Coding of smooth eye movements in three-dimensional space by frontal cortex

Through the development of a high-acuity fovea, primates with frontal eyes have acquired the ability to use binocular eye movements to track small objects moving in space. The smooth-pursuit system moves both eyes in the same direction to track movement in the frontal plane (frontal pursuit), whereas the vergence system moves left and right eyes in opposite directions to track targets moving towards or away from the observer (vergence tracking). In the cerebral cortex and brainstem, signals related to vergence eye movements--and the retinal disparity and blur signals that elicit them--are coded independently of signals related to frontal pursuit. Here we show that these types of signal are represented in a completely different way in the smooth-pursuit region of the frontal eye fields. Neurons of the frontal eye field modulate strongly during both frontal pursuit and vergence tracking, which results in three-dimensional cartesian representations of eye movements. We propose that the brain creates this distinctly different intermediate representation to allow these neurons to function as part of a system that enables primates to track and manipulate objects moving in three-dimensional space.     

Selective gating of visual signals by microstimulation of frontal cortex

Several decades of psychophysical and neurophysiological studies have established that visual signals are enhanced at the locus of attention. What remains a mystery is the mechanism that initiates biases in the strength of visual representations. Recent evidence argues that, during spatial attention, these biases reflect nascent saccadic eye movement commands. We examined the functional interaction of saccade preparation and visual coding by electrically stimulating sites within the frontal eye fields (FEF) and measuring its effect on the activity of neurons in extrastriate visual cortex. Here we show that visual responses in area V4 could be enhanced after brief stimulation of retinotopically corresponding sites within the FEF using currents below that needed to evoke saccades. The magnitude of the enhancement depended on the effectiveness of receptive field stimuli as well as on the presence of competing stimuli outside the receptive field. Stimulation of non-corresponding FEF representations could suppress V4 responses. The results suggest that the gain of visual signals is modified according to the strength of spatially corresponding eye movement commands.     

Representation of a perceptual decision in developing oculomotor commands

Behaviour often depends on the ability to make categorical judgements about sensory information acquired over time. Such judgements require a comparison of the evidence favouring the alternatives, but how the brain forms these comparisons is unknown. Here we show that in a visual discrimination task, the accumulating balance of sensory evidence favouring one interpretation over another is evident in the neural circuits that generate the behavioural response. We trained monkeys to make a direction judgement about dynamic random-dot motions and to indicate their judgement with an eye movement to a visual target. We interrupted motion viewing with electrical microstimulation of the frontal eye field and analysed the resulting, evoked eye movements for evidence of ongoing activity associated with the oculomotor response. Evoked eye movements deviated in the direction of the monkey's judgement. The magnitude of the deviation depended on motion strength and viewing time. The oculomotor signals responsible for these deviations reflected the accumulated motion information that informed the monkey's choices on the discrimination task. Thus, for this task, decision formation and motor preparation appear to share a common level of neural organization.     

An area for vergence eye movement in primate frontal cortex

To view objects at different distances, humans rely on vergence eye movements to appropriately converge or diverge the eyes and on ocular accommodation to focus the object. Despite the importance of these coordinated eye movements (the 'near response') very little is known about the role of the cerebral cortex in their control. As near-response neurons exist within the nucleus reticularis tegmenti pontis, which receives input from the frontal eye field region of frontal cortex, and this cortical region is known to be involved in saccadic and smooth-pursuit eye movements, we propose that a nearby region might play a role in vergence and ocular accommodation. Here we provide evidence from rhesus monkeys that a region of frontal cortex located immediately anterior to the saccade-related frontal eye field region is involved in vergence and ocular accommodation, and in the sensorimotor transformations required for these eye movements. We conclude that the macaque frontal cortex is involved in the control of all voluntary eye movements, and suggest that the definition of the frontal eye fields should be expanded to include this region.     

Performance monitoring by the supplementary eye field

Intelligent behaviour requires self-control based on the consequences of actions. The countermanding task is designed to study self-control; it requires subjects to withhold planned movements in response to an imperative stop signal, which they can do with varying success. In humans, the medial frontal cortex has been implicated in the supervisory control of action. In monkeys, the supplementary eye field in the dorsomedial frontal cortex is involved in producing eye movements, but its precise function has not been clarified. To investigate the role of the supplementary eye field in the control of eye movements, we recorded neural activity in macaque monkeys trained to perform an eye movement countermanding task. Distinct groups of neurons were active after errors, after successful withholding of a partially prepared movement, or in association with reinforcement. These three forms of activation could not be explained by sensory or motor factors. Our results lead us to put forward the hypothesis that the supplementary eye field contributes to monitoring the context and consequences of eye movements.     

