The influence of ambient room lighting on the pattern electroretinogram (PERG)

Cortical Interactions between Prosthetic and Natural Vision

Outer retinal degenerative diseases, such as retinitis pigmentosa (RP) and age-related macular degeneration (AMD), are among the leading causes of incurable blindness in the Western world [1]. Retinal prostheses have been shown to restore some useful vision by electrically stimulating the remaining retinal neurons [2]. In contrast to inherited retinal degenerative diseases (e.g., RP), typically leading to a complete loss of the visual field, in AMD patients the disease is localized to the macula, leaving the peripheral vision intact. Implanting a retinal prosthesis in the central macula in AMD patients [3, 4] leads to an intriguing situation where the patient's central retina is stimulated electrically, whereas the peripheral healthy retina responds to natural light stimulation. An important question is whether the visual cortex responds to these two concurrent stimuli similarly to the interaction between two adjacent natural light stimuli projected onto healthy retina. Here, we investigated the cortical interactions between prosthetic and natural vision based on visually evoked potentials (VEPs) recorded in rats implanted with photovoltaic subretinal implants. Using this model, where prosthetic and natural vision information are combined in the visual cortex, we observed striking similarities in the interactions of natural and prosthetic vision, including similar effect of background illumination, linear summation of non-patterned stimuli, and lateral inhibition with spatial patterns [5], which increased with target contrast. These results support the idea of combined prosthetic and natural vision in restoration of sight for AMD patients.

Comparison of the uniform-field electroretinogram and the pattern electroretinogram to checkerboard and bar gratings

PURPOSE

To compare the electroretinal response associated with the uniform-field electroretinogram (UF-ERG) to that of the pattern electroretinogram (PERG) to checkerboard and bar-grating stimuli.

METHODS

UF-ERG and PERG to bars and checkerboard were recorded for 18 visually normal subjects (36 eyes) of mean age 45 years (range 20-75). UF-ERG was recorded to the increment and decrement of a 200-ms duration luminance modulation. Luminance onset and offset UF-ERG responses were averaged to produce a simulation of the PERG response. The mean amplitude and implicit time for the P₅₀ and N₉₅ potentials of actual and simulated PERG responses were recorded for each eye in the cohort.

RESULTS

The simulated PERG waveform resulting from arithmetic averaging of the UF-ERG to luminance increment and decrement was characterized by prominent positive and negative components resembling those of the P₅₀ and N₉₅ PERG potentials. Implicit timing of the P₅₀ potential was lengthened in the actual PERG to bars and checks relative to that of the simulation (P < 0.05, P < 0.001). Amplitude of the N95 potential was greater in the PERG to bars than in the PERG to checks (P < 0.05) or the simulated PERG (P < 0.001). The amplitude and implicit timing of all waveform components were significantly correlated between the actual and simulated PERG.

CONCLUSIONS

The UF-ERG to light onset and offset can be reliably recorded in human subjects. The extent to which the simulated PERG recapitulates the actual PERG response is better with checkerboard rather than bar-grating stimuli.

Next Generation PERG Method: Expanding the Response Dynamic Range and Capturing Response Adaptation

Purpose

To compare a new method for steady-state pattern electroretinogram (PERGx) with a validated method (PERGLA) in normal controls and in patients with optic neuropathy.

Methods

PERGx and PERGLA were recorded in a mixed population (n = 33, 66 eyes) of younger controls (C1; n = 10, age 38 ± 8.3 years), older controls (C2; n = 11, 57.9 ± 8.09 years), patients with early manifest glaucoma (G; n = 7, 65.7 ±11.6 years), and patients with nonarteritic ischemic optic neuropathy (N; n = 5, mean age 59.4 ± 8.6 years). The PERGx stimulus was a black-white horizontal grating generated on a 14 × 14 cm LED display (1.6 cycles/deg, 15.63 reversals/s, 98% contrast, 800 cd/m² mean luminance, 25° field). PERGx signal and noise were averaged over 1024 epochs (∼2 minutes) and Fourier analyzed to retrieve amplitude and phase. Partial averages (16 successive samples of 64 epochs each) were also analyzed to quantify progressive changes over recording time (adaptation).

