The ability to see clear images, to resolve fine spatial detail in faint objects, to see and appr... more The ability to see clear images, to resolve fine spatial detail in faint objects, to see and appreciate small color differences, to discriminate motion direction and to detect rapidly flickering lights over a very large range of natural illumination is something we often take for granted. Good vision is our expectation, but as we grow older, we become aware of very gradual visual deficits that in some cases can lead to severe loss of vision. These deficits are often caused by changes in the optics of the eye and/or the retina. A normal supply of oxygen and nutrients to the retina is essential to maintain good vision, but when the supporting mechanisms fail, local hypoxia can trigger structural changes that affect normal vision. In this article, we review endogenous and exogenous factors that cause changes in the supply of oxygen to the eye and how these can affect color vision when oxygen demands vary with light adaptation level. We review the latest color vision assessment techniques and show how the level of retinal oxygenation can affect color vision in normal trichromats and in subjects with congenital color deficiencies. We also discuss acquired loss of color vision in relation to hypoxia and how it may soon become possible to use advanced vision and optometric tests to detect the earliest signs of disease that precede clinical diagnosis and to monitor disease progression and effectiveness of treatment. We explore how knowledge of normal differences between photopic and mesopic visual performance and the expected limits of normal variability as a function of age can help achieve this goal.
Proceedings of the National Academy of Sciences of the United States of America, Aug 19, 1997
Following striate cortex damage in monkeys and humans there can be residual function mediated by ... more Following striate cortex damage in monkeys and humans there can be residual function mediated by parallel visual pathways. In humans this can sometimes be associated with a ``feeling'' that something has happened, especially with rapid movement or abrupt onset. For less transient events, discriminative performance may still be well above chance even when the subject reports no conscious awareness of the stimulus. In a previous study we examined parameters that yield good residual visual performance in the ``blind'' hemifield of a subject with unilateral damage to the primary visual cortex. With appropriate parameters we demonstrated good discriminative performance, both with and without conscious awareness of a visual event. These observations raise the possibility of imaging the brain activity generated in the ``aware'' and the ``unaware'' modes, with matched levels of discrimination performance, and hence of revealing patterns of brain activation associated with visual awareness. The intact hemifield also allows a comparison with normal vision. Here we report the results of a functional magnetic resonance imaging study on the same subject carried out under aware and unaware stimulus conditions. The results point to a shift in the pattern of activity from neocortex in the aware mode, to subcortical structures in the unaware mode. In the aware mode prestriate and dorsolateral prefrontal cortices (area 46) are active. In the unaware mode the superior colliculus is active, together with medial and orbital prefrontal cortical sites.
We have extended the experiments of McCann et al. by incorporating their Mondrian stimulus into a... more We have extended the experiments of McCann et al. by incorporating their Mondrian stimulus into a dynamic color matching (DCM) technique that allows the subject to match accurately the color of any test patch under sequential changes of illuminant. Changes in the perceived color of a central test stimulus as a result of surround illuminant changes are investigated in a
Adaptive Optics for Industry and Medicine - Proceedings of the Sixth International Workshop, 2008
An ocular adaptive optics system was used to investigate the effects of higher-order ocular aberr... more An ocular adaptive optics system was used to investigate the effects of higher-order ocular aberrations on everyday functional vision. The system comprised a Shack-Hartmann wavefront sensor, a Badal optometer and cylindrical lenses to statically pre-correct refractive errors, and a 35 element bimorph mirror from AOptix to dynamically compensate for higher-order aberrations. Measurements of contrast acuity with and without correction of
While coeliac disease is primarily a disease of the digestive system, there have been several rep... more While coeliac disease is primarily a disease of the digestive system, there have been several reports of neurological effects, both motor and cognitive. Here, we present the case of a woman with coeliac disease, under dietary control, in whom there is profound long-standing visual disturbance including reduction of visual fields, loss of rapid flicker and colour sensitivity and severe deficits in acuity. Structural magnetic resonance imaging (MRI) indicates large regions of calcification and abnormal tissue that is restricted to the occipital cortex, particularly the posterior region. Functional MRI indicates an absence of normal visual activation in the primary visual cortex, but at least in one hemisphere, there is neural activity to moving stimuli in visual motion area hMT+. White matter microstructure in the pathway between the lateral geniculate nucleus and hMT+ is normal compared to healthy control subjects, but is severely abnormal between the lateral geniculate nucleus and primary visual cortex. This case study illustrates the very specific nature of cortical deficit that can arise in association with coeliac disease, and highlights the importance of early dietary control for the disease.
