Impaired axonal regeneration in acrylamide intoxication

Acrylamide is a neurotoxin known to impair regeneration of axons following nerve crush and to produce structurally abnormal regenerating sprouts. To investigate the mechanism of these abnormalities, protein synthesis and fast axonal transport were studied in acrylamide-intoxicated and control rats 2 weeks after sciatic nerve crush. Using an in vitro preparation of sciatic nerve-dorsal root ganglion, there was no difference in ganglion ³H-leucine incorporation between the two groups. In these preparations of sensory axons, as well as in motor axons studied in vivo, a smaller proportion of rapidly transported radioactivity was carried beyond the crush in the acrylamide-regenerating nerves compared to the control-regenerating nerves. Correlative ultrastructural studies demonstrated that this difference reflected the impaired outgrowth of the acrylamide-regenerating nerves, rather than an abnormality in fast transport. The acrylamide-treated sprouts often developed swellings filled with whorls of neurofilaments; in addition, many sprouts ended in massively enlarged growth cones containing membranous organelles. EM autoradiography showed labeled, rapidly transported organelles accumulated in the neurofilamentous whorls, and therefore suggested that these organelles might be “trapped” or impeded in passage through these regions. However, there was no evidence that the growth cones received insufficient amounts of transported protein; in fact, the distended endings were densely labeled and apparently “ballooned” by transported organelles. These results suggest that acrylamide intoxication does not impair regeneration by diminishing the delivery of rapidly transported materials to the growing tip. Rather, the marked distention of the growth cones is interpreted as the morphological consequence of continued delivery of rapidly transported organelles into sprouts unable to utilize them in outgrowth.     

Evolution of colour vision in mammals

Colour vision allows animals to reliably distinguish differences in the distributions of spectral energies reaching the eye. Although not universal, a capacity for colour vision is sufficiently widespread across the animal kingdom to provide prima facie evidence of its importance as a tool for analysing and interpreting the visual environment. The basic biological mechanisms on which vertebrate colour vision ultimately rests, the cone opsin genes and the photopigments they specify, are highly conserved. Within that constraint, however, the utilization of these basic elements varies in striking ways in that they appear, disappear and emerge in altered form during the course of evolution. These changes, along with other alterations in the visual system, have led to profound variations in the nature and salience of colour vision among the vertebrates. This article concerns the evolution of colour vision among the mammals, viewing that process in the context of relevant biological mechanisms, of variations in mammalian colour vision, and of the utility of colour vision.     

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 uses of colour vision: behavioural and physiological distinctiveness of colour stimuli

Colour and greyscale (black and white) pictures look different to us, but it is not clear whether the difference in appearance is a consequence of the way our visual system uses colour signals or a by-product of our experience. In principle, colour images are qualitatively different from greyscale images because they make it possible to use different processing strategies. Colour signals provide important cues for segmenting the image into areas that represent different objects and for linking together areas that represent the same object. If this property of colour signals is exploited in visual processing we would expect colour stimuli to look different, as a class, from greyscale stimuli. We would also expect that adding colour signals to greyscale signals should change the way that those signals are processed. We have investigated these questions in behavioural and in physiological experiments. We find that male marmosets (all of which are dichromats) rapidly learn to distinguish between colour and greyscale copies of the same images. The discrimination transfers to new image pairs, to new colours and to image pairs in which the colour and greyscale images are spatially different. We find that, in a proportion of neurons recorded in the marmoset visual cortex, colour-shifts in opposite directions produce similar enhancements of the response to a luminance stimulus. We conclude that colour is, both behaviourally and physiologically, a distinctive property of images.     

Visual acuity and colour vision tests--a preliminary report.

Results are reported of a preliminary survey of colour vision changes in fifteen patients with central serous retinopathy. Colour vision was monitored with the HRR plates, 100-hue test and Nagel anomaloscope. In those patients revealing an acquired dyschromatopsia the defect had a tritan-like response. However, diagnosis is made difficult because of the positively correlated trend between 100-hue error score and visual acuity.     

Ultraviolet colour vision and ornamentation in bluethroats

Many birds see in the ultraviolet (300–400 nm), but there is limited evidence for colour communication (signalling by spectral shape independently of brightness) in this ''hidden'' waveband. Such data are critical for the understanding of extravagant plumage colours, some of which show considerable UV reflectance. We investigated UV colour vision in female social responses to the male UV/violet ornament in bluethroats, Luscinia s. svecica. In an outdoor aviary at the breeding grounds, 16 females were each presented with a unique pair of males of equal age. In UVR (UV reduction) males, sunblock chemicals reduced only the UV reflectance and thereby the spectral shape (colour) of the throat ornament. In NR (neutral reduction) males, an achromatic pigment in the same base solvent (preen gland fat) was used for a corresponding but uniform brightness reduction. Both colour and brightness effects were invisible to human eyes, and were monitored by spectrometry. In 13 of the 16 trials, the female associated most with the NR male, a preference that implies that UV colour vision is used in mate choice by female bluethroats. Reflectance differences between one-year-old and older males were significant only in UV, suggestive of a UV colour cue in age-related mate preferences.     

Deficient colour vision and interpretation of histopathology slides: cross sectional study.

OBJECTIVE: To determine whether histopathologists with deficient colour vision make more errors in slide interpretation than those with normal colour vision. DESIGN: Examination of projected transparencies of histopathological slides under standardised conditions by subjects whose colour discriminating ability was accurately assessed. SETTING: Departments of histopathology in 45 hospitals in the United Kingdom. SUBJECTS: 270 male histopathologists and medical laboratory scientific officers. MAIN OUTCOME MEASURES: Number of slides correctly identified by subjects whose colour vision was measured on the Ishihara, City University, and Farnsworth-Munsell 100 hue tests. RESULTS: Mean (SD) scores (out of 10) for doctors with colour deficient vision were 9.4 (0.7) v 9.9 (0.4) for controls (P < 0.01) and 7.5 (1.6) v 9.4 (0.7) for scientific officers (P < 0.001). When subjects with colour deficient vision were categorised into severe, moderate, or mild, there was a significant trend towards those with severe deficiency making more mistakes (P < 0.001). CONCLUSIONS: Histopathologists and medical laboratory scientific officers should have their colour vision tested; if they are found to have a severe protan or deutan deficiency, they should be advised to adopt a safe system of working.     

The design of diagnostic tests for defective colour vision.

Pseudoisochromatic plates are among the most popular tests for defective colour vision. They are particularly good for screening but are less good in assessing the degree and type of the colour vision defect. To select colours for use in diagnostic plates a large number of colour defective subjects have made colour matches with the Lovibond Tintometer and the isochromatic data collected. Pseudoisochromatic plates have been printed using pairs of colours only and incorporating both a random dot and a regular dot format. These plates have proved effective in a clinical trial. Not only must pairs of inks be carefully selected to lie upon appropriate isochromatic lines but the luminance contrast between the two colours must be kept within 5%. Failure to control luminance contrast is as much a source of error in currently available pseudoischromatic tests as the inappropriate use of colour.