Tactile sensory coding in the glabrous skin of the human hand

32
has been attempted lately is to electrically
stimulate a single afferent nerve fibre in
conscious human subjects and analyse the
sensation that might result ~a24. In these
studies, impulses from a single tactile unit
is first recorded and the functional proper-
ties of the unit are assessed. The electrode is
then reconnected to a stimulator and a train
of weak pulses (0.2-2.0/zA) is delivered.
It has been found that a sensation of light
mechanical deformation is often experi-
enced within the receptive field of the par-
ticular afferent unit that was recorded
immediately before. The extent over the
skin surface where this sensation is experi-
enced is usually in the same order of mag-
nitude as the extent of the receptive field.
When stimulus intensity was successively
raised, one or two additional sensations of
similar type appeared usually well sepa-
rated and often at a considerable distance on
the skin surface from the primary sensation,
suggesting that other units were stimulated.
At still higher intensities, the subjects
usually reported either a localized painful
sensation or a paraesthetic sensation over
a larger area. Differential atlributes of the
sensations were usually reported when the
electrode saw a SA I unit and a FA I unit.
For instance, with a SA I unit a light and
uniform pressure may be felt, similar to the
pressure of a soft water-colour brush stead-
ily held against the skin. With a FA I unit, a
buzzing, wobbling or fluttery sensation
may appear. The findings obtained so far
suggest that studies with this method may
shed light on a set of fundamental problems
in the border-line between neurophysiology
and psychophysics.
This contribution has briefly been con-
cerned with some recent findings regarding
the tactile sensory mechanisms of the
human hand. As to the future, we believe
that these results may serve as a basis for
further studies of the interactions between
the hand and the brain during integrative
hand functions. Such studies will most cer-
tainly yield progress when carried out on
conscious human subjects, able tO partici-
pate in sophisticated and co-operative
experimental tests.
Acknowledgements
We would like to thank Mrs MargLouise
Svensson for invaluable help with the illustra-
tions. The financial support by the Swedish
Medical Research Council (project 3548) and
the Swedish Work Envirorm~nt Fund (project
79/104) is gratefully acknowledged.
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Roland S. Johansson and ke B. Vallbo are at the
Department of Physiology, University of Ume~,
S-901 87 Urned, Sweden.
Po-s;'tion and proximity in
deve nt of
stripes
M. Constantine-Paton
Recent evidence indicates that neural-map formation is a multiphase process in which
presynaptic proximity-dependent sorting of terminals is superimposed on relatively
weaker interactions that match neuronal processes by virtue of their positions in the
pre- and post-synaptic populations. This biphasic mechanism will produce highly
ordered contacts with little 'pre-specified' cell surface information. Exactly the same
process will produce periodic patterns of segregated afferents when two different
afferent populations map within a single target zone.
In recent years considerable interest has
been generated by the widespread occur-
rence of periodic subdivisions within many
terminal zones of the vertebrate brain.
These regular patterns can take the form of
either patches or alternating source-specific
stripes. They have been observed in path-
ways as diverse as the pyramidal tract input
to the dorsal-column nuclei, inputs to the
cerebellum, to the neoeortex and several of
the different projections which converge
upon the mammalian superior colliculus.
Thus, this afferent ordering is likely to
reflect principles of fundamental import-
ance to brain organization, but even in the
best-studied instances, such as the ocular
dominance columns of the cat and primate
cortex, the reason for such terminal segre-
gation is not at all clear.
Some insight into the determinants of
periodic input patterning was unexpectedly
obtained in my laboratory several years ago
as a result of work on the retinotectal path-
way of the common leopard frog Rana
pipiens. In the frog, axons from only one
retina normally form a continuous map
within the superficial neuropfl of the con-
tralateral optic tectum. However, we forced
two retinas to co-innervate one tectal lobe,
either by implanting a third eye primordium
into a young embryo s. 21 or by removing one
optic tecturn in a late tadpole or a young
post-metamorphic frog aa. We invariably
obtained the same end result. Axon terrm-