Neural population dynamics during reaching

Most theories of motor cortex have assumed that neural activity represents movement parameters. This view derives from what is known about primary visual cortex, where neural activity represents patterns of light. Yet it is unclear how well the analogy between motor and visual cortex holds. Single-neuron responses in motor cortex are complex, and there is marked disagreement regarding which movement parameters are represented. A better analogy might be with other motor systems, where a common principle is rhythmic neural activity. Here we find that motor cortex responses during reaching contain a brief but strong oscillatory component, something quite unexpected for a non-periodic behaviour. Oscillation amplitude and phase followed naturally from the preparatory state, suggesting a mechanistic role for preparatory neural activity. These results demonstrate an unexpected yet surprisingly simple structure in the population response. This underlying structure explains many of the confusing features of individual neural responses.     

Optimal eye movement strategies in visual search

To perform visual search, humans, like many mammals, encode a large field of view with retinas having variable spatial resolution, and then use high-speed eye movements to direct the highest-resolution region, the fovea, towards potential target locations. Good search performance is essential for survival, and hence mammals may have evolved efficient strategies for selecting fixation locations. Here we address two questions: what are the optimal eye movement strategies for a foveated visual system faced with the problem of finding a target in a cluttered environment, and do humans employ optimal eye movement strategies during a search? We derive the ideal bayesian observer for search tasks in which a target is embedded at an unknown location within a random background that has the spectral characteristics of natural scenes. Our ideal searcher uses precise knowledge about the statistics of the scenes in which the target is embedded, and about its own visual system, to make eye movements that gain the most information about target location. We find that humans achieve nearly optimal search performance, even though humans integrate information poorly across fixations. Analysis of the ideal searcher reveals that there is little benefit from perfect integration across fixations--much more important is efficient processing of information on each fixation. Apparently, evolution has exploited this fact to achieve efficient eye movement strategies with minimal neural resources devoted to memory.     

Temporal dynamics of a neural solution to the aperture problem in visual area MT of macaque brain

A critical step in the interpretation of the visual world is the integration of the various local motion signals generated by moving objects. This process is complicated by the fact that local velocity measurements can differ depending on contour orientation and spatial position. Specifically, any local motion detector can measure only the component of motion perpendicular to a contour that extends beyond its field of view. This "aperture problem" is particularly relevant to direction-selective neurons early in the visual pathways, where small receptive fields permit only a limited view of a moving object. Here we show that neurons in the middle temporal visual area (known as MT or V5) of the macaque brain reveal a dynamic solution to the aperture problem. MT neurons initially respond primarily to the component of motion perpendicular to a contour's orientation, but over a period of approximately 60 ms the responses gradually shift to encode the true stimulus direction, regardless of orientation. We also report a behavioural correlate of these neural responses: the initial velocity of pursuit eye movements deviates in a direction perpendicular to local contour orientation, suggesting that the earliest neural responses influence the oculomotor response.     

Control of visual cortical signals by prefrontal dopamine

The prefrontal cortex is thought to modulate sensory signals in posterior cortices during top-down attention, but little is known about the underlying neural circuitry. Experimental and clinical evidence indicate that prefrontal dopamine has an important role in cognitive functions, acting predominantly through D1 receptors. Here we show that dopamine D1 receptors mediate prefrontal control of signals in the visual cortex of macaques (Macaca mulatta). We pharmacologically altered D1-receptor-mediated activity in the frontal eye field of the prefrontal cortex and measured the effect on the responses of neurons in area V4 of the visual cortex. This manipulation was sufficient to enhance the magnitude, the orientation selectivity and the reliability of V4 visual responses to an extent comparable with the known effects of top-down attention. The enhancement of V4 signals was restricted to neurons with response fields overlapping the part of visual space affected by the D1 receptor manipulation. Altering either D1- or D2-receptor-mediated frontal eye field activity increased saccadic target selection but the D2 receptor manipulation did not enhance V4 signals. Our results identify a role for D1 receptors in mediating the control of visual cortical signals by the prefrontal cortex and suggest how processing in sensory areas could be altered in mental disorders involving prefrontal dopamine.     