The presence of a bright light in the visual field has two main effects on the retinal image: red... more The presence of a bright light in the visual field has two main effects on the retinal image: reduced contrast and increased retinal illuminance because of scattered light; the latter can, under some conditions, lead to an improvement in retinal sensitivity. The combined effect remains poorly understood, particularly at low light levels. A psychophysical flicker-cancellation test was used to measure the amount and angular distribution of scattered light in the eye for 40 observers. Contrast thresholds were measured using a functional contrast sensitivity test. Pupil-plane glare-source illuminances (i.e., 0, 1.35, and 19.21 lm/m<sup>2</sup>), eccentricities (5°, 10°, and 15°), and background luminances (1, 2.6, and 26 cd/m<sup>2</sup>) were investigated. Visual performance was better than predicted, based on a loss of retinal image contrast caused by scattered light, particularly in the mesopic range. Prediction accuracy improved significantly when the expected increase in retinal sensitivity in the presence of scattered light was also incorporated in the model.
The presence of a bright light source in the visual field can generate visual discomfort. Based o... more The presence of a bright light source in the visual field can generate visual discomfort. Based on empirical observations we can predict to a reasonable degree of accuracy how uncomfortable a given lighting installation is likely to be; yet very little is known about the mechanism or physiological underpinnings that lead to visual discomfort. This study attempts to elucidate some of the underlying mechanisms by controlling the amount of light reaching the retina and by varying photometric properties of the glare source. The participants were required to view a source of light presented against a simulated residential street background in the form of uniform flashes of light of varying intensity. Discomfort-glare thresholds were estimated using a staircase procedure; the dependent variable was retinal illuminance. The size of the glare source and the luminance of the surrounding background were varied systematically. Across glare-source sizes or background luminances the discomfort-g...
The spectral sensitivity of the eye was investigated using reaction times to broadband chromatic ... more The spectral sensitivity of the eye was investigated using reaction times to broadband chromatic stimuli over a range of background luminances. Relative sensitivity was determined from the nonlinear reaction time curve by converting reaction times to a linear measure that was independent of spectral sensitivity. Two models for mesopic spectral sensitivity were compared. The first was a linear combination of V(lambda) and V'(lambda), and the second included input from the L-M colour-opponent mechanism and the S-cones. The second model produced a significantly better fit to the data. The chromatic mechanisms appear to contribute to reaction time when there is an appreciable chromatic signal but luminance contrast is low.
A small circular target of high retinal illuminance level can have a comet-like appearance when p... more A small circular target of high retinal illuminance level can have a comet-like appearance when presented moving continuously with a speed as low as 0.2 deg/s. This perceived lengthening of the circular target increases with the speed of movement and is only observed for target presentations outside the foveal region. Data on the parametric properties of the "comet effect" are presented together with related results on the time-course of recovery of retinal sensitivity following brief exposure to intense stimuli. Measurement of target spectral irradiance levels which are just sufficient to yield the comet effect suggests that the lengthening of the circular target reflects a rod-cone interaction and therefore it may be due to unsuppressed, saturated rod responses at high retinal illuminance levels. The restriction of the comet effect to areas outside the foveal region is used to produce spatial maps of what appears to be the rod-free area of the retina. A model simulation ...
All forms of corneal refractive surgery can sometimes cause an increase in optical aberrations an... more All forms of corneal refractive surgery can sometimes cause an increase in optical aberrations and scattered light, which can affect visual performance. The purpose of this study was to develop a suitable test that was sensitive to retinal image degradation in subjects who have undergone excimer laser refractive surgery and that was also relevant to visual demands in commercial aviation. Assessment of the visual environment and the tasks involved in piloting a commercial aircraft formed the basis for the selection of the test parameters. The new contrast acuity assessment (CAA) test covers a functional visual field of +/-5 degrees and is based on minimum spatial vision requirements for commercial pilots. Data measured in 100 normal subjects were used to define the 'standard normal observer' and the range of variation for the parameters of the test. This approach makes it possible to quickly establish whether a given subject's performance falls within the range of the sta...
A comparison of the results obtained with the Nagel anomaloscope and the Holmes-Wright Type A, Sp... more A comparison of the results obtained with the Nagel anomaloscope and the Holmes-Wright Type A, Spectrolux, and Beyne aviation color vision lanterns was undertaken. The Joint Aviation Requirements (JAR) specify pass/fail limits for these four secondary color vision tests and the Ishihara screening test. The results for individuals on all five tests were studied. The color vision of 55 color-vision deficient and 24 color-vision normal subjects, mostly applicant pilots, was assessed using a battery of tests, including the Ishihara plates, the Nagel anomaloscope, and three lanterns. The testing methods and characteristics of the lanterns and anomaloscope were compared. Of the color-deficient applicants, only deuteranomalous trichromats passed more than one of the four secondary JAR tests, but a pass on one test did not reliably predict a pass on another test. Three out of nine protanomalous trichromats passed the Nagel anomaloscope but failed all three lantern tests. Of the normal trich...
