Aids for visual impairment.

This article provides only a flavour of the type and range of aids available to the visually impaired person. Many other aids for leisure, learning, and daily living are illustrated in the RNIB equipment and games catalogue.     

Oscillatory synchrony and human visual cognition.

Oscillatory synchrony could be used to establish dynamic links between the various cortical areas participating in the same cognitive process. Is it possible to detect oscillatory synchrony in humans, and is it relevant to behavior? There is now converging evidence for the existence of a transient oscillatory activity in the gamma range (30-60 Hz), obtained in response to static visual objects, and having only a loose temporal relationship to stimulus onset. This so-called "induced" gamma response is much larger in response to coherent static or moving objects. However, functional variations of gamma and/or beta (15-20 Hz) oscillations are not restricted to perceptive, bottom-up mechanisms, but are also observed during visual imagery or short-term memory maintenance. Oscillations at the scalp level thus seem to reflect large-scale neural cooperativity in a variety of task-dependent networks. Human intra-cranial recordings in a short-term memory paradigm further reveal the existence and the task-dependency of oscillatory synchrony in the beta range, between focal sites separated by several centimeters and with a few milliseconds time-lag. These findings thus confirm experimentally the hypothesis of a functional role of synchronized oscillatory activity in the coordination of distributed neural activity in humans, and support Hebb's concept of short-term memory maintenance by reentrant activity within the activated network. In addition, the intra-cranial data obtained in humans and monkeys also help to better understand the neural mechanisms generating scalp-recorded oscillations.     

Saccadic suppression precedes visual motion analysis.

There is now good evidence that perception of motion is strongly suppressed during saccades (rapid shifts of gaze), presumably to blunt the disturbing sense of motion that saccades would otherwise elicit. Other aspects of vision, such as contrast detection of high-frequency or equiluminant gratings, are virtually unaffected by saccades [1] [2] [3] [4] [5]. This has led to the suggestion that saccades may suppress selectively the magnocellular pathway (which is strongly implicated in motion perception), leaving the parvocellular pathway unaffected [5] [6]. Here, we investigate the neural level at which perception of motion is suppressed. We used a simple technique in which an impression of motion is generated from only two frames, allowing precise control over the stimulus [7] [8]. One frame has a certain fixed contrast, whereas the contrast of the other (the test frame) is varied to determine the threshold for motion discrimination (that is, the lowest test-frame contrast level at which the direction of motion can be correctly guessed). Contrast thresholds of the test depended strongly and non-monotonically on the contrast of the fixed-contrast frame, with a minimum at medium contrast. To study the effect of saccadic suppression, we triggered the two-frame sequence by a voluntary saccade. Thresholds during saccades increased in a way that suggested that saccadic suppression precedes motion analysis: when the test frame was first in the motion sequence there was a general depression of sensitivity, whereas when it was second, the contrast response curve was shifted to a higher contrast range, sometimes even resulting in higher sensitivity than without a saccade. The dependence on presentation order suggests that saccadic suppression occurs at an early stage of visual processing, on the single frames themselves rather than on the combined motion signal. As motion detection itself is thought to occur at an early stage, saccadic suppression must take place at a very early phenomenon.     

Binocular visual processing in the owl's telencephalon.

Single neurons recorded from the owl's visual Wulst are surprisingly similar to those found in mammalian striate cortex. The receptive fields of Wulst neurons are elaborated, in an apparently hierarchical fashion, from those of their monocular, concentrically organized inputs to produce binocular interneurons with increasingly sophisticated requirements for stimulus orientation, movement and binocular disparity. Output neurons located in the superficial laminae of the Wulst are the most sophisticated of all, with absolute requirements for a combination of stimuli, which include binocular presentation at a particular horizontal binocular disparity, and with no response unless all of the stimulus conditions are satisfied simultaneously. Such neurons have the properties required for 'global stereopsis', including a receptive field size many times larger than their optimal stimulus, which is more closely matched to the receptive fields of the simpler, disparity-selective interneurons. These marked similarities in functional organization between the avian and mammalian systems exist in spite of a number of structural differences which reflect their separate evoluntionary origins. Discussion therefore includes the possibility that there may exist for nervous systems only a very small number of possible solutions, perhaps a unique one, to the problem of stereopsis.     

Functional visual streams.

The idea that visual signals relayed by the parvocellular and magnocellular subdivisions of the lateral geniculate nucleus remain segregated in the cerebral cortex has attracted considerable attention. It has been proposed that parvocellular contributions dominate in the temporal visual cortex, and that magnocellular contributions dominate in the parietal cortex. Recent experiments have shown that the organization of primate visual pathways is not this simple.     

