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The Journal of Neurophysiology Vol. 81 No. 6 June 1999, pp. 2711-2719
Copyright ©1999 by the American Physiological Society
Department of Neuroscience and Krieger Mind/Brain Institute, Johns Hopkins University, Baltimore, Maryland 21218
Vega-Bermudez, F. and
K. O. Johnson.
Surround Suppression in the Responses of Primate SA1 and RA
Mechanoreceptive Afferents Mapped with a Probe Array. J. Neurophysiol. 81: 2711-2719, 1999.
Surround suppression in the responses of primate SA1 and RA
mechanoreceptive afferents mapped with a probe array.
Twenty-four slowly adapting type 1 (SA1) and 26 rapidly adapting (RA)
cutaneous mechanoreceptive afferents in the rhesus monkey were studied
with an array of independently controlled, punctate probes that covered an entire fingerpad. Each afferent had a receptive field (RF) on a
single fingerpad and was studied at 73 skin sites (50 mm2).
The entire array was lowered to 1.6 to 3.0 mm below the point of
initial skin contact (the background indentation) before delivering indentations with one to seven probes. Indentations were generally limited to 100 µm to minimize gross mechanical interactions. There were two major, new findings. 1) The discharge rates of both
SA1 and RA afferents were strongly affected by the number of probes indenting the RF simultaneously. The effect was exponential. Each increase in probe number reduced the response by 24% in SA1 and 12%
in RA afferents on average. When seven probes indented the skin
simultaneously, the impulse rates in SA1 and RA afferents were reduced
to 20 and 40% of the rates evoked by a single probe at the hot spot
(all indentations were 100 µm). This shows that before any synaptic
interaction in the CNS there is already a mechanism analogous to
surround inhibition that suppresses an afferent's responses to uniform
indentation and makes it especially sensitive to deviations from
spatial uniformity. 2) The responses of both SA1 and RA
afferents were independent of background array depth over the range
from 1.6 to 3 mm below the point of initial skin contact. This shows
that the neural responses to elements raised above a background are
independent of the applied force over a wide range of forces. To relate
the background depths to indentation force and to compare humans and
monkeys, we studied the biomechanics of indentation with a uniform
surface. A remarkable result is that the force-displacement
relationships in humans and monkeys were the same; the skin is highly
compliant for the first 2-3 mm of indentation and then becomes much
stiffer. The results were the same in alert humans and monkeys and in
monkeys anesthetized with pentobarbital. Ketamine anesthesia made the skin much stiffer and reduced the compliant range substantially.
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