Cognitive Neuroscience
Reaching and Grasping and the brain
compare the eye movement and reaching/grasping effector systems. Explain how reaching and grasping use different visual information.
Each perceptual system possesses its own specific procedures and processes for exploration. Visual system has got the most complex and specialised visual system. Reaching, perceiving, controlling the gaze, grasping, everything is based on the specific type of eye movement associated with it.
Reach movements and grasp movements are some of the most common actions that happen in everyday life. These types of movements can be separated into two components. A reach component that reflects the movement of the hand towards the target object as well as a grasp component that helps in grasping. The reach component is influenced by the object’s spatial position. The grasp component is influenced mainly by the size of the object
A portion of the anterior intraparietal (AIP) sulcus is stimulated during visually-guided grasping actions which require data about the size and shape of the object in order to make further actions like the movement of hands and other body parts. Macaque AIP is an area containing neurons which are activated as a result of the viewing and grasping of different shapes. Human AIP is an essential homologue of macaque AIP. Human AIP is very active and lively throughout the vision and action periods of delayed grasping trial, as the macaque AIP. Human AIP cannot be activated through the visual presentation of two-dimensional object images and does not appear as an all-purpose object processing area. On the other hand, the lateral occipital complex (which is a ventral stream area reacting towards 2D object images) does not bring out any activation for grasping (when comparing with what it does for reaching). In short AIP in human parietal cortex is the major neural substrate for analyzing the shape of the object for carrying out actions but not for recognizing.
Grasping and reaching have common ‘transport’ component of the movement of body (hand movement) towards the target object. But grasping need a ‘grip’ part which considers object orientation as well as size details to preshape the hand. Reaching actions and grasping actions (with respect to an intertrial interval baseline) jointly activate a major portion of parietal cortex through the intraparietal sulcus apart from various other motor, somatosensory, premotor and visual portions. But some of the areas are more activated as a result of grasping than reaching. However, no area is more activated because of reaching than what happens because of grasping.
The most significant difference between reaching and grasping is the usage of object properties for the preshape of the grip. But it is possible that other dissimilarities between the two tasks (especially the variations in the degree of digit control as well as the somatosensory feedback to the fingers) could help for the activity found in the anterior IPS.
Grasping (comparing with reaching) generates activation in a contiguous region which has the postcentral sulcus as well as the anterior intraparietal sulcus. Comparisons done with somatosensory finger stimulation and finger movement however suggests that the continuous area of activation should be comprised of two subregions.
Information about object shape is processed independently through two pathways, the ventral and dorsal streams (Goodale and Milner 1992). Object shape and size computations are needed to preshape the hand while grasping, compared to reaching that seldom needs preshaping. Human AIP does not show any response to 2D images for which grasping response is not required. Computations of object properties hand preshaping and recognition purposes happen independently in the dorsal and ventral streams. AIP activation is greater in grasping than reaching during the vision and action phases.
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