HHF Part6 - Prehension

Part 6 Prehension

08/18/2024

"Reaching to Grasp"

**1. Reaching to Grasp:

• Keywords:
  • Reaching Movements: Movements involving the arm moving to grasp an object.
  • Grasp Phase: The stage where the hand adjusts to grasp the object.
  • Manipulation Phase: The final phase involving actual interaction with the object.
  • Velocity Profile: The speed and acceleration of the hand during movement.
• Takeaways:
  • Reaching to grasp an object involves multiple phases, each characterized by specific kinematic features like movement time, velocity, and grip aperture.
  • The kinematics of the wrist and hand are crucial in coordinating the reaching and grasping phases.

 

**2. Kinematic Features of Reaching:

• Keywords:
  • Wrist Movement: The trajectory and velocity of the wrist during reaching.
  • Grip Aperture: The maximum opening of the hand during a grasp, influenced by object size and distance.
  • Peak Velocity: The maximum speed attained during reaching.
• Takeaways:
  • The velocity curve during reaching typically follows a bell-shaped pattern, with a single acceleration and deceleration peak.
  • The grip aperture increases with object size, and the timing of the maximum aperture occurs during the deceleration phase of reaching.

**3. Hand Posture and Object Interaction:

• Keywords:
  • Hand Posture: The configuration of the hand as it prepares to grasp an object.
  • Tactile and Proprioceptive Information: Sensory feedback from the hand about object size and position.
  • Visual Illusions: Visual distortions that can affect grip aperture and grasping accuracy.
• Takeaways:
  • Hand posture changes dynamically as the hand approaches the object, influenced by tactile and proprioceptive cues.
  • Visual illusions have limited impact on grasping, suggesting some dissociation between visual perception and action.

**4. Effect of Tactile Feedback on Grasping:

• Keywords:
  • Tactile Feedback: Sensory information from the fingertips during object interaction.
  • Finger-Closure Time: The time taken for the fingers to close around the object.
  • Object Slippage: The tendency of the object to slip if tactile feedback is impaired.
• Takeaways:
  • Tactile feedback from the fingertips is crucial for precise grasping and avoiding object slippage.
  • Anesthetizing the fingertips leads to more conservative movement strategies, with increased grip aperture and potential for object slippage.

**5. Multivariate Analysis of Grasping:

• Keywords:
  • Finger Joint Angles: The angles at which the fingers are positioned during grasping.
  • Concave and Convex Objects: Different shapes that require specific hand postures.
  • Degrees of Freedom: The independent movements that the hand can control during grasping.
• Takeaways:
  • Despite the complexity of hand movements, the actual number of actively controlled degrees of freedom is smaller, indicating a highly coordinated system.
  • The hand posture evolves gradually during reaching, becoming more defined as the hand gets closer to the object.

Multivariate statistical analyses of the changes in finger joint angles during the latter part of the reaching movement indicate that it is possible to distinguish between concave and convex target objects from the relative amount of flexion in the index and little fingers (together) as compared to the ring and middle fingers (Santello & Soechting, 1998). These analyses reveal that despite the large number of mechanical degrees of freedom available during grasping, the number of independent degrees of freedom actively controlled is considerably smaller.

"Visuomotor and Digit-channel Theories of Reaching"

Visuomotor Channel Hypothesis: This theory posits that reaching and grasping involve separate but temporally coupled visuomotor mechanisms. The transport channel handles spatial information (extrinsic properties) and motor commands for arm movement, while the manipulation channel deals with the size and shape of the object (intrinsic properties) and commands for grasping.

Critiques and Alternatives: Studies have questioned the independence of these channels, showing that both are affected by the same task constraints. The Digit Channel Hypothesis offers an alternative, focusing on the separate control of the thumb and finger during reaching and grasping.

"Visual Feedback and Reaching"

Role of Visual and Proprioceptive Feedback: Visual feedback is crucial for guiding reaching and grasping, but proprioceptive cues can compensate when visual information is unavailable. This suggests that hand posture can be pre-configured and adjusted during movement based on available sensory information.

Impact of Object Properties: The trajectory and kinematics of reaching and grasping movements are influenced by the spatial location, size, shape, and symmetry of the object. Visual and proprioceptive cues both play important roles in ensuring the accuracy of these movements.

Hand Aperture and Movement Dynamics: The maximum aperture of the hand and its timing are consistent features of reaching and grasping, reflecting the integration of visual and proprioceptive feedback for efficient movement planning and execution.

When visual feedback of the arm is eliminated as people reach to grasp a visible object under normal illumination, the duration of the movement increases in comparison to movements made with full visual feedback, as shown in figure 6.3 (Connolly & Goodale, 1999). The maximum aperture of the hand and the relative time taken to reach maximum aperture remains unchanged, even though the arm is occluded from view (Connolly & Goodale, 1999). This suggests that the posture of the hand can be controlled without direct visual feedback, presumably on the basis of proprioceptive feedback from the hand and visual cues from the object.

