Movement selection in advance of action in the superior colliculus.

The primate superior colliculus contains a map of saccadic eye movements. Saccades are high-velocity eye movements to selected targets in the visual field, but little is known about the neural mechanisms responsible for target selection or the related problem of choosing a particular movement from the oculomotor repertoire. Two classes of neurons have been described in the superior colliculus which show bursts of activity before the saccade: discrete bursters display a vigorous pre-saccadic burst and prelude bursters show low-frequency activity as a prelude to burst onset. We have designed experiments to test whether prelude activity is related to saccade selection. Our tasks use a cue to specify which of two physically identical visual stimuli is the goal of an impending saccade. This cue is spatially and temporally isolated from the potential targets as well as from visual cues signalling movement initiation. Our results show that prelude activity occurs shortly after information is available for correct saccade selection and, more importantly, the activity is predictive of saccade choice. The results thus suggest that the superior colliculus participates in the process of saccade selection.     

Dynamic coding of behaviourally relevant stimuli in parietal cortex.

A general function of cerebral cortex is to allow the flexible association of sensory stimuli with specific behaviours. Many neurons in parietal, prefrontal and motor cortical areas are activated both by particular movements and by sensory cues that trigger these movements, suggesting a role in linking sensation to action. For example, neurons in the lateral intraparietal area (LIP) encode both the location of visual stimuli and the direction of saccadic eye movements. LIP is not believed to encode non-spatial stimulus attributes such as colour. Here we investigated whether LIP would encode colour if colour was behaviourally linked to the eye movement. We trained monkeys to make an eye movement in one of two directions based alternately on the colour or location of a visual cue. When cue colour was relevant for directing eye movement, we found a substantial fraction of LIP neurons selective for cue colour. However, when cue location was relevant, colour selectivity was virtually absent in LIP. These results demonstrate that selectivity of cortical neurons can change as a function of the required behaviour.     

Dissociation between hand motion and population vectors from neural activity in motor cortex

The population vector hypothesis was introduced almost twenty years ago to illustrate that a population vector constructed from neural activity in primary motor cortex (MI) of non-human primates could predict the direction of hand movement during reaching. Alternative explanations for this population signal have been suggested but could not be tested experimentally owing to movement complexity in the standard reaching model. We re-examined this issue by recording the activity of neurons in contralateral MI of monkeys while they made reaching movements with their right arms oriented in the horizontal plane-where the mechanics of limb motion are measurable and anisotropic. Here we found systematic biases between the population vector and the direction of hand movement. These errors were attributed to a non-uniform distribution of preferred directions of neurons and the non-uniformity covaried with peak joint power at the shoulder and elbow. These observations contradict the population vector hypothesis and show that non-human primates are capable of generating reaching movements to spatial targets even though population vectors based on MI activity do not point in the direction of hand motion.     

一种山区急弯路段驾驶人熟练度甄别方法

为探究驾驶人眼动行为与驾驶熟练程度以及道路线型之间的响应特性,设计并组织山区公路实车驾驶行为试验。招募20名驾驶人,实时采集被试驾驶人在急弯路段的6个注视及扫视行为表征参数。试验表明:眼睛注视点在左转急弯路段的水平方向分布范围是-0.5~0.1 m,在右转急弯路段的水平方向分布范围是-0.2~0.5 m;眼睛水平注视角变化的主要范围是0°~30°;扫视速度的变化范围区间是0~1(°)·ms~(-1),扫视幅度的变化范围是5°~25°。基于采集的眼动行为参数,构建驾驶人视觉搜索模态矩阵。采用主成分分析法对影响因子进行降维,并建立驾驶人眼动特征综合评价模型。结果显示,眼动综合评分与驾驶里程呈正相关关系,表明该模型可用于量化甄别驾驶人驾驶熟练程度,并为驾驶能力评估提供有效的理论依据。     

Illusory perceptions of space and time preserve cross-saccadic perceptual continuity.

When voluntary saccadic eye movements are made to a silently ticking clock, observers sometimes think that the second hand takes longer than normal to move to its next position. For a short period, the clock appears to have stopped (chronostasis). Here we show that the illusion occurs because the brain extends the percept of the saccadic target backwards in time to just before the onset of the saccade. This occurs every time we move the eyes but it is only perceived when an external time reference alerts us to the phenomenon. The illusion does not seem to depend on the shift of spatial attention that accompanies the saccade. However, if the target is moved unpredictably during the saccade, breaking perception of the target's spatial continuity, then the illusion disappears. We suggest that temporal extension of the target's percept is one of the mechanisms that 'fill in' the perceptual 'gap' during saccadic suppression. The effect is critically linked to perceptual mechanisms that identify a target's spatial stability.     

