December 21, 2004
In-depth: Reaching and Grasping
How do we decide how to pick up a glass of water? Many factors are involved in the decision:
- We must select the relevant target (the glass of water)
amongst many stimuli (e.g. my lovely Mac)
- We must get information about the location of the target
relative to our hand position
- We must determine the grip strength required to grasp and
lift it, and open our grip to the appropriate size.
So; your brain asks 'what is it', 'where is it', and 'how shall I get it'.
Your senses, particularly sight, provide you with answers to these questions, and so the study of reaching and grasping can also be thought of as the study of how visual areas of the brain connect to the motor areas to result in the sensory guidance of movement.
In tackling the problem of the sensory guidance of movements, we need to think about the pathways that control these movements. As we mentioned at the start, what may seem to be a simple action (picking up the glass) may consist of several independent factors working together. Some neurones may be extracting visual data from the scene and sending this to a motor area for processing. Here, then, is the big question: we know all about visual areas, we know all about motor areas, but how are the two linked?
ARTICLE SUMMARY
This article is based largely on Mitchell Glickstein's excellent review paper .
a quick history
It was Munk in the 19th century who first identified the primary sensory and motor areas of the cerebral cortex. He noticed that injury to one side of the stritate (visual) cortex of the occipital lobe (called a unilateral lesion) caused the monkey to become blind in the visual field opposite to the one the lesion was in (known as hemianopia - "half-blindness").
Henschen clarified further by stating that the left occipital lobe recieves input from the left half of each retina (and so the right half of the visual field), and vice versa. He also localised the upper bank of the calcarine fissures as the region that receives its input from the upper retina (and hence the lower visual field).
Inouye constructed a more detailed map of what part of the visual cortex deals with what part of the visual field. The central visual fields seemed to be represented caudally (towards the back of the visual cortex) and furthermore seemed to occupy alot more cortical space than the peripheral visual field did. Holmes and Lister confirmed this in more detail in 1916.
What about the motor cortex? It was later shown that if you weakly stimulated a bit of a dog's frontal cerebral cortex then you could cause movement of the face or of a limb on the contralateral (opposite) side of the body. Nice. If you cut out an area that elicited a particular limb movement, then that limb's movement would be impaired. Betz then descibed the pyramidal cells in layer V of cortex that we now associate with the motor area.
So what of our big question, how do visual and motor areas link to produce sensory control of movement?
Well, our main assumptions today are broadly similar to our assumptions in 1900, with, um, different terminology.
The assumptions are:
the dorsal stream
There are thought to be two main classes that the extrastriate visual areas can be segregated into: a dorsal stream of information and a ventral stream. This was based on anatomical and behavioural studies. It was suggested that the dorsal stream was used more for the coding of an object's location, while the ventral stream was concerned with identification of the object.
This theory was further reinforced by findings that these dorsal and ventral areas had very different projections: the dorsal, paretial areas were found to project heavily to the cerebellum via the pontine nuclei, while the ventral areas were found to show no such projection. It was suggested that the connections of the dorsal paretial areas to the cerebellum indicated that these areas might be important for the visual control of movement.
To test this, they got a bunch of monkeys and lesioned their brains in different areas to see the results.
When they lesioned the dorsal stream, they found that monkeys were seriously impaired in their ability to guide their hands and wrists to pick up objects. Other lesioned areas had little or no effect. This was clear evidence that the first pathway between motor and visual areas was via the dorsal stream of extrastriate visual areas, as loss of these areas would severely limit ability to visually guide movement.
Similarly, lesioning appropriate ventral stream areas will impair recognition of objects (or faces) but will not affect visually-guided coordination.
So our next question is how this dorsal stream then goes on to control movement. To do this it has to somehow connect to the cortical and subcortical structures and interact with them, which will themselves control movement.
Article to be completed later today... check back soon.
REFERENCES & FURTHER READING
This article is based on an excellent review by Mitchell Glickstein of UCL.
Books
Principles of Neural Science, p777: two pages, a basic introduction
Papers
How are visual
areas of the brain connected to motor areas for the sensory guidance of
movement?
Glickstein, Trends Neurosci. 2000, 23:12, 613-617
Posted at 9:33 pm
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