The function of alpha-crystallin in vision

The alpha-crystallins account for approximately one-third of the total soluble protein in the lens, contributing to its refractive power. In addition, alpha-crystallin also has a chaperone-like function and thus can bind unfolding lens proteins. Alpha B-crystallin is also found outside the lens, having an extensive tissue distribution. It is over-expressed in response to stresses of all kinds, where it is thought to serve a general protective function. Recently, it has been shown in humans that naturally occurring point mutations in the alpha-crystallins result in a deficit in chaperone-like function, and cause cataracts as well as a desmin-related myopathy. This review summarizes much of the past and current knowledge concerning the structure and functions of alpha-crystallin.     

The saltatory effect in vision.

Events in vision triggered by brief flashes in train prove to be strictly analogous to those generated in the skin by a succession of taps. A requisite condition for their appearance in vision is projection of the flashes into the peripheral retinal field. Experiments are described that establish the general relation between extent of saltatory leaping and degree of retinal eccentricity and between leaping and retinal subtense of stimulus patches. Additionally, a curious "dip" phenomenon and several qualitative experiments in which color has been used to "tag" the saltatory image are reported.     

The persistences of vision

Human observers continue to experience a visual stimulus for some time after the offset that stimulus. The neural activity evoked by a visual stimulus continues for some time after its offset. The information extracted from a visual stimulus continues to be registered in a visual form of memory ('iconic memory') for some time after its offset. We may thus distinguish three distinct senses in which a visual stimulus may be said to persist after its physical offset: there is phenomenological persistence, neural persistence and informational persistence. Various assumptions have been made about the relation between these forms of visual persistence. The most frequent assumption is that they correspond simply to three different methods for studying a single entity. Detailed consideration of what is known about the properties of these three forms of persistence suggests, however, that this assumption is not correct. It can reasonably be proposed that visible persistence is the phenomenological correlate of neural persistence occurring at various stages of the visual system: photoreceptors, ganglion cells and the stereopsis system. Iconic memory on the other hand, does not correspond to visible persistence, nor to neural persistence in any stage of the visual system. Recent work, in fact, suggests that iconic memory is a property of some relatively late stage in the visual information-processing system, rather than being a peripheral sensory buffer store. This suggestion raises some fundamental theoretical issues concerning the psychology of visual perception, issues with which cognitive psychology has yet to come to grips.     

有机污染物生物降解的机制研究进展

介绍了主要的有机污染物：脂肪烃、芳香烃、卤代烃、芳香酸及其酯类化合物在好氧和厌氧条件下的生物降解机理,以期为从事环保工作研究和环境生物技术的教学提供基础知识,促进环境科学的普及和发展。     

The Medawar Lecture 2001 knowledge for vision: vision for knowledge

An evolutionary development of perception is suggested-from passive reception to active perception to explicit conception-earlier stages being largely retained and incorporated in later species. A key is innate and then individually learned knowledge, giving meaning to sensory signals. Inappropriate or misapplied knowledge produces rich cognitive phenomena of illusions, revealing normally hidden processes of vision, tentatively classified here in a 'periodic table'. Phenomena of physiology are distinguished from phenomena of general rules and specific object knowledge. It is concluded that vision uses implicit knowledge, and provides knowledge for intelligent behaviour and for explicit conceptual understanding including science.     

The unsolved mystery of vision

Vision looms large in neuroscience--it is the subject of a gigantic literature and four Nobel prizes--but there is a growing realization that there are problems with the textbook explanation of how mammalian vision works. Here we will summarize the evidence behind this disquiet. In effect, we shall present a portrait of a field that is 'stuck'. Our initial focus, because it is our area of expertise, is on evidence that the early steps of mammalian vision are more diverse and more interesting than is usually imagined, so that our understanding of the later stages is in trouble right from the start. But we will also summarize problems, raised by others, with the later stages themselves.     

The machinery of colour vision

Some fundamental principles of colour vision, deduced from perceptual studies, have been understood for a long time. Physiological studies have confirmed the existence of three classes of cone photoreceptors, and of colour-opponent neurons that compare the signals from cones, but modern work has drawn attention to unexpected complexities of early organization: the proportions of cones of different types vary widely among individuals, without great effect on colour vision; the arrangement of different types of cones in the mosaic seems to be random, making it hard to optimize the connections to colour-opponent mechanisms; and new forms of colour-opponent mechanisms have recently been discovered. At a higher level, in the primary visual cortex, recent studies have revealed a simpler organization than had earlier been supposed, and in some respects have made it easier to reconcile physiological and perceptual findings.     

