The perception of heading during eye movements.

When a person walks through a rigid environment while holding eyes and head fixed, the pattern of retinal motion flows radially away from a point, the focus of expansion (Fig. 1a). Under such conditions of translation, heading corresponds to the focus of expansion and people identify it readily. But when making an eye/head movement to track an object off to the side, retinal motion is no longer radial (Fig. 1b). Heading perception in such situations has been modelled in two ways. Extra-retinal models monitor the velocity of rotational movements through proprioceptive or efference information from the extraocular and neck muscles and use that information to discount rotation effects. Retinal-image models determine (and eliminate) rotational components from the retinal image alone. These models have been tested by measuring heading perception under two conditions. First, observers judged heading while tracking a point on a simulated ground plane. Second, they fixated a stationary point and the flow field simulated the effects of a tracking eye movement. Extra-retinal models predict poorer performance in the simulated condition because the eyes do not move. Retinal-image models predict no difference in performance because the two conditions produce identical patterns of retinal motion. Warren and Hannon observed similar performance and concluded that people do not require extra-retinal information to judge heading with eye/head movements present, but they used extremely slow tracking eye movements of 0.2-1.2 deg s-1; a moving observer frequently tracks objects at much higher rates (L. Stark, personal communication). Here we examine heading judgements at higher, more typical eye movement velocities and find that people require extra-retinal information about eye position to perceive heading accurately under many viewing conditions.     

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.     

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.     

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.     

Regulation of the gain of visually guided smooth-pursuit eye movements by frontal cortex

In studies of the neural mechanisms giving rise to behaviour, changes in the neural and behavioural responses produced by a given stimulus have been widely reported. This 'gain control' can boost the responses to sensory inputs that are particularly relevant, select among reflexes for execution by motoneurons or emphasize specific movement targets. Gain control is also an integral part of the smooth-pursuit eye movement system. One signature of gain control is that a brief perturbation of a stationary target during fixation causes tiny eye movements, whereas the same perturbation of a moving target during the active state of accurate pursuit causes large responses. Here we show that electrical stimulation of the smooth-pursuit eye movement region in the arcuate sulcus of the frontal lobe ('the frontal pursuit area', FPA) mimics the active state of pursuit. Such stimulation enhances the response to a brief perturbation of target motion, regardless of the direction of motion. We postulate that the FPA sets the gain of pursuit, thereby participating in target selection for pursuit.     

A limit on spin-charge separation in high-Tc superconductors from the absence of a vortex-memory effect.

There is a long-standing debate about whether spin-charge separation is the root cause of the peculiar normal-state properties and high superconducting transition temperatures of the high-Tc materials. In the proposed state of matter, the elementary excitations are not electron-like, as in conventional metals, but rather the electron 'fractionalizes' to give excitations that are chargeless spin-1/2 fermions (spinons) and charge +e bosons (chargons). Although spin-charge separation has been well established in one dimension, the theoretical situation for two dimensions is controversial and experimental evidence for it in the high-Tc materials is indirect. A model with sharp experimental tests for a particular type of separation in two dimensions has recently been proposed. Here we report the results of those experimental tests, placing a conservative upper limit of 190 K on the energy of the proposed topological defects known as visons. There is still debate about the extent to which this experiment can settle the issue of spin-charge separation in the high-Tc copper oxides, because some forms of the separation are able to avoid the need for visons. But at least one class of theories that all predict a vortex-memory effect now are unlikely models for the copper oxides.     

Subatomic movements of a domain wall in the Peierls potential

The discrete nature of crystal lattices plays a role in virtually every material property. But it is only when the size of entities hosted by a crystal becomes comparable to the lattice period--as occurs for dislocations, vortices in superconductors and domain walls--that this discreteness is manifest explicitly. The associated phenomena are usually described in terms of a background Peierls 'atomic washboard' energy potential, which was first introduced for the case of dislocation motion in the 1940s. This concept has subsequently been invoked in many situations to describe certain features in the bulk behaviour of materials, but has to date eluded direct detection and experimental scrutiny at a microscopic level. Here we report observations of the motion of a single magnetic domain wall at the scale of the individual peaks and troughs of the atomic energy landscape. Our experiments reveal that domain walls can become trapped between crystalline planes, and that they propagate by distinct jumps that match the lattice periodicity. The jumps between valleys are found to involve unusual dynamics that shed light on the microscopic processes underlying domain-wall propagation. Such observations offer a means for probing experimentally the physics of topological defects in discrete lattices--a field rich in phenomena that have been subject to extensive theoretical study.     

