The effects of image degradation on retinal illuminance and pattern responses to checkerboard stimuli

The electrophysiological response to polarization-modulated patterned visual stimuli

Recent reports indicate that the subjective ability of humans to discriminate between polarization E-vector orientations approaches that of many invertebrates. Here, we show that polarization-modulated patterned stimuli generate an objectively recordable electrophysiological response in humans with normal vision. We investigated visual evoked potential (VEP) and electroretinographic (ERG) responses to checkerboard patterns defined solely by their polarization E-vector orientation alternating between ± 45°. Correcting for multiple comparisons, paired-samples t-tests were conducted to assess the significance of post-stimulus deflections from baseline measures of noise. Using standard check pattern sizes for clinical electrophysiology, and a pattern-reversal protocol, participants showed a VEP response to polarization-modulated patterns (PolVEP) with a prominent and consistent positive component near 150 ms (p < 0.01), followed by more variable negative components near 200 ms and 300 ms. The effect was unrecordable with visible wavelengths >550 nm. Further, pseudo-depolarization negated the responses, while control studies provided confirmatory evidence that the PolVEP response was not the product of luminance artefacts. Polarization-modulated patterns did not elicit a recordable ERG response. The possible origins of the PolVEP signals, and the absence of recordable ERG signals, are discussed. We conclude that evoked cortical responses to polarization-modulated patterns provide an objective measure of foveal function, suitable for both humans and non-human primates with equivalent macular anatomy.

Focal macular photopic negative response in patients with optic neuritis

PURPOSE

To investigate, by focal macular electroretinography (ERG), the change of photopic negative response (PhNR) in the recovery of visual function in patients with optic neuritis.

METHODS

Focal macular ERG was recorded from nine patients with acute optic neuritis (38.6 ± 10.2 years). The photostimulator device projected 15° visual angle spotlight onto the macula. Focal macular ERG recording was performed at the onset and at 1 month and 6 months after the onset of optic neuritis. The results were compared between each recording for seven of the patients.

RESULTS

All patients decreased in the vision below 20/100 and had central scotoma. Vision improved more than 20/20 within 1 month and full-visual field recovered within 6 months after the onset in all patients. The amplitude of the a-wave, b-wave, and PhNR of focal macular ERG at the onset was significantly attenuated in eyes with optic neuritis (66.8 ± 15.5, 65.8 ± 17.7, and 65.2 ± 14.4% of normal control, respectively). The amplitude of the a-wave and b-wave increased gradually after steroid pulse therapy. The increase in a-wave amplitude was significant at 6 months (P = 0.046), whereas the PhNR amplitude did not show any significant change over 6 months after the onset of optic neuritis.

CONCLUSIONS

Our results suggest that inflammation at the onset of optic neuritis leads to functional deficits that extend to at least the inner nuclear layers of the retina, and that all but the ganglion cell layers of retina recover.

S-cone, L + M-cone, and pattern, electroretinograms in ocular hypertension and glaucoma

Silent substitution and selective adaptation techniques were used to obtain full field S-cone and L + M-cone electroretinograms from 18 patients with ocular hypertension (OHT), 9 with normotensive glaucoma (NTG), 18 with early primary open angle glaucoma (POAG) and 19 normal controls. Pattern electroretinograms were also recorded, using a reduced check size to increase the contribution of retinal ganglion cells. In the OHT and POAG groups, statistically significant reductions (P = 0.05-0.001) were observed in the amplitudes, most notably in the late negative waves of all three types of ERG compared to the controls. These are thought to reflect ganglion cell activity. The results imply a diffusely distributed loss of activity (20-35%) affecting many retinal pathways to a similar extent in OHT and early POAG, with an additional amount (<5%) in POAG corresponding approximately to the loss associated with local field defects. The electrophysiology indicated that virtually all cases of untreated OHT have greater retinal dysfunction than the least affected cases of POAG. The NTG group showed a different pattern of loss in that the PERG was markedly affected but the S-cone ERG was not significantly reduced.

Baseline characteristics of the transient pattern electroretinogram in non-human primates: inter-ocular and inter-session variability

This study assessed the inter-ocular and inter-session variability of the transient pattern electroretinogram (PERG) in a group of non-human primates. The transient PERG was measured both eyes of 29 non-human primates, and again after three months in 23 eyes of 23 of these animals. Signals were elicited using a contrast (90%, 75 cdm(-2)) reversing (5 reversals sec(-1)) checkerboard pattern (0.56 cpd). PERGs were also measured for stimuli of varied spatial frequency (n=8, 0.07-2.22 cpd), contrast (n=4, 20-100%), mean luminance (n=4, 4.7-75 cdm(-2)) and defocus (n=5, +1, +2, +3 diopters). The inter-eye and inter-session limits-of-agreement (LOA; 95%) were determined for each PERG parameter. Variability was also compared with previous studies using the coefficient-of-variability (COV). Pharmacological blockade of the inner retinal contributions to the PERG measured under these conditions was conducted in one animal using intravitreal injection of tetrodotoxin (approximately 6 microM) and N-methyl-D-aspartic acid (approximately 6 microM). The N95 component of the primate transient PERG showed spatial tuning, with a peak between 0.14 and 0.28cpd. This spatial tuning was not as apparent for the P50 component. A linear relationship between P50 and N95 amplitude was found with contrast and mean luminance. Both components were attenuated with the introduction of +2 diopters or more of defocus. The inter-session COV for the P50 and N95 components were 23.8 and 19.2%, respectively, while the LOA were 58 and 46%, respectively. The N95:P50 ratio had smaller inter-session variability, was robust to changes in contrast, mean luminance and defocus, and was effective for characterization of inner-retinal dysfunction after pharmacologic block.

