CW) Or Counterclockwise Research Articles

Wavelength-tunable quasi-unidirectional spatiotemporal mode-locked solitons in an isolator-free Er-doped fiber laser

In this letter, we have demonstrated the realization of the wavelength-tunable spatiotemporal mode-locked (STML) operation in an Er-doped partial multimode fiber laser without the intracavity isolator. The laser adopts an all-fiber structure and the saturable absorption effect is attributed to nonlinear multimode interference (NL-MMI) from the single mode fiber-graded index multimode fiber-single mode fiber (SMF-GIMF-SMF) design. The STML laser can operate in the quasi-unidirectional regime with an emission strength difference of larger than 30 dB for the two counter-propagating lasers. And the switching between the dominating operating directions in the clockwise (CW) or counter-clockwise (CCW) directions can be easily achieved by adjusting the MM polarization controller (PC). Both STML conventional solitons (CSs) and the STML soliton molecules are obtained with a similar tunable wavelength from ∼1.56 μm to ∼1.58 μm for the opposite operating directions. Furthermore, the spatial dynamics of the STML soliton are also investigated in detail. The experimental results are of great help for us to explore nonlinear spatiotemporal dynamics and provide potential applications in fields of all-optical signal processing, fiber sensing and information coding.

Microbial stir bars: Light-activated rotation of tethered bacterial cells to enhance mixing in stagnant fluids.

Microfluidics devices are gaining significant interest in biomedical applications. However, in a micron-scale device, reaction speed is often limited by the slow rate of diffusion of the reagents. Several active and passive micro-mixers have been fabricated to enhance mixing in microfluidic devices. Here, we demonstrate external control of mixing by rotating a rod-shaped bacterial cell. This rotation is driven by ion transit across the bacterial flagellar stator complex. We first measured the flow fields generated by rotating a single bacterial cell rotationally locked to rotate either clockwise (CW) or counterclockwise (CCW). Micro-particle image velocimetry (μPIV) and particle tracking velocimetry results showed that a bacterial cell of ∼ 2.75 μm long, rotating at 5.75 ± 0.39 Hz in a counterclockwise direction could generate distinct micro-vortices with circular flow fields with a mean velocity of 4.72 ± 1.67 μm/s and maximum velocity of 7.90 μm/s in aqueous solution. We verified our experimental data with a numerical simulation at matched flow conditions, which revealed vortices of similar dimensions and speed. We observed that the flow-field diminished with increasing z-height above the plane of the rotating cell. Lastly, we showed that we could activate and tune rotational mixing remotely using strains engineered with proteorhodopsin, where rotation could be activated by controlled external illumination using green laser light (561 nm).

Active foam: the adaptive mechanics of 2D air-liquid foam under cyclic inflation.

Foam is a canonical example of disordered soft matter where local force balance leads to the competition of many metastable configurations. We present an experimental and theoretical framework for "active foam" where an individual voxel inflates and deflates periodically. Local periodic activity leads to irreversible and reversible T1 transitions throughout the foam, eventually reaching a reversible limit cycle. Individual vertices displace outwards and subsequently return back to their approximate original radial position; this radial displacement follows an inverse law. Surprisingly, each return trajectory does not retrace its outbound path but encloses a finite area, with a clockwise (CW) or counterclockwise (CCW) direction, which we define as a local swirl. These swirls form coherent patterns spanning the scale of the material. Using a dynamical model, we demonstrate that swirl arises from disorder in the local micro-structure. We demonstrate that disorder and strain-rate control a crossover between cooperation and competition between swirls in adjacent vertices. Over 5-10 cycles, the region around the active voxel structurally adapts from a higher-energy metastable state to a lower-energy state, locally ordering and stiffening the structure. The coherent domains of CW/CCW swirl become smaller as the system stabilizes, indicative of a process similar to the Hall-Petch effect. Finally, we introduce a statistical model that evolves edge lengths with a set of rules to explore how this class of materials adapts as a function of initial structure. Adding activity to foam couples structural disorder and adaptive dynamics to encourage the development of a new class of abiotic, cellularized active matter.

