Kinematics of Visually-Guided Eye Movements

One of the hallmarks of an eye movement that follows Listing’s law is the half-angle rule that says that the angular velocity of the eye tilts by half the angle of eccentricity of the line of sight relative to primary eye position. Since all visually-guided eye movements in the regime of far viewing follow Listing’s law (with the head still and upright), the question about its origin is of considerable importance. Here, we provide theoretical and experimental evidence that Listing’s law results from a unique motor strategy that allows minimizing ocular torsion while smoothly tracking objects of interest along any path in visual space. The strategy consists in compounding conventional ocular rotations in meridian planes, that is in horizontal, vertical and oblique directions (which are all torsion-free) with small linear displacements of the eye in the frontal plane. Such compound rotation-displacements of the eye can explain the kinematic paradox that the fixation point may rotate in one plane while the eye rotates in other planes. Its unique signature is the half-angle law in the position domain, which means that the rotation plane of the eye tilts by half-the angle of gaze eccentricity. We show that this law does not readily generalize to the velocity domain of visually-guided eye movements because the angular eye velocity is the sum of two terms, one associated with rotations in meridian planes and one associated with displacements of the eye in the frontal plane. While the first term does not depend on eye position the second term does depend on eye position. We show that compounded rotation - displacements perfectly predict the average smooth kinematics of the eye during steady- state pursuit in both the position and velocity domain.     

Cursive writing with smooth pursuit eye movements

The eyes never cease to move: ballistic saccades quickly turn the gaze toward peripheral targets, whereas smooth pursuit maintains moving targets on the fovea where visual acuity is best. Despite the oculomotor system being endowed with exquisite motor abilities, any attempt to generate smooth eye movements against a static background results in saccadic eye movements [1, 2]. Although exceptions to this rule have been reported [3-5], volitional control over smooth eye movements is at best rudimentary. Here, I introduce a novel, temporally modulated visual display, which, although static, sustains smooth eye movements in arbitrary directions. After brief training, participants gain volitional control over smooth pursuit eye movements and can generate digits, letters, words, or drawings at will. For persons deprived of limb movement, this offers a fast, creative, and personal means of linguistic and emotional expression.     

眼球运动相关EOG信号的波形分析与特征提取

人-计算机交互系统(HCI)是当前计算机与人自然交流的研究热点之一。从眼眶周围记录的眼电图信号(EOG)可以转化为坐标去控制计算机。本文讨论了与眼球扫视运动相关的EOG波形在时域、频域内的分析及特征的提取,并应用简单贝斯分类算法建立原型系统。通过对实验数据的统计分析,初步验证了将眼电图信号转化为坐标点的可行性。     

A burst-based "Hebbian" learning rule at retinogeniculate synapses links retinal waves to activity-dependent refinement

Patterned spontaneous activity in the developing retina is necessary to drive synaptic refinement in the lateral geniculate nucleus (LGN). Using perforated patch recordings from neurons in LGN slices during the period of eye segregation, we examine how such burst-based activity can instruct this refinement. Retinogeniculate synapses have a novel learning rule that depends on the latencies between pre- and postsynaptic bursts on the order of one second: coincident bursts produce long-lasting synaptic enhancement, whereas non-overlapping bursts produce mild synaptic weakening. It is consistent with “Hebbian” development thought to exist at this synapse, and we demonstrate computationally that such a rule can robustly use retinal waves to drive eye segregation and retinotopic refinement. Thus, by measuring plasticity induced by natural activity patterns, synaptic learning rules can be linked directly to their larger role in instructing the patterning of neural connectivity.     

A Burst-Based “Hebbian” Learning Rule at Retinogeniculate Synapses Links Retinal Waves to Activity-Dependent Refinement

Patterned spontaneous activity in the developing retina is necessary to drive synaptic refinement in the lateral geniculate nucleus (LGN). Using perforated patch recordings from neurons in LGN slices during the period of eye segregation, we examine how such burst-based activity can instruct this refinement. Retinogeniculate synapses have a novel learning rule that depends on the latencies between pre- and postsynaptic bursts on the order of one second: coincident bursts produce long-lasting synaptic enhancement, whereas non-overlapping bursts produce mild synaptic weakening. It is consistent with “Hebbian” development thought to exist at this synapse, and we demonstrate computationally that such a rule can robustly use retinal waves to drive eye segregation and retinotopic refinement. Thus, by measuring plasticity induced by natural activity patterns, synaptic learning rules can be linked directly to their larger role in instructing the patterning of neural connectivity.     