The ability to see clear images, to resolve fine spatial detail in faint objects, to see and appr... more The ability to see clear images, to resolve fine spatial detail in faint objects, to see and appreciate small color differences, to discriminate motion direction and to detect rapidly flickering lights over a very large range of natural illumination is something we often take for granted. Good vision is our expectation, but as we grow older, we become aware of very gradual visual deficits that in some cases can lead to severe loss of vision. These deficits are often caused by changes in the optics of the eye and/or the retina. A normal supply of oxygen and nutrients to the retina is essential to maintain good vision, but when the supporting mechanisms fail, local hypoxia can trigger structural changes that affect normal vision. In this article, we review endogenous and exogenous factors that cause changes in the supply of oxygen to the eye and how these can affect color vision when oxygen demands vary with light adaptation level. We review the latest color vision assessment techniques and show how the level of retinal oxygenation can affect color vision in normal trichromats and in subjects with congenital color deficiencies. We also discuss acquired loss of color vision in relation to hypoxia and how it may soon become possible to use advanced vision and optometric tests to detect the earliest signs of disease that precede clinical diagnosis and to monitor disease progression and effectiveness of treatment. We explore how knowledge of normal differences between photopic and mesopic visual performance and the expected limits of normal variability as a function of age can help achieve this goal.
Proceedings of the National Academy of Sciences of the United States of America, Aug 19, 1997
Following striate cortex damage in monkeys and humans there can be residual function mediated by ... more Following striate cortex damage in monkeys and humans there can be residual function mediated by parallel visual pathways. In humans this can sometimes be associated with a ``feeling'' that something has happened, especially with rapid movement or abrupt onset. For less transient events, discriminative performance may still be well above chance even when the subject reports no conscious awareness of the stimulus. In a previous study we examined parameters that yield good residual visual performance in the ``blind'' hemifield of a subject with unilateral damage to the primary visual cortex. With appropriate parameters we demonstrated good discriminative performance, both with and without conscious awareness of a visual event. These observations raise the possibility of imaging the brain activity generated in the ``aware'' and the ``unaware'' modes, with matched levels of discrimination performance, and hence of revealing patterns of brain activation associated with visual awareness. The intact hemifield also allows a comparison with normal vision. Here we report the results of a functional magnetic resonance imaging study on the same subject carried out under aware and unaware stimulus conditions. The results point to a shift in the pattern of activity from neocortex in the aware mode, to subcortical structures in the unaware mode. In the aware mode prestriate and dorsolateral prefrontal cortices (area 46) are active. In the unaware mode the superior colliculus is active, together with medial and orbital prefrontal cortical sites.
We have extended the experiments of McCann et al. by incorporating their Mondrian stimulus into a... more We have extended the experiments of McCann et al. by incorporating their Mondrian stimulus into a dynamic color matching (DCM) technique that allows the subject to match accurately the color of any test patch under sequential changes of illuminant. Changes in the perceived color of a central test stimulus as a result of surround illuminant changes are investigated in a
Adaptive Optics for Industry and Medicine - Proceedings of the Sixth International Workshop, 2008
An ocular adaptive optics system was used to investigate the effects of higher-order ocular aberr... more An ocular adaptive optics system was used to investigate the effects of higher-order ocular aberrations on everyday functional vision. The system comprised a Shack-Hartmann wavefront sensor, a Badal optometer and cylindrical lenses to statically pre-correct refractive errors, and a 35 element bimorph mirror from AOptix to dynamically compensate for higher-order aberrations. Measurements of contrast acuity with and without correction of
While coeliac disease is primarily a disease of the digestive system, there have been several rep... more While coeliac disease is primarily a disease of the digestive system, there have been several reports of neurological effects, both motor and cognitive. Here, we present the case of a woman with coeliac disease, under dietary control, in whom there is profound long-standing visual disturbance including reduction of visual fields, loss of rapid flicker and colour sensitivity and severe deficits in acuity. Structural magnetic resonance imaging (MRI) indicates large regions of calcification and abnormal tissue that is restricted to the occipital cortex, particularly the posterior region. Functional MRI indicates an absence of normal visual activation in the primary visual cortex, but at least in one hemisphere, there is neural activity to moving stimuli in visual motion area hMT+. White matter microstructure in the pathway between the lateral geniculate nucleus and hMT+ is normal compared to healthy control subjects, but is severely abnormal between the lateral geniculate nucleus and primary visual cortex. This case study illustrates the very specific nature of cortical deficit that can arise in association with coeliac disease, and highlights the importance of early dietary control for the disease.