Results

PERGLA and PERGx amplitudes and latencies were correlated (Amplitude R² = 0.59, Latency R² = 0.39, both P < 0.0001) and were similarly altered in disease. Compared to PERGLA, however, PERGx had shorter (16 ms) latency, higher (1.39×) amplitude, lower (0.37×) noise, and higher (4.2×) signal-to-noise ratio. PERGx displayed marked amplitude adaptation in C1 and C2 groups and no significant adaptation in G and N groups.

Conclusions

The PERGx high signal-to-noise ratio may allow meaningful recording in advanced stages of optic nerve disorders. In addition, it quantifies response adaptation, which may be selectively altered in glaucoma and optic neuropathy.

Translational Relevance

A new PERG method with increased dynamic range allows recording of retinal ganglion cell function in advanced stages of optic nerve disorders. It also quantifies the response decline during the test, an autoregulatory adaptation to metabolic challenge that decreases with age and presence of disease.

Pattern electroretinogram progression in glaucoma suspects

PURPOSE

To prospectively monitor progressive changes of retinal ganglion cell function in early glaucoma using the pattern electroretinogram (PERG).

METHODS

Fifty-nine patients enrolled as glaucoma suspects were observed untreated over an average of 5.7±1.4 years, during which they were tested with PERG (PERGLA paradigm) and standard automated perimetry (SAP) 2 times per year. PERG amplitude and phase were normalized for physiological age-related changes, and linear regressions fitted to the data to calculate progression slopes (signal), slope SE (noise), and corresponding signal-to-noise ratios (SNR=slope÷SE). Linear regressions were also used to fit SAP global indices mean deviation (MD) and pattern standard deviation (PSD).

RESULTS

On average, progression slopes of PERG amplitude/phase were skewed toward negative values, their mean being significantly (P<0.01) different from zero. In contrast, mean slopes of SAP-MD and PSD were not significantly different from zero. SNRs were higher for PERG than SAP (P<0.01). A substantial number of eyes displayed significant (P<0.05) progression of PERG amplitude (15% to 20%) or PERG phase (16% to 25%). Fewer eyes displayed significant progression of SAP-MD (0% to 2%) or SAP-PSD (4% to 8%).

CONCLUSIONS

The PERG displayed clear longitudinal loss of signal (diminished amplitude, phase delay, or both) in a substantial number of eyes of patients, indicating progressive deterioration of retinal ganglion cell function. Progression of SAP global indices MD and PSD was found in a relatively smaller number of eyes. It remains to be established whether PERG progression has predictive value for developing visual dysfunction.

Pattern electroretinogram association with spectral domain-OCT structural measurements in glaucoma

PURPOSE

To describe the association between pattern electroretinogram (PERG) amplitude and spectral domain-optical coherence tomography (SD-OCT) macular thickness, retinal nerve fibre layer (RNFL) thickness and optic disc topography measurements.

SUBJECTS AND METHODS

Both eyes (n = 132) of 66 glaucoma patients (mean age = 67.9 years) enrolled in the University of California, San Diego, CA, USA, Diagnostic Innovations in Glaucoma Study (DIGS) were included. Eyes were tested with PERG (Glaid PERGLA, Lace Elettronica, Pisa, Italy), RTVue SD-OCT (Optovue Inc., Fremont, CA, USA) GCC, and NHM4 protocols on the same day. Of the 66 enrolled patients, 43 had glaucoma defined by repeated abnormal standard automated perimetry (SAP) results in at least one eye and 23 were glaucoma suspects defined by a glaucomatous-appearing optic disc by physicians' examination in at least one eye and normal SAP results in both eyes. Associations (R(2)) were determined between PERG amplitude (μV) and SD-OCT macular ganglion cell complex (GCC) thickness (μm), macular thickness (μm), macular outer retinal thickness (macular thickness minus GCC thickness) (μm), RNFL thickness (μm), neuroretinal rim area (mm(2)), and rim volume (mm(3)).

RESULTS

PERG amplitude was significantly associated with GCC thickness (R(2) = 0.179, P < 0.001), RNFL thickness (R(2) = 0.174, P < 0.001), and macular thickness (R(2) = 0.095, P<0.001). R(2) associations with other parameters were not significant (all P > 0.624). Significant associations remained for GCC and average RNFL thickness when age and intraocular pressure at the time of testing were included in multivariate models (both P ≤ 0.030).

CONCLUSIONS

PERG amplitude is significantly (but weakly) associated with macular GCC thickness, RNFL thickness, and macular thickness. The lack of association between PERG amplitude and macular outer retinal thickness supports previous results, possibly suggesting that that the PERG is driven primarily by retinal ganglion cell (inner retinal) responses.

