David Whitaker

Neural contribution to spatiotemporal contrast sensitivity decline in healthy ageing eyes.

Contrast sensitivity thresholds were measured over a range of spatial and temporal frequencies in both a group of young and older observers. Results demonstrate a significant reduction in contrast sensitivity of the older age group at all but the lowest combinations of spatial and temporal frequencies investigated. The senile miosis and reduced optical transmission of the older eye was then mimicked using the younger observers as subjects. This combined effect of reducing retinal illumination produced no significant change in sensitivity. These findings are discussed in terms of neural loss within the visual pathways with increasing age.

Contrast sensitivity and glare sensitivity changes with three types of cataract morphology: are these techniques necessary in a clinical evaluation of cataract?

LogMAR visual acuity, contrast sensitivity and glare sensitivity measurements were made on 39 eyes of 18 cataractous subjects and compared against normative data. Only cataracts of one of the main three morphological cataract types were used‐cortical, nuclear and posterior subcapsular. Results indicate that contrast sensitivity decline with cataract is an intermediate and high spatial frequency loss. For nuclear and cortical cataracts with a LogMAR visual acuity of less than 0.5 (Snellen equivalent better than 6/18), there was no loss of contrast sensitivity at the lowest spatial frequency (1 c/deg). For posterior subcapsular cataracts, low spatial frequency contrast sensitivity loss did occur but was unrelated to visual acuity. Glare sensitivity increased for all cataract types. This was related to visual acuity for both cortical and nuclear cataracts but was not for the posterior subcapsular type. It was concluded that contrast and glare sensitivity measurements are a useful part of the assessment of visual function in patients with posterior subcapsular cataract.

Differences in the legibility of letters at contrast threshold using the Pelli-Robson chart.

One disadvantage of using high‐contrast letters as test objects when measuring visual acuity is the fact that they are not of equal legibility. A number of charts are now commercially available that assess contrast sensitivity using letter targets. This study attempted to assess the legibility of letters at contrast threshold on the Pelli‐Robson letter contrast sensitivity chart by determining the percentage of correct responses for each of the ten Sloan letters at contrast threshold. Results of 493 contrast sensitivity measurements taken in optometric practice indicated that there is a definite difference in legibility between letters at contrast threshold as for letters at acuity threshold. The data suggest that the probability of correctly identifying two out of a group of three letters at threshold on the Pelli‐Robson chart varies between 67% and 97% due to letter type alone. Because of the very regular and pronounced miscalling of the letter C as an O, we suggest that this should be accepted as a correct call during threshold measurements on the Pelli‐Robson chart. This helps to balance the legibility of different groups of letters.

The influence of eccentricity on position and movement acuities as revealed by spatial scaling.

The rate of decline with increasing eccentricity of several position and movement acuities was measured using a method of spatial scaling. In this method all stimuli at each visual field location are simply magnified versions of each other. The influence of separation and eccentricity were dissociated by presenting stimuli on an iso-eccentric arc. For each task, the rate of decline in performance was quantified by the parameter E2 which represents the eccentricity at which stimulus size must double in order to maintain performance equivalent to that at the fovea. All tasks were found to obey the concept of spatial scaling in that performance at a given field location could be equated with performance at any other location simply by a change of scale. However, the rate at which performance deteriorated with eccentricity varied over an enormous range (over 100-fold) depending on the task itself. The advantage of such diverse peripheral gradients is clear; the goal is to establish the physiological mechanisms which underlie this phenomenon.

Modelling of orientation discrimination across the visual field.

In many visual tasks thresholds can be made equal across the visual field by increasing the size of peripherally presented stimuli by appropriate magnification. The parameter E2 represents the eccentricity at which foveal stimulus size must double in order to maintain performance equivalent to that at the fovea. We measured orientation discrimination for a set of line sizes at various eccentricities. Thresholds were successfully scaled by applying a scaling function estimated by a technique that requires no prior assumptions regarding the size of peripherally presented stimuli. E2 of 1.95 deg was found for orientation discrimination. From our results we were able to develop a single equation which predicts orientation discrimination thresholds for an average observer over a range of different eccentricities and line lengths.

Spatial scaling of vernier acuity tasks.

Vernier acuity thresholds for two abutting lines and for a two-dot stimulus were measured as a function of stimulus magnification at eccentricities of 0, 5, 10 and 15 deg using a spatial scaling technique in which all stimuli are simply magnified versions of each other. The advantage of such a technique is that no prior knowledge of a suitable magnification factor with which to increase the size of peripheral stimuli is required. Thresholds for the line stimulus could be successfully scaled by the application of a magnification factor with an E2 value of 1.23-1.78 deg. Further, provided that the effects of dot separation and eccentricity were dissociated, spatial scaling with an E2 value of 1.06-1.96 deg was also successful in removing eccentricity dependence for two-dot vernier thresholds.

Clinical contrast sensitivity chart evaluation

Three different types of contrast sensitivity chart were used on normal patients by six optometrists in clinical practice. The charts were the Vistech, the Pelli ‐Robson and the Cambridge low‐contrast gratings test. We examine the data in terms of the differences between optometrists and the variation of contrast sensitivity with the age of the patient. There was a highly significant difference between the scores from different optometrists for all three charts. We attribute this to variability in measurement technique. There was also a highly significant effect of age for all three charts, with older observers tending to exhibit lower contrast sensitivity. On the Vistech chart, this sensitivity deficit was most pronounced at higher spatial frequencies. The level of redundant information in the tests is discussed.

Use of displacement threshold hyperacuity to isolate the neural component of senile vision loss

Displacement threshold hyperacuity is a visual task which should remain unaffected by the optical changes within the aging human eye. As such, it represents a method by which the neural component of senile vision loss may be studied. Displacement thresholds were measured in eighty subjects in the 18-85-yr age range. Results demonstrate a marked reduction in sensitivity with increasing age with the rate of deterioration being proportional to the age of the subject. This can only be attributed to senile changes within the neural system.

Simulating age-related optical changes in the human eye

The decline in visual function observed in older adults has been attributed to a deterioration in optical quality, decreased neural function, or a combination of both of these factors. One way of separating their contribution is to design a simulation of the optics of the elderly eye and examine its effect on the visual performance of younger observers. The age-related reduction in pupil size was simulated by the administration of an ophthalmic miotic drug, whilst a neutral density filter was included to account for the increased absorption of the ocular media. An optical cell containing a critical concentration of 500 nm diameter polystyrene microspheres in suspension was used to simulate light scatter. The spatiotemporal contrast sensitivity of an older group of observers was then compared with that of a younger group with and without the optical simulation. The contrast sensitivity of the younger group was consistently better than that of the older, and the presence of the optical simulation produced no significant effect on performance. This suggests that, under normal viewing conditions, it is primarily neural factors which underlie the deterioration in visual quality experienced by older observers.

What part of a vernier stimulus determines performance

Vernier acuity was measured using a pair of long, abutting vertical sinusoidal luminance gratings. Performance was also measured for pairs of thin horizontal strips of grating whose vertical separation was varied in order to represent different parts of the original grating. At small strip separations, performance was equivalent to that for the long gratings only at high spatial frequencies. At moderate spatial frequencies, intermediate strip separations resulted in equivalent performance. This suggests that different parts of a vernier stimulus can mediate performance depending upon the underlying spatial frequency content. Implications for the effect of contrast upon different vernier configurations are examined.