Profound reductions of flicker sensitivity in the elderly: can glaucoma involve the retina distal to ganglion cells?

Clinical impact of migraine for the management of glaucoma patients

Migraine is a common and debilitating primary headache disorder that affects 10-15% of the general population, particularly people of working age. Migraine is relevant to providers of clinical eye-care because migraine attacks are associated with a range of visual sensory symptoms, and because of growing evidence that the results of standard tests of visual function necessary for the diagnosis and monitoring of glaucoma (visual fields, electrophysiology, ocular imaging) can be abnormal due to migraine. These abnormalities are measureable in-between migraine events (the interictal period), despite patients being asymptomatic and otherwise healthy. This picture is further complicated by epidemiological data that suggests an increased prevalence of migraine in patients with glaucoma, particularly in patients with normal tension glaucoma. We discuss how migraine, as a co-morbidity, can confound the results and interpretation of clinical tests that form part of contemporary glaucoma evaluation, and provide practical evidence-based recommendations for the clinical testing and management of patients with migraine who attend eye-care settings.

Behavioural assessment of flicker fusion frequency in chicken Gallus gallus domesticus

To interact with its visual environment, an organism needs to perceive objects in both space and time. High temporal resolution is hence important to the fitness of diurnally active animals, not least highly active aerial species such as birds. However, temporal resolution, as assessed by flicker fusion frequency (FFF; the stimulus frequency at which a flickering light stimulus can no longer be resolved and appears continuous) or critical flicker fusion frequency (CFF; the highest flicker fusion frequency at any light intensity) has rarely been assessed in birds. In order to further our understanding of temporal resolution as a function of light intensity in birds we used behavioural experiments with domestic chickens (Gallus gallus domesticus) from an old game breed 'Gammalsvensk dvärghöna' (which is morphologically and behaviourally similar to the wildtype ancestor, the red jungle fowl, G. gallus), to generate an 'Intensity/FFF curve' (I/FFF curve) across full spectrum light intensities ranging from 0.2 to 2812 cd m⁻². The I/FFF curve is double-branched, resembling that of other chordates with a duplex retina of both rods and cones. Assuming that the branches represent rod and cone mediated responses respectively, the break point between them places the transition between scotopic and photopic vision at between 0.8 and 1.9 cd m⁻². Average FFF ranged from 19.8 Hz at the lowest light intensity to a CFF 87.0 Hz at 1375 cd m⁻². FFF dropped slightly at the highest light intensity. There was some individual variation with certain birds displaying CFFs of 90-100 Hz. The FFF values demonstrated by this non-selected breed appear to be considerably higher than other behaviourally derived FFF values for similar stimuli reported for white and brown commercial laying hens, indicating that the domestication process might have influenced temporal resolution in chicken.

Short-wavelength automated perimetry in patients with migraine

BACKGROUND

The aim was to investigate short-wavelength sensitivity deficits in patients with migraine.

METHODS

Fifteen migraine and 18 age-matched healthy volunteers with normal ophthalmologic examination participated in this study. Migraine characteristics were graded by the Migraine Disability Assessment Questionnaire (MIDAS). All participants underwent SWAP (short wavelength amplitude perimetry) testing using a Humphrey field analyzer; there was a 30-2 presentation pattern.

RESULTS

Short wavelength amplitude perimetry parameters for mean deviation (MD; p<0.0001) and pattern standard deviation (PSD; p<0.0001) were significantly worse in the migraine group. In the migraine group 53.3%. of eyes had glaucoma hemi-field tests (GHT) outside normal limits and 10 of these had early glaucomatous visual field loss. Statistically significant correlations were found between frequency of migraine attacks and MD (p=0.02; r=0.56) and PSD (p=0.03; r=0.41) and also between the MIDAS score and MD (p=0.03; r=0.49) and PSD (p=0.04; r=0.51). In all migraine cases with early glaucomatous visual field defect a corresponding site of the head was predominantly involved in headache (p=0.03).

CONCLUSION

Some patients with severe migraine have earlier defects on SWAP suggesting a common vascular insult of glaucoma and migraine, and all migraine cases with high MIDAS scores should be further evaluated for early glaucomatous visual field defects using SWAP.

