The pigeon pattern electroretinogram is not affected by massive loss of cell bodies in the ganglion layer induced by chronic section of the optic nerve

Retinal ganglion cell ablation in guinea pigs

Guinea pigs are a common model of human ocular conditions; however, their visual function has not been fully characterized. The purpose of this study was to determine the contributions of retinal ganglion cells to structural and functional measures in guinea pigs. Healthy adult guinea pigs (n = 12) underwent unilateral optic nerve crush. Retinal structure was assessed with spectral domain optical coherence tomography (OCT), and thickness of the ganglion cell/nerve fiber layer (GC/NFL) was determined. Visual function was assessed with optomotor tracking of a drifting grating and light adapted electroretinograms (ERGs). From flash ERGs, a-wave, b-wave, oscillatory potentials (OPs), and photopic negative response (PhNR) were analyzed. From pattern ERGs, N1P1 and P1N2 were analyzed. Histological studies were done at various time points for ganglion cell quantification. Optomotor tracking was absent in optic nerve crush eyes following optic nerve crush. Significant thinning of the GC/NFL was evident four weeks following the crush. Flash ERGs revealed a significant reduction in the OP1 amplitude two weeks following crush (P < 0.01) and in the PhNR amplitude six weeks following crush (P < 0.01). There were no significant changes in a-wave, b-wave, or pattern ERG responses (P > 0.05 for all). In vivo OCT imaging showed progressive thinning of inner retinal layers. Ganglion cell density, quantified histologically, was significantly reduced by 75% in the optic nerve crush eye compared to the control eye at four weeks following crush. These findings indicate that retinal ganglion cells contribute to the PhNR and OP1 components of the full field flash ERG, but not significantly to the pattern ERG in guinea pigs. This study demonstrates that OCT imaging and full field flash ERGs are valuable in assessing retinal ganglion cell loss in vivo in guinea pigs and will help to further establish the guinea pig as a model of human ocular pathologies.

The pattern ERG in chicks - Stimulus dependence and optic nerve section

The chick is widely used in studies of eye growth regulation and myopia. The aim of this study was to explore the utility of pattern (p)ERG as a tool to assess retinal function in such studies. Effects of optical defocus and diffusing blur, manipulations used to alter eye growth experimentally, were evaluated. PERGs were recorded from White-Leghorn chickens, using a checkerboard pattern, including 8 spatial frequencies (0.05-2.2c/d SF), 13 contrast levels (1-100%), and 8 temporal reversal frequencies (0.5-20Hz). The acute effects of defocus and diffusing blur were examined. Flash- and pERGs were also recorded from chicks that underwent monocular optic nerve section (ONS), to explore the contribution of retinal ganglion cells (RGCs). Measurements were made up to 6weeks post-ONS, complemented with SD-OCT imaging. In normal chicks, the response to 1Hz, 100% contrast stimuli showed positive- and negative-going waveforms at 43ms (P1) and 75ms (N95), respectively, with 0.06-0.1c/d SF eliciting the largest P1 amplitudes of 21.9±2.5μV. Contrast levels above 5% yielded measurable P1 responses. Responses were transient and monophasic for 0.5-5Hzreversal rates, with higher temporal frequencies yielding steady state responses. Defocus and diffusing blur decreased pERG amplitude across all SFs. pERG responses remained normal after ONS, despite the loss of RGCs. In conclusion, chicks show robust pERG responses, which are attenuated by defocus and diffusing blur. The pERG response is not affected by ONS, suggesting that RGCs do not contribute to the chick pERG.

Contributions from lateral interaction mechanisms to the human ERG can be studied with a two-frequency method

We present a two-frequency method to investigate lateral interaction components (liERGs) in the human electroretinogram. Adjacent half cycles of sinusoidal gratings were modulated sinusoidally with different temporal frequencies f1 and f2. The liERGs were defined by the Fourier components at the intermodulation frequencies /f1 - f2/ and f1 + f2 which indicate nonlinear interactions between the half cycles. Significant liERGs were found in all subjects with a monotonic increase of the liERG magnitude in the spatial frequency range from f(s)=0.07 to 2.4 cpd. When /f1 - f2/ was below 5 Hz, liERGs were masked by noise intrusions. In a control experiment we demonstrated that the liERGs were not evoked by stray light artifacts. The liERGs may help to further differentiate the responses that are evoked by patterned stimuli within the retina.

