Afferent fibers from mystacial vibrissae of cats and seals

1. Studies of vibrissae of cats and seals revealed minor species-specific differences in functional characteristics of the afferent fibers: vibrissal fibers from the seal showed a significantly higher rate (P smaller than 0.05) and frequency of occurrence (P smaller than 0.01) of spontaneous activity, and a significantly greater percentage (P smaller than 0.01) responded to vibrations greater than 256 Hz. None of these differences could be interpreted as an adaptation to the aquatic environment. 2. Of 455 afferent fibers obtained from the infraorbital nerve, a large proportion (66% in cats and 85% in seals) served vibrissae. A population study of 357 such fibers showed that two-thirds were rapidly adapting (RA) and the remainder were slowly adapting (SA). Neither group displayed preferential directional sensitivity, but for 75% of the individual fibers in cats and 71% in seals, a particular direction elicited maximal response. 3. On the basis of their tuning curves, the RA fibers were divisible into three major groups: insensitive, sensitive, and those with frequency-dependent tuning curves. Single RA fibers provided little information about vibratory stimulus amplitude. However, it is postulated that the range of thresholds in the RA fiber group within each follicle represents a mechanism of encoding the intensity of vibratory stimuli. 4. By contrast, single SA fibers encoded precisely the intensity of steady stimuli. The average fiber conveyed 2.7 bits of information about the magnitude of vibrissal deflections of smaller than 17 degrees (1.5 mm, at 5 mm from skin). 5. Each type of fiber abstracted one or more features from the environment, and conveyed these abstractions by its individual pattern of neural activity. It is postulated that vibrissae provide fine textural information about surfaces.