Temporal organization of multi-whisker contact in rats

Previous work has established that during exploration and discrimination, rats move their whiskers at frequencies between 6 and 12 Hz and that whisking frequency changes during contact. One critical component of any tactile system is contact. In the rat whisker system, such contacts may involve one or more vibrissa in the whisker array and contact duration of each whisker may vary over a considerable range, depending upon the behavioral context. However, little is known about the variables controlling contact duration or about the temporal relationships among contacts by adjacent whiskers. To address these issues head fixed rats were trained to touch a piezo-contact-sensor with the shaft of their whiskers (Bermejo and Zeigler, Somatosens Mot Res 17: 373-377, 2000). During the task, whisker movements and contacts were monitored with a high-speed camera at 500 frames/s and stored on videotape. To facilitate analysis, animals had their whiskers selectively trimmed. Data are reported from animals with C1 & C2, D1 & D2, or Arc2 (E2, D2, C2, B2) whiskers intact. For both row and arc animals, when just a single whisker touched the sensor the duration of contact was significantly shorter than when multiple whiskers made contact. When multiple whiskers made contact, onset was rarely simultaneous. Furthermore, in row-intact animals, contact progressed in an orderly fashion such that the rostral whisker in a row made contact first followed 24 ms (SE = 1.9 ms) later by the caudal whisker. When contact reversed the caudal whisker lifted off first, followed by the rostral whisker. Thus, the order in which whiskers touch an object regulates contact duration: the first whisker to touch the sensor stays in contact longer than any other whisker. The temporal discharge properties of neurons in the trigeminal system are expected to reflect position of whiskers on the nose.