Timing and Consistency

• Timing: The point in time when your hand reaches its maximum aperture is quite consistent, typically occurring at around 70% of the total movement time, regardless of the object's size. This means that whether you're reaching for a tennis ball or a pen, your hand will reach its maximum opening at a similar point in your reach.
• Consistency: This process is very consistent across different objects and movements. Your brain automatically adjusts the maximum aperture based on the size of the object and ensures that this maximum occurs at the right moment for a smooth and accurate grasp.

What is proprioceptive feedback? How is it different from extroceptive and introceptive feedbacks?

Proprioceptive Feedback: Proprioceptive feedback refers to the body's ability to sense its position, movement, and orientation in space. This feedback comes from receptors located in muscles, tendons, and joints, which detect changes in muscle length, tension, and joint angle. Proprioceptive feedback helps us coordinate movements, maintain balance, and perform tasks like reaching for an object without directly looking at our limbs. For example, when you close your eyes and touch your nose with your finger, you rely on proprioception to guide your hand accurately.

Extroceptive Feedback: Extroceptive feedback involves information about the external environment that is processed through the sensory systems, particularly vision, hearing, touch, smell, and taste.

Introceptive Feedback: Introceptive feedback refers to the internal sensations within the body that relate to the physiological state. This includes signals from organs, such as hunger, thirst, heart rate, or the need to breathe.

"Grasping"

 

• Research Focus on Grasping: Grasping research has explored various factors influencing the grasp, including object properties and task constraints, focusing on the coordination of fingertip forces.
• Experimental Protocol: A widely used experimental protocol developed by Johansson and Westling (1984) involves grasping and lifting an object, breaking down the process into distinct phases (preload, loading, transitional, static, replacement, and unloading).
• Grip and Load Forces: During grasping, grip (normal) and load (tangential) forces increase in a coordinated manner, maintaining a constant ratio that stabilizes the object in a secure position.
• Slip Force and Safety Margin: The minimum force required to prevent an object from slipping (slip force) varies based on the friction between the skin and the object. The difference between grip and slip forces, termed the "safety margin," tends to be smaller in more dexterous individuals.
• Automatic Grip Adjustment: If an object begins to slip, the grip force is automatically adjusted within 70 ms, a reflexive response mediated by tactile mechanoreceptors in the fingers.
• Mechanoreceptor Involvement: Mechanoreceptors (FA I, FA II, and SA I) in the fingerpads play a crucial role in detecting slips and adjusting grip force, even when these slips are not consciously perceived.

 

"Effect of Friction on Grip Forces"

• Friction and Grip Force: The friction between the skin and the object being grasped is crucial for maintaining a stable grip. Different materials, such as sandpaper, suede, and silk, have varying coefficients of friction, which influence the required grip force.
• Effect of Surface Material: As friction decreases (e.g., with silk), higher normal forces are necessary to prevent the object from slipping, indicating that the hand adjusts its grip force based on the frictional properties of the surface.
• Force Coordination: The coordination of grip (normal) and load (tangential) forces is affected by the friction of the surface. Low-friction surfaces require rapid adjustments in force to maintain stability during grasping.
• Role of Tactile Feedback: Tactile feedback from cutaneous mechanoreceptors in the skin is essential for adapting grip forces to the frictional conditions of the object. Anesthetizing the fingertips, which removes this feedback, results in less optimized and larger grip forces, increasing the risk of dropping the object.
• Independent Digit Control: The forces applied by each digit (e.g., thumb and index finger) are adjusted independently based on the frictional conditions at each contact point, ensuring safety margins against slips.
• Centrally Mediated Modulation: Even in the absence of tactile feedback, such as when fingertips are anesthetized, the temporal coupling of grip and load forces is preserved, suggesting central nervous system involvement in force modulation.

Is load force a result of grip force or is it a type of force that humans intetionally apply with fingers?

오히려 grip force가 파생적인 느낌.

The load force is not a direct result of the grip force, but rather a separate force that humans intentionally apply when manipulating an object.

Explanation:

• Load Force: This is the force that is tangential to the object's surface and acts in the direction of movement, such as when you lift, push, or pull an object. It is the force that needs to overcome the weight of the object and any other resistance (like friction or inertia) to move it. When you lift an object, for example, the load force is what counteracts gravity to raise the object.
• Grip Force: This is the normal force applied perpendicular to the object’s surface by the fingers (usually between the thumb and another finger). The purpose of grip force is to prevent the object from slipping out of the hand by ensuring that there is enough friction between the fingers and the object.

 

Relationship Between Grip and Load Forces:

• When grasping and lifting an object, the load force and grip force are coordinated. The grip force must be strong enough to prevent the object from slipping while you apply the load force to lift or move the object.
• The load force is intentional and varies depending on the task (lifting, moving, or holding an object). The grip force is adjusted based on the load force and the frictional properties of the object to maintain a stable grasp.

