Dopaminergic modulation of photopic temporal transfer properties in goldfish retina investigated with the ERG

The influence of dopamine (DA) through either D1- or D2-dopamine receptors (D1-/D2-R) onto temporal transfer properties of the retina has been investigated using the ERG. Single flash responses and flicker responses were measured in the vitreous under photopic illumination conditions after application of either D1-/D2-R agonists or antagonists. All DA-R drugs did change the single flash responses, but only blockade of D2-R or activation of D1-R also changed the temporal transfer properties. In the Bode plot the gain characteristic was changed and thereby the upper limit frequency reduced. The action of DA is discussed on the base of a membrane resonance model in the outer retina versus a feed-forward inhibition model in the inner retina.     

Evaluating macular function using the focal ERG

A stimulus consisting of 96 red LEDs mounted in the rear of a ganzfeld bowl was used to elicit focal electroretinograms (FERG) from the central 9 degrees of the retina in human subjects. The luminance of the stimulus was driven sinusoidally at frequencies from 10-60 Hz. The temporal responsiveness and response phase lags of normal subjects and patients with retinal disease were measured. Normal subjects produced maximum amplitude FERG responses to stimuli between 30-40 Hz. Patients with retinitis pigmentosa showed a low-pass pattern of amplitude loss, with an additional frequency independent loss in sensitivity in those with poorer visual acuity. Patients with macular degeneration showed general amplitude loss associated with a relative sparing of the mid-temporal frequencies. The response phase lags in both patient groups were not significantly different from the normals. These findings point to a loss in temporal responsiveness accompanied by a secondary loss of sensitivity in these heredoretinal degenerations.     

Clinical application of temporal modulation transfer function

The temporal modulation transfer function (temporal MTF) was measured in normal controls and in patients with retinal diseases, optic neuritis and amblyopia. The temporal MTF of controls showed "band-pass" filter properties, having a peak sensitivity at about 15 Hz at 1,000 trolands of mean retinal illuminance. As the retinal illuminance was reduced, the sensitivity decreased, first at high temporal frequencies, then the peak sensitivity decreased. Finally at low temporal frequencies the sensitivity decreased and the peak shifted toward the lower frequency. In retinal diseases, abnormal temporal MTF was detected, and the curves were very similar to those of normal controls when retinal illuminance was reduced. These patterns were not specific to any retinal diseases, and the severity of the condition seemed to determine the pattern. In optic neuritis, an attenuated sensitivity was detected in all frequency ranges; however, the degree of attenuation was more severe than that found in retinal diseases even when the visual acuity was similar. At the convalescent stage, the low frequency sensitivity was restored first, and high frequency loss was observed even when the visual acuity was recovered to 1.0. Two amblyopic patients showed an attenuated sensitivity in the high and intermediate frequency ranges, and in one case sensitivity was attenuated in all frequencies. Three other patients showed normal sensitivity. By occlusion of the nonamblyopic eye, recovery of the sensitivity was observed in one patient whose visual acuity improved.     

Evaluation of the LKC stimulator for focal ERG testing

Numerous studies have demonstrated the ability of focal ERGs to diagnose abnormally functioning maculae in the absence of funduscopic and angiographic signs, as well as to confirm diagnoses of clinically suspected macular disease. We present normative data on the second commercially available focal ERG stimulator (FCS-500, LKC Technologies), which may be added to the standard full field ERG systems currently used in many laboratories. The stimulator uses a monocular indirect ophthalmoscope to present a 5° white stimulus flickering at 31.25 Hz in a 20° background field. We have established a range of normal amplitudes (747–3000 nV) and implicit times (30.5–37.5 ms) for the instrument based on tests of 45 eyes of 45 normal patients. To confirm the validity of this test for diagnosis of ocular dysfunction we compared these normals to 46 eyes of 46 patients with macular disease and decreased acuity, and to 49 eyes of 49 patients with retinitis pigmentosa. Using Focal ERG amplitude alone, we found overall accuracy of 85% in diagnosing macular disease associated with decreased acuity. These findings confirm the validity and efficacy of the system we have evaluated.     

Asymmetry of focal ERG in human macular region

Electroretinograms (ERGs) were elicited by hemicircular (half-disc) stimuli to the upper, lower, temporal and nasal maculas of 26 normal subjects, and the amplitudes and implicit times of the ERGs from opposing macular regions were compared. The amplitudes of a-wave, b-wave and oscillatory potentials (OPs) were significantly larger in the upper macular region than in the lower macular region (P less than 0.05). The amplitudes of a- and b-waves did not differ significantly between temporal and nasal macular regions, but OPs showed enormous asymmetry, with significantly larger amplitudes in the temporal retina than in the nasal retina (P less than 0.001). The implicit times of a-waves, b-waves and OPs did not differ significantly between upper and lower retina, or between temporal and nasal retina. These findings aided analysis of the ERG of a patient with a retinal defect.     

