Oxygenation and gender effects on photopic frequency-doubled contrast sensitivity

Hypoxia impairs reaction time but not response accuracy in a visual choice reaction task

We investigated the effect of hypoxia on the reaction time (RT) and response accuracy of pilots performing a visual choice reaction task that corresponded to the scanning of helmet mounted display (HMD) symbology. Eighteen male military pilots performed the task in a hypobaric chamber at two simulated altitudes (92 m and 4572 m) in a single-blinded repeated measures and counter-balanced design. The visual stimuli were displayed in low and high contrast and at a 30- and 50-degree field of view (FoV). We measured the pilots' RT and response accuracy. Using an eye tracker, we measured the pilot's glance time at each stimulus location. Finally, we collected subjective ratings of alertness. The results show that hypoxia increased the RT and glance time. Lowering the stimulus contrast and increasing the FoV further increased the RT, independent of hypoxia. These findings provide no evidence for hypoxia-induced changes in visual contrast sensitivity or visual field. Instead, hypoxia seemed to affect RT and glance time by reducing alertness. Despite the increased RT, the pilots maintained their accuracy on the visual task, suggesting that visual scanning of HMD symbology may be resistant to the effects of acute hypoxia.

The Effect of Systemic Hyperoxia and Hypoxia on Scotopic Thresholds in People with Early and Intermediate Age-related Macular Degeneration

Purpose: Morphological retinal changes combined with functional evidence implicate hypoxia in the pathogenesis of age-related macular degeneration (AMD). However, the role of hypoxia in the scotopic threshold deficit reported in AMD has not been investigated. This study compared scotopic thresholds in participants with early and intermediate AMD recorded under conditions of systemic hypoxia, hyperoxia and normoxia. Materials and Methods: Over two sessions scotopic thresholds were measured with participants breathing 21% and 60% oxygen (n = 12 early AMD, n = 11 age-similar controls) or 21% and 14% oxygen (n = 16 early AMD, n = 20 age-similar controls). Thresholds were measured using a 'white', annular 12 degrees stimulus, using a QUEST procedure. Results: There was no statistically significant change in scotopic thresholds within the AMD or control group when breathing the hyperoxic gas mixture (60% oxygen) or the hypoxic gas mixture (14% oxygen) when compared to the normoxic condition (21% oxygen). There was also no statistically significant difference in scotopic thresholds between groups under the hyperoxic or hypoxic gas conditions. The difference between groups under the normoxic condition was not statistically significant for the hyperoxia study (p = .70), but did reach significance in the hypoxia study (p = .05). Conclusion: This study provided no evidence that breathing that breathing 14% or 60% oxygen altered scotopic thresholds in those with early AMD when compared to controls. However, the lack of elevated scotopic thresholds in the AMD group of the hyperoxia study is of note, as it is unlikely that hyperoxia would reduce thresholds which were not significantly raised at baseline, regardless of whether hypoxia was a factor in the disease pathogenesis. The findings of this study do not rule out a role for hypoxia in early AMD, but this needs to be assessed in future experiments using measures that differ significantly between people with AMD and controls.

The effect of hypercapnia on the sensitivity to flicker defined stimuli

BACKGROUND/AIMS

To investigate the effect of increased CO2 levels on flicker defined stimuli.

METHODS

The sensitivity of two flicker defined tasks was measured in nine healthy, trained observers using the Flicker Defined Form (FDF) stimulus of the Heidelberg Edge Perimeter (HEP; Heidelberg Engineering) and Frequency Doubling Technology (FDT) stimulus of the Matrix perimeter (Carl Zeiss Meditec) during normoxia and 15% hypercapnia (end-tidal CO2 increased by 15% relative to baseline). HEP-FDF and Matrix-FDT sensitivities were analysed for the global field, superior and inferior hemifields and at specific matched eccentricities, using repeated measures analysis of variance. The main effect of hypercapnia on flicker sensitivity was analysed using regression models.

