Gradient of Tactile Properties in the Rat Whisker Pad

The array of vibrissae on a rats face is the first stage in a high-resolution tactile sensing system. Progressing from rostral to caudal in any vibrissae row results in an increase in whisker length and thickness. This in turn may provide a systematic map of separate tactile channels governed by the mechanical properties of the whiskers. To examine whether this map is expressed in a location-dependent transformation of tactile signals into whisker vibrations and neuronal responses, we monitored whiskers movements across various surfaces and edges. We found a robust rostral-caudal gradient of tactile information transmission in which rostral shorter vibrissae displayed a higher sensitivity and a bigger differences in response to the different textures, whereas longer caudal vibrissae were less sensitive to the different textures. Nonetheless, we found that texture identity is not represented spatially across the whisker pad. Based on the responses of first-order sensory neurons, we found that neurons innervating rostral whiskers were better suited for textures discrimination, whereas neurons innervating caudal whiskers were more suited for edge detection. To examine the functional role of this organization, we monitored the whisking activity of awake rats foraging for food. We found a caudal-rostral gradient of whisking angle movements in which the longer caudal vibrissae spanned a larger space making them more suitable for object localization, whereas the rostral shorter vibrissae hardly moved. These results suggest that the whisker array in rodents forms a sensory structure in which different facets of tactile information are transmitted through location-dependent gradient of vibrissae on the rats face.