Research Lecture Advance Programme 2001/02

Vision Science and its Clinical Applications

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Temporal frequency and chromatic contrast adaptation

Chromatic contrast adaptation (i.e., exposure to a field whose chromaticity is temporally modulated along a line in colour space) is a valuable technique for delineating cardinal and higher-order colour mechanisms. Most studies of chromatic contrast adaptation have modulated test lights at 1 Hz on the assumption that second-stage mechanisms respond best to low temporal frequencies. I will discuss evidence that suggests that second-order adaptation is a relatively fast process.

The chromatic selectivity of global motion processing

I will present two experiments that examine the chromatic selectivity of global motion processing in humans.

Experiment 1 uses a structure-from-motion (SFM) task and observers have to extract the 3-D shape of an object defined by local speed differences. This SFM task is performed in various colour directions, an isoluminant red-green, the tritanopic confusion line, and an achromatic direction. We find that S cones do not contribute to SFM but that there is a significant input of the red-green mechanism to SFM. We also measured speed discrimination thresholds and find that the relative superiority in performance of the luminance versus the red-green mechanism is not accounted for by the difference in speed discrimination performance.

Experiment 2 assesses the chromatic selectivity of global motion processing by using a random dot kinematogram. Observers were asked to detect coherent motion in the various colour directions. We find again that the S cones do not contribute to global motion processing for the parameter range we used. In the isoluminant plane, global motion seems to be mediated only by the red-green mechanism. Adding chromatic noise with identical projections onto the red-green mechanism yields the same thresholds as pure red or green stimuli. This provides further evidence that in the isoluminant plane, global motion is mediated exclusively by a red-green mechanism.

Conclusions:

• For a wide range of parameters (low spatial frequencies and low speeds) the S cones do not contribute to global motion processing (assessed either by a structure from motion task or by using random dot kinematograms)
• There is a significant input of the red-green mechanism to global motion processing (for both tasks)
• The red-green mechanism does not seem to be limited by the extraction of the local speed signals (in the SFM task) but by the fast integration of the local motion signals
• For the majority of our observers (8 out of 10), global motion is mediated exclusively by the red-green mechanisms: when chromatic noise is added, the projection onto the red-green mechanism determines human performance.

The epidemiology of refractive errors

There are at least three reasons why this topic deserves attention:-

1. Comparative studies may help to throw light on environmental factors, thereby influencing public health care.
2. Preventive measures may evolve, which are likely to be of long-term benefit to eye health.
3. Greater insight is likely to be gained into basic underlying biological mechanisms, leading to the discovery of new information.

Illustrations of these ideas will be provided.

Sensory basis of reading problems

10% of children have unexpected difficulty learning to read and spell properly despite having normal intelligence; this is known as developmental dyslexia and it is now known to have a neurodevelopmental basis. We have found that deficits in visual and auditory transient processing that is normally mediated by magnocellular neuronal systems may account for many of the difficulties. This has helped us to generate new, highly effective, means of helping dyslexics.

Prof. John Stein FRCP is a Professor of Physiology at Oxford University. He studied medicine at New College, Oxford and St Thomas's Hospital, London. He anticipated a career in Neurology, continuing his training in London, Leicester and Oxford. But he soon decided that basic research into the visual guidance of movement might be more useful, and he was appointed tutor in medicine at Magdalen College in 1970. Since then he has been studying normal and abnormal eye and limb movement control in animals, neurological patients and dyslexics. He began to study the role of eye control in dyslexics in 1978, and has been pursuing the hypothesis that many of dyslexics' problems may result from impaired low level perceptual visuomotor and auditory processing that is caused by abnormal development of magnocellular neurones in the brain.

Using compound eyes to discover how human vision works

During their evolution compound eye have solved most the basic problems of vision. As first demonstrated by Heffer Hartline with Limulus, their relative simplicity allows us to look more deeply at the design principles that govern vision. I will discuss the relevance of our studies of flies' eyes to three aspects of human vision - the regulation of pupil diameter, motion adaptation (e.g. the waterfall effect) and the constraints imposed on retinal and cortical function by the metabolic demands of photoreceptors and neurons.

The role of the retinal pigment epithelium in regulating development of the neural retina.

Albinos have a diverse series of deficits including abnormal chiasmatic pathways, reduced rod numbers and an underdeveloped central retina. Hence a melanin associated product plays a major role in sculpting retinal development. Current research is focused on the identification of this agent and establishing its mode of action.

