Stimulus contrast in tactile resolutlon

Abstract

The nature of the adequate stimulus for tactile sensitivity remains a controversial issue. Several theories have been proposed to define the stimulus, the earliest one probably being that of Meissner (1859), who suggested that tactile sensitivity depends on the contrast between stimulated and adjacent unstimulated area, and not on the stimulus per se, While Meissner derived his theory from introspective analysis, von Frey and Kiesow (1899), who used absolute threshold measurements, concluded that tension is the basic determinant of the sensitivity of the skin to mechanical stimuli. Holway and Crozier (1937) came to a different conclusion through the use of the differential threshold technique, by finding linear relations between pressure (and not tension) and tactile sensitivity. Unfortunately, the use of discrete additions of the stimulus force by von Frey and Kiesow vs a continuous addition of it by the latter authors does not permit a direct comparison of their opposing results. This point was stressed by Jenkins and Stone (1941) and is highlighted by numerous investigations which demonstrated the influence of the rate of application of the stimulus on the threshold (Grindley 1934 a,b; Wolf, 1937) and by the psychophysical and physiological experiments of Nafe and Wagoner (1941) and Nafe and Kenshalo (1958) who found pressure perception and cutaneous nerve activity only during progressive mechanical deformation.In spite of the fact that the three above-mentioned theories differ, they have in common an attempt to correlate the areal distribution of the mechanical stimulus with its perception. These theories were formulated on the basis of psychophysical observations alone without consideration of the neurophysiological events in the receptors. Recently an attempt has been made by Ratliff and Hartline (1959) to investigate the effect of concomitant stimulation of several receptor units on the areal distribution of the intensity of the local response. In a set of experiments done on the compound eye of the Limulus they have found that neighboring receptors exert inhibition on each other. This inhibition is an inverse function of the distance between the receptors and a direct function of the intensity of the stimulation applied to the inhibiting receptor. Ratliff and Hartline inferred that a similar state of affairs holds for a sensory organ endowed with more complex connections between the receptors, such as the vertebrate eye. For example, if a limited area of the eye is under uniform illumination, the amount of excitation in this area will not be uniform. The inner receptors in the area will be subjected to more inhibition than the peripheral ones due to the geometrical arrangement. Thus the effect of the stimulation will be maximal in periphery of the area, accounting for the apparent brightness of the edges. Moreover, since the inhibition in the surround is maximal around the edges, the resulting contrast will be enhanced ever further at this point, contributing to the apparent brightness of the edge...

Parametric aspects of tactile resolution

“…The inhibition is inversely related to the distance between the receptors, while also being directly related to the intensity of excitation of the inhibiting receptor (Ratliff & Hartline, 1959). The model of lateral inhibition was used by one of us (Carmon, 1968) to investigate the effect of stimulus configuration on spatiotactile resolution. According to the model, dispersion of a constant amount of intensity over more receptors will result in a decrease of the mutual inhibition within the stimulated area, and in an increase of the stimulation gradient between the stimulated areas and the gap between them.…”

“…This lack was encountered during the course of other studies in this laboratory (Carmon, 1968), which utilized the tactile resolution function in investigating the spatial interaction of tactile inhibition and excitation. Hence, in order to establish some necessary parametric data with regard to spatial resolution in the somatosensory system, a series of studies on the influence of some stimulus variables on the two-point threshold was undertaken, the results of which are reported here.…”