Spatial tuning of the pattern ERG across temporal frequency

The spatial response function of the electroretinogram (ERG) to contrast checkerboard pattern reversal at several check sizes was determined at a fixed contrast. The influence of the rate of modulation on the spatial response function was assessed: Reversing square wave patterns were presented at eight temporal frequencies ranging from 1 to 25 reversals per sec; The waveform consisted of an initial positive and a subsequent negative deflection. Irrespective of the temporal frequency, the spatial response function of the positive component did not show a spatial tuning. The amplitude of the negative component exhibited a pronounced attenuation of the response at check sizes larger than optimal. Mean maximal amplitude was found at an optimal check size between 25 and 50 min of arc. A distinction between a positive or negative component was not made for temporal frequencies higher than 10 reversals per sec, since the wave-form at these modulation rates consisted merely of a sinusoidal steady-state response. The spatial response function obtained at 14 reversals per sec; resembling that of the negative component, exhibited a prominent spatial tuning. The results demonstrate that the pattern ERG has at least two components: a positive component which is not specific to changes in retinal distribution of contrast, followed by a negative wave showing spatial tuning across temporal frequency.     

Development of a technique for recording the focal rod ERG

The scotopic ERG recorded in response to a focal stimulus has a double b-wave. The first wave results from direct focal stimulation of the retina, and the second originates in the peripheral retina in response to scattered light. The aim of this study was to assess two possible protocols for the isolation of the focal rod response, namely 'Subtraction' and 'Background Adaptation' techniques. The Subtraction technique involved the recording of a full-field response, which matched the b-wave elicited by intraocular light scatter, and the subtraction of this full-field response from the initial trace to isolate the focal component. In the Background Adaptation technique an adapting surround was used to suppress the response from the peripheral retina. Focal rod responses were isolated with both techniques. However, the Background Adaptation technique was found to more reliably elicit a focal response with a measurable a-wave, and was also considerably less time consuming than the Subtraction technique.     

ERG assessment of zebrafish retinal development

Research has shown that adult zebrafish have a complex visual system, with two possible opponent mechanisms. Anatomically, zebrafish retina develops in a sequential manner and is immature at hatching. The purpose of the present study was to assess zebrafish retinal development using the electroretinogram (ERG). ERG responses to visual stimuli were obtained from 4-5, 6-8, 13-15, and 21-24 days postfertilization (dpf) zebrafish. Individual waveforms were assessed and compared across the four age groups. Spectral-sensitivity functions were calculated for the a- and b-wave components of the ERG response. Results showed that the ERG waveforms and spectral-sensitivity functions varied with age. While the 21-24 dpf subjects had an ERG waveform that was similar to that of adults, the younger subjects did not. Although there were modest differences in the a-wave spectral sensitivity, substantial differences were found in the b-wave spectral sensitivities across the ages. There was a consistent strong response to ultraviolet wavelengths, while across the remaining parts of the spectrum, there was a gradual increase in sensitivity with age. Also, the 21-24 dpf subjects appear to have adult-like U- and S-cone functions, but were missing the L-M and the M-S opponent mechanisms found in the adult. These results support the findings of the anatomical studies and demonstrate that the zebrafish is a useful model for examining the development of retinal function.     

Variation in ocular modulation and phase transfer functions with grating orientation

For circular pupils of diameters < or = 2 mm the optical performance of the eye in monochromatic light is effectively diffraction-limited, so that its modulation transfer function (MTF) is essentially independent of grating orientation and its phase transfer function (PTF) is close to zero. For larger pupils, however, the wave aberration of the eye usually exceeds the Rayleigh quarter-wavelength limit. It is shown that the monochromatic MTF may then display marked variation with orientation and the associated PTFs can be non-zero. The exact effects vary markedly with the individual subject. In white light additional asymmetries may occur due to transverse chromatic aberration.     

An instrument for measuring the modulation transfer functions of low power telescopes and telemicroscopes

An instrument using an electro-optical Fourier method for measuring the modulation transfer function (MTF) of low power telescopes and telemicroscopes is described. Because these devices are afocal, or nearly so, relay optics are needed to form real images at the detection section of the apparatus. The system is capable of measuring the MTF in monochromatic and white light, at any target azimuth, across the field of view, and through focus. The target system contains 14 square-wave gratings with spatial frequencies that range from 2.5 to 156 cpd. Images of these gratings are scanned across a slit. The output data are fed to a first-order recursive digital Butterworth bandpass filter for MTF analysis. The apparatus is diffraction limited at f/31.4. Therefore, it negligibly affects the measurement of the MTF of telescopes and telemicroscopes tested with exit pupils of up to 6.4 mm.     