Movement pattern and upper extremity muscle activation during fast and slow continuous steering movement

Continuous steering movement (CSM) is an essential component of the upper extremity (UE) task during vehicle driving, and could be a suitable candidate for multi-joint rehabilitation programs for patients with UE disabilities. This study aims to evaluate the UE muscle activation during CSM and how the rotating speed and direction affect CSM's kinematic and kinetic performance. Surface electromyography (EMG), hand contact information, and steering torque were measured under fast (180°/s) and slow (60°/s) constant-velocity CSM to reveal the activation of shoulder and elbow muscles, temporal characteristics, and force exertion during the stance and swing phases of a CSM cycle. Data from 24 normal young adults showed that shorter contact duration but higher force exertion occurred in the hand moving in an outward steering direction during only fast CSM in either the clockwise (CW) or counterclockwise (CCW) direction. During a steering cycle (either fast or slow speed), the triceps brachii, sternal part of the pectoralis major (PS), and posterior deltoid play major roles in generating steering torque in the CW direction of the CSM. In contrast, the PS, clavicular part of the pectoralis major (PC), and anterior deltoid (AD) largely contribute to torque generation during the CCW CSM. During the swing phase of CSM, AD, PC, and PS are the major muscles that move the hand for the next grasping of the steering wheel in all four conditions. Using the mean activation profiles of the major contributing muscles, the functional roles of these elbow and shoulder muscles were analyzed and are discussed herein. These findings help us to further understand the activation patterns of UE muscles and the kinematic and kinetic changes during two rotating directions and two speeds of CSM, and suggest important implications for future practice in clinical training.

Effects of Occlusal Plane Inclination on the Temporomandibular Joint Stress Distribution: A Three-Dimensional Finite Element Analysis.

Background Sudden changes in masticatory loads and occlusal conditions contribute to temporomandibular disorders. Clockwise (CW) or counterclockwise (CCW) rotation of the occlusal plane is one of the factors that alter the direction of the occlusal forces transmitted to the temporomandibular joint structures. Finite element analysis was used to identify possible regions of high stress in the temporomandibular joint. Materials and Methods A computer-aided design model of a symmetrical edentulous maxillomandibular bony complex with a temporomandibular joint was manually generated using Rhinoceros 4.0 freeform modeling software. Three-dimensional discrete mesh generation was performed using VRMesh Studio. The reference occlusal plane angle was accepted as 8° in the sagittal plane, and by modifying 4° in the CW and CCW directions, CW and CCW models were obtained, respectively. The present study aimed to evaluate the changes in stress distribution in the condylar cartilage and temporomandibular disc using the von Mises and maximum-minimum principal stress evaluations of three different occlusal plane inclinations. The null hypothesis of this three-dimensional finite element analysis was that “occlusal plane inclination does not change the stress distribution on the temporomandibular joint structures.” Results The compressive stress on the condyle increased with CW rotation of the occlusal plane. The von Mises equivalent stress of the temporomandibular disc shifted to the medial, posterior, and superior directions after CW and CCW rotations of the occlusal plane. The CW rotation of the occlusal plane increased the von Mises equivalent.

Differences in behavioral and cortical indices in pianists and non-musicians during a non-musical motor planning task: An event-related potential study

Psychological studies have shown that music has an impact on human cognitive function. We aimed to compare the performance and neural activity of pianists and non-musicians during a non-musical motor-planning task. In addition, we investigated the effect of task complexity on the characteristics of the behavioral and neural responses. The participants had to grasp a hexagonal knob with their right hand and rotate it 60° or 180° clockwise (CW) or counterclockwise (CCW). We examined the groups in terms of the amplitude of the P2 component in the event-related potential (at the neural level) and the planning time, grasping time, releasing time, and planning pattern for initial grip selection (at the behavioral level). At the behavioral level, we observed no significant difference between groups, while at the neural level; we found an interaction between direction and group indicating that pianists showed lower P2 amplitude in the CW directions. However, there was no significant difference between groups in the CCW direction. A significant main effect of rotation was revealed at both the neural and behavioral levels; increasing the rotation angle led to an increase in the planning time and the P2 amplitude, indicating a complexity effect. In conclusion, we observed that pianists had lower P2 amplitude in lateral movements than non-musicians; however, due to the lack of behavioral group differences, further research is warranted to support the far-transfer theory in this field.

