RESUMEN

Multiple hominin species used and produced stone tools, and the archaeological record provides evidence that stone tool behaviors intensified among later members of the genus Homo. This intensification is widely thought to be the product of cognitive and anatomical adaptations that enabled later Homo taxa to produce stone tools more efficiently relative to earlier hominin species. This study builds upon recent investigations of the knapping motions of modern humans to test whether aspects of our upper limb anatomy contribute to accuracy and/or efficiency. Knapping kinematics were captured from eight experienced knappers using a Vicon motion capture system. Each subject produced a series of Oldowan bifacial choppers under two conditions: with normal wrist mobility and while wearing a brace that reduced wrist extension (∼30°-35°), simulating one aspect of the likely primitive hominin condition. Under normal conditions, subjects employed a variant of the proximal-to-distal joint sequence common to throwing activities: subjects initiated down-swing upper limb motion at the shoulder and proceeded distally, increasing peak linear and angular velocities from the shoulder to the elbow to the wrist. At the wrist, subjects utilized the 'dart-thrower's arc,' the most stable plane of radiocarpal motion, during which wrist extension is coupled with radial deviation and flexion with ulnar deviation. With an unrestrained wrist, subjects achieved significantly greater target accuracy, wrist angular velocities, and hand linear velocities compared with the braced condition. Additionally, the modern wrist's ability to reach high degrees of extension (≥28.5°) following strike may decrease risk of carpal and ligamentous damage caused by hyperextension. These results suggest that wrist extension in humans contributes significantly to stone tool-making performance.

Asunto(s)

RESUMEN

We report measurements of nonlinear magneto-optical rotation (NMOR) for the D2 line of 87Rb atoms in an antirelaxation-coated vapor cell in the presence of a radio-frequency (rf) field. The experimental NMOR signals as a function of rf field frequency for various rf field powers are compared to a theoretical model based on the density-matrix formalism. The comparison between experiment and theory enables understanding of the ground-state atomic spin polarization dynamics, illustrated using plots of the probability distribution of the atomic angular momentum.

Asunto(s)

RESUMEN

Past studies have hypothesized that aspects of hominin upper limb morphology are linked to the ability to produce stone tools. However, we lack the data on upper limb motions needed to evaluate the biomechanical context of stone tool production. This study seeks to better understand the biomechanics of stone tool-making by investigating upper limb joint kinematics, focusing on the role of the wrist joint, during simple flake production. We test the hypotheses, based on studies of other upper limb activities (e.g., throwing), that upper limb movements will occur in a proximal-to-distal sequence, culminating in rapid wrist flexion just prior to strike. Data were captured from four amateur knappers during simple flake production using a VICON motion analysis system (50 Hz). Results show that subjects utilized a proximal-to-distal joint sequence and disassociated the shoulder joint from the elbow and wrist joints, suggesting a shared strategy employed in other contexts (e.g., throwing) to increase target accuracy. The knapping strategy included moving the wrist into peak extension (subject peak grand mean = 47.3 degrees) at the beginning of the downswing phase, which facilitated rapid wrist flexion and accelerated the hammerstone toward the nodule. This sequence resulted in the production of significantly more mechanical work, and therefore greater strike forces, than would otherwise be produced. Together these results represent a strategy for increasing knapping efficiency in Homo sapiens and point to aspects of skeletal anatomy that might be examined to assess potential knapping ability and efficiency in fossil hominin taxa.

Asunto(s)

RESUMEN

We show that a system characterized by long-lived coherent population oscillations (CPOs), such as a two-level system that decays via a shelving state, can be used to construct a spatial optical memory. In the presence of a cw control field, a field is generated at the four-wave mixing (FWM) frequency with the same spatial profile as the initial probe. When the control field is switched off, these spatial profiles are encoded in the CPOs of the ground and storage states. When the control field is switched on again, the probe and FWM fields are retrieved simultaneously with the same spatial profiles as the initial probe.

RESUMEN

We show analytically and numerically that the application, during the storage time, of an additional weak field connecting the lower levels (LLs) of a Lambda system can modify the spin coherence and the amplitude of the retrieved probe without causing distortion. The LL coupling field adds an additional phase-dependent term to the probe propagation equation. The retrieved probe is amplified when the total relative phase Phi=0 or pi and decreased when Phi=pi/2.

RESUMEN

We show analytically and numerically that a double-Lambda system, characterized by parametric amplification of cross-coupled probe and four-wave mixing pulses, is an excellent medium for producing both slow and stored light. Moreover, this system can compensate for the absorption and broadening processes that usually occur without destroying the coherent properties of the medium. The stored probe signal pulse is written by means of a simple Lambda system and is retrieved and amplified, at the same frequency, by a double-Lambda system that also produces a pulse at the four-wave mixing frequency.

Asunto(s)

RESUMEN

This study describes a human foot bone assemblage from prehistoric Mangaia, Cook Islands in the context of diaphyseal cross-sectional strength measures. We use this sample to test the hypothesis that habitually unshod individuals who walk over rugged terrain will have stronger foot bones than a sample of habitually shod industrialized people. Specifically, we examine whether the Mangaian sample has a stronger size-adjusted metatarsal (MT) and phalangeal cross-sectional properties than the industrial sample, drawn from the Terry Collection. Contrary to expectations, residual analyses showed that most values of cross-sectional area (CA) and torsional resistance (J) of MTs 1-4 and the hallucal proximal phalanx (HPP) of the Mangaians are among those in the lower range of the Terry Collection sample. However, the bending strength ratios (Zy/Zx) of the Mangaian HPP are significantly greater than those of the Terry Collection. While characteristics such as forefoot shape variation between the sexes and among geographic populations cannot be ruled out as influential factors, cross-sectional properties of the hallucal proximal phalanges, but not the MTs, indicate terrain complexity in prehistoric populations.