Chemotropic effect of specific target epithelium in the developing mammalian nervous system

Developing nerve fibres are guided to their targets by specific directional cues which are thought to be expressed in the tissues along the route and may involve the extracellular matrix. Another possibility, that directional cues emanate from the target itself, is consistent with the recent demonstration of homing behaviour by ectopic retinal ganglion axons and our previous demonstration that early trigeminal neurites grow directly to their virgin peripheral target in vitro. Here we show that this chemotropic effect is precisely limited to the trigeminal system; trigeminal ganglion neurites grow directly to their own target field but not to the adjoining field, normally innervated by the geniculate ganglion; furthermore, the trigeminal field does not influence the growth of geniculate neurites. Also, when trigeminal ganglia are co-cultured with isolated tissue layers of their target, neurites grow only towards the epithelial and not the mesenchymal component. These findings suggest that trigeminal epithelium is specified to attract correct innervation and that pathway mesenchyme, in which preformed guidance cues have been postulated, may provide favourable conditions for nerve fibre growth but not govern its direction.     

Advanced optics in a jellyfish eye

Cubozoans, or box jellyfish, differ from all other cnidarians by an active fish-like behaviour and an elaborate sensory apparatus. Each of the four sides of the animal carries a conspicuous sensory club (the rhopalium), which has evolved into a bizarre cluster of different eyes. Two of the eyes on each rhopalium have long been known to resemble eyes of higher animals, but the function and performance of these eyes have remained unknown. Here we show that box-jellyfish lenses contain a finely tuned refractive index gradient producing nearly aberration-free imaging. This demonstrates that even simple animals have been able to evolve the sophisticated visual optics previously known only from a few advanced bilaterian phyla. However, the position of the retina does not coincide with the sharp image, leading to very wide and complex receptive fields in individual photoreceptors. We argue that this may be useful in eyes serving a single visual task. The findings indicate that tailoring of complex receptive fields might have been one of the original driving forces in the evolution of animal lenses.     

加强对学生感性认识的培养

让一年级的学生在入学的第一学期就来实验室做一些简单的基础性实验,对培养学生科学求实的学习态度,保持良好的工作作风,增强安全意识,以及激发学生浓厚的学习兴趣,给后续实验打下良好的基础等方面均收到了良好的效果。     

思维分类理论研究述评

分类是一项非常重要的认知活动。通过分类活动,人们能够在一定程度上推知某一事物的未知特征,并作出预测。因而分类研究一直是现代心理学研究的重点和热点。回顾分类研究的历程,可把分类研究划分为两个领域。一是从实证的角度探讨分类活动内部的心理机制;一是从理论的角度概括分类活动的研究成果,提出分类的理论依据和模型。这些理论和模型主要围绕相似观和解释观来建构的,这两种观点都有一些实验证据的支持,但也都存在一些不足。因此,近年来,研究者又提出一些新的分类理论与模型。     

汉语耳语音库的建立与听觉实验研究

耳语音的识别和转换是个全新的课题,可应用于公共场合下的通讯和公安司法工作的某些特殊需要等方面.首先建立了一个包含1172个字和98个近音词的单人女声的汉语耳语音库.通过对两个听觉测试实验数据的统计分析,研究了人耳对汉语耳语音字和近音词声调的辨认率特征,得出人耳对孤立字四个声调的辨认率由高到低的排序为三声>四声>二声>一声.同时也得出人耳对词声调的辨认能力比字要强得多.幅值包络和音长这两个特征参量能够反映出汉语耳语音声调的特性,基于此参数对汉语耳语音字进行声调识别实验,其声调识别率已达到了人耳的平均辨认率,为连续耳语音声调识别研究打下了基础.