The role of background movement in goldfish vision

In experiments described in the literature objects presented to restrained goldfish failed to induce eye movements like fixation and/or tracking. We show here that eye movements can be induced only if the background (visual surround) is not stationary relative to the fish but moving. We investigated the influence of background motion on eye movements in the range of angular velocities of 5–20° s⁻¹. The response to presentation of an object is a transient shift in mean horizontal eye position which lasts for some 10 s. If an object is presented in front of the fish the eyes move in a direction such that it is seen more or less symmetrically by both eyes. If it is presented at ±70° from the fish's long axis the eye on the side of the object moves in the direction that the object falls more centrally on its retina. During these object induced eye responses the typical optokinetic nystagmus of amplitude of some 5° with alternating fast and slow phases is maintained, and the eye velocity during the slow phase is not modified by presentation of the object. Presenting an object in front of stationary or moving backgrounds leads to transient suppression of respiration which shows habituation to repeated object presentations. Accepted: 14 April 2000     

The evolution of color vision in the primates

There will be no difficulty in seeing how and by what mixtures the colors are made … He, however, who should attempt to verify all this by experiment would forget the difference of the human and the divine nature. For God only has the knowledge and also the power which are able to combine many things into one and again resolve the one into many. But no man either is or ever will be able to accomplish either the one or the other operation.The law of proportion according to which the several colors are formed, even if a man knew he would be foolish in telling, for he could not give any necessary reason, nor indeed any tolerable or probable explanation of them (Jowett, 1871).     

The dynamics of velocity adaptation in human vision

Since Barlow and Hill's classic study of the adaptation of the rabbit ganglion cell to movement [1], there have been several reports that motion adaptation is accompanied by an exponential reduction in spike rate, and similar estimates of the time course of velocity adaptation have been found across species [2-4]. Psychophysical studies in humans have shown that perceived velocity may reduce exponentially with adaptation [5,6]. It has been suggested that the reduction in firing of single cells may constitute the neural substrate of the reduction in perceived speed in humans [1,5-7]. Although a model of velocity coding in which the firing rate directly encodes speed may have the advantage of simplicity, it is not supported by psychophysical research. Furthermore, psychophysical estimates of the time course of perceived speed adaptation are not entirely consistent with physiological estimates. This discrepancy between psychophysical and physiological estimates may be due to the unrealistic assumption that speed is coded in the gross spike rate of neurons in the primary visual cortex. The psychophysical data on motion processing are, however, generally consistent with a model in which perceived velocity is derived from the ratio of two temporal channels [8-14]. We have examined the time course of speed adaptation and recovery to determine whether the observed rates can be better related to the established physiology if a ratio model of velocity processing is assumed. Our results indicate that such a model describes the data well and can accommodate the observed difference in the time courses of physiological and psychophysical processes.     

The mechanism of mental scanning in foveal vision

In order to perceive a visual pattern which includes several elemental pictures, the perceiver must allot his cognitive resources to suitably selected parts of the pattern and scan them in sequence. Even when the visual field is small and eye-movement is not required, such scanning is found. We called it mental scanning and performed psychological experiments to investigate the mechanism. The tasks were to discern whether the elemental pictures in a pattern are all the same (SP) or not (DP). The per cents correct of the task were measured for various exposure durations. We defined the threshold as the exposure duration at which 75% correct answers were obtained. Our main findings are as follows. The threshold for SP is proportional to the number of picture elements, while the threshold for DP is constant. It appears that two modes of mental scanning exist. One is serial processing for SP, and the other is parallel processing for DP. We proposed a two-layered neural network model having the following characteristics. 1) Information is transmitted as two types of signals through two separate channels; one is the transient signals to the Y layer and the other is the sustained signals slowly conducted to the X layer. 2) Interactions among neurons in the Y layer are lateral inhibitory, while those in the X layer are self-excitatory and lateralinhibitory. 3) Every neuron in the Y layer sends inhibitory signals to every neuron in the X layer except one with the same receptive field. Under these conditions, the dynamics of neurons in the X layer is represented by a set of certain equations. From phase plane analysis and numerical integration, the model appears to have an ability to account for various experimental results.