Diagnostic features of emotional expressions are processed preferentially

Diagnostic features of emotional expressions are differentially distributed across the face. The current study examined whether these diagnostic features are preferentially attended to even when they are irrelevant for the task at hand or when faces appear at different locations in the visual field. To this aim, fearful, happy and neutral faces were presented to healthy individuals in two experiments while measuring eye movements. In Experiment 1, participants had to accomplish an emotion classification, a gender discrimination or a passive viewing task. To differentiate fast, potentially reflexive, eye movements from a more elaborate scanning of faces, stimuli were either presented for 150 or 2000 ms. In Experiment 2, similar faces were presented at different spatial positions to rule out the possibility that eye movements only reflect a general bias for certain visual field locations. In both experiments, participants fixated the eye region much longer than any other region in the face. Furthermore, the eye region was attended to more pronouncedly when fearful or neutral faces were shown whereas more attention was directed toward the mouth of happy facial expressions. Since these results were similar across the other experimental manipulations, they indicate that diagnostic features of emotional expressions are preferentially processed irrespective of task demands and spatial locations. Saliency analyses revealed that a computational model of bottom-up visual attention could not explain these results. Furthermore, as these gaze preferences were evident very early after stimulus onset and occurred even when saccades did not allow for extracting further information from these stimuli, they may reflect a preattentive mechanism that automatically detects relevant facial features in the visual field and facilitates the orientation of attention towards them. This mechanism might crucially depend on amygdala functioning and it is potentially impaired in a number of clinical conditions such as autism or social anxiety disorders.     

Branch angle and flow into a symmetric bifurcation

Arterial branches are found to be a major site for formation of arterial plaque. In this study, we investigate the role of the bifurcation angle on the flow into a symmetric bifurcation. Specially, how the changes in the bifurcation angle influences the distribution of axial wall shear in the bifurcation model. The flow in a range of branch opening half-angle of pi/25< or =theta< or =pi/4 are numerically simulated. The flow in the above models is calculated for the inlet flow Reynolds numbers of 250, 500, 1000, and 2000. It is found that at higher values of the opening angle of the bifurcation, the possibility and severity of flow separation at the appropriate wall location increases.     

The Viscoelastic Properties of Passive Eye Muscle in Primates. III: Force Elicited by Natural Elongations

We have recently shown that in monkey passive extraocular muscles the force induced by a stretch does not depend on the entire length history, but to a great extent is only a function of the last elongation applied. This led us to conclude that Fung''s quasi-linear viscoelastic (QLV) model, and more general nonlinear models based on a single convolution integral, cannot faithfully mimic passive eye muscles. Here we present additional data about the mechanical properties of passive eye muscles in deeply anesthetized monkeys. We show that, in addition to the aforementioned failures, previous models also grossly overestimate the force exerted by passive eye muscles during smooth elongations similar to those experienced during normal eye movements. Importantly, we also show that the force exerted by a muscle following an elongation is largely independent of the elongation itself, and it is mostly determined by the final muscle length. These additional findings conclusively rule out the use of classical viscoelastic models to mimic the mechanical properties of passive eye muscles. We describe here a new model that extends previous ones using principles derived from research on thixotropic materials. This model is able to account reasonably well for our data, and could thus be incorporated into models of the eye plant.     

Nanosecond time-resolved fluorescence investigations of temperature-induced conformational changes in cytochrome oxidase in phosphatidylcholine vesicles and solubilized systems

Intrinsic and lipid phase transition-induced conformational changes in cytochrome oxidase in phosphatidylcholine vesicle and solubilized systems were examined by the fluorescence lifetime of N-(1-anilinonaphthyl-4)-maleimide conjugated with the enzyme. The time-dependent fluorescence intensity of N-(1-anilinonaphthyl-4)-maleimide attached to cytochrome oxidase was described as a triple exponential decay. Both the intrinsic and lipid phase transition-induced conformational changes were detectable in plots of the average lifetime against temperature. In most cases a peak occurred at the temperature of the conformational change. The time-dependent emission anisotropy showed that N-(1-anilinonaphthyl-4)-maleimide embedded in cytochrome oxidase in phosphatidylcholine vesicles underwent a rapid restricted wobbling within a cone. The half-angle of the cone was around 30 degrees for cytochrome oxidase in dimyristoyl phosphatidylcholine vesicles.     