矩形光栅法视网膜调制传递函数测定技术的研究

目的:证明利用矩形光栅法测定人眼视网膜-大脑调制传递函数(MTF)的有效性,从而可以开发出一种结构小巧、移动性好、价格便宜、性能稳定的MTF测量仪。方法:在实验室搭建实验装置,分别利用矩形光栅法的实验装置和平板法视网膜MTF测量仪器,来测定正视眼组和屈光不正组的不同空间频率条纹分辨的阈值,算出并分析比较所测MTF值。结果:用本实验装置所测量的MTF曲线特征符合人眼MTF的山型特点。在六个空间频率下用本实验装置所测的正视眼和屈光不正组的MTF值之间没有显著性差异,P >0.05。对于同一样本组,用本实验装置和平板法微机控制视网膜MTF测定仪所测量的视网膜MTF值之间比较没有发现显著性差异,P >0.05。结论:本实验装置经过试用证明,其性能稳定、结果正确、可避开屈光因素的影响、能有效反映人眼的MTF曲线特征,与以往方法所测MTF值的符合性好,从而证明了利用矩形光栅测量视网膜MTF的原理的正确性。     

Analysis of focal macular ERG in idiopathic central serous chorioretinopathy

Focal macular electroretinogram (MERG), was analyzed in 33 patients with unilateral central serous chorioretinopathy (CSC) of recent onset. The stimulus spots were 5 degrees, 10 degrees and 15 degrees in diameter. Nonaffected fellow eyes served as controls. When macular detachment was present, the MERG showed significantly reduced amplitude and delayed peak latency in a-wave, b-wave and oscillatory potentials (OPs) in all stimulus spots. The reduction of amplitude in 5 degree and 10 degree spots was more significant in b-wave and OPs than in a-wave. In the convalescent stage, the a-wave and b-wave recovered to nearly normal levels, however OPs showed selective delay of recovery. These abnormalities shown by MERG strongly suggest that CSC may involve functional disturbances in the inner retinal layer as well as photoreceptors. It has been assumed that the pathogenic properties of CSC are receptor disorientation and a disturbance in the rate of photopigment regeneration. The present study showed that the abnormal pattern of MERG in CSC cannot be explained simply by the assumption of receptor disorientation or a disturbance of photopigment regeneration by analyzing similar conditions in normal eyes.     

Objective assessment of the human visual attentional state

Primary objective

The purpose of this study was to develop an objective way to assess human visual attention using the alpha-band component of the visual-evoked potential (VEP).

Design and methods

Six different attentional conditions were tested: eyes-open, eyes-closed, eyes-closed with backwards number counting, and three rapid-serial visual presentation (RSVP) tasks. Eighteen visually normal, young-adult subjects (ages 21–28 years) were tested binocularly at 1 m for each condition on two separate days. The Diopsys? NOVA-TR system was used to obtain the visual-evoked potential (VEP) and extracted alpha wave and its related power spectrum. Additionally, the Visual Search and Attention Test (VSAT) was administered as a subjective measure of visual attention.

Results

Subjects exhibited significant decreases in power in the alpha band when comparing the eyes-closed with the eyes-open conditions, with power in the eyes-closed condition being, on average, twice as large. The response from the other four conditions did not reflect the differential attentional demands. The ratio of the power in the eyes-closed condition to the eyes-open condition in the lower-alpha frequencies (8–10 Hz) was found to be significantly correlated with the group’s performance on the VSAT, especially the 10-Hz component.

Conclusions

An individual’s ability to attenuate their alpha component during visual processing may be a predictor of their visual attentional state. These findings solidify the role of the VEP alpha subcomponent as an objective electrophysiological correlate of visual attention, which may be useful in the diagnosis and treatment of human visual attention disorders in the future.     

Reproducibility of multifocal ERG using the scanning laser ophthalmoscope

BACKGROUND: Prior to application of multifocal electroretinography (mfERG) using the scanning laser ophthalmoscope (SLO) in a clinical routine setting it is important to determine the reproducibility of this technique in normals during the use of monochromatic red HeNe laser light. METHODS: Thirty-five eyes of 35 volunteers (visual acuity > or =0.8) were examined using the SLO for mfERG with two test patterns in a 20-deg diameter field. Multifocal ERG was performed using the RETIscan system with a binary m-sequence technique. Evaluation was performed for the first-order kernel concentric ring segments. Three measurements were performed for each type of sequence (six sequences for seven hexagons, eight for 19 hexagons) with removal of electrodes between the recordings. Stimulus intensity for all subjects was set to 235 cd/m2 (background illumination 20 cd/m2). We used gold-foil electrodes to obtain good signal quality while not disturbing corneal transmission and refraction. RESULTS: The coefficient of variation for the P1 amplitudes for the center and surrounding ring(s) was 10.4% and 9.3% for seven hexagons and 15.2%, 9.3% and 7.6%, for 19 hexagons, respectively. The corresponding intraclass correlation coefficients varied between 0.59 and 0.77. The CV for the P1 peak times varied between 1.3% and 1.5% for ring analysis in seven and 19 hexagons. CONCLUSION: Our data shows that the setup of mfERG with the SLO provides a retest variability comparable with conventional CRT examinations. One advantage is the simultaneous visualization of the fundus with observation of fixation during examination, thereby allowing more confidence in pathological results.     