High blood pressure and visual sensitivity

The study had two main purposes: (1) to determine whether the foveal visual sensitivities of people treated for high blood pressure (vascular hypertension) differ from the sensitivities of people who have not been diagnosed with high blood pressure and (2) to understand how visual adaptation is related to standard measures of systemic cardiovascular function. Two groups of middle-aged subjects--hypertensive and normotensive--were examined with a series of test/background stimulus combinations. All subjects met rigorous inclusion criteria for excellent ocular health. Although the visual sensitivities of the two subject groups overlapped extensively, the age-related rate of sensitivity loss was, for some measures, greater for the hypertensive subjects, possibly because of adaptation differences between the two groups. Overall, the degree of steady-state sensitivity loss resulting from an increase of background illuminance (for 580-nm backgrounds) was slightly less for the hypertensive subjects. Among normotensive subjects, the ability of a bright (3.8-log-td), long-wavelength (640-nm) adapting background to selectively suppress the flicker response of long-wavelength-sensitive (LWS) cones was related inversely to the ratio of mean arterial blood pressure to heart rate. The degree of selective suppression was also related to heart rate alone, and there was evidence that short-term changes of cardiovascular response were important. The results suggest that (1) vascular hypertension, or possibly its treatment, subtly affects visual function even in the absence of eye disease and (2) changes in blood flow affect retinal light-adaptation processes involved in the selective suppression of the flicker response from LWS cones caused by bright, long-wavelength backgrounds.

Flashed stimuli and the suppression of flicker response from long-wavelength-sensitive cones: integrating two separate approaches

The selective suppression of flicker response from LWS cones has been investigated with two approaches. One approach has emphasized the use of light-adaptation conditions, and the other has emphasized the use of dark-adaptation conditions. In both cases, stimuli are arranged to restrict or exceed the ability of adaptation processes to maintain an afferent flicker response, and long-wavelength stimuli are used to overload spectrally opponent processes. By integrating these two approaches, this study shows that diverse manifestations of flicker response suppression can be closely related mechanistically. For instance, the steep flicker TVI slopes that resulted from superimposing temporally modulated (100% contrast) test stimuli on flashed backgrounds corresponded to the disappearance of flicker that resulted from increasing the time-averaged illuminance of temporally modulated stimuli (contrast x < 100%) that were flashed alone in an otherwise dark field. For the stimulus parameters of this study, flicker response suppression was more evident for small (19' diameter) than for large (1 degree diameter) stimuli. However, flicker response suppression was elicited reliably for both sizes by adding a spatially coincident short-wavelength stimulus to the interstimulus interval between presentations of the long-wavelength stimuli. By showing that temporal contrast can be treated as an independent variable for an important set of test/background stimulus combinations, the results of this study make it possible to investigate the means by which changes of contrast gain help to maintain flicker response as assessed in a conventional flicker TVI paradigm. The reduced degree of suppression for relatively large stimuli probably is related to the increased action of spatially extensive contrast gain-control processes. These contrast gain-control processes might not act independently of spectrally opponent processes.

Multiple processes mediate flicker sensitivity

By systematically manipulating the luminance of a flickering spot and the area immediately surrounding it, we investigated why thresholds from flickering stimuli that cause a change in average luminance are elevated relative to those from stimuli with no luminance change. Threshold elevation resulted from local light adaptation and from temporal-frequency-specific interactions between the spot and its surround: at low frequencies, the contrast between the spot and the surround elevated thresholds, whereas at high frequencies, dark adaptation within the surround elevated thresholds. Our findings suggest that two common ways of determining temporal sensitivity may give markedly different outcomes.

Aliasing for rapidly counterphasing stimuli: a failure to demonstrate an M-cell sampling limit to resolution

We investigated whether resolution is sampling limited for stimuli optimized for detection by magnocellular mechanisms. We measured peripheral (15 degrees and 30 degrees) spatial detection and resolution thresholds using 50% and 90% contrast flicker-defined gratings (25 Hz) and 90% contrast counterphasing sinusoidal gratings (25 Hz). Direction-discrimination performance for 90% contrast counterphasing sinusoidal gratings (25 Hz) was measured foveally. Our results indicate that resolution of rapidly counterphasing stimuli is sampling limited in peripheral vision but is consistent with limiting of performance by parvocellular mechanisms. Also, undersampling may not be necessary to account for motion reversals observed with gratings that both drift and flicker.

The diagnostic value of automated flicker threshold perimetry

Automated perimetry techniques have advanced from the standard white-on-white threshold perimetry to a myriad of perimetric models. These models include motion detection, frequency-doubling contrast sensitivity, and spatial contrast sensitivity perimetry. The research findings for the more popular of the automated perimetry models, in particular those of blue-on-yellow and critical fusion frequency perimetry, are discussed and compared with findings for flicker threshold perimetry. Flicker threshold perimetry demonstrates resistance to such factors as test variability and retinal image blur and has great promise in its ability to detect early visual field loss in the presence of primary open-angle glaucoma.

