Bayesian modeling of dynamic motion integration

The quality of the representation of an object's motion is limited by the noise in the sensory input as well as by an intrinsic ambiguity due to the spatial limitation of the visual motion analyzers (aperture problem). Perceptual and oculomotor data demonstrate that motion processing of extended objects is initially dominated by the local 1D motion cues, related to the object's edges and orthogonal to them, whereas 2D information, related to terminators (or edge-endings), takes progressively over and leads to the final correct representation of global motion. A Bayesian framework accounting for the sensory noise and general expectancies for object velocities has proven successful in explaining several experimental findings concerning early motion processing [Weiss, Y., Adelson, E., 1998. Slow and smooth: a Bayesian theory for the combination of local motion signals in human vision. MIT Technical report, A.I. Memo 1624]. In particular, these models provide a qualitative account for the initial bias induced by the 1D motion cue. However, a complete functional model, encompassing the dynamical evolution of object motion perception, including the integration of different motion cues, is still lacking. Here we outline several experimental observations concerning human smooth pursuit of moving objects and more particularly the time course of its initiation phase, which reflects the ongoing motion integration process. In addition, we propose a recursive extension of the Bayesian model, motivated and constrained by our oculomotor data, to describe the dynamical integration of 1D and 2D motion information. We compare the model predictions for object motion tracking with human oculomotor recordings.     

三唑锡亚致死浓度对土耳其斯坦叶螨运动速率的影响

【目的】通过研究三唑锡亚致死浓度LC10、LC20及致死中浓度LC50对土耳其斯坦叶螨Tetranychus turkestani运动速率的影响,为杀螨剂对土耳其斯坦叶螨在行为学方面影响提供科学依据,并将此作为评价药剂的作用机理的理论依据。【方法】采用叶片浸渍法,明确致死中浓度和亚致死浓度,使用LC-100昆虫行为记录仪对土耳其斯坦叶螨在受到三唑锡亚致死浓度LC10、LC20和LC50刺激下其运动速率进行记录,观察其运动速率在三唑锡不同浓度、不同天数、不同时间段的变化规律。【结果】在三唑锡不同亚致死浓度作用下,第1天到第3天的10:00―12:00,其运动速率比对照低,从第3天10:00―12:00期间到第5天20:00―22:00期间,其运动速率与对照没有显著性差异。在相同亚致死浓度作用下,第1天10:00―12:00期间,土耳其斯坦叶螨的运动速率显著高于15:00―17:00期间和20:00―22:00期间,而在第3天和第5天的3个时间段,对土耳其斯坦叶螨的运动速率没有显著影响。当作用的时段不同时,LC10第1天10:00―12:00显著高于第3天和第5天,而LC20、LC50在10:00―12:00没有显著性差异。三种浓度的第1天15:00―17:00显著低于第3天和第5天。第3及第5天20:00―22:00没有显著性差异。【结论】总体来看,3唑锡亚不同致死浓度对土耳其斯坦叶螨的运动速率影响受浓度、时间影响,浓度越高土耳其斯坦叶螨运动速率越慢;随着时间延长,农药亚致死浓度对土耳其斯坦叶螨的运动速率基本没有影响。     

What you feel is what you see: inverse dynamics estimation underlies the resistive sensation of a delayed cursor

How our central nervous system (CNS) learns and exploits relationships between force and motion is a fundamental issue in computational neuroscience. While several lines of evidence have suggested that the CNS predicts motion states and signals from motor commands for control and perception (forward dynamics), it remains controversial whether it also performs the ‘inverse’ computation, i.e. the estimation of force from motion (inverse dynamics). Here, we show that the resistive sensation we experience while moving a delayed cursor, perceived purely from the change in visual motion, provides evidence of the inverse computation. To clearly specify the computational process underlying the sensation, we systematically varied the visual feedback and examined its effect on the strength of the sensation. In contrast to the prevailing theory that sensory prediction errors modulate our perception, the sensation did not correlate with errors in cursor motion due to the delay. Instead, it correlated with the amount of exposure to the forward acceleration of the cursor. This indicates that the delayed cursor is interpreted as a mechanical load, and the sensation represents its visually implied reaction force. Namely, the CNS automatically computes inverse dynamics, using visually detected motions, to monitor the dynamic forces involved in our actions.     