RESULTS

Higher flicker sensitivity outcomes with increasing CO2 values were found for HEP-FDF and Matrix-FDT with a statistically significant main effect for HEP-FDF global, superior and inferior hemifields (p<0.01 for all) as well as 6°, 18°, 12° and 24° eccentricities (p=0.03, 0.04, 0.01, 0.05, respectively). When comparing mean sensitivity values between normocapnia and hypercapnia conditions, no statistically significantly different results were found for HEP-FDF and Matrix-FDT (p>0.05).

CONCLUSIONS

As CO2 levels were increased in healthy young individuals, there was an associated increase in visual sensitivity that was only significant for HEP-FDF stimuli, highlighting the different mechanisms involved in processing each of HEP-FDF and Matrix-FDT stimuli. Mean visual sensitivity outcomes were found to be similar for normocapnia and hypercapnia suggesting that a capability to compensate for a mild and stable increase in systemic CO2 levels may exist.

Mild systemic hypoxia and photopic visual field sensitivity

PURPOSE

Flickering stimuli increase the metabolic demand of the retina, making it a sensitive perimetric stimulus to the early onset of retinal disease. We determine whether flickering stimuli are a sensitive indicator of vision deficits resulting from acute, mild systemic hypoxia when compared to standard static perimetry.

METHODS

Static and flicker visual perimetry were performed in 14 healthy young participants while breathing 12% oxygen (hypoxia) under photopic illumination. The hypoxia visual field data were compared with the field data measured during normoxia. Absolute sensitivities (in dB) were analysed in seven concentric rings at 1°, 3°, 6°, 10°, 15°, 22° and 30° eccentricities as well as mean defect (MD) and pattern defect (PD) were calculated. Preliminary data are reported for mesopic light levels.

RESULTS

Under photopic illumination, flicker and static visual field sensitivities at all eccentricities were not significantly different between hypoxia and normoxia conditions. The mean defect and pattern defect were not significantly different for either test between the two oxygenation conditions.

CONCLUSION

Although flicker stimulation increases cellular metabolism, flicker photopic visual field impairment is not detected during mild hypoxia. These findings contrast with electrophysiological flicker tests in young participants that show impairment at photopic illumination during the same levels of mild hypoxia. Potential mechanisms contributing to the difference between the visual fields and electrophysiological flicker tests including variability in perimetric data, neuronal adaptation and vascular autoregulation are considered. The data have implications for the use of visual perimetry in the detection of ischaemic/hypoxic retinal disorders under photopic and mesopic light levels.

Oxygenation state and mesopic sensitivity to dynamic contrast stimuli

PURPOSE

Comparative studies suggest that increasing photoreceptor oxygen consumption in dim light, relative to bright light, may make the outer retina susceptible to hypoxia at light levels relevant to aviation at night. Accordingly, this study investigates effects of relevant oxygenation states on sensitivity to a dynamic contrast stimulus at low photopic and mesopic light levels experienced during night flying.

METHODS

Threshold sensitivity to frequency-doubled contrast stimuli was assessed under mild hypoxia (breathing 14.1% oxygen), hyperoxia (100% oxygen), and normoxia (air) using frequency doubling perimetry, viewing at background fields of approximately 10 cd/m2 and approximately 1 cd/m2. Data were analyzed by retinal eccentricity and visual field quadrant.

RESULTS

At low photopic luminance (approximately 10 cd/m2), sensitivity was marginally enhanced when breathing 100% oxygen. At mesopic luminance (approximately 1 cd/m2), sensitivity was consistently poorest with hypoxia and greatest with supplementary oxygen at all eccentricities and in all field quadrants, suggesting oxygen-dependent performance.

CONCLUSIONS

The known effects of oxygenation state on pupil size are likely to influence frequency doubling perimetry thresholds, but oxygen-dependent changes in mesopic sensitivity are greater than expected from altered retinal illumination alone and support outer retinal (photoreceptor) susceptibility to hypoxia under twilight viewing.