Coding of stimulus location and intensity in populations of mechanosensitive nerve fibers of the raccoon: II. Across-fiber response patterns

Abstract

The major aim of this study was to determine whether information about stimulus location and intensity can be precisely encoded in the population response profiles of mechanosensitive nerve fibers. Across-fiber patterns (AFPs) were reconstructed using data obtained previously (58) from 129 single median nerve fibers and 61 posterior tibial nerve fibers of the raccoon. Punctate mechanical stimuli of varying intensities were presented to six standard test sites on the glabrous skin of the forepaw and hindpaw, and the AFP produced by each unique stimulus was determined. Innervation density (ID) at each of the test sites was estimated from the integrated voltage of compound action potentials evoked by constant current stimulation of the skin at each site. Differences in ID between corresponding regions of the forepaw and hindpaw were estimated by counts of myelinated fibers and measurements of skin areas supplied by the median and tibial nerves, respectively. All AFPs were adjusted to take into account the differences in ID among the various skin sites on the two paws. The data indicated that different stimulus locations and intensities on each paw could be precisely and unambiguously encoded by AFPs; location was represented primarily by the shape of the AFP, whereas intensity was represented by the height of the AFP. The discriminability of location and intensity was shown to be directly related to the magnitude of the difference between the AFPs elicited by any two stimuli. Consistent with expectations, AFPs for stimuli delivered distally on each paw showed greater differences as a function of stimulus location and intensity than AFPs for stimuli applied proximally. Furthermore, AFPs produced by stimuli to the forepaw showed greater changes than AFPs for stimuli to comparable sites on the hindpaw. It was concluded that systematic differences in the population response profiles of median and tibial nerve fibers could account for regional variations in tactile acuity across each paw and differences between the two paws.