Diploid-dependent regulation of gene expression: a genetic cause of abnormal development in fish haploid embryos

A diploid-dependent regulatory mechanism of gene expression for spatial patterning of the eye in vertebrates has been determined by analyzing the phenotypes of haploid goldfish embryos. There are two gene loci in charge of eye spatial patterning during embryonic morphogenesis. The expressional probability for each copy of the two genes in a set of chromosomes is 50%. A pair of genes in two sets of homologous or heterologous chromosomes is 100% and essential for normal gene expression. The haploid condition itself would result in the obstruction of gene expression and abnormal development because the diploid-dependent regulatory apparatus will regulate gene expression in a haploid embryo according to the same rule as in the diploid embryo.     

Topography and ocular dominance: a model exploring positive correlations

The map from eye to brain in vertebrates is topographic, i.e. neighbouring points in the eye map to neighbouring points in the brain. In addition, when two eyes innervate the same target structure, the two sets of fibres segregate to form ocular dominance stripes. Experimental evidence from the frog and goldfish suggests that these two phenomena may be subserved by the same mechanisms. We present a computational model that addresses the formation of both topography and ocular dominance. The model is based on a form of competitive learning with subtractive enforcement of a weight normalization rule. Inputs to the model are distributed patterns of activity presented simultaneously in both eyes. An important aspect of this model is that ocular dominance segregation can occur when the two eyes are positively correlated, whereas previous models have tended to assume zero or negative correlations between the eyes. This allows investigation of the dependence of the pattern of stripes on the degree of correlation between the eyes: we find that increasing correlation leads to narrower stripes. Experiments are suggested to test this prediction.     

Effects of temperature on cell size and number in the yellow dung fly Scathophaga stercoraria

(1) A number of hypotheses suggest that the temperature-size rule (larger at cooler temperatures), and consequently Bergmann clines in whole-organism body size (larger at higher latitudes), may be a mere consequence of processes at the cellular level, i.e., a physiological constraint.(2) We show that in the yellow dung fly, Scathophaga stercoraria (Diptera: Scathophagidae), the temperature-size rule holds for wing cell and ommatidia size. Increases in cell number made up two-thirds (eye) to three-quarters (wing) of the increase in organ size. Temperature effects on body size can be fully explained by its effects on cell size and number.(3) Our study adds to the generality of previous results in Drosophila spp. The physiological constraint hypothesis remains viable as a proximate, non-adaptive explanation for the temperature-size rule in ectotherms.     

A neural computation for visual acuity in the presence of eye movements

Humans can distinguish visual stimuli that differ by features the size of only a few photoreceptors. This is possible despite the incessant image motion due to fixational eye movements, which can be many times larger than the features to be distinguished. To perform well, the brain must identify the retinal firing patterns induced by the stimulus while discounting similar patterns caused by spontaneous retinal activity. This is a challenge since the trajectory of the eye movements, and consequently, the stimulus position, are unknown. We derive a decision rule for using retinal spike trains to discriminate between two stimuli, given that their retinal image moves with an unknown random walk trajectory. This algorithm dynamically estimates the probability of the stimulus at different retinal locations, and uses this to modulate the influence of retinal spikes acquired later. Applied to a simple orientation-discrimination task, the algorithm performance is consistent with human acuity, whereas naive strategies that neglect eye movements perform much worse. We then show how a simple, biologically plausible neural network could implement this algorithm using a local, activity-dependent gain and lateral interactions approximately matched to the statistics of eye movements. Finally, we discuss evidence that such a network could be operating in the primary visual cortex.     

