This movement co-varies with small changes in elevation. The largest spatial displacement of the vibrissae tips is due to azimuthal rotation, which moves the vibrissae back and forth along the longitudinal axis. Vibrissae movements relative to the head are described by three angles of rotation (azimuth, \(\theta\) elevation, \(\phi\) and torsion, \(\zeta\)) and translation of the vibrissa base (horizontal and vertical Fig. Vibrissae positions relative to the head are typically measured by high-speed videography (Knutsen et al., 2005), in conjunction with head and body movements (Mitchinson et al., 2007) or while an animal has been partially immobilized (Bermejo et al., 1998). Vibrissae movements are produced by contractions of facial musculature, head movements and locomotion. 5 Neural representations of whisking kinematics. 1.3 Comparisons with saccadic eye movements.1.1 Kinematic profile of the whisk cycle.Whisking has been studied primarily in rats, mice and shrews. Following contact, motor control of the vibrissae is modulated on several time scales as the animal approaches an object with head, nose and micro-vibrissae. Whisking is coordinated with head and body movements, which enables rapid sampling of the proximal environment during spatial exploration. Non-rhythmic vibrissae movements also serve many behavioral processes, such as social interactions (Wolfe et al., 2011) and discrimination of lateral gaps (Krupa et al., 2001). The primary functions of whisking are spatial search and tactile exploration of objects and surfaces (see Vibrissal Behavior and Function). Whisking refers to a behavioral process, whereby motile facial vibrissae are repeatedly and rhythmically moved back and forth in order to sample the proximal environment ( Fig. Middle: Vibrissa angular velocity (\(\theta'\)). Top: Vibrissa angle (azimuth, \(\theta\)). The azimuth (\(\theta\)), elevation (\(\phi\)) and torsion (\(\zeta\)) angles are measured with respect to the skull. The shape and position of the vibrissa at the protraction set-point (green lines) is projected down and back onto the transverse (light red) and coronal (light orange) planes, respectively. After a protraction, the follicle is displaced a distance \(x\) anterior and \(z\) dorsal with respect to the skull (green dot). The origin is arbitrarily set to the retraction set-point of the follicle in the previous whisk cycle (blue dot). \(X\) indicate direction of nose (anterior), \(Y\) points lateral and \(Z\) upwards (dorsal). Conventions for representing vibrissa position, translation and rotation in head-centered Cartesian coordinates. Dotted lines indicate the vibrissa azimuth, estimated as the tangent at the base of the shaft. Blue and green lines display the vibrissa at retraction and protraction set-points respectively. Representation of the vibrissa azimuth angle (\(\theta\)), as seen from a top-down perspective. Data was obtained from a head restrained rat, and is adapted from (Pietr et al., 2010) with permission. The angle envelope (gray area) is indicated by the retraction (blue) and protraction (green) set-points (dotted lines) showing that the vibrissa position fluctuates on a slower timescale than the protraction (up) and retraction (down) whisk cycles. Solid black line is the time-varying azimuth angle of the C2 vibrissa relative to the head.
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