Influence of the thalamus on spatial visual processing in frontal cortex

Each of our movements activates our own sensory receptors, and therefore keeping track of self-movement is a necessary part of analysing sensory input. One way in which the brain keeps track of self-movement is by monitoring an internal copy, or corollary discharge, of motor commands. This concept could explain why we perceive a stable visual world despite our frequent quick, or saccadic, eye movements: corollary discharge about each saccade would permit the visual system to ignore saccade-induced visual changes. The critical missing link has been the connection between corollary discharge and visual processing. Here we show that such a link is formed by a corollary discharge from the thalamus that targets the frontal cortex. In the thalamus, neurons in the mediodorsal nucleus relay a corollary discharge of saccades from the midbrain superior colliculus to the cortical frontal eye field. In the frontal eye field, neurons use corollary discharge to shift their visual receptive fields spatially before saccades. We tested the hypothesis that these two components-a pathway for corollary discharge and neurons with shifting receptive fields-form a circuit in which the corollary discharge drives the shift. First we showed that the known spatial and temporal properties of the corollary discharge predict the dynamic changes in spatial visual processing of cortical neurons when saccades are made. Then we moved from this correlation to causation by isolating single cortical neurons and showing that their spatial visual processing is impaired when corollary discharge from the thalamus is interrupted. Thus the visual processing of frontal neurons is spatiotemporally matched with, and functionally dependent on, corollary discharge input from the thalamus. These experiments establish the first link between corollary discharge and visual processing, delineate a brain circuit that is well suited for mediating visual stability, and provide a framework for studying corollary discharge in other sensory systems.     

Predictable eye-head coordination during driving.

Large changes in the direction of gaze are made with a combination of fast saccadic eye movements and rather slower head movements. Since the first study on freely moving subjects, most authors have agreed that the head movement component of gaze is very variable, with a high 'volitional' component. But in some circumstances head and eye movements can be quite predictable, for example when a subject is asked to shift gaze as quickly as possible. Under these conditions, laboratory studies have shown that the eye and head motor-systems both receive gaze-change commands, although they execute them in rather different ways. Here I reconsider the way gaze direction is changed during free movement, but in the performance of a task where the subject is too busy to exert conscious control over head or eye movements. Using a new portable and inexpensive method for recording head and eye movements, I examine the oculomotor behaviour of car drivers, particularly during the large gaze changes made at road junctions. The results show that the pattern of eye and head movements is highly predictable, given only the sequence of gaze targets.     

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.     

Illusory shifts in visual direction accompany adaptation of saccadic eye movements.

A central problem in human vision is to explain how the visual world remains stable despite the continual displacements of the retinal image produced by rapid saccadic movements of the eyes. Perceived stability has been attributed to 'efferent-copy' signals, representing the saccadic motor commands, that cancel the effects of saccade-related retinal displacements. Here we show, by means of a perceptual illusion, that traditional cancellation theories cannot explain stability. The perceptual illusion was produced by first inducing adaptive changes in saccadic gain (ratio of saccade size to target eccentricity). Following adaptation, subjects experienced an illusory mislocalization in which widely separated targets flashed before and after saccades appeared to be in the same place. The illusion shows that the perceptual system did not take the adaptive changes into account. Perceptual localization is based on signals representing the size of the initially-intended saccade, not the size of the saccade that is ultimately executed. Signals representing intended saccades initiate a visual comparison process used to maintain perceptual stability across saccades and to generate the oculomotor error signals that ensure saccadic accuracy.     

Saccadic oscillations facilitate ocular perfusion from the avian pecten

THE evolution of the eye is constrained by two conflicting requirements--good vascular perfusion of the retina, and an optical path through the retina that is unobstructed by blood vessels. Birds are interesting in that they have higher metabolic rates and thicker retinas than mammals, but have no retinal blood vessels. Nutrients and oxygen must thus reach the neurons of the inner retina either from the choroid through 300 micron of metabolically very active retina, or from the pecten, a pleated vascular structure protruding from the head of the optic nerve into the vitreous chamber, and more than a centimetre away from some retinal neurons. Despite the diffusional distance involved, several lines of evidence indicate that the pecten is the primary source of nutrients for the inner retina: the presence of an oxygen gradient from pecten to retina, the large surface area produced by macroscopic folds and by microscopic infoldings of the luminal and external surfaces of the capillary endothelium, extrusion of circulating fluorescein, high content of carbonic anhydrase and alkaline phosphatase, and retinal impairments after pecten ablation. Another peculiarity of birds, their saccadic oscillations, occur with a large cyclotor-sional component during every saccadic eye movement. In different species, saccades, which occur at intervals of 0.5-40 s, have up to 13 oscillations with frequencies of 15-30 Hz and ampliá-tudes of about 10 degrees. Therefore, as much as 12% of some birds' total viewing time may be subject to the image instability caused by the oscillations. Using fluorescein angiography, we show here that during every saccade, the pecten acts as an agitator which propels perfusate towards the central retina much more effectively than is observed during intersaccadic intervals.     