Primary motor cortex underlies multi-joint integration for fast feedback control

A basic difficulty for the nervous system is integrating locally ambiguous sensory information to form accurate perceptions about the outside world. This local-to-global problem is also fundamental to motor control of the arm, because complex mechanical interactions between shoulder and elbow allow a particular amount of motion at one joint to arise from an infinite combination of shoulder and elbow torques. Here we show, in humans and rhesus monkeys, that a transcortical pathway through primary motor cortex (M1) resolves this ambiguity during fast feedback control. We demonstrate that single M1 neurons of behaving monkeys can integrate shoulder and elbow motion information into motor commands that appropriately counter the underlying torque within about 50 milliseconds of a mechanical perturbation. Moreover, we reveal a causal link between M1 processing and multi-joint integration in humans by showing that shoulder muscle responses occurring ～50 milliseconds after pure elbow displacement can be potentiated by transcranial magnetic stimulation. Taken together, our results show that transcortical processing through M1 permits feedback responses to express a level of sophistication that rivals voluntary control; this provides neurophysiological support for influential theories positing that voluntary movement is generated by the intelligent manipulation of sensory feedback.     

Population coding of saccadic eye movements by neurons in the superior colliculus

The deeper layers of the superior colliculus are involved in the initiation and execution of saccadic (high velocity) eye movements. A large population of coarsely tuned collicular neurons is active before each saccade. The mechanisms by which the signals that precisely control the direction and amplitude of a saccade are extracted from the activity of the population are unknown. It has been assumed that the exact trajectory of a saccade is determined by the activity of the entire population and that information is not extracted from only the most active cells in the population at a subsequent stage of neural processing. The trajectory of a saccade could be based on vector summation of the movement tendencies provided by each member of the population of active neurons or be determined by a weighted average of the vector contributions of each neuron in the active population. Here we present the results of experiments in which a small subset of the active population was reversibly deactivated with lidocaine. These results are consistent with the predictions of the latter population-averaging hypothesis and support the general idea that the direction, amplitude and velocity of saccadic eye movements are based on the responses of the entire population of cells active before a saccadic eye movement.     

A neural correlate of response bias in monkey caudate nucleus

Primates are equipped with neural circuits in the prefrontal cortex, the parietal cortex and the basal ganglia that predict the availability of reward during the performance of behavioural tasks. It is not known, however, how reward value is incorporated in the control of action. Here we identify neurons in the monkey caudate nucleus that create a spatially selective response bias depending on the expected gain. In behavioural tasks, the monkey had to make a visually guided eye movement in every trial, but was rewarded for a correct response in only half of the trials. Reward availability was predictable on the basis of the spatial position of the visual target. We found that caudate neurons change their discharge rate systematically, even before the appearance of the visual target, and usually fire more when the contralateral position is associated with reward. Strong anticipatory activity of neurons with a contralateral preference is associated with decreased latency for eye movements in the contralateral direction. We conclude that this neuronal mechanism creates an advance bias that favours a spatial response when it is associated with a high reward value.     

A sensory source for motor variation

Suppose that the variability in our movements is caused not by noise in the motor system itself, nor by fluctuations in our intentions or plans, but rather by errors in our sensory estimates of the external parameters that define the appropriate action. For tasks in which precision is at a premium, performance would be optimal if no noise were added in movement planning and execution: motor output would be as accurate as possible given the quality of sensory inputs. Here we use visually guided smooth-pursuit eye movements in primates as a testing ground for this notion of optimality. In response to repeated presentations of identical target motions, nearly 92% of the variance in eye trajectory can be accounted for as a consequence of errors in sensory estimates of the speed, direction and timing of target motion, plus a small background noise that is observed both during eye movements and during fixations. The magnitudes of the inferred sensory errors agree with the observed thresholds for sensory discrimination by perceptual systems, suggesting that the very different neural processes of perception and action are limited by the same sources of noise.     

基于唇重构与三维耦合CNN的多视角音唇一致性判别

针对传统音唇一致性判别方法主要对正面唇动视频进行处理,未考虑视频采集角度变化对结果的影响,且容易忽略唇动过程中的时空特性等不足,文中以唇部角度变化对一致性判别的影响为研究重心,结合三维卷积神经网络在非线性表示和时空维度特征提取上的优势,提出了基于正面唇重构与三维耦合卷积神经网络的多视角音唇一致性判别方法。该方法先通过在生成器中引入自映射损失来提高正面重建效果,并采用基于自映射监督循环一致性生成对抗网络（SMS-CycleGAN）的唇重构方法对多视角唇图进行角度分类及正面重构;然后设计两个异构三维卷积神经网络,分别用来描述音频和视频信号,并提取包含长时时空关联信息的三维卷积特征;最后引入对比损失函数作为音视频信号匹配的相关度鉴别度量,将音视频网络输出耦合到同一表示空间,并进行一致性判别。实验结果表明,文中方法能重建出更高质量的正面唇图,一致性判别性能优于多种不同类型的比较方法。     

Feature-based attention influences motion processing gain in macaque visual cortex.

