The Advantages of a Tapered Whisker

The role of facial vibrissae (whiskers) in the behavior of terrestrial mammals is principally as a supplement or substitute for short-distance vision. Each whisker in the array functions as a mechanical transducer, conveying forces applied along the shaft to mechanoreceptors in the follicle at the whisker base. Subsequent processing of mechanoreceptor output in the trigeminal nucleus and somatosensory cortex allows high accuracy discriminations of object distance, direction, and surface texture. The whiskers of terrestrial mammals are tapered and approximately circular in cross section. We characterize the taper of whiskers in nine mammal species, measure the mechanical deflection of isolated felid whiskers, and discuss the mechanics of a single whisker under static and oscillatory deflections. We argue that a tapered whisker provides some advantages for tactile perception (as compared to a hypothetical untapered whisker), and that this may explain why the taper has been preserved during the evolution of terrestrial mammals

728-1 Determination of Stenosis Length by Magnetic Resonance Coronary Angiography

Magnetic resonance coronary angiography (MRA) can identify significant stenoses (>50%) as regions with reduced signal intensity due to disturbed intraluminal flow. To determine whether MRA stenosis length reflects true lesion length by x-ray angiography (XA). 12 patients (10 male and 2 female: age 65±8 years) underwent both MRA and XA with an average of 2.1±1.7 days between procedures. MRA was performed with the patient prone on an elliptical spine coil using a fat suppressed. TurboFlash, breath hold, segmented k-space sequence during late diastole. MRA defects were quantified off-line by manual tracing of digital images. XA stenoses were analyzed from 35 mm cine films using an electronic caliper which reported defect length, stenosis diameter and area. Matched MRA-XA stenoses included 5 in the left anterior descending (LAD), 8 right coronary artery (RCA) and 2 left circumflex (LCX). MRA and XA results were highly correlated (r=0.96). MRA reported a slightly longer stenoses length 9.65±2.0 mm versus 8.32±2.1 mm for XA (p<0.05 paired t-test). The ratio (MRA stenosis length/XA stenosis length) was not dependent on stenosis severity, absolute stenosis lumen diameter or area (MANOVA). Thus, in addition to detecting the presence of coronary lesions. MRA allows quantification of stenosis length

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Towards wave extraction in numerical relativity: the quasi-Kinnersley frame

The Newman-Penrose formalism may be used in numerical relativity to extract coordinate-invariant information about gravitational radiation emitted in strong-field dynamical scenarios. The main challenge in doing so is to identify a null tetrad appropriately adapted to the simulated geometry such that Newman-Penrose quantities computed relative to it have an invariant physical meaning. In black hole perturbation theory, the Teukolsky formalism uses such adapted tetrads, those which differ only perturbatively from the background Kinnersley tetrad. At late times, numerical simulations of astrophysical processes producing isolated black holes ought to admit descriptions in the Teukolsky formalism. However, adapted tetrads in this context must be identified using only the numerically computed metric, since no background Kerr geometry is known a priori. To do this, this paper introduces the notion of a quasi-Kinnersley frame. This frame, when space-time is perturbatively close to Kerr, approximates the background Kinnersley frame. However, it remains calculable much more generally, in space-times non-perturbatively different from Kerr. We give an explicit solution for the tetrad transformation which is required in order to find this frame in a general space-time.Comment: 13 pages, 3 figure