Broadband tunable orbital angular momentum mode converter based on a conventional single-mode all-fiber configuration.

A broadband tunable orbital angular momentum (OAM) mode converter based on a helical long-period fiber grating (HLPFG) inscribed in a conventional single-mode fiber (SMF) is experimentally demonstrated. The proposed all-fiber OAM mode converter is based on the core-cladding mode dual resonance near the dispersion turning point (DTP). The converter can operate with a bandwidth of 303.9 nm @ -3 dB and 182.2 nm @ -10 dB, which is, as far as we know, the widest bandwidth for a conventional SMF. Furthermore, the bandwidth of the OAM mode can be dynamically tuned within a large dynamic range (>80 nm) by simply twisting the fiber clockwise (CW) or counterclockwise (CCW). The dynamic tunability of the bandwidth of the proposed OAM mode generator may find vital applications in large-capacity optical fiber communication systems.

Single-Line Bidirectional Optical Add/Drop Multiplexer for Ring Topology Optical Fiber Networks.

A new type of passive single-line bidirectional optical add/drop multiplexer (SBOADM) is proposed and experimentally demonstrated. When the proposed SBOADM is placed as a node of a ring topology optical fiber network, the special routing function of the SBOADM can always drop down the desired downstream signals whether the signals are injected into the SBOADM in either the clockwise (CW) or counterclockwise (CCW) direction and can upload and send back the upstream signals via the reversed optical pathway of the downstream signals. Once fiber link failure occurs in the optical network, the blocked network connections can be recovered immediately by sending out the downstream signals in both the CW and CCW directions of the fiber ring. As in all passive devices, the SBOADM needs no power supply or complicated network management to achieve the bidirectional function. Thus, the proposed device is an optimal solution to enhance the stability and reliability of rapidly developed optical fiber networks.

Control of magnetic vortex circulation in one-side-flat nanodisk pairs by in-plane magnetic filed

In a nanodisk made of soft ferromagnet, the magnetic vortex structure are highly stabilized, and the circulation directions of the vortices are naturally binary (either clockwise (CW) or counter-clockwise (CCW)), which can be associated with one bit of information, and thus the magnetic vortices have been of great interest recently. A vortex-circulation-based memory requires the perfect controllability of the circulation direction. From the circulation point of view, there are four possible ground states in a nanodisk pair: (CCW, CCW), (CCW, CW), (CW, CCW) and (CW, CW). In a perfect circular nanodisk, CW and CCW states are degenerate because of the high symmetry of the system. However, the circulation of the magnetic vortex is known to be controlled by introducing the asymmetry. It has been reported that the magnetic vortices with opposite (the same) circulations are realized in one-side-flat disk pair. That means in one-side-flat nanodisk pair only the control of two of these four ground states is possible, eg., (CCW, CW), (CW, CCW) or (CCW, CCW), (CW, CW). We found that the reversal of the magnetic vortex circulation is affected by the nanodisk thickness as well. By further introducing another asymmetry, different thickness, the control of the four circulation ground states is achieved in a nanodisk pair. In this work, the controllability of the four ground states in a nanodisk pair was numerically investigated via micromagnetic simulations. The results show that in a single one-side-flat nanodisk, there exists a preferred rotational sense at the remanent state after the nanodisk is saturated by the external magnetic field, applied parallel to the flat edge of the nanodisk. The shape anisotropy is the primary cause of this phenomenon. We further found that the obtained rotational senses of the magnetization in the vortex state in nanodisks with the same geometrical parameters but different thickness (20 nm and 50 nm) are opposite for the same direction of the externally applied field. This is attributed to the competition between the demagnetization field energy and the exchange energy during the vortex formation. The method we proposed provides a simple means of controlling the vortex state that can thus become a useful tool for designing vortex-based devices.