Keeping an eye on the time

The eye has been one of the best-studied parts of the central nervous system and its fundamental functions are considered well understood. All photoreception within the eye of mammals was considered the province of the rods and cones. However, studies over the past decade have provided the substrate for the heretical view that the mammalian eye contains a novel photoreceptor system. Mice with eyes but lacking rod and cone photoreceptors can detect light to regulate their circadian rhythms, suppress pineal melatonin, modify locomotor activity, and regulate pupil size. This lecture will outline the discovery of this novel photoreceptor system of the eye and discuss some recent experiments detailing how these photoreceptors might function.

Lantern Tests for North of the 49th Parallel and West of the Prime Meridian

The recent challenges to colour vision standards have brought about the desire to have occupationally-relevant colour vision tests. This is especially true in the transportation industry where movement of equipment is governed by coloured signal lights. This presentation will overview the development of a lantern test for the railway industry and discuss how individuals with colour vision defects perform on this test and other lantern tests used in Canada.

Electrophysiological approaches to the early diagnosis of glaucoma

Raised intraocular pressure (IOP) is a major risk factor for glaucoma, but only approximately 1% of patients with pressure of 25mm Hg or above develop the condition each year. Significant retinal ganglion cell loss may occur prior to the development of a visual field defect, and it is therefore of potential benefit to identify those patients with elevated IOP at risk of field defect prior to the occurrence of the visual field loss.

Electrophysiology offers a wide spectrum of tools to assess visual pathway function (e.g. ERG, EOG, pattern-ERG, multifocal ERG, VEP, motion VEP, multifocal VEP). Nearly all of these have been reported to be sensitive to glaucomatous damage, usually by comparing a group of patients with advanced glaucoma to a group of normal subjects. However, rather than inter-group differences, measures are needed that allow accurate diagnosis and management in an individual patient. Further, the electrophysiological characteristics of advanced glaucoma may differ from those in early disease.

The pattern electroretinogram (PERG), a direct indicator of retinal ganglion cell function, is markedly affected by glaucoma, and in longitudinal studies has correctly identified eyes at risk before the development of the visual field defect. These data will be fully discussed. Less proven but promising measures such as the photopic negative response, the motion VEP and the multifocal VEP will also be addressed.

Topographic visual maps in the occipital lobe of human albinos

The retinal projection to the thalamus is abnormal in albinos and causes conflicting visual information to converge at the level of the visual cortex. In the majority of animals studied this sensory mismatch is resolved either by reordering the cortical projection or by suppressing the abnormal cortical input. Using fMRI retinotopic mapping techniques, we demonstrate that neither of these solutions is adopted by the human visual system. We find that the sensory conflict is resolved but it is through reorganization of intracortical connections.

Prime-sight in a Blindsight Patient

D.B., the subject of the book 'Blindsight', has recently returned for follow-up testing. Many of the original capacities remain, but in addition he now experiences after-images on the termination of the non-experienced stimuli in his blind field. These will be described, together with some recent event-related potential recordings.

A description of some of the after-image phenomena appears in Nature Neuroscience, Feb. 2002, Weiskrantz, Cowey & Hodinott-Hill.

The use of pupil perimetry in clinical practice

The pupil light reflex has been used by clinicians for over two thousand years to evaluate patients with visual loss. Advances in recording techniques and analysis tools have led recently to the development of pupil perimetry, a technique for testing sensitivity in different parts of the visual field using pupil responses to perimetric stimuli. Results of performing pupil perimetry in patients with a variety of neuro-ophthalmological problems will be presented.

The Tale of Two Tests: Functional vs. Clinical Vision Assessment

The vast majority of tests of acuity, contrast sensitivity and visual field have been designed to support clinical diagnosis and treatment. However, with the advent of legislation protecting the rights of individuals with disabilities, tests of functional vision are becoming increasingly important. Differences in these two assessment techniques will be discussed along with some results from recent studies conducted in our laboratory.

Visual psycho-physics at the Colour and Multimedia Laboratory

To generate the visual stimulus, computer-controlled output devices are used: colour monitors, projectors, and printers. In addition, modern light sources (eg., white LED clusters) are also applied. An important issue is the characterisation of these light sources including spectral measurements to find mathematical models to predict the dependence of their light output on their input current or voltage values. Computer-controlled projectors will provide real-life mesopic driving situations on the wall plus mesopic targets with flashing LEDs and Landolt rings of superimposed quasi-monochromatic radiation. Reaction time and search time will be measured. Other research concerns the fields of:

1. colour rendering of modern light sources;
2. "cognitive" colour - colour memory;
3. the so-called colour size effect - the colour appearance of large coloured surfaces.

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