Localized scotomata detected with temporal modulation perimetry in central serous chorioretinopathy

AIM

Flicker deficits have been reported in various maculopathies, including age-related macular degeneration. We test whether flicker losses exist in patients with central serous chorioretinopathy (CSC) and whether the size and flicker frequency of the target is important in detecting such losses.

METHODS

We examined four CSC patients with temporal modulation (flicker perception) perimetry using the Medmont auto-flicker module (Medmont Pty Ltd, Melbourne, Vic. Australia), as well as static perimetry and colour vision. One case was examined using sophisticated laboratory equipment to precisely measure their temporal contrast sensitivity function (temporal CSF or de Lange curve) using larger targets to consider the effect of target frequency and size. Two patients were followed longitudinally and tested after resolution of the maculopathy. We compared our patients with an age-matched control group of 11 people.

RESULTS

Temporal modulation perimetry detected larger and more localized defects in all cases of active CSC compared with static perimetry. There appeared to be size and frequency tuning to the deficit, with greatest loss being found at 16 Hz with small (0.5 degree) targets. The losses resolved in one case where the retina recovered in 4 weeks, but remained to a lesser degree in another case who suffered a 2 year long fluctuating course before the CSC subsided.

CONCLUSIONS

Temporal modulation perimetry detects a loss of flicker sensitivity in patients with CSC. Deeper and more clearly defined scotomata are found with a flickering stimulus compared with a steady state one. The greatest losses of flicker sensitivity are found with 16 Hz modulation and with small targets located directly over the lesion. The duration of the disease may be important for recovery of flicker sensitivity. Temporal modulation perimetry appears to be a valuable tool for the confirmation of functional loss due to CSC.

Comparison of red-green, blue-yellow and achromatic losses in glaucoma

Achromatic losses in glaucoma would be expected to be greater than, or equal to, red-green chromatic losses if the following assumptions are made: (1) the function of the remaining axons is either unchanged or non-selectively reduced; (2) red-green chromatic information is signaled by the midget ganglion cell system; and (3) the function of the magnocellular system is reduced at least as much as that of the midget ganglion cells. This prediction was tested by measuring red-green (along with blue-yellow) mixture thresholds for 1 deg, 0.2 sec test spots presented on a color monitor on a white background of 50 cd/m2. Ellipses were fitted to plots of green contrast as a function of red contrast (or yellow as a function of blue), and major and minor axes of these ellipses were taken as measures of chromatic and achromatic thresholds, respectively. The study population consisted of 29 eyes in 29 patients with early glaucoma; control data were derived from a data bank of 83 normal eyes. Red-green losses were significantly (P < 0.05) greater than achromatic losses in 6 out of the 11 eyes which showed significant losses of either chromatic or achromatic sensitivity (or both). It is concluded that, for these eyes, at least one of the above three assumptions is incorrect.

Foveal adaptation abnormalities in early glaucoma

Foveal sensitivities were measured after onset of adapting background fields for each of the following four groups of subjects aged 40-70 years: (1) low-tension glaucoma subjects with minimal field loss in the test eye, (2) primary open-angle glaucoma subjects with minimal field loss in the test eye, (3) normal control subjects, and (4) subjects originally enrolled as control subjects but subsequently found, on the basis of masked clinical evaluation, to be suspect for glaucoma despite ostensibly normal intraocular pressures. We found that the desensitization of a short-wavelength-sensitive-cone-mediated response after onset of a 580-nm background field was diminished from that of normal observers for low-tension glaucoma subjects but not for primary open-angle glaucoma subjects. The desensitization was also diminished for a glaucoma-suspect subjects aged 60-70 years. In contrast, the flicker sensitivity instabilities that persisted after onset of a long-wavelength background field for the majority of subjects with primary open-angle glaucoma [J. Glaucoma Suppl. 3, S19 (1994)] occurred only infrequently among the other subject groups. These results imply that glaucoma often involves the fovea, probably by affecting retinal subtractive adaptation processes, although with different consequences for different types of glaucoma. The results also suggest that undiagnosed low-tension glaucoma may not be rare in the general aging population.

