| 1. |
- Bowman, Miles C, et al.
(author)
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Eye-hand coordination in a sequential target contact task
- 2009
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In: Experimental Brain Research. - : Springer Science and Business Media LLC. - 0014-4819 .- 1432-1106. ; 195:2, s. 273-283
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Journal article (peer-reviewed)abstract
- Most object manipulation tasks involve a series of actions demarcated by mechanical contact events, and gaze is typically directed to the locations of these events as the task unfolds. Here, we examined the timing of gaze shifts relative to hand movements in a task in which participants used a handle to contact sequentially five virtual objects located in a horizontal plane. This task was performed both with and without visual feedback of the handle position. We were primarily interested in whether gaze shifts, which in our task shifted from a given object to the next about 100 ms after contact, were predictive or triggered by tactile feedback related to contact. To examine this issue, we included occasional catch contacts where forces simulating contact between the handle and object were removed. In most cases, removing force did not alter the timing of gaze shifts irrespective of whether or not vision of handle position was present. However, in about 30% of the catch contacts, gaze shifts were delayed. This percentage corresponded to the fraction of contacts with force feedback in which gaze shifted more than 130 ms after contact. We conclude that gaze shifts are predictively controlled but timed so that the hand actions around the time of contact are captured in central vision. Furthermore, a mismatch between the expected and actual tactile information related to the contact can lead to a reorganization of gaze behavior for gaze shifts executed greater than 130 ms after a contact event.
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| 2. |
- Burstedt, Magnus K, et al.
(author)
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Coordination of fingertip forces during human manipulation can emerge from independent neural networks controlling each engaged digit.
- 1997
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In: Experimental Brain Research. - : Springer Science and Business Media LLC. - 0014-4819 .- 1432-1106. ; 117:1, s. 67-79
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Journal article (peer-reviewed)abstract
- We investigated the coordination of fingertip forces in subjects who lifted an object (i) using the index finger and thumb of their right hand, (ii) using their left and right index fingers, and (iii) cooperatively with another subject using the right index finger. The forces applied normal and tangential to the two parallel grip surfaces of the test object and the vertical movement of the object were recorded. The friction between the object and the digits was varied independently at each surface between blocks of trials by changing the materials covering the grip surfaces. The object's weight and surface materials were held constant across consecutive trials. The performance was remarkably similar whether the task was shared by two subjects or carried out unimanually or bimanually by a single subject. The local friction was the main factor determining the normal:tangential force ratio employed at each digit-object interface. Irrespective of grasp configuration, the subjects adapted the force ratios to the local frictional conditions such that they maintained adequate safety margins against slips at each of the engaged digits during the various phases of the lifting task. Importantly, the observed force adjustments were not obligatory mechanical consequences of the task. In all three grasp configurations an incidental slip at one of the digits elicited a normal force increase at both engaged digits such that the normal:tangential force ratio was restored at the non-slipping digit and increased at the slipping digit. The initial development of the fingertip forces prior to object lift-off revealed that the subjects employed digit-specific anticipatory mechanisms using weight and frictional experiences in the previous trial. Because grasp stability was accomplished in a similar manner whether the task was carried out by one subject or cooperatively by two subjects, it was concluded that anticipatory adjustments of the fingertip forces can emerge from the action of anatomically independent neural networks controlling each engaged digit. In contrast, important aspects of the temporal coordination of the digits was organized by a "higher level" sensory-based control that influenced both digits. In lifts by single subjects this control was mast probably based on tactile and visual input and on communication between neural control mechanisms associated with each digit. In the two-subject grasp configuration this synchronization information was based on auditory and visual cues.
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| 3. |
- Häger-Ross, Charlotte, et al.
(author)
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Grip-force responses to unanticipated object loading : load direction reveals body- and gravity-referenced intrinsic task variables
- 1996
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In: Experimental Brain Research. - : Springer. - 0014-4819 .- 1432-1106. ; 110:1, s. 142-150
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Journal article (peer-reviewed)abstract
- 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.
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| 4. |
- Häger-Ross, Charlotte, et al.
(author)
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Nondigital afferent input in reactive control of fingertip forces during precision grip
- 1996
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In: Experimental Brain Research. - : Springer. - 0014-4819 .- 1432-1106. ; 110:1, s. 131-141
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Journal article (peer-reviewed)abstract
- 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.
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| 5. |
- Johansson, Roland S, et al.
(author)
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Somatosensory control of precision grip during unpredictable pulling loads. I. Changes in load force amplitude.
- 1992
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In: Experimental Brain Research. - 0014-4819 .- 1432-1106. ; 89:1, s. 181-191
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Journal article (peer-reviewed)abstract
- 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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| 6. |
- Johansson, Roland S, et al.
(author)
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Somatosensory control of precision grip during unpredictable pulling loads. II. Changes in load force rate.
