Lecturer's Précis - Mesulam (1995)
Copyright Notice: This material was written and published in Wales by Derek J. Smith (Chartered Engineer). It forms part of a multifile e-learning resource, and subject only to acknowledging Derek J. Smith's rights under international copyright law to be identified as author may be freely downloaded and printed off in single complete copies solely for the purposes of private study and/or review. Commercial exploitation rights are reserved. The remote hyperlinks have been selected for the academic appropriacy of their contents; they were free of offensive and litigious content when selected, and will be periodically checked to have remained so. Copyright © 2010, High Tower Consultants Limited.
First published online 14:50 GMT 30th January 2003, Copyright Derek J. Smith (Chartered Engineer). This version [HT.1 - transfer of copyright] dated 09:00 13th January 2010
Mesulam (1995) Lecturer's Précis from Conference Notes
In this conference paper, Professor Mesulam offered some new insights into the phenomenon of hemineglect. To begin with, he posed the question why the left hand is as dumb as it is when it has just as many fingers as the right and a lifetime to learn to do what the right hand has been doing. It just seems not to have bothered, and this is one of the (many) factors leading to the conventional view of there being a dominant-linguistic cerebral hemisphere assisted by a non-dominant-non-linguistic one.
But Mesulam then went on to argue that the minor hemisphere was perhaps not so minor after all. He reviewed literature using the Wada amytal technique of hemispheric suppression. Where the left hemisphere is momentarily suppressed, then typically there is no resulting hemineglect (although this effect is traditionally difficult to test in isolation because the linguistic person is simultaneously suppressed). On the other hand, where the right hemisphere is suppressed, there is a severe left neglect, and a mild right neglect.
With naturally occurring CVAs, however, few lesions involve an entire cerebral hemisphere. Single processes can be damaged within a single hemisphere, giving rise to "clinical subtypes" of neglect. Thus if the sensory representation of the world (and that includes the attentional mechanisms which support sensation) is damaged you get a sensory neglect. Similarly, if the motor representation of the world is damaged you get an exploratory neglect, and if the motivational processes are damaged you get a motivational neglect.
And with normals, PET scanning shows evidence of right hemisphere involvement in the planning of right hand exploration. This is highly counter-intuitive, because only the left hemisphere's motor cortex is in a position to issue the final muscle commands to the right hand. And as to why this should be, he showed the following diagram:
Mesulam's (1995) Model of the Lateralisation of the Internal Mapping of External Space: Here is a coronal section through the two cerebral hemispheres. The sensory representation of space is shown as "FOCAL SENSATION", and is carried out in the posterior parietal areas. Dealing with the motor aspects of space is shown as "MOTOR PLANNING", and is carried out in a number of frontal premotor fields such as the frontal eye fields. And the motivation to act within space (that is to say, processing which relates to where the subject wants or needs to be) is shown as "WILLING", and is carried out in the anterior portion of the cingulate gyrus. Note that all proposed processes are contralaterally organised except one (see the red exclamation mark), which seems to plan bilateral action. Hence the central thesis of this paper, namely that whilst the left hemisphere maps the spatial coordinates of the right half of the world, the right hemisphere maps the spatial coordinates of both halves. Right hemisphere lesions are therefore much more likely to give rise to left body neglect than left hemisphere lesions are to give rise to right body neglect.
This graphic Copyright © 2003, Derek J. Smith, attempting to represent the scholarship of Marcel Mesulam.
Mesulam, M.M. (1995). Right cerebral dominance for neglect and spatial awareness. Paper presented to the joint meeting of the Association of British Neurologists and the British Neuropsychological Society, The Commonwealth Institute, London, 29th September 1995.