Check size tuning of the pattern electroretingoram: a reappraisal

The pattern electroretinogram was recorded to checkerboard stimuli with a wide range of check sizes and two stimulus field sizes. Check sizes ranged from 0.25 degree to 7 degrees (field size, 16 degrees x 14 degrees) and 0.25 degree to 15 degrees (field size, 32 degrees x 27 degrees) in 14 and seven subjects, respectively. Reversal rate was 4.5/s. For minimal intrusion of blink artifacts the interrupted stimulation technique was employed. The P50 and N95 components of the pattern electroretinogram were evaluated separately. With both stimulus field sizes amplitude of P50 and N95 was maximal between 0.75 degree and 1 degree. With smaller check sizes the amplitude dropped monotonically. With larger check sizes field size played a role: with the 16 degrees x 14 degrees field, P50 gradually dropped to 89% from 1 degree to 7 degrees, which was paralleled by N95 only up to 7 degrees, where N95 dropped to 81% (p < 0.05). With the 32 degrees x 27 degrees field, there was no significant difference in size dependency between P50 and N95 for large check, both components staying constant from 1 degree to 15 degrees. We conclude that there is only minor large-check attenuation of the pattern electroretinogram, especially with a large field. The apparent field-size dependency may explain previous discrepancies in the literature.

Electroretinograms to checkerboard pattern reversal in cats: physiological characteristics and effect of retrograde degeneration of ganglion cells

Pattern electroretinograms (P-ERGs) evoked by alternating checks were studied in cats. Spatial frequency functions for transient and steady-state P-ERG waves showed a bimodal distribution with a preferred frequency at 0.6-0.75 c/deg and a second amplitude increase at frequencies lower than 0.5 c/deg. Decreasing the pattern luminance by 0.5 log units produced a shift of the spatial tuning curve toward lower spatial frequencies. No temporal tuning was noted in the temporal frequency functions. The bimodal distribution suggests that at spatial frequencies higher than 0.5 c/deg, the tuning reflects a 'contrast response' originating in cells with center surround organization. At spatial frequencies below 0.5 c/deg, the 'luminance response' becomes predominant and is generated in cells sensitive to mean luminance changes. Transient and steady-state P-ERGs to medium and high spatial frequencies were abolished by section of the optic nerve, while low spatial frequency stimuli at or below 0.3 c/deg continued to evoke P-ERGs at 1, 5 and 10 months after surgery. Quantitative whole mount retina microscopic examination confirmed the retrograde degeneration of the ganglion cells. It is concluded that both transient and steady-state P-ERGs to small and medium spatial frequencies checks are predominantly related to ganglion cell activity, while P-ERGs to low spatial frequencies reflect preganglionic cell activity.

Evoked responses to sinusoidal gratings in the pigeon optic tectum

Tectal evoked potentials (TEPs) in response to sinusoidal gratings of different contrast, spatial and temporal frequency have been recorded from the tectal surface of the pigeon. Responses have been digitally filtered in order to isolate transient oscillatory (fast) potentials (50-150 Hz), transient slow potentials (1-50 Hz), and the steady-state second-harmonic component (16.6 Hz). Transient slow potentials, as well as the steady-state second-harmonic component, are band-pass spatially tuned with a maximum at 0.5 cycles/deg and attenuation at higher and lower spatial frequencies. The high spatial frequency cutoff is 4-5 cycles/deg. Both transient slow potentials and the steady-state second-harmonic component increase in amplitude as a function of log contrast and saturate at about 20% contrast. The contrast sensitivity, as determined by extrapolating TEP amplitude to 0 V is 0.1-0.2%. These characteristics of spatial-frequency selectivity and contrast sensitivity are similar to those reported for single tectal cells. Unlike slow potentials, oscillatory potentials are not band-pass spatially tuned. In addition, their contrast response function does not saturate at moderate contrast. These differences suggest that tectal evoked slow and fast potentials reflect the activity of different neuronal mechanisms.

The pattern electroretinogram and visual-evoked potential in glaucoma

The pattern electroretinogram (PERG) may reflect ganglion cell or inner retinal layer activity. The most sensitive spatial and temporal variables for testing patients with glaucoma have not yet been identified. Fifty-two glaucoma suspects, 51 glaucoma patients, and 28 normal subjects were studied with the PERG and VEP, using three repetition rates and three spatial frequencies. Fast Fourier transforms were calculated at each spatial frequency and reversal rate. An analysis of variance revealed that normals could be differentiated from ocular hypertension and glaucoma patients using the amplitude of the PERG (second and fourth harmonic). Abnormalities in phase of the PERG between groups were also detected. A discriminant analysis of all amplitude and phase data revealed that the phase shift of the response of the second harmonic at 11 alternations/s (15-min checks) and at 5.5 alternations/s (15-min checks) correctly identified 81% of the normal and 75% of the glaucoma patients. The phase shift determinations of the VEP revealed significant abnormalities using 2 and 1/2 standard deviation confidence limits. There was significant overlap in the pattern ERG amplitude and phase shift in all three groups.