Application Of Contrast Sensitivity Function In The Evaluation Of Visual Function

An outstanding pilot must have excellent visual function when flying missions.With the development of the third and fourth generation fighter,unmanned aerial vehicle and night vision technology,visual function selection and medical assessment of pilots need to meet new requirements.Comprehensive and detailed visual function evaluation for pilots also become more and more important.But the existing evaluation methods are relatively insufficient.Visual acuity can only reflect the limiting spatial frequency of human eyes,and dark adaptation curve can only reflect dark adaptation time and light reaction threshold.These methods can't evaluate the visual quality of pilots in low contrast environment such as heavy fog and sand storm,or in low light environment such as twilight and night.Contrast sensitivity function(CSF),reflecting the contrast sensitivity characteristics of eyes at the different space frequencies,is suitable to evaluate visual function for pilots.Optical and neural factors are two components of CSF.Modulation transfer function(MTF)can describe the optical factor.Neural contrast sensitivity function(NCSF)can describe the neural factor.The relationship is NCSF=CSF/MTF.Our study is to explore the change of NCSF and CSF with different pupil diameters in photopic,mesopic and scotopic vision environment or with glare,and the influence of mild hypoxia on CSF.The long-term optical quality after laser in situ keratomileusis(LASIK)is evaluate by MTF.The function of the retina-brain neural system after dark adaptation or with night vision goggles is evaluate by NCSF.Based on our studies,we provide experimental basis for the evaluation of visual function for pilots and remote pilots.Materials and methodsOur study has four parts.All the volunteers were recruited from one military medical university.1.Effects of surround luminance and pupil diameter on NCSF and CSF.Ten healthy male volunteers(10 eyes)were recruited in this study,and the right eye was tested in this experiment.Different surround luminance levels and glare were set by Vision Monitor System(Monpack3),to measure CSF curves of normal pupil,3.0mm pupil and 5.0mm pupil.MTF curves of 3.0mm pupil and 5.0mm pupil were measured by Optical Quality Analysis System(OQAS?).NCSF is the ratio of CSF and MTF.2.Effects of mild hypoxia on CSF.Eleven healthy male volunteers(11 eyes)were recruited in this study,response ability and CSF were evaluated at altitudes of 300,3000,4500 meters in low pressure oxygen cabin after 5~10min,heart rate and oxygen saturation were monitored all along.3.Evaluation of long-term(?5 years)optical quality after LASIK.70 eyes from 35 student volunteers after LASIK and 140 eyes from 70 normal student volunteers were recruited from one military medical university.The volunteers after LASIK were divided into recent observation group(2 ~ 4 years after LASIK)and long-term observation group(5~10 years after LASIK).The normal volunteers were divided into recent control group and long-term control group,to analysis the influence of confounding factors such as age and eyes-using habit on the optical quality.The observation groups and the control groups were compared respectively.The objective scattering index(OSI),modulation transfer function cutoff frequency(MTF cutoff),Strehl ratio(SR),OQAS value in 100% contrast(OV100%),OQAS value in 20% contrast(OV20%),OQAS value in 9% contrast(OV9%)were analyzed.4.Evaluation of visual performance after dark adaptation and with night vision goggles.Ten healthy male volunteers(8 eyes)were recruited in this study,and the right eye was tested in this experiment.To measure CSF curves of light adaptation,dark adaptation and night vision goggle by Monpack3 in different surround luminance levels(10-1~10-4cd/m2).MTF curves were measured by OQAS?.NCSF is the ratio of CSF and MTF.Results1.NCSF and CSF curves go downward considerably and the curve waves shift towards low spatial frequency as the surround luminance decreases.Compared with NCSF,the curve wave of CSF shifts towards lower spatial frequency.The change of CSF is affected by NCSF and MTF.When pupil diameter increases,MTF curve go downward,while NCSF and CSF curves mount up at low spatial frequency in the mesopic and photopic visual environment and show downward trend at high spatial frequency in excessive bright environment or with glare.In the scotopic visual environment,the slight pupil diameter change cannot considerably affect CSF and NCSF curves.2.At an altitudes of 3000 meters,CSF goes downward considerably at 1.5cpd(P<0.05).At an altitudes of 4500 meters,CSF goes downward considerably at 1.5~3.0cpd(P<0.01)and error rate increases considerably(P<0.01).3.The OSI of observation group improved significantly but the MTF cutoff,SR,OV100%,OV20% and OV9% decreased significantly in several years after LASIK(P<0.05).The optical quality is generally stable after LASIK.The OSI had a tendency to reduce and the MTF cutoff,SR,OV100%,OV20% and OV9% had a tendency to improve,but showed no significant difference.4.In the region of lower spatial frequency,dark adaptation made NCSF and CSF mount up considerably at the spatial frequency of 0.6~3.4cpd in the scotopic vision environment.Night vision goggle made NCSF and CSF mount up considerably at the spatial frequency of 1.1~7.1cpd in the mesopic and scotopic vision environment,better than dark adaptation.In the region of higher spatial frequency,No obvious difference was found neither after dark adaptation nor with night vision goggles.Conclusions1.CSF is influenced by MTF and NCSF.The curve shape of CSF is similar to NCSF,but the curve wave is at lower spatial frequency.NCSF and CSF curves go downward as the surround luminance decreases.As the pupil diameter increases,NCSF and CSF curves mount up at low spatial frequency but show downward trend at high spatial frequency in excessive bright environment or with glare.CSF,MTF and NCSF can evaluate the visual quality of pilots from different perspectives.2.Mild hypoxia,which can influence visual quality and response ability,increases error rate and decreases CSF respectively.The sensitivity of hypoxia is increased at low spatial frequency.Mild hypoxia should be paid more attention in low contrast or low light environment for pilots.3.MTF can describe the quality of the eye optics.LASIK,one of the most frequent operation methods for myopia correction,can reduce the optical quality in a certain extent.The optical quality has a tendency to recover in several years after LASIK,but not completely.It should be cautious to select pilots from those who accepted myoporthosis operation.4.NCSF can describe the function of the retina-brain neural system.Both dark adaptation and night vision goggles can improve visual performance in the scotopic vision environment,but night vision goggle is better.Visual performance in higher spatial frequency and mesopic vision environment can be better improved by night vision goggle.Because of its bad effects on visual field and stereo vision,the related training and evaluation system should be improved.