Performance and polarization response of slit homogenizers for the GeoCarb mission

The observing strategy of the Geostationary Carbon Observatory (GeoCarb), which is a “step and stare” approach, can lead to distortions in the instrument spectral response function (ISRF) when there are gradients in brightness across instrument field of view. These distortions induce errors in the retrieved trace gases. In order to minimize these errors, the GeoCarb instrument design was modified to include a “slit homogenizer” whose purpose is to scramble the pattern of the incoming light and effectively remove the ISRF distortions caused by the variations in illumination across the slit. As a risk reduction, GeoCarb procured six different homogenizers and had them tested for performance in a benchtop optical system. The major finding is that the homogenizer performance depends strongly on the polarization of the incoming light, with the sensitivity growing as a function of wavelength. The width of the ISRF is substantially smaller when the light is vertically polarized (orthogonal to the slit length) compared to horizontally polarized (parallel to the slit length), and the throughput is accordingly reduced. These effects are due to the effects of the gold coating and high incidence angles present in the GeoCarb homogenizer design, which was verified using a polarization-dependent model generalized from previous homogenizer modeling work. The results strongly recommend controlling the polarization of the light entering a similar implementation using a polarizer, depolarizer, or polarization scrambler for other instruments attempting to mitigate scene illumination non-uniformity effects, as well as a robust characterization of the polarization sensitivity of all key subsystems.</p

The role of ipsilateral motor areas in hand motor function in healthy ageing – insights from online TMS

EPV

The role of contralesional motor areas in early motor recovery – evidence from event-related (“online”) TMS

BackgroundTMS studies examining the role of the unaffected hemisphere in motor function after stroke have mainly focused on the contralesional primary motor cortex (M1), revealing both supporting (Lotze et al., 2006 and Rehme et al., 2011) as well as disturbing (Vollmer et al., 2015 and Nowak et al., 2008) influence for recovery of function. However, the relevance of other contralesional motor areas for paretic hand function has rarely been examined, especially with respect to motor recovery in the first days and weeks after stroke. Therefore, we tested the effects of online-TMS applied to contralesional primary motor cortex, anterior intraparietal sulcus (aIPS) and dorsal premotor cortex (dPMC) while stroke patients performed different motor tasks with their paretic hand.MethodsPatients with first-ever ischemic left hemispheric stroke performed motor tasks of different complexities with their paretic right hand while being stimulated with ‘online TMS’: (i) index finger tapping, (ii) hand tapping, (iii) visuomotor pointing task (pointing back and forth between two defined targets) and (iv) simple reaction time task. Performance was measured using a three-dimensional motion analyzer system (Zebris). Patients were measured in the acute stage after stroke.ResultsCompared to healthy controls, acute stroke patients showed reduced finger tapping amplitudes during TMS interference with ipsilateral (=contralesional) aIPS. Likewise, reaction times deteriorated upon contralesional aIPS stimulation in stroke patients but not in controls. Finally, accuracy in the 3D pointing task not only deteriorated upon aIPS stimulation (like in controls) but in addition also during interference with contralesional M1, which was exclusively found in the stroke patients group.ConclusionWe here found evidence that especially anterior intraparietal cortex –an area critically engaged in hand motor function by means of parieto-frontal loops– has a relevant influence on motor performance of the paretic hand early after stroke. Therefore, aIPS might constitute an interesting stimulation target for excitability enhancing brain stimulation protocols in order to support recovery of function after stroke

Interhemispheric Structural Connectivity Underlies Motor Recovery after Stroke

Objective: Although ample evidence highlights that the ipsilesional corticospinal tract (CST) plays a crucial role in motor recovery after stroke, studies on cortico-cortical motor connections remain scarce and provide inconclusive results. Given their unique potential to serve as structural reserve enabling motor network reorganization, the question arises whether cortico-cortical connections may facilitate motor control depending on CST damage.Methods: Diffusion spectrum imaging (DSI) and a novel compartment-wise analysis approach were used to quantify structural connectivity between bilateral cortical core motor regions in chronic stroke patients. Basal and complex motor control were differentially assessed.Results: Both basal and complex motor performance were correlated with structural connectivity between bilateral premotor areas and ipsilesional primary motor cortex (M1) as well as interhemispheric M1 to M1 connectivity. Whereas complex motor skills depended on CST integrity, a strong association between M1 to M1 connectivity and basal motor control was observed independent of CST integrity especially in patients who underwent substantial motor recovery. Harnessing the informational wealth of cortico-cortical connectivity facilitated the explanation of both basal and complex motor control.Interpretation: We demonstrate for the first time that distinct aspects of cortical structural reserve enable basal and complex motor control after stroke. In particular, recovery of basal motor control may be supported via an alternative route through contralesional M1 and non-crossing fibers of the contralesional CST. Our findings help to explain previous conflicting interpretations regarding the functional role of the contralesional M1 and highlight the potential of cortico-cortical structural connectivity as a future biomarker for motor recovery post-stroke. ANN NEUROL 2023

Recovery from apraxic deficits and its neural correlate

Background and Objective: Apraxia is a deficit of motor cognition leading to difficulties in actual tool use, imitation of gestures, and pantomiming object use. To date, little data exist regarding the recovery from apraxic deficits after stroke, and no statistical lesion mapping study investigated the neural correlate of recovery from apraxia. Accordingly, we here examined recovery from apraxic deficits, differential associations of apraxia task (imitation vs. pantomime) and effector (bucco-facial vs. limb apraxia) with recovery, and the underlying neural correlates. Methods: We assessed apraxia in 39 patients with left hemisphere (LH) stroke both at admission and approximately 11 days later. Furthermore, we collected clinical imaging data to identify brain regions associated with recovery from apraxic deficits using voxel-based lesion-symptom mapping (VLSM). Results: Between the two assessments, a significant recovery from apraxic deficits was observed with a tendency of enhanced recovery of limb compared to bucco-facial apraxia. VLSM analyses revealed that within the lesion pattern initially associated with apraxia, lesions of the left insula were associated with remission of apraxic deficits, whereas lesions to the (inferior) parietal lobe (IPL; supramarginal and angular gyrus) and the superior longitudinal fasciculus (SLF) were associated with persistent apraxic deficits. Conclusions: Data suggest that lesions affecting the core regions (and white matter) of the fronto-parietal praxis network cause more persistent apraxic deficits than lesions affecting other regions (here: the left insula) that also contribute to motor cognition and apraxic deficits