Suppression of flicker response with increasing test illuminance: roles of temporal waveform, modulation depth, and frequency

This study examined the detectability of flicker for small foveal long-wavelength test stimuli centered within surrounding long-wavelength annular adaptation stimuli. Flicker threshold-versus-illuminance (tvi) curves were analyzed for four different test-stimulus waveforms--sine-wave, square-wave, and rapid-on sawtooth and rapid-off sawtooth flicker--at temporal frequencies ranging from 12 to 21 Hz and at temporal modulation depths ranging from approximately 50% to 100%. For all stimulus combinations that were examined involving temporal frequencies above 12 Hz, the resultant flicker tvi curves shared the following characteristic features: First, at operationally dim surround illuminances, there was always a single elevated threshold for detection of flicker. Second, some surround illuminance always could be found for which flicker threshold decreased abruptly, typically by approximately 1.5 log units within 0.1 log unit of surround illuminance increase. Third, when test illuminance was incremented above this lower flicker threshold, flicker always vanished; when test illuminance was incremented still further, flicker reappeared. Finally, at sufficiently bright surround illuminances flicker did not disappear with increasing test illuminance. Although these effects held for all waveforms, the abrupt decrease of flicker threshold occurred at brighter surround illuminances for sawtooth than for sine-wave flicker, and at brighter surround illuminances for sine-wave than for square-wave flicker, at least for fully modulated waveforms (of a given temporal frequency). Moreover, when modulation depth was adjusted so that any two different waveforms had the same first-harmonic contrast, the resultant flicker tvi curves became identical when plotted as first-harmonic amplitude versus surround illuminance. This identity held for any given temporal frequency, even though the flicker tvi curves for 12-Hz fully modulated sine-wave or square-wave flicker did not manifest flicker response suppression, whereas the flicker tvi curves for sawtooth flicker did. These and other results imply that the first-harmonic contrast of the test stimulus fully determines the shape of the entire flicker tvi curve and that the dc component of the test stimulus helps to cause flicker response suppression. The results also demonstrate that first-harmonic equivalence is only a necessary, not a sufficient, condition for linearity.

Nonmonotonic effects of test illuminance on flicker detection: a study of foveal light adaptation with annular surrounds

This study examined the detectability of flicker for small long-wavelength foveal test stimuli centered within larger long-wavelength surround stimuli. Flicker visibility was evaluated as a function of surround and test illuminance and as a function of test wavelength, of the time elapsed following test or surround onset, and of surround dimensions. Consistent with prior flicker threshold-versus-illuminance results [Vision Res. 26, 917 (1986)], flicker threshold decreased abruptly once the surround illuminance became sufficiently great. However, as test illuminance was increased above flicker threshold, flicker again vanished. Flicker reappeared at still higher test illuminances, as middle-wavelength-sensitive (M-) cone-mediated flicker threshold was exceeded. Meanwhile, the time required for the surround to render flicker visible increased at a rapidly accelerating rate with decreasing surround illuminance; it increased at a more sporadic rate with increasing test illuminance. At bright enough surround illuminances, flicker did not vanish with increasing test illuminance. These and other results are compatible with a framework derived from previous dark-adaptation data [Vision Res. 32, 1975 (1992)]. In that framework the test stimulus itself induces losses of flicker sensitivity by sufficiently perturbing retinal response during states or stages of adaptation that fail to cause spectrally antagonistic processes to redress that perturbation adequately. The relevant adaptation processes, which can require minutes, involve an adaptation pool that includes (and is affected by) the test stimulus.

Losses of flicker sensitivity during dark adaptation: effects of test size and wavelength

The losses of flicker sensitivity that occur during dark adaptation following extinction of a bleach depend greatly on the test stimuli that are used to measure those losses. Generally, the larger the test size is for a given test wavelength, the later is the loss of flicker sensitivity; for sufficiently large tests, losses become progressively more gradual. Similarly, the shorter the test wavelength is (between about 660 and 580 nm) for a given test size, the later is the loss of flicker sensitivity; for sufficiently short test wavelengths, losses become progressively more gradual. Although the pattern of flicker sensitivity loss as a function of test size is the same both foveally and parafoveally, foveal losses occur later and/or more gradually for a given test stimulus. The results suggest that losses of flicker sensitivity occur when spectrally antagonistic response(s) to the test are no longer adequate to maintain the flicker-response to that test. The results suggest also that the adequacy of such spectral antagonism depends on the local adaptation state of the retina, which becomes increasingly more light-adapted with increasingly large test size.

Non-invasive assessment of the visual system: introduction by the feature editors

This is an introduction to the papers that compose the fourth feature issue on noninvasive assessment of the visual system and discusses issues related to publishing in Applied Optics.