Transient and steady state focal and pattern electroretinogram nerve section losses in cats with unilateral optic

This study shows the ketamine/xylazine anaesthetised cat is a useful model for the effect of unilateral optic nerve section on pattern electroretinograms (PERGs), especially if stimuli extending to previously untested low spatial frequencies and preferably down to the focal ERG (FERG) are included. The transient reversal rate, seldom used in animals,has advantages over steady state recording. Transient PERGs had signs of true spatial tuning, a higher amplitude and signal noise ratio and showed the effect of optic atrophy at low spatial frequencies more rapidly.

The different contributions of local luminance decreases and increases to the pattern electroretinogram (PERG)

The typical pattern-onset-offset stimulus (stimulus A) consisting of local luminance increases and decreases was broken down into stimuli presenting only local luminance increases (stimulus B) or only local luminance decreases (stimulus C). With stimulus B the onset ERGs are luminance responses. With stimulus C the onset ERGs are pattern-related responses showing a spatial band-pass function. With stimulus A the response is a linear addition of responses to stimuli B and C. The simultaneously recorded VEP is a pattern-related response with all three stimuli (A-C).

Pattern electroretinograms in optic neuritis during the acute stage and after remission

A total of 20 patients with unilateral acute optic neuritis were studied. Each patient had experienced the recent onset of a decrease in visual acuity, a relative afferent pupillary defect, a relative or absolute central scotoma and a colour-vision defect. The pattern-reversal electroretinogram (PERG) of each patient was analysed with regard to the amplitude of the positive and negative components. During the acute stage the amplitude of the positive component was reduced in all patients and that of the negative, in 18 of 20 cases. Parallel to clinical recovery, a steady increase was observed in the amplitude of the positive component to normal values; no statistical differences between affected and fellow eyes was found. In contrast, the amplitude of the negative component remained significantly reduced after clinical recovery.

Differences between pattern onset and pattern reversal retinal responses

The pattern-evoked electroretinogram was recorded to pattern onset-offset and pattern reversal stimuli in two color-normal subjects with either luminance contrast of black-red (600 nm) and black-green (526 nm) square-wave stripe patterns or color contrast red-green patterns. The size of the onset response shows a spatial tuning with luminance contrast patterns and only a simple low-pass filter function with color contrast patterns. The peak latency of the response to luminance contrast increases with increasing spatial frequency but stays constant with color contrast patterns. The size of the reversal responses, however, shows only a low-pass filter function under both contrast conditions. The peak latency to luminance contrast shows a slight increase and to color contrast it remains constant with increasing spatial frequency. The differences noted under the various stimulus conditions must take into account the possible effects of different luminance modulation depths of onset and reversal stimuli, the modulation transfer function of the eye, and the activity of luminance-antagonistic and color-antagonistic receptive fields.

Distortion of spatial selectivity by pattern onset stimulation

Spatial selectivity of pattern evoked potentials has been thought to provide evidence of lateral inhibition. However, spatial tuning functions may be distorted by pattern onset stimulation that is applied repeatedly to the same area of retina so an after-image is formed. This only applies at low spatial frequencies because of the randomizing effects of eye movements. Low-frequency attenuation may therefore be exaggerated. Pattern reversal stimulation has the opposite effect and this is reflected in the literature by fewer reports of bandpass functions. A new method has therefore been devised to provide the true spatial response function. The spatial phase is reversed after every two consecutive presentations. By combining this paradigm with a correction for the optical transfer function of the eye, the true neural response function is obtained. Ten subjects participated in this study to evaluate the distortion of spatial selectivity in the pattern electroretinogram. The new stimulus paradigm reduced the low spatial frequency attenuation to a barely significant level giving an almost flat amplitude response for the + ve and - ve transients of the pattern electroretinogram for check sizes from 222' to 7' angular subtense. However, correction for optical degradation produces bandpass curves, which closely correspond to those predicted from recent data on receptive fields of primate retinal ganglion cells.

The effect of stimulus contrast on the latency and amplitude of the pattern electroretinogram

Our studies have verified that a linear relationship exists between the amplitude of the pattern onset ERG and retinal contrast, but indicate that there is no systematic variation in peak latency with contrast. The optical degradation of high spatial frequency patterns cannot therefore provide an explanation for the variation in peak latency with spatial frequency.

The pattern electroretinogram

Physiological experiments and the exploitation of clinical conditions have provided compelling evidence that retinal ganglion cells and other inner retinal structures generate the pattern ERG (PERG). As an increasing number of clinical reports have been published some contradictory findings have been reported. These may be ascribed to variation in recording and measuring techniques. The PERG consists of two major portions, the early positive and the following negative component which can be investigated separately if the stimulus conditions allow isolated (or "transient") responses to be recorded. Care has to be taken in positioning the reference electrode, maintaining accurate refraction, and the influence of pupil size must be considered. Furthermore the PERG is contaminated by a luminance component which may be generated in the outer retina. The size of this increases with low spatial frequency (large check-sizes) and high mean luminance. The PERG permits the examination of an additional level of the retina and helps the understanding of pathophysiology of various eye diseases, and is of clinical importance in routine diagnosis and assessment. In glaucoma the PERG amplitude is often reduced before it is possible to detect a scotoma and it is therefore an important prognostic indicator in patients with ocular hypertension. In diabetic retinopathy, retinal ischaemia sufficient to lead to the pre-proliferative state can be demonstrated. The PERG also has a major clinical role in examining localised retinal pathology. If combined with VECP recording, it greatly extends the interpretations possible, since not only can damage to the optic nerve be detected by both tests, but the normal PERG in the presence of an abnormal PVECP implies that the losses are confined to the central pathway.