The perceived image: efficient modelling of visual inhomogeneity.

When we fixate on a certain location in a scene, the image that our visual system provides us with is not like a snapshot of the scene. Because of visual inhomogeneity, the image we perceive is sharp and clean only around the fixation location, and it gradually blurs and loses detail with increasing distance from that location. In this paper, a procedure is presented that allows one to obtain the luminance distribution in the perceived image of a scene. The procedure is based on a stack distribution model of spatial visual processing, with parameters obtained experimentally. An application of this procedure is presented which allows an explanation of data on the decrease of visual acuity with eccentricity. The advantages of this procedure for taking visual inhomogeneity into account when seeking explanations for perceptual phenomena are also discussed.     

An owl's eye: Schematic optics and visual performance inStrix aluco L.

Summary The schematic eye ofStrix aluco, a nocturnal owl species, is described. A comparative and ecological context is used to examine the relationships between optical parameters of the eye and its light gathering and resolving powers. It is concluded that the essentially nocturnal feature of the owl eye does not lie in either its light gathering power or the sensitivity of individual rod receptors. Differences in visual performance at low light levels between the owl and the diurnal pigeon appear to be attributable to differences in the retinal neural integration mechanisms of the two species. However, it is hypothesised that the neural mechanisms which mediate the extraction of spatial information from the retinal image throughout the nocturnal luminance range, can function in the owl eye only because of its absolutely large sized retinal image. Thus the primarily nocturnal feature of the owl eye is its absolutely large posterior nodal distance, retinal image brightness is maximised only as a secondary feature.Abbreviation PND posterior nodal distance     

Measurement of the 2D affine Lie group parameters for visual motion analysis.

A class of linear operators is presented for estimating the local components of 2D translation, dilatation, rotation, and the shear/deformations which span the six degrees of freedom of motion of arbitrarily textured surfaces. This results in a model of visual motion analysis which proposes that the local transformations in the image are analysed by decomposing them into the six one-parameter subgroups of the 2D affine group. Each of the required invariant integral operators are easily specified by the characters of these six subgroups. The 2D affine group, however, does not have a simple structure. It is a Lie group which possesses a semi-direct product manifold, and classical harmonic analysis cannot proceed unless some mechanism is prescribed to isolate the 2D 'general linear' transformations from the 2D translations. It must also do so using measures which receive only local support from the image, since the global affine group model is only valid tangentially. A form of 'active perception' is thereby implicated; it is proposed that spatial indexing and 2D tracking is needed in order to form reliable estimates of 3D motion parameters using local operators in a data-driven fashion.     

Investigation of the visual cytoscreening of conventional gynecologic smears. II. Analysis of eye movement

The perception of visual information in cytoscreening was studied: eye movements were recorded while the cytotechnologist was screening cervical smears by means of a projection screen. Four phases of eye movement could be distinguished: small, aimless movements during the stage movement; a latency period with a duration of about 180 milliseconds; saccadic movement to the position of an object; and fixation on an object. These components explain the two-phase behavior of cytoscreening found in our previous investigations of the stage movement. Visual perception during the period of latency was found to be the most important since only those objects that are recognized by peripheral vision during this period can trigger the necessary saccadic movement before fixation takes place. The scanpath of search in the stationary field of view is determined by the conspicuousness of the objects; the main features of conspicuousness are size and contrast. Even with the comparatively small fields of view (24 degrees and 29 degrees in diameter) used in these experiments, it was found that the detection threshold of peripheral vision increases towards the margin of the field of view. This raises the question of whether the use of large-field binoculars (with 40-degree visual angles) may cause higher false-negative rates for samples with only a few atypical cells.     

Biochemical aspects of the visual process. XXVIII. Classification of sulfhydryl groups in phodopsin and other photoreceptor membrane proteins.

Reaction of isolated bovine rod outer segment membrane with radioactive N-ethylmaleimide, both in the presence and absence of 1% dodecyl sulfate followed by dodecyl sulfate-polyacrylamide gel electrophoresis, shows that six sulfhydryl groups (96% of total sulfhydryl in this membrane) are located on the rhodopsin molecule. On the basis of their reactivity towards rho-chloromercuribenzoate and rho-chloromercuribenzene sulfonate in suspensions of outer segment membranes, the sulfhydryl groups of rhodopsin can be divided into three pairs. One pair is rapidly modified, both in light and darkness. This modification does not impair the recombination capacity of opsin with 11-cis retinaldehyde under regeneration of rhodopsin. A second pair is modified upon prolonged interaction with the rho-chloromercuriderivatives in darkness. Modification of this pair leaves the typical rhodopsin absorbance at 500 nm intact, but a proportional loss of recombination capacity does occur. The third pair is only modified after illumination and isprobably located in the vicinity of the chromophoric center. The differences between these results and those obtained by modification with dithiobis-(2-nitrobenzoic acid) or N-ethylmaleimide in suspension, where even upon prolonged exposure to light as well as in darkness only two sulfhydryl groups of rhodopsin are modified, is explained by the detergent-like character of the rho-chloromercuri-derivatives.     

