Reach and grasp a willing colleague by the arm, now let them go, and pick up a pen or pencil instead. The first movement requires a power grip, flexing all the fingers together towards the palm, the second movement uses a precision grip involving the thumb and forefinger. Easy to do? Apparently yes, but the ease and accuracy with which we reach and grasp objects (or people!) belies the complexity of the neural processing underlying such movements. Now the journal Nature Reviews Neuroscience has published a comprehensive review on the neuroscience of grasping, by Umberto Castiello.
Castiello describes how studies on the ‘kinematics’ of grasping have shown there is a reliable ‘landmark’ during reaching movements: when the hand is between 60 to 70 per cent of the way towards its target, the gap between the thumb and fingers always reaches its largest point, the precise moment correlating highly with the size of the object to be grasped. Other object characteristics – its weight, texture, surface – also affect aspects of the grasping movement in a lawful way.
Most of our knowledge about the brain networks involved in grasping come from intrusive experiments on monkeys that are simply not possible or ethical in humans. These point to a circuit involving the primary motor cortex, the premotor cortex and the anterior intraparietal sulcus. How similar things are in the human brain is a matter of controversy and ongoing investigation using brain imaging and studies with brain-damaged patients. These suggest many of the grasping-related areas implicated in the monkey brain are activated in the human brain too, but that other regions are also involved, including the prefrontal, somatosensory and cerebellar areas.
Castiello describes one patient, A.T., with extensive damage to the parietal lobe and secondary visual areas, who had problems grasping neutral, laboratory objects but was okay at grasping familiar items such as a lipstick. This suggests that, in humans at least, brain areas involved in interpreting the meaning of an object also influence the brain’s grasping circuit.
Indeed, Castiello says more research is needed into whether and how the meaning of an object, and intentions for what to do with an object, affect grasping in monkeys in the same way research has shown these more ‘cognitive’ variables influence grasping in humans.
“It will only be through careful and thoughtful experimentation, using converging techniques from the brain and behaviour, that we might completely understand the grasping function of the human hand”, Castiello’s review concludes.
Link to abstract of the review.
Mind Hacks 