Relationship between subjective visual vertical and balance in individuals with multiple sclerosis.

Subjective visual vertical (SVV) deviations have been correlated to abnormal cerebellar function in individuals diagnosed with multiple sclerosis (MS). It has been shown that individuals with MS have increased incidence of SVV abnormalities, yet this is not routinely tested in this population during physical therapy evaluation. This study aims to determine if there is a relationship between SVV and balance performance in people with MS who have cerebellar involvement. We hypothesize that individuals with greater SVV deviations will have worse balance performance. Fifteen females and five males (mean age 54.5years [±7.03 SD]) with the diagnosis of MS and cerebellar involvement participated. Computerized SVV testing included rod and rod-and-frame conditions. None of the balance outcomes were correlated with the rod-only condition. Because there was a difference in magnitude of results within the rod-and-frame condition, based on whether the frame was rotated clockwise (CW) or counterclockwise (CCW), they were analysed independently. For all six of the balance outcomes, there was a statistically significant moderate correlation with SVV deviations when the frame was tilted CCW: Barthel Index (r=-0.47, p=0.018), Berg Balance Score (r=-0.59, p=0.003), gait velocity (r=-0.52, p=0.010), International Cooperative Ataxia Rating Scale (r=0.56, p=0.006), Scale for the Assessment and Rating of Ataxia (r=0.62, p=0.002), and Timed Up and Go (r=0.58, p=0.003). Interestingly, the Barthel Index was the only outcome that had statistical significance with a moderate correlation (r=-0.66, p=0.001) when the frame was rotated CW. In this cohort, greater deviations during the rod-and-frame condition of SVV testing correlated with worse functional outcomes, especially when the frame was tilted CCW. Individuals with MS who demonstrate decreased balance performance may rely more heavily on visual backgrounds. Implementation of SVV assessment for individuals with MS may provide clinicians with valuable information to identify clinical interventions.

Neural Determinants of Task Performance during Feature-Based Attention in Human Cortex.

Studies of feature-based attention have associated activity in a dorsal frontoparietal network with putative attentional priority signals. Yet, how this neural activity mediates attentional selection and whether it guides behavior are fundamental questions that require investigation. We reasoned that endogenous fluctuations in the quality of attentional priority should influence task performance. Human subjects detected a speed increment while viewing clockwise (CW) or counterclockwise (CCW) motion (baseline task) or while attending to either direction amid distracters (attention task). In an fMRI experiment, direction-specific neural pattern similarity between the baseline task and the attention task revealed a higher level of similarity for correct than incorrect trials in frontoparietal regions. Using transcranial magnetic stimulation (TMS), we disrupted posterior parietal cortex (PPC) and found a selective deficit in the attention task, but not in the baseline task, demonstrating the necessity of this cortical area during feature-based attention. These results reveal that frontoparietal areas maintain attentional priority that facilitates successful behavioral selection.

Neither Cholinergic Nor Dopaminergic Enhancement Improve Spatial Working Memory Precision in Humans.