Degradation of DIAP1 by the N-end rule pathway is essential for regulating apoptosis

Some members of the inhibitor of apoptosis (IAP) protein family block apoptosis by binding to and neutralizing active caspases. We recently demonstrated that a physical association between IAP and caspases alone is insufficient to regulate caspases in vivo and that an additional level of control is provided by IAP-mediated ubiquitination of both itself and the associated caspases. Here we show that Drosophila IAP 1 (DIAP1) is degraded by the 'N-end rule' pathway and that this process is indispensable for regulating apoptosis. Caspase-mediated cleavage of DIAP1 at position 20 converts the more stable pro-N-degron of DIAP1 into the highly unstable, Asn-bearing, DIAP1 N-degron of the N-end rule degradation pathway. Thus, DIAP1 represents the first known metazoan substrate of the N-end rule pathway that is targeted for degradation through its amino-terminal Asn residue. We demonstrate that the N-end rule pathway is required for regulation of apoptosis induced by Reaper and Hid expression in the Drosophila melanogaster eye. Our data suggest that DIAP1 instability, mediated through caspase activity and subsequent exposure of the N-end rule pathway, is essential for suppression of apoptosis. We suggest that DIAP1 safeguards cell viability through the coordinated mutual destruction of itself and associated active caspases.     

A Model of Self-Organizing Head-Centered Visual Responses in Primate Parietal Areas

We present a hypothesis for how head-centered visual representations in primate parietal areas could self-organize through visually-guided learning, and test this hypothesis using a neural network model. The model consists of a competitive output layer of neurons that receives afferent synaptic connections from a population of input neurons with eye position gain modulated retinal receptive fields. The synaptic connections in the model are trained with an associative trace learning rule which has the effect of encouraging output neurons to learn to respond to subsets of input patterns that tend to occur close together in time. This network architecture and synaptic learning rule is hypothesized to promote the development of head-centered output neurons during periods of time when the head remains fixed while the eyes move. This hypothesis is demonstrated to be feasible, and each of the core model components described is tested and found to be individually necessary for successful self-organization.     

Comparative studies on the eye morphology of lungless salamanders,family Plethodontidae,and the effect of miniaturization

In seven species of lungless salamanders, family Plethodontidae, ranging from medium to very small in body size and from small to very large in cell size, the morphology of the eye and the retina were investigated. Haller's rule was only partially corroborated. While the smallest species had the relatively largest eyes, the largest two species, having the largest cells, showed the third and fourth largest eyes of the series. An effect of cell size was also found with regard to eye morphology. Small species with small cells as well as large species with large cells had relatively small retinae and relatively large lenses. In contrast, small eyes with relatively large cells had absolutely and relatively large retinae and relatively small lenses. The retinae of all investigated plethodontids showed a morphology typical for land vertebrates with two fiber and three nuclear layers. Rods, cones and double cones could clearly be distinguished. A fovea or area centralis was not found. Retina ganglion cell and photoreceptor counts show that the number of these elements was lower than in salamandrids. However, determination of the resolution power of miniaturized eyes based on morphological and behavioral data shows that this does not seem to constitute a functional disadvantage. The morphological and functional properties and constraints of eyes of miniaturized salamanders are discussed.     

The anatomy of geniculocortical connections in monocularly deprived cats

In monocularly deprived (MD) cats, many cells in the lateral geniculate nucleus (LGN) but few cells in the visual cortex respond to input from the deprived eye, suggesting that the connections to visual cortex from the deprived geniculate laminae may have been disrupted. It has been known for some time that the afferents representing the deprived eye terminate over a smaller percentage of layer IV than do those representing the experienced eye, but it is becoming increasingly clear that this alone cannot explain the inability of the deprived pathway to activate cortical cells. 2-Deoxyglucose studies of ocular dominance columns in MD cats have shown that the columns are often (a) restricted to layer IV, suggesting that intracortical connections may be disrupted, and (b) very faint, suggesting that MD alters the efficacy of the deprived pathway in addition to restricting its territory. Electron microscopy has shown that both deprived and experienced afferents end in terminals that contain mitochondria and round synaptic vesicles and that make asymmetric contacts with dendritic profiles. However, the terminals of deprived afferents differ from those of experienced afferents: they are 25% smaller, contain 33% fewer mitochondria, are more likely to make synapses that are presynaptically convex (and thus, perhaps, immature), make fewer perforated synapses, and synapse onto smaller spines. Further, the geniculocortical axons from deprived laminae appear to end in fewer synaptic terminals, than do those from the experienced laminae. The finding that the synaptic terminals of deprived afferents are both abnormal morphologically and fewer in number can help to explain the reduced effectiveness of the deprived eye in driving cortical cells but does not rule out additional effects such as suppression and loss of intracortical connectivity.     

