Chapter 2. Prehension

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Chapter 7. Summary of Opposition Space Phases
"All my life's a circle".
A hammer sits on the table in front of you. You reach out, pick it
up, place the hammer on shelf, and continue with your daily tasks,
grasping and manipulating objects frequently. What was involved in
this two second task?
You probably turned your eyes and head to foveate the object,
though this was not necessary. Using vision, you perceived task-
relevant object characteristics, such as its orientation on the table, the
surface texture, length and width of the handle and the hammerhead.
Using proprioception and vision, you perceived your own body
configuration and its relation to the hammer. You 'saw' an opposition
vector on the object, a key driving variable in planning and control that
allowed you to envision an appropriate place to grasp the hammer,
given the task, the assumed cone of friction, weight, center and
distribution of mass. Movement began and, as your arm reached out,
anticipatory shaping of the fingers occurred; again, appropriate for the
task. Then, your fingers began to enclose around the hammer. After
initial contact, your hand captured the hammer and established a stable
grasp by complying with the hammer (and likely the table as well),
generating the necessary, functionally effective forces to achieve the
goal of the task, i.e., grasping the hammer for placement. Creating a
stable grasp meant taking into account the complex interactions
between the hammer's and your hand's surfaces. Without it, the object
would slip and even fall out of your hand. Your muscles contracted to
supply the necessary forces in the finger flexors with cocontraction in
the extensors, which caused torques at each joint in the hand. Your
skin deformed, and sensory signals from your cutaneous and
proprioceptive receptors were sent back to your spinal cord and brain
for further motor actions. Your hand had to resist small external
perturbations and generate restoring torques and forces to ensure
manipulative stability. While maintaining that stable grasp, you lifted
the hammer, transported it to a location above the shelf, and then
placed it on the shelf. As you let go of the hammer, your eyes and
head turned to focus on the next object to be grasped.