Asunto(s)

RESUMEN

The probe absorption spectra in single and multiple tripod systems formed when a weak sigma polarized pump and a tunable pi polarized probe interact with a Zeeman split F(g)-->F(e)=F(g)-1 atomic transition are characterized by two interfering stimulated Raman features separated by an electromagnetically induced absorption (EIA) peak at the line center. These Raman features can appear as either sharp stimulated emission peaks or electromagnetically induced transparency windows. In the multitripod systems, the EIA and stimulated emission peaks derive from the combined effects of interference between the stimulated Raman features and transfer of coherence from the excited to ground states.

RESUMEN

The optical properties, absorption (mua) and reduced scattering coefficient (mu's), of ex vivo human myometrium and leiomyoma (fibroid) have been determined by the Monte Carlo inversion technique over the wavelength range 600-1000 nm. This region is currently of interest for new, minimal-access, surgical laser procedures such as photodynamic therapy (PDT) for abnormalities of the uterus, and interstitial laser photocoagulation (ILP) for the thermal ablation of fibroids. In the region 630-675 nm (corresponding to PDT), the optical coefficients of myometrium are mua = 0.041+/-0.012 mm(-1) and mu's = 1.37+/-0.19 mm(-1). For the wavelength range 800-1000 nm (associated with infrared lasers for ILP), the optical coefficients of fibroid were found to be mua = 0.020+/-0.003 mm(-1) and mu's = 0.56+/-0.03 mm(-1). Overall, the optical properties of fibroid were found to be lower than myometrium, and this was attributed to the differences in both anatomy and vascularity. The results show that PDT for ablation of the uterine endometrium is most unlikely to affect any tissues beyond the myometrium, and that the region around 800 nm is the most effective for ablation of fibroids using ILP as the penetration depth of light is greatest at this wavelength.

Asunto(s)

RESUMEN

In investigations into the manner in which people store spatial information, experimental subjects are often asked to estimate the distances and bearings between various landmarks. Methodology is described for constructing the mental map underlying a subject's matrix of distances, and the mental map underlying a subject's matrix of bearings. Also presented are methods of comparing matrices of distances or bearings, and the mental maps constructed from them, with the geographical map; this enables an assessment of differences between them. Some statistical models of disorientation are considered, and a method of choosing between them is described.

Asunto(s)

RESUMEN

Pump-probe spectroscopy with copropagating pump and probe beams of an open two-level system where, as a result of inelastic collisions, the lower state of an optical transition relaxes faster to reservoir states than the upper state does, may yield an antihole in the probe spectral profile whose linewidth is subnatural and unaffected by Doppler broadening.

RESUMEN

In a gaseous system, energy can be transferred from an intense incoming beam to a counterpropagating probe beam at the same frequency. The effect may be interpreted in terms of Doppleron processes and arises because the medium undergoes interaction with beams of different effective frequencies owing to the Doppler effect.

RESUMEN

The polarization properties of stimulated three-photon scattering in a three-level atomic system are presented. In our investigation of the stimulated emission spectrum of Na, we find that interference between the amplitudes of the resonant contributions of the Na doublet states results in interesting polarization phenomena. Particularly striking is the behavior when the laser is tuned near the dispersion-free point. With the laser linearly polarized the stimulated three-photon scattering is polarized perpendicular to the laser polarization. For circularly polarized excitation, total extinction of three-photon scattering is observed.

RESUMEN

By using the Schenzle-Brewer model, in which the ground state of a two-level system also has a finite lifetime, it is shown that two pressure-induced extraresonant degenerate four-wave mixing signals are obtained. The first extra resonance is similar to that predicted and observed by Bogdan et al. [Opt. Lett. 6, 82, 348 (1981)]. It is characterized by a linewidth equal to the inverse lifetime of the upper state and appears only in the presence of collisional redistribution. The second extra resonance is characterized by a linewidth equal to the inverse lifetime of the ground state and does not depend on collisional redistribution for its appearance.

Asunto(s)

RESUMEN

A survey of the sexual attitudes and knowledge of general practitioners in Wessex found that GP trainees and those in practice for less than 10 years were less conservative and better informed than doctors in practice for 20 years or more, The results suggest that the attitudes of the doctors are determined by their early environmental influences rather than their clinical experience.

Asunto(s)

RESUMEN

Erythropoietic activity produced by exogenous erythropoietin (Ep) is markedly reduced in Swiss mice rendered uremic by bilateral nephrectomy or bilateral ureteral ligation. Exogenous Ep causes a greater increase in erythropoiesis in germfree Swiss mice as compared with non-germfree mice, but the germfree mice show a more drastic reduction of erythropoiesis in uremia. Heme production in blood, spleen, and femoral marrow retains its typical pattern in uremia; there is no shift of erythropoiesis among the three sites. Production of endogenous Ep in uremic mice is less by a factor of 2-3 than that in intact mice whereas the reduction is by a factor of almost 100 in anephric mice. The data suggest that the anemia associated with uremia is the result of two phenomena: (1) a decreased production of Ep and (2) a diminished erythropoietic response to Ep.

Asunto(s)