暗示性运动加工的认知神经机制

暗示性运动是指个体观看静止图片时从中知觉到的运动.研究者采用高低认知水平两类暗示性运动刺激材料,借助"冻结帧"、直接观看、运动后效和f MRI适应等任务范式,探讨了注意和意识在暗示性运动加工中的作用及其记忆特点;并借助脑成像等技术,考察了颞中区、颞上皮层区、颞上沟、镜像神经元系统等脑区在暗示性运动加工中的作用.但由于暗示性运动加工涉及"视觉腹侧通路与背侧通路功能的分离与整合"问题,目前对相关研究结果和解释还存在争议,暗示性运动加工的认知神经机制仍有待于进一步研究.     

Seasonal differences in the effects of oscillatory and uni‐directional flow on the growth and nitrate‐uptake rates of juvenile Laminaria digitata (Phaeophyceae)

The influence of oscillatory versus unidirectional flow on the growth and nitrate‐uptake rates of juvenile kelp, Laminaria digitata, was determined seasonally in experimental treatments that simulated as closely as possible natural environmental conditions. In winter, regardless of flow condition (oscillatory and unidirectional) or water velocity, no influence of water motion was observed on the growth rate of L. digitata. In summer, when ambient nitrate concentrations were low, increased water motion enhanced macroalgal growth, which is assumed to be related to an increase in the rate of supply of nutrients to the blade surface. Nitrate‐uptake rates were significantly influenced by water motion and season. Lowest nitrate‐uptake rates were observed for velocities <5 cm · s^?1 and nitrate‐uptake rates increased by 20%–50% under oscillatory motion compared to unidirectional flow at the same average speed. These data further suggested that the diffusion boundary layer played a significant role in influencing nitrate‐uptake rates. However, while increased nitrate‐uptake in oscillatory flow was clear, this was not reflected in growth rates and further work is required to understand the disconnection of nitrate‐uptake and growth by L. digitata in oscillatory flow. The data obtained support those from related field‐based studies, which suggest that in summer, when insufficient nitrogen is available in the water to saturate metabolic demand, the growth rate of kelps will be influenced by water motion restricting mass transfer of nitrogen.     

Phylogenetic eigenvectors and nonstationarity in the evolution of theropod dinosaur skulls

Despite the long‐standing interest in nonstationarity of both phenotypic evolution and diversification rates, only recently have methods been developed to study this property. Here, we propose a methodological expansion of the phylogenetic signal‐representation (PSR) curve based on phylogenetic eigenvectors to test for nonstationarity. The PSR curve is built by plotting the coefficients of determination R² from phylogenetic eigenvector regression (PVR) models increasing the number of phylogenetic eigenvectors against the accumulated eigenvalues. The PSR curve is linear under a stationary model of trait evolution (i.e. the Brownian motion model). Here we describe the distribution of shifts in the models R² and used a randomization procedure to compare observed and simulated shifts along the PSR curve, which allowed detecting nonstationarity in trait evolution. As an applied example, we show that the main evolutionary pattern of variation in the theropod dinosaur skull was nonstationary, with a significant shift in evolutionary rates in derived oviraptorosaurs, an aberrant group of mostly toothless, crested, birdlike theropods. This result is also supported by a recently proposed Bayesian‐based method (AUTEUR). A significant deviation between Ceratosaurus and Limusaurus terminal branches was also detected. We purport that our new approach is a valuable tool for evolutionary biologists, owing to its simplicity, flexibility and comprehensiveness.     

Mechanical resolution of chiral objects in achiral media: Where is the size limit?

Macroscopic chiral objects (boats and planes with turned rudders, shoes, etc.) get separated from their mirror‐image counterparts by motion in achiral media. However, chiral molecules are not enantio‐differentiated without the presence of a chiral environment, which may be due to other chiral molecules in the medium. This article explores the reasons of this micro/macro difference as well as the size borderline between the two regimes. There are two major demarcation lines, both related to the object's chaotic thermal motion. The first one is due to destruction of the necessary spatial orientation by the fast rotational diffusion. Only particles larger than 1 μm can maintain their original orientation for 1 sec or longer. For smaller particles, an additional external orienting factor, e.g., a strong electric field has to be applied. The second limitation is defined by the ratio of the hydrodynamic separation of the enantiomers (which is directly proportional to time) to their displacement due to the translational Brownian motion (which is proportional to square root of time). On the laboratory time scales (up to a year), the chiral objects have to be larger than 0.25 μm to be resolved. On evolutionary time scales, much smaller object could be resolved. For enantiomers approaching the molecular size, periods comparable to the age of the universe would be required. Chirality, 2011. © 2010 Wiley‐Liss, Inc.