The Journal of Neuroscience, September 15, 1999, 19(18):8057-8070
Effects of Nonuniform Fiber Sensitivity, Innervation Geometry,
and Noise on Information Relayed by a Population of Slowly Adapting
Type I Primary Afferents from the Fingerpad
Antony W.
Goodwin and
Heather E.
Wheat
Department of Anatomy and Cell Biology, University of Melbourne,
Parkville, Victoria 3052, Australia
The capacity of a population of primary afferent fibers to signal
information about a sphere indenting the fingerpad is limited by
factors such as the inhomogeneity of sensitivity among the afferents,
the pattern and density of innervation, and the effects of noise
(response variability). Using experimental data recorded from single
slowly adapting type I afferents (SAIs), we simulated the response of
the SAI population to such a stimulus. The human ability to
discriminate stimulus curvature, location, and force has been
quantified previously. We devised three neural measures, treating them
as surrogates for the real neural measures underlying human
performance, and explored how population parameters usually overlooked
in neural coding studies affect such measures. Variation in sensitivity
among SAIs is large; this distorts population response profiles
markedly but has no significant impact on the neural measures. Two
classes of noise were introduced, one dependent on and the other
independent of the level of neural activity. Resolution of the model
was compared with discrimination in humans. Correlation of noise among
neurons had different effects for the different measures. An increase
in correlation decreased resolution in the measure for force but
improved resolution in the measure for position. Increasing innervation
density (1) always increased resolution for position and (2) increased
resolution for force if noise was uncorrelated but had diminishing
effects as correlation increased. Correlation and innervation density
had complex effects on the measure for curvature, depending on the
class of noise. Nonuniformity in the pattern of innervation had
negligible effects on resolution.
Key words:
tactile resolution; population response; neural code; innervation density; neural noise; correlated noise; covariance; tactile shape; position on skin; contact force
Copyright © 1999 Society for Neuroscience 0270-6474/99/19188057-14$05.00/0
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