The presence of a bright light in the visual field has two main effects on the retinal image: red... more The presence of a bright light in the visual field has two main effects on the retinal image: reduced contrast and increased retinal illuminance because of scattered light; the latter can, under some conditions, lead to an improvement in retinal sensitivity. The combined effect remains poorly understood, particularly at low light levels. A psychophysical flicker-cancellation test was used to measure the amount and angular distribution of scattered light in the eye for 40 observers. Contrast thresholds were measured using a functional contrast sensitivity test. Pupil-plane glare-source illuminances (i.e., 0, 1.35, and 19.21 lm/m<sup>2</sup>), eccentricities (5°, 10°, and 15°), and background luminances (1, 2.6, and 26 cd/m<sup>2</sup>) were investigated. Visual performance was better than predicted, based on a loss of retinal image contrast caused by scattered light, particularly in the mesopic range. Prediction accuracy improved significantly when the expected increase in retinal sensitivity in the presence of scattered light was also incorporated in the model.
The presence of a bright light source in the visual field can generate visual discomfort. Based o... more The presence of a bright light source in the visual field can generate visual discomfort. Based on empirical observations we can predict to a reasonable degree of accuracy how uncomfortable a given lighting installation is likely to be; yet very little is known about the mechanism or physiological underpinnings that lead to visual discomfort. This study attempts to elucidate some of the underlying mechanisms by controlling the amount of light reaching the retina and by varying photometric properties of the glare source. The participants were required to view a source of light presented against a simulated residential street background in the form of uniform flashes of light of varying intensity. Discomfort-glare thresholds were estimated using a staircase procedure; the dependent variable was retinal illuminance. The size of the glare source and the luminance of the surrounding background were varied systematically. Across glare-source sizes or background luminances the discomfort-g...
The spectral sensitivity of the eye was investigated using reaction times to broadband chromatic ... more The spectral sensitivity of the eye was investigated using reaction times to broadband chromatic stimuli over a range of background luminances. Relative sensitivity was determined from the nonlinear reaction time curve by converting reaction times to a linear measure that was independent of spectral sensitivity. Two models for mesopic spectral sensitivity were compared. The first was a linear combination of V(lambda) and V'(lambda), and the second included input from the L-M colour-opponent mechanism and the S-cones. The second model produced a significantly better fit to the data. The chromatic mechanisms appear to contribute to reaction time when there is an appreciable chromatic signal but luminance contrast is low.
A small circular target of high retinal illuminance level can have a comet-like appearance when p... more A small circular target of high retinal illuminance level can have a comet-like appearance when presented moving continuously with a speed as low as 0.2 deg/s. This perceived lengthening of the circular target increases with the speed of movement and is only observed for target presentations outside the foveal region. Data on the parametric properties of the "comet effect" are presented together with related results on the time-course of recovery of retinal sensitivity following brief exposure to intense stimuli. Measurement of target spectral irradiance levels which are just sufficient to yield the comet effect suggests that the lengthening of the circular target reflects a rod-cone interaction and therefore it may be due to unsuppressed, saturated rod responses at high retinal illuminance levels. The restriction of the comet effect to areas outside the foveal region is used to produce spatial maps of what appears to be the rod-free area of the retina. A model simulation ...
All forms of corneal refractive surgery can sometimes cause an increase in optical aberrations an... more All forms of corneal refractive surgery can sometimes cause an increase in optical aberrations and scattered light, which can affect visual performance. The purpose of this study was to develop a suitable test that was sensitive to retinal image degradation in subjects who have undergone excimer laser refractive surgery and that was also relevant to visual demands in commercial aviation. Assessment of the visual environment and the tasks involved in piloting a commercial aircraft formed the basis for the selection of the test parameters. The new contrast acuity assessment (CAA) test covers a functional visual field of +/-5 degrees and is based on minimum spatial vision requirements for commercial pilots. Data measured in 100 normal subjects were used to define the 'standard normal observer' and the range of variation for the parameters of the test. This approach makes it possible to quickly establish whether a given subject's performance falls within the range of the sta...
A comparison of the results obtained with the Nagel anomaloscope and the Holmes-Wright Type A, Sp... more A comparison of the results obtained with the Nagel anomaloscope and the Holmes-Wright Type A, Spectrolux, and Beyne aviation color vision lanterns was undertaken. The Joint Aviation Requirements (JAR) specify pass/fail limits for these four secondary color vision tests and the Ishihara screening test. The results for individuals on all five tests were studied. The color vision of 55 color-vision deficient and 24 color-vision normal subjects, mostly applicant pilots, was assessed using a battery of tests, including the Ishihara plates, the Nagel anomaloscope, and three lanterns. The testing methods and characteristics of the lanterns and anomaloscope were compared. Of the color-deficient applicants, only deuteranomalous trichromats passed more than one of the four secondary JAR tests, but a pass on one test did not reliably predict a pass on another test. Three out of nine protanomalous trichromats passed the Nagel anomaloscope but failed all three lantern tests. Of the normal trich...
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