Acetylcholine and dopamine are neurotransmitters that play multiple important roles in perception and cognition. Pharmacological cholinergic enhancement reduces excitatory receptive field size of neurons in marmoset primary visual cortex and sharpens the spatial tuning of visual perception and visual cortical fMRI responses in humans. Moreover, previous studies show that manipulation of cholinergic or dopaminergic signaling alters the spatial tuning of macaque prefrontal cortical neurons during the delay period of a spatial working memory (SWM) task and can improve SWM performance in macaque monkeys and human subjects. Here, we investigated the effects of systemic cholinergic and dopaminergic enhancement on the precision of SWM, as measured behaviorally in human subjects. Cholinergic transmission was increased by oral administration of 5 mg of the cholinesterase inhibitor donepezil, and dopaminergic signaling was enhanced with 100 mg levodopa/10 mg carbidopa. Each neurotransmitter system was separately investigated in double-blind placebo-controlled studies. On each trial of the SWM task, a square was presented for 150 ms at a random location along an invisible circle with a radius of 12 degrees of visual angle, followed by a 900 ms delay period with no stimulus shown on the screen. Then, the square was presented at new location, displaced in either a clockwise (CW) or counterclockwise (CCW) direction along the circle. Subjects used their memory of the location of the original square to report the direction of displacement. SWM precision was defined as the amount of displacement corresponding to 75% correct performance. We observed no significant effect on SWM precision for either donepezil or levodopa/carbidopa. There was also no significant effect on performance on the SWM task (percent correct across all trials) for either donepezil or levodopa/carbidopa. Thus, despite evidence that acetylcholine and dopamine regulate spatial tuning of individual neurons and can improve performance of SWM tasks, pharmacological enhancement of signaling of these neurotransmitters does not substantially affect a behavioral measure of the precision of SWM in humans.

Manipulation of magnetization in GaMnAs films by spin-orbit-induced magnetic fields

We have investigated the effect of spin-orbit-induced (SOI) magnetic fields on magnetization switching in GaMnAs films. The sign of such SOI fields depends on the direction of the current flowing in the film, thus providing a handle for electrically manipulating magnetization in ferromagnetic GaMnAs films. Specifically, when an applied magnetic field is swept along the current direction, magnetization reversal occurs via rotations in opposite sense (i.e., clockwise (CW) or counterclockwise (CCW)) depending on the sign of the current, thus leading to opposite signs of the planar Hall resistance (PHR) measured on the film. The effect of SOI fields also manifests itself through hysteretic behavior of PHR for two opposite currents as a fixed magnetic field is rotated in the film plane. The width of the resulting hysteresis between two current directions then allows us to estimate the magnitude of the SOI field at current density of 1.0 × 105 A/cm2 as ∼1.2 Oe in our GaMnAs film. Such switching of magnetization between two magnetic easy axes induced by switching the sign of an applied current provides a means of electronically controlling the value of film resistance (in this case of PHR), a process that can be exploited in spintronic devices.

Instabilities of a rotating helical rod in a viscous fluid.

Bacteria such as Vibrio alginolyticus swim through a fluid by utilizing the rotational motion of their helical flagellum driven by a rotary motor. The flagellar motor is embedded in the cell body and turns either clockwise (CW) or counterclockwise (CCW), which may lead to straight forward or backward swimming, or reorientation of the cell. In this paper we investigate the dynamics of the helical flagellum by adopting the Kirchhoff rod theory in which the flagellum is described as a space curve associated with orthonormal triads that measure the degree of bending and twisting of the rod. The hydrodynamic interaction with the flagellum is described by the regularized Stokes formulation. We focus on two different types of instabilities: (1) whirling instability of a rotating helical filament in the absence of a hook and (2) buckling instability of a flagellum in the presence of a compliant hook that links the flagellar filament to the rotary motor. Our simulation results show that the helical filament without a hook displays three regimes of dynamical motions: stable twirling, unstable whirling, and stable overwhirling motions depending on the physical parameters, such as rotational frequency and elastic properties of the flagellum. The helical filament with a hook experiences buckling instability when the motor switches the direction of rotation and the elastic properties of the hook change. Variations of physical parameter values of the hook such as the bending modulus and the length make an impact on the buckling angle, which may subsequently affect the reorientation of the cell.

Expectations and visual aftereffects.