In summary, while the load force is a type of force that humans intentionally apply with their fingers (or hands) to move an object, the grip force is applied to ensure that the object does not slip while applying the load force. Both forces work together during object manipulation.

 The main effect of the different surface materials was on the rate with which the normal force changed during the preloading and loading phases, whereas the time course of the change in tangential force was similar for all three surfaces (see figure 6.7). Force coordination therefore varied as a function of surface material. This adjustment in force with surface material appeared to be based on friction per se, rather than the textural properties of the object; when the skin was washed and made less adhesive, the normal force was adapted to this change even though the surface material of the object remained unchanged (Johansson&Westling,1984).

 

"Grip- and Load-Force Coordination"

• Grip and Load Force Coordination: Grip forces adjust in response to the object's weight, with longer loading and unloading phases for heavier objects, though the balance between these forces remains stable under different weights.
• Anticipatory Control: The grip and load forces are programmed based on an estimate of the object's weight, especially for unfamiliar objects. Adjustments occur rapidly (within 100 ms) if the initial estimate is incorrect.
• Response to Active Objects: When handling active objects like a servo-controlled motor, grip forces are adjusted to match load forces. Higher safety margins are used due to the unpredictability of active objects.
• Adaptation to Load Force Rates: Grip-force responses are adapted to the rate of load-force changes, with quicker responses at higher load-force rates.
• Influence of External Forces: Grip forces are modulated in response to tangential loads, such as tilting an object or changing gravitational forces, maintaining stability despite these destabilizing forces.
• Tactile Mechanoreceptors' Role: FA I and SA I receptors in the fingertips play a critical role in signaling changes in force during the preloading and loading phases, with the contact area increasing rapidly at low grip forces.
• Directional Sensitivity: The direction of the applied load force influences grip-force modulation, with higher grip forces being produced in response to pulling forces, especially on slippery surfaces.
• Proprioception: SA II units, involved in proprioception, contribute to the control of finger forces during the static phase when the grip force remains constant.

What is Load Force Rates?

Load force rates refer to the rate at which the tangential (load) force applied to an object increases over time when it is being lifted or manipulated. It is a measure of how quickly the load force builds up as you apply force to lift or move an object.

What happens to grip and normal force if an object is tilted, compared with when it's not tilted?

When an object is tilted, both the grip force (normal force) and the load force (tangential force) increase, with the grip force being fine-tuned to ensure that the object remains securely in hand despite the altered orientation. The increase in grip force helps counteract the increased risk of slippage due to the new force components introduced by the tilt.

 

"Object's Shape and Size and Grip Forces" & "Object's Center of Mass and Grasping"

• Role of Visual and Tactile Cues: Visual cues are primarily used to anticipate and adjust grip forces before touching the object, while tactile cues fine-tune these forces during and after contact.
• Object Shape and Grip Forces: The shape of an object influences the distribution of grip forces. For example, tapered objects require increased normal and tangential forces, but the balance between these forces remains consistent.
• Impact of Surface Curvature: Curved surfaces, whether concave or convex, lead to higher safety margins in grip forces to prevent slipping, compared to flat surfaces.
• Size-Weight Illusion: Even though smaller objects are perceived as heavier than larger ones (size-weight illusion), people quickly adjust their grip forces to the actual weight rather than the perceived weight.
• Center of Mass in Grasping: The object's center of mass affects the distribution of load forces during grasping. When the center of mass is not aligned with the grip axis, grip forces must be adjusted to prevent rotation or tilt of the object.
• Adaptation and Anticipation: With practice, people can adapt their grip forces based on the object's properties, developing an internal representation that allows for precise control during repeated lifts.

 

"Anticipatory Models and Grasping"

• Internal Models and Grasping: Grasping actions are guided by internal models developed through previous experience, which represent the dynamic and kinematic properties of objects. These models help preprogram grip and load forces during object manipulation.
• Role of Sensory Feedback: Sensory information, especially from cutaneous mechanoreceptors in the fingertips, is crucial for maintaining effective grip forces. Without this feedback, as seen in cases of anesthesia or cortical lesions, precise control of grip is compromised.
• Adaptation and Memory: The internal model adapts to novel objects within a few trials and can retain this information for up to 24 hours. However, without tactile feedback, the model fails to adjust grip forces, leading to consistently high force application.
• Transfer of Object Properties: Some object properties, like texture and weight, are transferable between hands, allowing both hands to use the same internal model. However, properties like the object's center of mass, which require asymmetric force distribution, are not as easily transferred.
• Multiple Internal Models: The central nervous system likely holds multiple internal models for different objects, allowing for quick transitions between tasks in daily activities. These models can be combined and updated based on sensory input during object manipulation.
• Importance of Grip and Load Force Coordination: Effective object manipulation requires precise coordination between grip and load forces, adapted to the friction at the finger-object interface. This coordination ensures a stable grip and prevents slips during object handling.