Similarity of visual selectivity among clonally related neurons in visual cortex

Neurons in rodent visual cortex are organized in a salt-and-pepper fashion for orientation selectivity, but it is still unknown how this functional architecture develops. A recent study reported that the progeny of single cortical progenitor cells are preferentially connected in the postnatal cortex. If these neurons acquire similar selectivity through their connections, a salt-and-pepper organization may be generated, because neurons derived from different progenitors are intermingled in rodents. Here we investigated whether clonally related cells have similar preferred orientation by using a transgenic mouse, which labels all the progeny of single cortical progenitor cells. We found that preferred orientations of clonally related cells are similar to each other, suggesting that cell lineage is involved in the development of response selectivity of neurons in the cortex. However, not all clonally related cells share response selectivity, suggesting that cell lineage is not the only determinant of response selectivity.     

Motion-reversal visual evoked responses.

Motion-reversal visual evoked responses (VERs) have remarkable waveform variability. In our opinion this is caused by the alternative predominance of either motion or pattern-onset/offset related components. The motion dependent component of motion-reversal VER closely resembles motion-onset VER (main negative peak with the latency of about 170 ms), the first positive peak (with the latency of about 100 ms) corresponds to the pattern-onset component and the second non-constant positive peak (with the latency of about 130 ms) seems to be identical with the pattern-offset positivity. The differences in expression of these components are dependent on some stimulus characteristics (mainly on the contrast of a structure, velocity of motion, retinal localization of the stimulus) and on substantial differences in the sensitivity of subjects to motion stimulation.     

Influence of temporal lobe lesions on photic-evoked responses in occipital visual areas.

In 20 patients with temporal lobe lesions and 10 controls, the averaged photic-evoked responses (APERs) and their dispersion pattern (DP) were investigated in inion-vertex-lead and bilaterally in inion-parietal leads (I-P3 and I-P4). The patients with temporal lobe lesions, regardless of lesion location in temporal areas, displayed either absence of APER (the whole APER or only the initial components) or a latency increase without amplitude changes. The DP was generally abnormal. The role of the temporal lobe in the organization of APER in visual areas is discussed.     

Music alters visual perception

Background

Visual perception is not a passive process: in order to efficiently process visual input, the brain actively uses previous knowledge (e.g., memory) and expectations about what the world should look like. However, perception is not only influenced by previous knowledge. Especially the perception of emotional stimuli is influenced by the emotional state of the observer. In other words, how we perceive the world does not only depend on what we know of the world, but also by how we feel. In this study, we further investigated the relation between mood and perception.

Methods and Findings

We let observers do a difficult stimulus detection task, in which they had to detect schematic happy and sad faces embedded in noise. Mood was manipulated by means of music. We found that observers were more accurate in detecting faces congruent with their mood, corroborating earlier research. However, in trials in which no actual face was presented, observers made a significant number of false alarms. The content of these false alarms, or illusory percepts, was strongly influenced by the observers'' mood.

Conclusions

As illusory percepts are believed to reflect the content of internal representations that are employed by the brain during top-down processing of visual input, we conclude that top-down modulation of visual processing is not purely predictive in nature: mood, in this case manipulated by music, may also directly alter the way we perceive the world.     

Fucntional specialization and binocular interaction in the visual areas of rhesus monkey prestriate cortex.