The tilt aftereffect (TAE) is traditionally regarded as a consequence of orientation-selective sensory adaptation, a low-level stimulus-driven process. Adaptation has been recently suggested to be the outcome of predictive coding. Here, we tested whether the TAE is modulated by predictability, and specifically, whether TAE depends on the congruency of adapted and expected orientations. Observers were presented with successive pairs of oriented Gabor patches. Pairs were arranged in blocks, forming two conditions with the orientation of the second pair member either predictable or not. For all pairs, the orientation of the first Gabor was tilted clockwise (CW) or counterclockwise (CCW) (±20° relative to vertical, randomized). In the "Expected" conditions, the orientation of the second Gabor was fixed relative to the first Gabor (the same or a mirror orientation, blocked). In the "no-expectation" condition, the orientation of the second Gabor was independent of the first Gabor (randomized ±20°). Intermixed test pairs were used to measure observers' perceived vertical, with the second pair member serving as a target, oriented around the vertical, permitting an estimate of the TAE produced by the presentation of the first Gabor. Results show an increase in TAE with the expected orientation matching the inducing orientation, but a decrease with the expected mirror orientation, consistent with additivity of the adaptation and the expectation effects. A second experiment, with the first oriented Gabor replaced by a colored circular blob, showed that expectation alone does not modulate the perceived orientation. These findings indicate a role for expectation in generating the perceptual TAE and are in line with predictive coding models of perception. We suggest that orientation dependent adaptation is affected by both the mean orientation (first order statistics) and by temporal contingencies (second order statistics).

Selection of the Rotation Direction for a Camphor Disk Resulting from Chiral Asymmetry of a Water Chamber.

Self-motion of a camphor disk rotating inside a water chamber composed of two half-disks was investigated. The half-disks were joined along their diameter segments, and the distance between their midpoints (ds) was considered as the control parameter. Various types of camphor disk motions were observed depending on ds. When ds = 0, the chamber had a circular shape, so it was symmetric. A camphor disk showed either a clockwise (CW) or counterclockwise (CCW) rotation with the direction determined by its initial state. The symmetry of the chamber was broken for ds > 0. For moderate distances between the midpoints, a unidirectional orbital motion of the disk was observed. The preferred rotation direction was determined by the shape of the chamber, and it did not depend on the initial rotation direction. For yet larger ds, the unidirectional circular motion was no longer observed and the trajectory became irregular. A mathematical model coupling the camphor disk motion with the dynamics of the developed camphor molecular layer on water was constructed, and the numerical results were compared with the experimental results. The selection of motion type can be explained by considering the influence of camphor concentration on the disk trajectory through the surface tension gradient.

The decision, not the decision task, causes repulsive biases away from the decision boundary

The perceived direction of a random-dot kinematogram in a fine discrimination task is systematically biased away from the decision boundary. Stimulus uncertainty combined with a neural read-out that is optimized for that specific discrimination task was thought to cause these biases (Jazayeri and Movshon, 2007). Based on this explanation, we would expect the biases to disappear if subjects did not have to perform the discrimination task. We conducted two experiments to test this prediction. Experiment 1 (control) was aimed to reproduce the effects of the original study, using stimuli that consisted of an array of line segments (length of 0.5 degs). The orientation of each line segment was drawn from a Gaussian around the array's mean orientation (stimulus orientation). Stimulus orientation was uniformly sampled from a range +/−21 degrees relative to a reference orientation. Black marks indicated the randomly chosen reference orientation. Gray wedges around the reference mark indicated the total range of stimulus orientations (prior). Subjects first had to indicate whether the stimulus orientation was clockwise (CW) or counter-clockwise (CCW) relative to reference orientation. Subsequently they had to estimate the stimulus orientation. Experiment 2 was identical to Experiment 1 except that subjects were given the stimulus' category (CW/CCW) before stimulus presentation. The results show very similar repulsive biases in both experiments. Hence, we conclude that the biases are not the result of a specific, task-dependent neural read-out as previously suggested, but instead are consistent with a “self-consistent“ Bayesian observer model (Stocker and Simoncelli, 2008).

Stoichiometry and turnover of the bacterial flagellar switch protein FliN.