Encoding of movement time by populations of cerebellar Purkinje cells

One of the earliest computational principles attributed to the cerebellum was the measurement of time. This idea was originally suggested on anatomical grounds, and was taken up again to explain some of the deficits in cerebellar patients. The contribution of the cerebellum to eye movements, in contrast, has traditionally been discussed in the context of motor learning. This view has received support from the loss of saccade adaptation, one of the key examples of motor learning, following lesions of the posterior cerebellar vermis. However, the relationship between the properties of saccade-related vermal Purkinje cells and the behavioural deficits that follow lesions is unclear. Here we report results from single-unit recording experiments on monkeys that reconcile the seemingly unrelated concepts of timing and motor learning. We report that, unlike individual Purkinje cells, the population response of larger groups of Purkinje cells gives a precise temporal signature of saccade onset and offset. Thus a vermal population response may help to determine saccade duration. Modifying the time course of the population response by changing the weights of the contributing individual Purkinje cells, discharging at different times relative to the saccade, would directly translate into changes in saccade amplitude.     

不同抑制能力大学生图形推理的眼动特征

对160名大学生被试进行6个执行功能任务的测量,从中筛选出高抑制能力和低抑制能力被试各15人,考察被试在不同难易程度图形推理题目上的得分、反应时以及眼动模式的差别.研究发现抑制能力与推理得分之间的相关并不显著,但低抑制能力的被试注视频率显著高于高抑制能力的被试,而平均注视时间则显著低于,另外低抑制能力组倾向于采用选项剔除策略,而高抑制能力组倾向于采用规则构建策略.     

草原公路直线段路侧景观对驾驶员视觉特性的影响

 视觉系统是驾驶员获取行车信息的主要通道,而公路路侧景观是引起驾驶员视觉刺激的重要因素之一。以不同路侧景观的草原公路直线段作为研究对象,利用眼动仪采集5名驾驶员的眼动指标并进行分析。结果表明,驾驶员在景观相对复杂路段的瞳孔直径和扫视幅度较大,眨眼持续时间与注视持续时间较小;不同路侧景观对驾驶员的眨眼持续时间、注视持续时间、扫视幅度影响不显著;对驾驶员的瞳孔直径影响显著。本研究可为分析草原公路路侧景观对驾驶员疲劳的影响提供参考。     

基于运动相关脑电特征的手运动方向识别

为了研究如何从无创运动相关脑电中提取运动信息作为上肢主动康复训练的控制命令,通过设计实验,使右手完成左、上、右3个方向的运动,同时采集脑电数据和右手运动信息.通过小波时频分析确认与右手运动相关的脑电频带,并提取其小波分解系数作为特征,采用支持向量机进行特征分类,根据方向识别准确率分析提取特征的有效性.结果表明,运动脑电delta和theta频段的小波系数特征可以有效区分右手不同方向的运动,方向识别准确率的均值接近65%,并且用准备阶段特征分类的结果普遍优于运动阶段特征,因此,在手运动之前诱发的脑电活动含有丰富的运动信息,可用于脑-机接口系统提取上肢主动康复训练的控制命令.     

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.     

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.     

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.     

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.     

Neuronal switching of sensorimotor transformations for antisaccades

The influence of cognitive context on orienting behaviour can be explored using the mixed memory-prosaccade, memory-antisaccade task. A symbolic cue, such as the colour of a visual stimulus, instructs the subject to make a brief, rapid eye movement (a saccade) either towards the stimulus (prosaccade) or in the opposite direction (antisaccade). Thus, the appropriate sensorimotor transformation must be switched on to execute the instructed task. Despite advances in our understanding of the neuronal processing of antisaccades, it remains unclear how the brain selects and computes the sensorimotor transformation leading to an antisaccade. Here we show that area LIP of the posterior parietal cortex is involved in these processes. LIP's population activity turns from the visual direction to the motor direction during memory-antisaccade trials. About one-third of the visual neurons in LIP produce a brisk, transient discharge in certain memory-antisaccade trials. We call this discharge 'paradoxical' because its timing is visual-like but its direction is motor. The paradoxical discharge shows, first, that switching occurs already at the level of visual cells, as previously proposed by Schlag-Rey and colleagues; and second, that this switching is accomplished very rapidly, within 50 ms from the arrival of the visual signals in LIP.     

初中不同阅读能力学生的阅读眼动特点比较

用眼动记录仪对初中二年级不同阅读能力的学生(高阅读能力和低阅读能力各10名)在阅读说明文时的眼动过程进行了记录.结果发现:高阅读能力学生在阅读时间、注视点的持续时间、回视次数和注视次数上都显著地低于低阅读能力的学生,而注视频率显著地高于低阅读能力的学生;在眼跳角度和眼跳潜伏期2项眼动指标上没有显著差异.