These days I'm busy reading Vernon Mountcastle's 2005 book, The Sensory Hand. What a great read. What a great researcher. Here's a great review by someone named Edward Jones.
I'm only on page 9, but already he has me by my neurons. He states in a section containing his general hypothesis, his thoughts on small fibre relevance:
1. The small fibres are just as important to sensorimotor "understanding" and function as large ones:
"A general hypothesis I consider at several places in this book is that the small-fibered afferent systems, long known to contain the essential neural substrates for pain and temperature sensibilities, also contribute to the higher-order aspects of the several varieties of mechanosensitive sensibility. They are activated under many conditions by the same mechanical stimuli processed in the large-fibered system, and carry signals to the forebrain that evoke the overall affective components of the sensory experiences. These small-fibered afferent systems activate many distributed areas in the frontal lobe, the limbic areas of the cerebral cortex,and the insula."He got me with the word "insula." I've followed A.D. Craig, and Sandra Blakeslee's exposition of his work, for a long time now. I first got excited about the insular part of the brain when I found the Olausson paper (see link at end).
2. The small-fiber and the large-fiber afferent systems work together:
"At the same time signals in large-fibered afferents are processed in an elegant and quantitative manner and present to higher-order cortical systems signals that can be detected, discriminated and rated with precision, in quantitative correspondence with the parameters of the stimuli that evoke them."Indeed, how could they not?
3. Research on the two systems became separated:
"Until now we have learned very little about the final "integration" of these varied inputs in producing overall somesthetic experiences. This integration and its varieties are obvious in everyday life - the touch of a loved one's hand carries overtones not found in more ordinary tactile experiences. The hitherto perceived dichotomy of these two classes of systems has resulted in a parallel separation of investigators. Those talented individuals who have made such spectacular discoveries about the functions of the small-fibered systems in pain and temperature sensibilities now sense that the deepening knowledge of these systems, particularly the molecular aspects of the peripheral transducer mechanisms, will lead to chemical methods of blocking pain at its level of inception,with no effects on the function of the central nervous system. Yet, only a few have taken full account of the broader - or perhaps I should say, other - meanings for behavior of activity in the small-fibered systems. Of course, the same is true inversely for those investigators involved in quantitative studies of the brain mechanisms in mechanoreceptive sensibility. They have because of the constraints of experimental design and execution not been able to take into account the accompanying activity in the small-fibered systems and the powerful contributions they make to the overall somesthetic experience."Maybe we can lay the blame on Cartesian dichotomy.
4. Mountcastle is trying to bring the two fields of investigation together again: this book is his effort to provide a "sublation":
"This division now ends, and concerted efforts are directed to study of the somatic system, complete. That such a dichotomy has not occurred, or at least not to the same extent, in studies of the visual and auditory systems is attributable to the relative simplicity of those systems. Compare, for example, the 12 different sets of first-order fibers innervating the primate hand with the much smaller number of afferent sets leaving the eye or the ear. The visual and auditory systems are by no means simple, but only appear so when compared with the somatic afferent system, in which a number of afferent sets with congruent peripheral distributions feed many ascending systems. Moreover, at several transition stations of these systems there is a complex interaction with motor mechanisms. I make here an effort to begin the process of unifying these two major fields of research. The reader will find in several chapters descriptions of the small fiber systems, with some effort to show how they condition the overall mechanoreceptive sensory experiences."
Let's repeat something in there for emphasis:
That such a dichotomy has not occurred, or at least not to the same extent, in studies of the visual and auditory systems is attributable to the relative simplicity of those systems.What? Relative simplicity of vision and hearing?
This explains why touch has been ignored for so long. Production and perception of sensory physical contact is more complicated to study/make sense out of than either vision or hearing.
I looked for some good links to bring here on "somesthesis", but all I could find (so far) are simple definitions like this one.
I have high hopes for this book. I'm hoping to uncover material that will make sense out of human primate social grooming, make meaningful connections. I hope it will clarify a path that can be taken by people (like me) who want to see this form of human interaction not just lifted up out of the muck of institutionalized human greed and exploitation, but restored clean and shiny and science-based to those who want to bring hands-on care back into good use, good service, ethical service.
1. The Brain Voyager; includes a picture of Mountcastle in his retirement
2. See Sandra Blakeslee's article, A Small Part of the Brain and its Profound Effects (6th one down the list)
3. Unmyelinated tactile afferents signal touch and project to insular cortex; nature neuroscience 2002; Olausson et al.