Compound eyes of insects and crustaceans: Some examples that show there is still a lot of work left to be done

Similarities and differences between the 2 main kinds of compound eye (apposition and superposition) are briefly explained before several promising topics for research on compound eyes are being introduced. Research on the embryology and molecular control of the development of the insect clear‐zone eye with superposition optics is one of the suggestions, because almost all of the developmental work on insect eyes in the past has focused on eyes with apposition optics. Age‐ and habitat‐related ultrastructural studies of the retinal organization are another suggestion and the deer cad Lipoptena cervi, which has an aerial phase during which it is winged followed by a several months long parasitic phase during which it is wingless, is mentioned as a candidate species. Sexual dimorphism expressing itself in many species as a difference in eye structure and function provides another promising field for compound eye researchers and so is a focus on compound eye miniaturization in very small insects, especially those that are aquatic and belong to species, in which clear‐zone eyes are diagnostic or are tiny insects that are not aquatic, but belong to taxa like the Diptera for instance, in which open rather than closed rhabdoms are the rule. Structures like interommatidial hairs and glands as well as corneal microridges are yet another field that could yield interesting results and in the past has received insufficient consideration. Finally, the dearth of information on distance vision and depth perception is mentioned and a plea is made to examine the photic environment inside the foam shelters of spittle bugs, chrysales of pupae and other structures shielding insects and crustaceans.     

A discussion of the pediatric clinical guidelines

It is evident from both the AAO and AOA guideline position statements that a visual and eye examination is essential within the first few months of life. Thus, both groups recognize the importance of examining infants within the critical period of visual development. The adequacy of testing performed by lay screeners or primary care physicians to rule out all visual disorders is highly questionable. Photorefraction is a promising screening technique, but as yet needs to be validated by further study. The only adequate means of detecting early vision and eye disorders is through examination by an eye care professional. However, this conclusion immediately underscores the need for eye care professionals prepared to examine very young children. The primary eye care of children should not be considered the responsibility of a minority of ‘pediatric specialists’. Clinical techniques are currently available which would enable all practitioners to evaluate children in the first few months of life, a critical time for an initial evaluation. We believe it is essential for all eye care professionals to be trained in these basics of eye and vision care. The need for widespread pediatric training should be addressed when reviewing the clinical education of interns.Both guidelines yield valuable guidance and background information for the pediatric examination. The AOA guideline is more complete, and includes a useful bibliography. It is well researched, and should be particularly helpful to the newer practitioner. The AAO guideline provides less detailed guidance, and omits some newer and important examination techniques, such as preferential looking for visual acuity and spray application for administering drugs.The AOA and AAO pediatric clinical practice guidelines can serve as helpful examination tutorials for the practitioner. The guidelines do not address the decision making process of diagnosis and treatment options. The references included in the documents can help fill in the gaps, but would require a substantial commitment of time for research and study. We see the need for additional guidelines to help guide the process of difficult decision-making when the patient is ‘in the chair’. The publication of such second generation documents would serve eye care professionals in the total case management of the pediatric patient.     

Neural Activity in the Macaque Putamen Associated with Saccades and Behavioral Outcome

It is now widely accepted that the basal ganglia nuclei form segregated, parallel loops with neocortical areas. The prevalent view is that the putamen is part of the motor loop, which receives inputs from sensorimotor areas, whereas the caudate, which receives inputs from frontal cortical eye fields and projects via the substantia nigra pars reticulata to the superior colliculus, belongs to the oculomotor loop. Tracer studies in monkeys and functional neuroimaging studies in human subjects, however, also suggest a potential role for the putamen in oculomotor control. To investigate the role of the putamen in saccadic eye movements, we recorded single neuron activity in the caudal putamen of two rhesus monkeys while they alternated between short blocks of pro- and anti-saccades. In each trial, the instruction cue was provided after the onset of the peripheral stimulus, thus the monkeys could either generate an immediate response to the stimulus based on the internal representation of the rule from the previous trial, or alternatively, could await the visual rule-instruction cue to guide their saccadic response. We found that a subset of putamen neurons showed saccade-related activity, that the preparatory mode (internally- versus externally-cued) influenced the expression of task-selectivity in roughly one third of the task-modulated neurons, and further that a large proportion of neurons encoded the outcome of the saccade. These results suggest that the caudal putamen may be part of the neural network for goal-directed saccades, wherein the monitoring of saccadic eye movements, context and performance feedback may be processed together to ensure optimal behavioural performance and outcomes are achieved during ongoing behaviour.