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

Abstract

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.

Touch is a team effort: interplay of submodalities in cutaneous sensibility

“…In touch, intensity is determined both by the static and dynamic properties of a stimulus [11] but static indentations and skin vibrations can all be judged along a single intensive continuum. First, the perceived intensity of a skin deformation cannot be predicted from the responses of single afferents but rather requires that populations of afferents be taken into consideration [12,13]. Second, perceived intensity cannot be predicted from the response of any one population of afferents [14].…”

Human Ability to Scale and Discriminate Forces Typical of Those Occurring during Grasp and Manipulation

“…Population reconstructions have been performed for mechanoreceptive afferents innervating the glabrous skin on the terminal phalanx of the monkey's finger for a variety of stimuli. Taking such an approach enabled the testing of hypotheses about encoding mechanisms for the pertinent stimulus parameters (Johnson, 1974;Blake et al, 1997;Dodson et al, 1998;Khalsa et al, 1998;Goodwin and Wheat, 1999;Wheat and Goodwin, 2000). For example, Goodwin and Wheat (1999) showed how normal force can be perceived independently of the shape of the object and its position on the skin.…”

Cardiovascular and Respiratory Modulation of Tactile Afferents in the Human Finger Pad

“…The effect of this modulation would be to increase the variability in the afferent discharge. Recently, the effects of different types of noise on the responses of SA I afferents in the finger pad of the monkey have been modelled (Goodwin & Wheat, 1999). Although cardiovascular noise was not considered, according to their model this type of noise could be described as 'additive noise', the effect of which would be to decrease the resolution of the afferent signal.…”