Changes in neural responses based on spatial attention have been demonstrated in many areas of visual cortex, indicating that the neural correlate of attention is an enhanced response to stimuli at an attended location and reduced responses to stimuli elsewhere. Here we demonstrate non-spatial, feature-based attentional modulation of visual motion processing, and show that attention increases the gain of direction-selective neurons in visual cortical area MT without narrowing the direction-tuning curves. These findings place important constraints on the neural mechanisms of attention and we propose to unify the effects of spatial location, direction of motion and other features of the attended stimuli in a 'feature similarity gain model' of attention.     

基于遗传Hopfield混合神经网络的多轴控制系统设计与实现

采用ASIC核心的PCI多轴控制器,通过对步进电机的运动控制及状态反馈,设计完成了八轴控制系统.在汉字雕刻路径问题上,结合遗传算法和Hopfield神经网络算法的优点,提出遗传Hopfield混合神经网络算法对汉字雕刻路径进行优化,较好地解决了雕刻过程混乱、雕刻总路径过长的问题.给出了该系统的硬件结构以及遗传Hopfield混合神经网络优化算法和仿真结果.实际应用表明,该多轴运动控制系统具有很好的稳定性和控制精度.     

骨传导听觉脑机接口技术研究

听觉脑机接口技术为视觉通路存在障碍的闭锁综合征患者提供了与外界交流的新通道.为了使空气传导通路受损的病患也能应用此项技术,本文将声音骨传导方式引入到听觉脑机接口实验研究中.本研究设计了左右耳分别呈现不同声音序列的双耳分听实验范式,对骨传导及空气传导方式所诱发脑电信号进行了特征提取和分类比较.实验结果表明两种传导方式下靶刺激均可稳定的诱发出N200与P300电位,骨传导方式下,N200的幅值较空气传导方式更显著.两种方式都获得较高的分类正确率,均可用于听觉脑机接口系统,且骨传导方式优于空气传导方式.     

The influence of visual motion on fast reaching movements to a stationary object

One of the most important functions of vision is to direct actions to objects. However, every time that vision is used to guide an action, retinal motion signals are produced by the movement of the eye and head as the person looks at the object or by the motion of other objects in the scene. To reach for the object accurately, the visuomotor system must separate information about the position of the stationary target from background retinal motion signals-a long-standing problem that is poorly understood. Here we show that the visuomotor system does not distinguish between these two information sources: when observers made fast reaching movements to a briefly presented stationary target, their hand shifted in a direction consistent with the motion of a distant and unrelated stimulus, a result contrary to most other findings. This can be seen early in the hand's trajectory (approximately 120 ms) and occurs continuously from programming of the movement through to its execution. The visuomotor system might make use of the motion signals arising from eye and head movements to update the positions of targets rapidly and redirect the hand to compensate for body movements.     

LQR优化的BP神经网络PID控制器设计

针对传统神经网络PID(比例 积分 微分)控制器和传统线性二次调节器(LQR)优化型PID控制器对无刷直流电机转速控制恢复时间长及抗干扰性较差等问题, 提出一种LQR优化的BP神经网络PID控制器, 用于无刷直流电机的转速控制. 首先, 利用BP神经网络对PID增益进行调节, 提高控制器的动态适应性和鲁棒性； 然后, 采用LQR优化BP神经网络最优输出, 使其更接近目标PID增益. 仿真结果表明, 该控制器有效提高了响应速度, 减小了稳态误差并增强了抗干扰能力.     

座椅悬架和汽车悬架的集成变增益LQR控制

利用模糊控制理论和最优控制理论,提出了一种基于模糊控制的车辆主动悬架和座椅主动悬架的集成变增益LQR控制方法.在建立“车-椅”三自由度动力学模型的基础上,以底盘垂向加速度和座椅垂向加速度为控制目标,以车轮动态位移、车辆悬架动行程范围小于规定值为约束条件,设计出了车辆悬架和座椅悬架变增益LQR控制器,并用Matlab/Simulink进行了仿真实验分析与比较,得出该控制方法对座椅悬架和车辆悬架有较好的控制效果,验证了集成变增益LQR控制方法的有效性和可行性,为未来悬架系统控制的研究提供了参考.