- 1992
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In: Experimental Brain Research. - 0014-4819 .- 1432-1106. ; 89:1, s. 192-203
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Journal article (peer-reviewed)abstract
- In the previous paper regarding the somatosensory control of the human precision grip, we concluded that the elicited automatic grip force adjustments are graded by the amplitude of the imposed loads when restraining an 'active' object subjected to unpredictable pulling forces (Johansson et al. 1992a). Using the same subjects and apparatus, the present study examines the capacity to respond to imposed load forces applied at various rates. Grip and load forces (forces normal and tangential to the grip surfaces) and the position of the object in the pulling direction (distal) were recorded. Trapezoidal load force profiles with plateau amplitudes of 2 N were delivered at the following rates of loading and unloading in an unpredictable sequence: 2 N/s, 4 N/s or 8 N/s. In addition, trials with higher load rate (32 N/s) at a low amplitude (0.7 N) were intermingled. The latencies between the start of the loading and the onset of the grip force response increased with decreasing load force rate. They were 80 +/- 9 ms, 108 +/- 13 ms, 138 +/- 27 ms and 174 +/- 39 ms for the 32, 8, 4 and 2 N/s rates, respectively. These data suggested that the grip response was elicited after a given minimum latency once a load amplitude threshold was exceeded. The amplitude of the initial rapid increase of grip force (i.e., the 'catch-up' response) was scaled by the rate of the load force, whereas its time course was similar for all load rates. This response was thus elicited as a unit, but its amplitude was graded by afferent information about the load rate arising very early during the loading. The scaling of the catch-up response was purposeful since it facilitated a rapid reconciliation of the ratio between the grip and load force to prevent slips. In that sense it apparently also compensated for the varying delays between the loading phase and the resultant grip force responses. However, modification of the catch-up response may occur during its course when the loading rate is altered prior to the grip force response or very early during the catch-up response itself. Hence, afferent information may be utilized continuously in updating the response although its motor expression may be confined to certain time contingencies. Moreover, this updating may take place after an extremely short latency (45-50 ms).
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| 7. |
- Macefield, Vaughan G, et al.
(author)
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Control of grip force during restraint of an object held between finger and thumb : responses of cutaneous afferents from the digits
- 1996
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In: Experimental Brain Research. - 0014-4819 .- 1432-1106. ; 108:1, s. 155-171
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Journal article (peer-reviewed)abstract
- 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.
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| 8. |
- Macefield, Vaughan G, et al.
(author)
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Loads applied tangential to a fingertip during an object restraint task can trigger short-latency as well as long-latency EMG responses in hand muscles.
- 2003
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In: Experimental Brain Research. - : Springer Science and Business Media LLC. - 0014-4819 .- 1432-1106. ; 152:2, s. 143-149
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Journal article (peer-reviewed)abstract
- Electrical stimulation of the digital nerves can cause short- and long-latency increases in electromyographic activity (EMG) of the hand muscles, but mechanical stimulation of primarily tactile afferents in the digits generally evokes only a long-latency increase in EMG. To examine whether such stimuli can elicit short-latency reflex responses, we recorded EMG over the first dorsal interosseous muscle when subjects (n=13) used the tip of the right index finger to restrain a horizontally oriented plate from moving when very brisk tangential forces were applied in the distal direction. The plate was subjected to ramp-and-hold pulling loads at two intensities (a 1-N load applied at 32 N/s or a 2-N load applied at 64 N/s) at times unpredictable to the subjects (mean interval 2 s; trial duration 500 ms). The contact surface of the manipulandum was covered with rayon--a slippery material. For each load, EMG was averaged for 128 consecutive trials with reference to the ramp onset. In all subjects, an automatic increase in grip force was triggered by the loads applied at 32 N/s; the mean onset latency of the EMG response was 59.8 +/- 0.9 (mean +/- SE) ms. In seven subjects (54%) this long-latency response was preceded by a weak short-latency excitation at 34.6 +/- 2.9 ms. With the loads applied at 64 N/s, the long-latency response occurred slightly earlier (58.9 +/- 1.7 ms) and, with one exception, all subjects generated a short-latency EMG response (34.9 +/- 1.3 ms). Despite the higher background grip force that subjects adopted during the stronger loads (4.9 +/- 0.3 N vs 2.5 +/- 0.2 N), the incidence of slips was higher--the manipulandum escaped from the grasp in 37 +/- 5% of trials with the 64 N/s ramps, but in only 18 +/- 4% with the 32-N/s ramps. The deformation of the fingertip caused by the tangential load, rather than incipient or overt slips, triggered the short-latency responses because such responses occurred even when the finger pad was fixed to the manipulandum with double-sided adhesive tape so that no slips occurred.
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| 9. |
- Reichelt, Andreas F, et al.
(author)
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Adaptation of lift forces in object manipulation through action observation.
- 2013
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In: Experimental Brain Research. - : Springer. - 0014-4819 .- 1432-1106. ; 228:2, s. 221-234
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Journal article (peer-reviewed)abstract
- The ability to predict accurately the weights of objects is essential for skilled and dexterous manipulation. A potentially important source of information about object weight is through the observation of other people lifting objects. Here, we tested the hypothesis that when watching an actor lift an object, people naturally learn the object's weight and use this information to scale forces when they subsequently lift the object themselves. Participants repeatedly lifted an object in turn with an actor. Object weight unpredictably changed between 2 and 7 N every 5th to 9th of the actor's lifts, and the weight lifted by the participant always matched that previously lifted by the actor. Even though the participants were uninformed about the structure of the experiment, they appropriately adapted their lifting force in the first trial after a weight change. Thus, participants updated their internal representation about the object's weight, for use in action, when watching a single lift performed by the actor. This ability presumably involves the comparison of predicted and actual sensory information related to actor's actions, a comparison process that is also fundamental in action.
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