Journal of Neuroscience, Vol 3, 1572-1585, Copyright © 1983 by Society for Neuroscience
The neural signal for skin indentation depth. I. Changing indentations
PR Burgess, J Mei, RP Tuckett, KW Horch, CM Ballinger and DA Poulos
Psychophysical tests on human subjects showed that judgments of skin
indentation depth made when the fingertip was indented at rates from 0.2 to
16 mm/sec were quite insensitive to changes in indentation velocity.
Similar results were obtained on the forearm at indentation velocities of
0.4 to 16 mm/sec. Recordings were made from mechanoreceptors in the
monkey's hand that were able to respond over the same range of velocities
and at comparable depths to determine how skin indentation depth might be
signaled (coded) at the receptor level and to examine the rate sensitivity
of the possible depth codes. It was found that most of the receptors with
foci under the stimulator were recruited relatively early during an
indentation, especially at velocities of 1.6 mm/sec and higher, making it
improbable that the full range of indentation depths is signaled by the
"subsurface" recruitment of different receptors at different indentation
depths. A subsurface recruitment code involving subcutaneous receptors is
not likely since subjects could feel virtually none of the stimuli after
skin anesthesia. Progressive recruitment with depth of receptors whose foci
lie further and further away from the stimulator ("lateral" recruitment)
was considered an unlikely depth code because changing the area of the
stimulator had little effect on its perceived depth. Also, it was shown
that subjects could sense the curvature of the indentation (the profile of
the depth at right angles to the skin surface), which requires information
about the depth of individual patches of skin beneath the stimulator. There
is no obvious way that a lateral recruitment code can provide this
information. Thus it is probable that the discharge rate of some or all of
the receptors excited by the indentation is involved in indicating its
depth. Both impulse frequency and receptor recruitment at any given depth
increased as the velocity of the indentation increased. The demonstrated
reliability of information about skin indentation depth in humans indicates
that the central neural circuitry responsible for judgments of skin
indentation depth is able to compensate for the rate-sensitive receptor
signals.