Scale-Invariance and the Strong Coupling Problem

The effective theory of adiabatic fluctuations around arbitrary Friedmann-Robertson-Walker backgrounds - both expanding and contracting - allows for more than one way to obtain scale-invariant two-point correlations. However, as we show in this paper, it is challenging to produce scale-invariant fluctuations that are weakly coupled over the range of wavelengths accessible to cosmological observations. In particular, requiring the background to be a dynamical attractor, the curvature fluctuations are scale-invariant and weakly coupled for at least 10 e-folds only if the background is close to de Sitter space. In this case, the time-translation invariance of the background guarantees time-independent n-point functions. For non-attractor solutions, any predictions depend on assumptions about the evolution of the background even when the perturbations are outside of the horizon. For the simplest such scenario we identify the regions of the parameter space that avoid both classical and quantum mechanical strong coupling problems. Finally, we present extensions of our results to backgrounds in which higher-derivative terms play a significant role.Comment: 17 pages + appendices, 3 figures; v2: typos fixe

Validity of Robot-based Assessments of Upper Extremity Function

Objective To examine the validity of 5 robot-based assessments of arm motor function post-stroke. Design Cross sectional. Setting Outpatient clinical research center. Participants Volunteer sample of 40 participants, age \u3e18 years, 3-6 months post-stroke, with arm motor deficits that had plateaued. Intervention None. Main Outcome Measures Clinical standards included the Fugl-Meyer Arm Motor Scale (FMA), and 5 secondary motor outcomes: hand/wrist subsection of the FMA; Action Research Arm Test (ART); Box & Blocks test (B/B); hand subscale of Stroke Impact Scale-2 (SIS); and the Barthel Index (BI). Robot-based assessments included: wrist targeting; finger targeting; finger movement speed; reaction time; and a robotic version of the (B/B) test. Anatomical measures included percentage injury to the corticospinal tract (CST) and primary motor cortex (M1, hand region) obtained from MRI . Results Subjects had moderate-severe impairment (arm FMA scores = 35.6±14.4, range 13.5-60). Performance on the robot-based tests, including speed (r=0.82, p\u3c0.0001), wrist targeting (r=0.72, p\u3c0.0001), and finger targeting (r=0.67, p\u3c0.0001) correlated significantly with the FMA scores. Wrist targeting (r=0.57 - 0.82) and finger targeting (r=0.49 - 0.68) correlated significantly with all 5 secondary motor outcomes and with percent CST injury. The robotic version of the B/B correlated significantly with the clinical B/B test but was less prone to floor effect. Robot-based assessments were comparable to FMA score in relation to percent CST injury and superior in relation to M1 hand injury. Conclusions The current findings support using a battery of robot-based methods for assessing the upper extremity motor function in subjects with chronic stroke