Objective assessment of pseudoaccommodation in pseudophakia

The paper considers the results of studies into an accommodative response in 75 patients with aphakia and pseudophakia following 3 months of cataract surgery. In pseudophakia, there is no objective recording of changes in both refraction and accommodation on producing an accommodative stimulus of varying size and at varying rates. Nevertheless, astigmatism subgroup patients could read, by using the capacities of the ocular focal region.     

Detection of glaucomatous damage using multifocal ERG

The first‐order kernel analysis in multifocal electroretinogram (mfERG) using low contrast stimulation is suggested as a way to detect the inner retinal responses in animal studies. In this case report, this protocol is applied to human patients with glaucoma to demonstrate the possibility of using mfERG as a tool to detect glaucomatous damage. Two patients with glaucoma were recruited and had mfERG measurements with the 103‐scaled hexagonal stimulus pattern at low (50 per cent) contrast. Their responses were analysed and compared with those from normal subjects with the mfERG measured under the same condition. In the normal subjects, there were obvious oscillatory components on the ascending and descending limbs of the first‐order kernel response to 50 per cent contrast. In the glaucomatous patients, the oscillatory component on the descending limb was obviously diminished. In addition, this component was significantly diminished in the quadrant with a glaucomatous visual field defect. This suggests that the low‐contrast stimulation condition in mERG measurement may provide a good way to detect glaucomatous damage and this may help in clinical diagnosis of glaucoma.     

Nonlinear analysis of spatial vision using first-and-second-order volterra transfer functions measurement

The harmonic input method of nonlinear system identification is modified to allow the Volterra series approach to be used for psychophysical investigation of various aspects of human pattern vision in the spatial frequency domain. While it is well known that only one modulation transfer function provides a complete characterization of a linear system, a number of multidimensional transfer functions are needed to identify a nonlinear system. We have shown, that so far as the contrast sensitivity to sine-wave gratings may be used for an empirical estimate of the first-order modulation transfer function of the human visual system, the contrast sensitivity to difference harmonics may be used as an empirical estimate of the second-order modulation transfer function.

A difference harmonic arises from a mixture of two sine-wave gratings resulting from the nonlinearity of the visual system. Difference harmonic, experienced as some periodic beatlike structure, may still be observed if frequencies of the component gratings are higher than the maximum visual acuity.

The visibility of the low-frequency beatlike pattern produced by pairs of sine-wave gratings, which themselves are of spatial frequencies too high to be resolved, could be accounted for either by a difference frequency distortion product (Burton, 1973) or by a special beat detector (Derrington & Badcock, 1985). We found that increasing the contrast of one component grating may be compensated for by reducing the contrast of the other component grating, the beatlike pattern being at threshold. This is exactly what would be expected if the beatlike pattern is detected because of the difference harmonics produced by nonlinearity of the visual system.

We have determined contrast thresholds for the difference harmonics which occur between two unresolved different spatial frequencies. The contrast sensitivity function for difference harmonics was found to have a marked similarity both in the shape and position of peak sensitivity to the contrast sensitivity function for single sine-wave gratings. Another important characteristic of the contrast sensitivity function for difference harmonics is that it depends only on the frequency difference, Δf = f₁ − f₂, rather than on the value of either f₁ or f₂.

All this indicates that a difference harmonic arises from local nonlinearities in the visual system. More specifically, the visual system may be represented as a cascade system, composed of a linear system with transfer function O(f) followed by a nonlinear element, r(·), without spatial spread in cascade with another linear system with transfer function P(f). The nth order transfer function of this cascade system, H_n(f₁,...,f_n) can be expressed in the following way: H_n(f₁,...,f_n) = a_nO(f₁)P(f₁ + ... + f_n) where a_n is the nth coefficient in the Taylor series expansion for the nonlinear function r(·). It follows from this that the measurement of the first- and second-order transfer functions is sufficient to determine O(f) and P(f). We have derived the estimates of cO(f) and P(f) from contrast sensitivity functions for single sine-wave gratings and difference harmonics by the least squares method.