In active sensation, sensory information is acquired via movements of sensory organs;
rats move their whiskers repetitively to scan the environment, thus detecting, localizing,
and identifying objects. Sensory information, in turn, affects future motor movements.
How this motor-sensory-motor functional loop is implemented across anatomical loops of the
whisker system is not yet known. While inducing artificial whisking in anesthetized rats,
we recorded the activity of individual neurons from three thalamic nuclei of the whisker system,
each belonging to a different major afferent pathway: paralemniscal, extralemniscal
(a recently discovered pathway), or lemniscal. We found that different sensory signals related
to active touch are conveyed separately via the thalamus by these three parallel afferent
pathways. The paralemniscal pathway conveys sensor motion (whisking) signals, the extralemniscal
conveys contact (touch) signals, and the lemniscal pathway conveys combined whisking-touch
signals. This functional segregation of anatomical pathways raises the possibility that
different sensory-motor processes, such as those related to motion control, object localization,
and object identification, are implemented along different motor-sensory-motor loops.