If is is believed that neural mechanisms mediating stereoscopic vision may be localized in specific areas of the visual cortex, then it becomes necessary to be able to define these areas adequately. This is no easy matter in the rhesus monkey, an animal close to man, where the cytoarchitecturally uniform prestriate cortex is folded into deep sulci with secondary gyri. One way around this awkward problem is to use the callosal connections of the prestriate cortex as the anatomical landmarks. Callosal connections are restricted to regions at which the vertical meridian is represented. Since the visual fields, including the vertical meridian, are separately represented in each area, each has its own callosal connections. These are of great help in defining some of the boundaries of these areas, since the boundaries often coincide with the representation of the vertical meridian. With the visual areas thus defined anatomically, it becomes relatively easy to assign recordings to particular areas. Studies of binocular interactions in these areas reveal that most cells in all prestriate areas are binocularly driven. Hence, theoretically, all of the prestriate areas are candidates for stereoscopic mechanisms. The degree of binocular interaction varies from cell to cell. At the two extremes are cells which either respond to monocular stimulation only and are inhibited by binocular stimulation or ones which respond to binocular stimulation only. Changing, as opposed to fixed, disparity is signalled by two types of cells. In one category are cells activated in opposite directions for the two eyes. Such cells are always binocularly driven. In the other category are cells, some of which are monocularly activated, that are capable of responding to changing image size. In the monkey, both these categories of cells have so far been found in the motion area of the superior temporal sulcus only.     

Photolysis intermediates of the artificial visual pigment cis-5,6-dihydro-isorhodopsin.

The photolysis intermediates of an artificial bovine rhodopsin pigment, cis-5,6-dihydro-isorhodopsin (cis-5,6,-diH-ISORHO, lambda max 461 nm), which contains a cis-5,6-dihydro-9-cis-retinal chromophore, are investigated by room temperature, nanosecond laser photolysis, and low temperature irradiation studies. The observations are discussed both in terms of low temperature experiments of Yoshizawa and co-workers on trans-5,6-diH-ISORHO (Yoshizawa, T., Y. Shichida, and S. Matuoka. 1984. Vision Res. 24: 1455-1463), and in relation to the photolysis intermediates of native bovine rhodopsin (RHO). It is suggested that in 5,6-diH-ISORHO, a primary bathorhodopsin intermediate analogous to the bathorhodopsin intermediate (BATHO) of the native pigment, rapidly converts to a blue-shifted intermediate (BSI, lambda max 430 nm) which is not observed after photolysis of native rhodopsin. The analogs from lumirhodopsin (LUMI) to meta-II rhodopsin (META-II) are generated subsequent to BSI, similar to their generation from BATHO in the native pigment. It is proposed that the retinal chromophore in the bathorhodopsin stage of 5,6-diH-ISORHO is relieved of strain induced by the primary cis to trans isomerization by undergoing a geometrical rearrangement of the retinal. Such a rearrangement, which leads to BSI, would not take place so rapidly in the native pigment due to ring-protein interactions. In the native pigment, the strain in BATHO would be relieved only on a longer time scale, via a process with a rate determined by protein relaxation.     

Cortical connections and early visual function: intra- and inter-columnar processing.

While it is widely assumed that the long-range horizontal connections in V1 are present to support contour integration, there has been only limited consideration of other possible relationships between anatomy and physiology (the horizontal connections) and visual function beyond contour integration. We introduce the possibility of other relationships directly from the perspective of computation and differential geometry by identifying orientation columns in visual physiology with the (unit) tangent bundle in differential geometry. This suggests abstracting early vision in a space that incorporates both position and orientation, from which we show that the physiology is capable of supporting a number of functional computations beyond contour integration, including texture-flow and shading-flow integration, as well as certain relationships between them. The geometric abstraction emphasizes the role of curvature, which necessitates a coupled investigation into how it might be estimated. The result is an elaboration of layer-to-layer interactions within an orientation column, with non-linearities possibly implemented by shunting inhibition. Finally, we show how the same computational framework naturally lends itself to solving stereo correspondence, with binocular tangents abstracting curves in space.     

Asymmetries in visual texture discrimination

Recent results have shown that texture discrimination is an asymmetrical process; texture A within texture B may be much easier to detect than texture B within texture A. Two questions regarding discrimination asymmetries are addressed: (i) what sorts of textural properties are associated with discrimination asymmetries; and (ii) what sort of architecture would yield asymmetries. Two experiments show that discrimination asymmetries obtain when textures comprise circles of different sizes (large circles are easier to detect in small than vice versa) and when circles differ only in the regularity of their placement (irregularly placed circles are easier to detect in a background of regularly placed circles than vice versa). A plausible account of texture discrimination would involve the decomposition of images via a set orientation and scale selective filters followed by a second layer of filtering to detect energy differences between adjacent regions in the original convolutions. Discrimination asymmetries provide prima facie evidence against such a model because it involves only local measurements and comparisons. We propose that discrimination asymmetries are elegantly explained if it is assumed that the responses of the orientation and scale selective filters are normalized by the degree to which similarly tuned operators are responding elsewhere in the image; viz., global normalization of filter responses. However, there are cases where such global normalization is not required to explain asymmetrical discrimination.