consisting of a force increase (loading ramp), a phase of maintained force (static phase), and a force decrease (unloading ramp) were applied to the receptor bearing tooth, which was most often an incisor. The static forces applied ranged between 0.05 and 5 N, and the rate of force applied during the loading ramps ranged between 0.4 and 70 N/s. The forces were primarily applied in one of six directions (lingual, labial, mesial, distal, upward, or downward) that evoked the greatest discharge activity. 2. For each force application, the steady-state response was defined as the mean discharge rate during a 1-s period starting 0.5 s after the end of the loading ramp. Most afferents (15/19) exhibited a "hyperbolic" (viz., negatively accelerating) relationship between the amplitude of the stimulation force and the steady-state response, featuring a pronounced saturation tendency: the highest sensitivity to changes in static force was observed at force levels below 1 N. At higher force levels the sensitivity gradually diminished. Moreover, the dynamic sensitivity similarly decreased with increasing amplitude of static background force. For a subsample of afferents studied, comparable stimulus-response relationships were obtained in directions other than the most responsive one, but the discharge rates were lower. 3. In contrast to the response of most afferents, four (4/19) differed in that they consistently exhibited a nearly linear relationship between force amplitude and the steady-state response. Moreover, these afferents maintained their dynamic sensitivity as the amplitude of the background force was increased. 4. The steady-state response of all afferents was well described as a constant times F/ (F + c), where F represents the steady-state force, and c the force generating one-half the estimated maximum discharge rate that could be evoked by steady-state force stimulation. The c-parameter was on average 0.42 N (range 0.05-1.1 N) for the afferents exhibiting hyperbolic stimulus-response relationships. In contrast it ranged between 5 and 22 N for those exhibiting "nearly linear" relationships. A hypothetical model of the mechanics of the periodontal ligament supporting the F/(F + c) transform is proposed. 5. A general transfer function was developed to predict the instantaneous discharge rate of an individual afferent to arbitrary force profiles applied to the receptor bearing tooth.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗

PubMed1993

Friction at the digit-object interface scales the sensorimotor transformation for grip responses to pulling loads.

When restraining a mechanically "active" object (one that exerts unpredictable changes in loading forces) with a precision grip of the digits, we maintain a stable grasp by modulating our grip force using somatosensory information related to the loading forces. The response to ramp load increases consists of an initial fast rise in grip force ("catch-up") followed by a secondary response that steadily increases the grip force in parallel with the load force ("tracking"). The sizes of these response components scale in proportion to the loading rate. However, maintaining a stable grasp without employing an exceedingly large grip force may require further scaling of this load-to-grip sensorimotor transformation based on two additional factors: (1) the friction at the digit-object interface and (2) the grip force present at the start of the load increase. The present experiments sought to determine whether such scaling occurs and to characterize its control. Subjects restrained a manipulandum held between the tips of the thumb and index finger. At unpredictable times a pulling force appeared, directed away from the subject's hand. Each pull had a trapezoidal load profile beginning and ending at 0 N with 4-N/s ramps; each ramp was 1 s in duration. The texture of the gripped surfaces varied among sandpaper, suede, and rayon, which represented increasingly slippery surfaces. The grip force at the start of the load ramp (intertrial grip force), and the amplitudes of the catch-up and secondary grip responses scaled in proportion to the inverse friction. We interpret these results to indicate a uniform scaling of the transformations controlling the intertrial grip force, the catch-up response, and the secondary response. Initial-state information from tactile cues available upon object contact appeared to update the frictional scaling value. This conclusion is based on observations of immediate changes in the intertrial grip force upon contact with a new surface, and because differences in force-rate profiles appeared virtually by the onset of the catch-up response. Similarly, the intertrial grip force also constituted initial-state information. The size of the catch-up and secondary grip force responses varied inversely with the size of the intertrial grip force. These scalings of the load-to-grip-force sensorimotor transformation for friction and intertrial grip force level appear to be functionally adaptive, because they contribute to a stable grasp (prevent object slips) while avoiding exceedingly large safety margins.

Adolescent↗

PubMed1993

Memory representations underlying motor commands used during manipulation of common and novel objects.

