John L. Barbur

Variation of chromatic sensitivity across the life span

Thresholds were measured along three directions in color space for detecting an equiluminant color change of a set of bars embedded in a larger field of spatio-temporal achromatic noise for observers ranging in age from 3 months to 86 years. Pre-verbal observers were assessed with a forced-choice preferential-looking technique while older observers responded orally or manually. Over the life span, thresholds could be described along each color axis tested by a curve with two trends. Thresholds decreased with each doubling of age by nearly a factor of two until adolescence. Thereafter, thresholds increased by a factor of 1.4-2 with each doubling of age. Sensitivity to chromatic differences varied similarly along all three axes tested, suggesting uniformity in the sensitivity of chromatic mechanisms across the life span.

Insights into the Different Exploits of Colour in the Visual Cortex

A new method that allows controlled masking of luminance contrast has been developed to study the use of chromatic signals in human vision. The method also makes it possible to examine the different uses of chromatic signals (e. g. the generation of perceived colour, or the construction and representation of object structure and form). By using this technique, we studied the threshold detection of chromatic signals in normal trichromats. The results show that chromatic signals are virtually unaffected by ongoing, randomly varying, luminance contrast changes. These findings suggest that chromatic signals are either processed independently or can be separated completely from any confounding luminance contrast components in the stimulus. Thresholds for detection of colour changes only, and for extraction of stimulus structure from chromatic signals in normal trichromats, in subjects with single cone receptor deficiency (i.e. dichromats) and in three subjects with abnormal colour vision caused by bilateral damage to ventromedial, extra-striate visual cortex (i. e. subjects with cerebral achromatopsia) have also been measured. No significant difference in thresholds for the two conditions was observed either in normal trichromats or in dichromats. Subjects with cerebral achromatopsia, however, reveal markedly different thresholds. The results suggest that chromatic signals are processed independently to generate perceived object colour or to construct spatially structured objects, and that these functions involve different neural substrates. The results help to explain, at least in part, why cerebral achromatopsia is a heterogeneous disorder, and why there can be significant differences in the effective use of chromatic signals in subjects described as cerebral achromatopsics.

Supplementation with the carotenoids lutein or zeaxanthin improves human visual performance

Background:  Macular pigment (MP) is found in diurnal primate species when vision spans a range of ambient illumination and is mediated by cone and rod photoreceptors. The exact role of MP remains to be determined. In this study we investigate two new hypotheses for possible MP functions.

Pupillary responses to stimulus structure, colour and movement

Pupillary responses to stimuli which favour the preferential stimulation of neural mechanisms invoked in the detection of visual attributes such as colour, spatial structure, movement and light flux changes on the retina have been measured and compared. Pupil responses in a decrement m stimulus luminance (i.e., a flash of darkness), suggest that at least three components are involved in this response, their relative contribution being determined largely by stimulus size. contrast and presentation time. A comparison of pupil responses to gratings of equal and lower space‐averaged luminance shows that the amplitude of pupillary constriction at grating onset for the equal luminance condition is about twice that measured with similar gratings in the lower luminance condition. Pupillary responses to chromatic isoluminant gratings are in general of longer latency when compared to responses of similar amplitude elicited by achromatic gratings. Small pupillary constrictions elicited by the onset of coherent movement in dynamic, random dot patterns are also demonstrated under stimulus conditions which eliminate pupillary responses to sudden light flux changes on the retina. The results support an earlier hypothesis which suggests that the onset of sudden changes in neural activity in the visual cortex when a visual stimulus is presented to the eye causes an overall perturbation which weakens transiently the regulatory inhibitory input to the pupillomotor nucleus. This, in turn, results in a transient increase in the efferent parasympathetic innervation of the iris sphincter muscle and hence the observed constriction of the pupil. The characteristics of the pupillary response reflect the properties of the mechanisms and the number of neurones which participate in the detection of each simulus attribute.

Exploring eye movements in patients with glaucoma when viewing a driving scene.

Background Glaucoma is a progressive eye disease and a leading cause of visual disability. Automated assessment of the visual field determines the different stages in the disease process: it would be desirable to link these measurements taken in the clinic with patients actual function, or establish if patients compensate for their restricted field of view when performing everyday tasks. Hence, this study investigated eye movements in glaucomatous patients when viewing driving scenes in a hazard perception test (HPT). Methodology/Principal Findings The HPT is a component of the UK driving licence test consisting of a series of short film clips of various traffic scenes viewed from the drivers perspective each containing hazardous situations that require the camera car to change direction or slow down. Data from nine glaucomatous patients with binocular visual field defects and ten age-matched control subjects were considered (all experienced drivers). Each subject viewed 26 different films with eye movements simultaneously monitored by an eye tracker. Computer software was purpose written to pre-process the data, co-register it to the film clips and to quantify eye movements and point-of-regard (using a dynamic bivariate contour ellipse analysis). On average, and across all HPT films, patients exhibited different eye movement characteristics to controls making, for example, significantly more saccades (P<0.001; 95% confidence interval for mean increase: 9.2 to 22.4%). Whilst the average region of ‘point-of-regard’ of the patients did not differ significantly from the controls, there were revealing cases where patients failed to see a hazard in relation to their binocular visual field defect. Conclusions/Significance Characteristics of eye movement patterns in patients with bilateral glaucoma can differ significantly from age-matched controls when viewing a traffic scene. Further studies of eye movements made by glaucomatous patients could provide useful information about the definition of the visual field component required for fitness to drive.