ABSTRACTSome proteins in biological complexes exchange with pools of free proteins while the complex is functioning. Evidence is emerging that protein exchange can be part of an adaptive mechanism. The bacterial flagellar motor is one of the most complex biological machines and is an ideal model system to study protein dynamics in large multimeric complexes. Recent studies showed that the copy number of FliM in the switch complex and the fraction of FliM that exchanges vary with the direction of flagellar rotation. Here, we investigated the stoichiometry and turnover of another switch complex component, FliN, labeled with the fluorescent protein CyPet, in Escherichia coli. Our results confirm that, in vivo, FliM and FliN form a complex with stoichiometry of 1:4 and function as a unit. We estimated that wild-type motors contained 120 ± 26 FliN molecules. Motors that rotated only clockwise (CW) or counterclockwise (CCW) contained 114 ± 17 and 144 ± 26 FliN molecules, respectively. The ratio of CCW-to-CW FliN copy numbers was 1.26, very close to that of 1.29 reported previously for FliM. We also measured the exchange of FliN molecules, which had a time scale and dependence upon rotation direction similar to those of FliM, consistent with an exchange of FliM-FliN as a unit. Our work confirms the highly dynamic nature of multimeric protein complexes and indicates that, under physiological conditions, these machines might not be the stable, complete structures suggested by averaged fixed methodologies but, rather, incomplete rings that can respond and adapt to changing environments.

Manipulating visual–motor experience to probe for observation-induced after-effects in adaptation learning

Observers can learn to move in novel, adapted environments after watching a learning or expert model. Although this is an effective practice technique, it is unclear how this learning is achieved and if observers update an internal model of their visual-motor environment, as shown through the presence of after-effects (i.e., negative carry-over effects when aiming in a normal environment following exposure to perturbed conditions). For such updating to occur via observational practice, it has been reasoned that the observer requires the motor capabilities to perform the task they are observing. To test this, we first trained three groups to physically move in clockwise (CW) or counterclockwise (CCW) rotated environments. When immediately returned to a normal environment, after-effects were seen. We then attempted to wash out these effects before allowing two of these groups (CW and CCW), and a naïve observation only group, to watch a video of an actor performing in a CW environment. This observation phase was immediately followed by another test for after-effects and a direct test of learning when aiming in the rotated environment. Consistent with previous data, there were direct learning effects due to observation. Although after-effects increased for the experienced observers, these were small and were not significantly different from a physical practice only group that did not undergo the observation phase. Therefore, even with a motor repertoire for the rotated environment, there was a lack of evidence that observational practice results in implicit (re)updating of an internal model for aiming.

The assessment of subjective visual vertical: comparison of two psychophysical paradigms and age-related performance

Perception of the subjective visual vertical (SVV) is usually assessed by asking to subjects, in complete darkness, to adjust the position of a luminous rod that is variably tilted (i.e., by the method of adjustment [ADJ]). Conversely, the two-alternative forced choice (2AFC) method requires subjects to categorize, as tilted either clockwise (CW) or counterclockwise (CCW), stimuli that are presented on a computer screen and are variably tilted from vertical. In this study, we aimed to compare the results of these two methods and investigate age-related effects on the SVV. SVV was assessed in 102 healthy individuals, 50 women and 52 men, with a mean age of 45.7 (range 20-91), using both ADJ (ten trials, initial 1°, 2°, 4°, 8°, or 12° bar tilts both CW and CCW) and 2AFC (120 stimuli with a 1°-32° variable tilt). Also, 50 of the subjects performed the ADJ test twice, with different bar lengths. We estimated bias and threshold for the two methods, and found that neither measure differed across the methods. Age was a significant predictor of threshold (2AFC, R (2) = .141; ADJ, R (2) = .190; p < .001), implying lower sensitivity with increasing age. Moreover, the ADJ method showed a significant increase of bias when the initial tilt was farthest from vertical, whereas the rod length was irrelevant. SVV measures obtained with the ADJ and 2AFC methods were comparable, but the latter measures were more resistant to artifacts that might affect the measurement. The lower sensitivity found in older persons may have an influence on their ability to interact with the environment and may contribute to impairment of postural control.