1. While subjects lifted a variety of commonly handled objects of different shapes, weights, and densities, the isometric vertical lifting force opposing the object's weight was recorded from an analog weight scale, which was instrumented with high-stiffness strain gauge transducers. 2. The force output was scaled differently for the various objects from the first lift, before sensory information related to the object's weight was available. The force output was successfully specified from information in memory related to the weight of common objects, because only small changes in the force-rate profiles occurred across 10 consecutive lifts. This information was retrieved during a process related to visual identification of the target object. 3. The amount of practice necessary to appropriately scale the vertical lifting and grip (pinch) force was also studied when novel objects (equipped with force transducers at the grip surfaces) of different densities were encountered. The mass of a test object that subjects had not seen previously was adjusted to either 300 or 1,000 g by inserting an appropriate mass in the object's base without altering its appearance. This resulted in either a density that was in the range of most common objects (1.2 kg/l) or a density that was unusually high (4.0 kg/l). 4. Low vertical-lifting and grip-force rates were used initially with the high-density object, as if a lighter object had been expected. However, within the first few trials, the duration of the loading phase (period of isometric force increase before lift-off) was reduced by nearly 50% and the employed force-rate profiles were targeted for the weight of the object.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed1992

Development of human precision grip. II. Anticipatory control of isometric forces targeted for object's weight.

The development of anticipatory control during lifts with the precision grip was examined in 100 children aged 1 to 15 years and in 15 adults. The children were instructed to lift an instrumented test object by using the precision grip between the thumb and index finger. The employed grip force, load force (vertical lifting force), vertical position and their corresponding time derivatives (i.e., grip and load force rates and acceleration) were recorded. The weight of the object was varied between trials to access the influence of the object's weight in the previous trial on the isometric force output. Already by the second year, children began to use information pertaining to the object's weight in the previous lift, i.e., they began to use an anticipatory control strategy. This occurred concomitant to the development of mainly bell shaped force rate profiles (Forssberg et al. 1991). The succeeding development of a more mature anticipatory control was gradual and adult-like capacity was not reached until 8-11 years of age.

Adolescent↗

PubMed1992

Development of human precision grip. III. Integration of visual size cues during the programming of isometric forces.

Recent evidence has shown that visual and haptical size information can be used by adults to estimate the weight of the object, forming the basis of the force programming during precision grip (Gordon et al. 1991a, b,). The present study examined the development of the capacity to use visual size information. In the first experiment, 30 children (age 1-7 years) and 10 adults performed a series of lifts with two boxes presented in an unpredictable order. The boxes were equal in weight but unequal in size and were attached to an instrumented grip handle which measured the employed grip force, load force, position and their corresponding time derivatives. The isometric force development was not influenced by the box size before the age of 3. However, the children aged 3 years and older demonstrated greater visual influences on the force programming than adults. To determine more precisely when children began to use visual size information, a second experiment in which the size and weight covaried was performed on 15 children. Children still did not use the size information during the force programming until the later half of the third year. It is concluded that this ability, probably involving associative transformations between the size and weight of objects, emerges around one year after anticipatory control based on somatosensory information pertaining to the weight of the object.

Adolescent↗

PubMed1992

Somatosensory control of precision grip during unpredictable pulling loads. I. Changes in load force amplitude.

In manipulating 'passive' objects, for which the physical properties are stable and therefore predictable, information essential for the adaptation of the motor output to the properties of the current object is principally based on 'anticipatory parameter control' using sensorimotor memories, i.e., an internal representation of the object's properties based on previous manipulative experiences. Somatosensory afferent signals only intervene intermittently according to an 'event driven' control policy. The present study is the first in a series concerning the control of precision grip when manipulating 'active' objects that exert unpredictable forces which cannot be adequately represented in a sensorimotor memory. Consequently, the manipulation may be more reliant on a moment-to-moment sensory control. Subjects who were prevented from seeing the hand used the precision grip to restrain a manipulandum with two parallel grip surfaces attached to a force motor which produced distally directed (pulling) loads tangential to the finger tips. The trapezoidal load profiles consisted of a loading phase (4 N/s), plateau phase and an unloading phase (4 N/s) returning the load force to zero. Three force amplitudes were delivered in an unpredictable sequence; 1 N, 2 N and 4 N. In addition, trials with higher load rate (32 N/s) at a low amplitude (0.7 N), were superimposed on various background loads. The movement of the manipulandum, the load forces and grip forces (normal to the grip surfaces) were recorded at each finger. The grip force automatically changed with the load force during the loading and unloading phases. However, the grip responses were initiated after a brief delay. The response to the loading phase was characterized by an initial fast force increase termed the 'catch-up' response, which apparently compensated for the response delay--the grip force adequately matched the current load demands by the end of the catch-up response. In ramps with longer lasting loading phases (amplitude greater than or equal to 2 N) the catch-up response was followed by a 'tracking' response, during which the grip force increased in parallel with load force and maintained an approximately constant force ratio that prevented frictional slips. The grip force during the hold phase was linearly related to the load force, with an intercept close to the grip force used prior to the loading. Likewise, the grip force responses evoked by the fast loadings superimposed on existing loads followed the same linear relationship.(ABSTRACT TRUNCATED AT 400 WORDS)

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