The effect of age on the light scattering characteristics of the eye

The rising mean age of our population has increased the need for understanding the physiologic consequences of ageing on visual function. Changes due to age were evaluated using a new scatter test implemented on theP_SCAN 100 pupillometer apparatus (Barbur, 1991). The test yields the full scatter function of the eye and also permits simultaneous measurement of pupil size (Barbur et al., 1995). In addition, contrast sensitivity was measured using sine wave gratings. The 28 subjects had a spherical refractive error between +0.50 DS and −0.25 DS, and astigmatism of less than −0.50 DC, V.A. of at least 6/6, and were ophthalmologically normal. To facilitate statistical analysis, subjects were classified into five groups according to age. For younger subjects (under 45 years),k′, the integrated straylight parameter, varied little with age. For this group,k′ ranged from 4.9 to 8.1. For subjects aged over 45 years,k′ increased with age, ranging from 10.7 to 19.7. One way analysis of variance showed Group 5 (60 year olds) to have significantly greaterk′ than 20, 30, 40 and 50 year olds (P=0.000). A slight downward shift in the contrast sensitivity function was seen over the age of 45. Significant differences between older and younger subjects were found at spatial frequencies of 3, and 10 cpd (P=0.081,P=0.002, respectively). Pupil diameter was found to reduce with age, but there was no significant difference between groups. Therefore, above 45 years, the ageing eye reveals a more rapid increase in forward scatter, and a reduction in contrast sensitivity, despite apparently good visual acuity.

Visual processing levels revealed by response latencies to changes in different visual attributes

Visual latencies, and their variation with stimulus attributes, can provide information about the level in the visual system at which different attributes of the image are analysed, and decisions about them made. A change in the colour, structure or movement of a visual stimulus brings about a highly reproducible transient constriction of the pupil that probably depends on visual cortical mechanisms. We measured this transient response to changes in several attributes of visual stimuli, and also measured manual reaction times to the same stimulus changes. Through analysis of latencies, we hoped to establish whether changes in different stimulus attributes were processed by mechanisms at the same or different levels in the visual pathway. Pupil responses to a change in spatial structure or colour are almost identical, but both are ca. 40 ms slower than those to a change in light flux, which are thought to depend largely on subcortical pathways. Manual reaction times to a change in spatial structure or colour, or to the onset of coherent movement, differ reliably, and all are longer than the reaction time to a change in light flux. On average, observers take 184 ms to detect a change in light flux, 6 ms more to detect the onset of a grating, 30 ms more to detect a change in colour, and 37 ms more to detect the onset of coherent motion. The pattern of latency variation for pupil responses and reaction times suggests that the mechanisms that trigger the responses lie at different levels in cortex. Given our present knowledge of visual cortical organization, the long reaction time to the change in motion is surprising. The range of reaction times across different stimuli is consistent with decisions about the onset of a grating being made in V1 and decisions about the change in colour or change in motion being made in V4.

The effects of supplementation with lutein and/or zeaxanthin on human macular pigment density and colour vision.

Background:  Both yellow‐blue (YB) discrimination thresholds and macular pigment optical density (MPOD) measurements in the eye exhibit large variability in the normal population. Although it is well established that selective absorption of blue light by the macular pigment (MP) can significantly affect trichromatic colour matches, the extent to which the MP affects colour discrimination (CD) sensitivity remains controversial.

New method based on random luminance masking for measuring isochromatic zones using high resolution colour displays

A new method of measuring normal hue discrimination ellipses and dichromatic zones using a high resolution colour monitor is described. The lesl involves the detection of chromatic bars on a grey background (x= 0.305, y= 0.323) having a luminance of 34 cd m−2. Elements of the background matrix of square cheeks are varied randomly in luminance in space and time to provide random luminance masking (RLM) which compensates for differences in the relative luminous efficiency of different observers. The measurement technique provides a rapid and comprehensive colour vision test. Typical results are presented for normal trichromats. protanopes and deuteranopes without RLM and with the RLM sel of 25%. The size of the discrimination ellipse in normal observers is the same in both viewing conditions, hut the use of the RLM technique reveals the extent of the isochromatic zones in colour deficient observers.

Imaging reveals optic tract degeneration in hemianopia.

PURPOSE To investigate whether there is transsynaptic degeneration in the human optic tract in hemianopia. To consider how the degeneration varies with duration of hemianopia and location of insult. METHODS Seven patients with damage to the primary visual cortex (V1), the lateral geniculate nucleus (LGN), or the optic tract were scanned with structural MRI. The volume and cross-sectional area of the left and right optic tracts were computed based on the intensity values of the T1-weighted image. High values correspond to voxels with high white matter content, and the values decrease as the white matter content drops (indicating degeneration). A laterality index to compare the tract size in the two hemispheres was calculated at different intensity values. RESULTS The three hemianopic patients with longstanding damage to either V1 or LGN showed laterality indices greater than 0.5 at the highest intensity values, indicating significant optic tract degeneration. Those with recent damage to the optic tract had even higher laterality indices due to direct degeneration. Even 18 months after V1 lesion, there was a significant correlation between the cross-section and volume indices at different intensity thresholds, whereas no control subject showed any correlation. CONCLUSIONS Transsynaptic degeneration had already begun 18 months after lesion. Although there was no visible decrease in volume at this stage, the white matter integrity was compromised. Significant decrease in volume could be visualized at longer durations of hemianopia. This method of objectively assessing structural images provides an effective, noninvasive approach to monitor the timescale of optic tract degeneration.