Pattern reversal ERG and VEP--comparison of stimulation by LED, monitor and a Maxwellian-view system

PURPOSE

Pattern stimulation is widely used to detect inner retinal dysfunction. In this work we describe a pattern stimulation technique with LEDs and compare the results with conventional methods.

METHODS

PERG and VEP were derived from three normal subjects. Three different techniques were used to generate a checkerboard pattern reversal stimulus: a 70 Hz monitor, a Maxwellian-view system equipped with a Xenon-arc lamp and a mechanical mirror system, and a LED array (Roland Consult) consisting of 100 white LEDs. Two kinds of luminance (125 and 340 cd/m2) and four temporal frequencies (4, 8, 12 and 24 reversals per second) were studied on three healthy subjects. Additionally, a luminance tuning experiment (30, 60, 90, 125 and 340 cd/m2) was performed on one subject.

RESULTS

Comparison of different stimulation techniques shows reproducible responses of PERG and VEP with all three methods. The LED array leads to slightly smaller amplitudes than both other techniques, which we ascribe to the design of the LED field. No difference of peak times or phases was noticed between different stimulation techniques. A luminance dependency of PERG and VEP is noticeable using stimulation with LED: with decreasing luminance we measured increasing peak times of PERG and VEP and decreasing amplitude of PERG.

CONCLUSION

We conclude that central retinal stimulation with checkerboard pattern reversal is possible with LED. It gives comparable results to monitor and Maxwellian-view system.

Acute effects of cigarette smoking on pattern electroretinogram

In this study, acute effects of cigarette smoking on the pattern electroretinogram (PERG) were investigated. First, variability of the PERG was studied in a group of young male smokers (26 right eyes of 26 subjects). Then PERGs were investigated in a group of habitual smokers (17 right eyes of 17 subjects) in separate real smoking and sham smoking sessions. On each session PERGs were recorded pre-smoking (PS), immediately after smoking (IAS) and 5 min after smoking (5th) conditions. Real smoking significantly increased P50 amplitudes and decreased N95 latencies. Regarding P50 amplitudes in the real smoking sessions, the differences were significant between PS and IAS (PS: 3.3 +/- 0.5 muV, IAS: 3.7 +/- 0.7microV, P = 0.015) and between PS-5th (PS: 3.3 +/- 0.5microV, 5th: 4.1 +/- 0.9microV, P = 0.039). There was significant difference (P = 0.024) between N95 latencies of PS (98.5 +/- 6.9 ms) and IAS (94.7 +/- 5.1 ms) in the real smoking sessions. No statistically significant difference was observed in sham smoking sessions. Our results indicated, for the first time, that cigarette smoking may influence PERG amplitude and latency significantly in habitual smokers.

Pattern reversal ERG with LED-stimulation using cyclic summation technique

PURPOSE

Multifocal pattern reversal stimulation can be used to detect inner retinal dysfunction. Commonly, the stimulus is generated on a monitor using m-sequence technique. We describe a pattern reversal ERG evoked by LED arrays using cyclic summation (CS).

METHODS

One eye of eight healthy subjects was examined with an arrangement of 13 LED arrays. Each array consisted of 100 LEDs separated by thin walls. One of the fields was placed centrally, three fields each were placed above, below, left and right of the central field. CS technique at a temporal frequency of 16 reversals per second (RPS) was used for stimulation. Viewing distance was 30 cm, check size was 0.58 cyc/deg. Luminance of the bright fields was 340 cd/m2.

RESULTS

Fourier analysis was performed. Centrally, the amplitude of the 2nd harmonic wave was highest (0.87 microV). In the first paracentral fields, amplitudes were 0.28 microV (nasally), 0.21 microV (superior, inferior and temporally). In the second paracentral fields, amplitudes were 0.11 microV (nasally), 0.09 microV (superior), 0.13 microV (inferior) and 0.15 microV (temporally). With exception of the temporal field (0.1 microV), in the outermost fields no reproducible ERG response could be recorded.

CONCLUSION

Peripheral ERG responses to a pattern reversal stimulus can be recorded with LED stimulation using CS technique up to an eccentricity of 30 degrees. Responses are highest centrally and decrease with increasing distance to the centre.

The Multifocal Pattern Electroretinogram in Chloroquine Retinopathy

PURPOSE

Optimal screening for ocular toxicity caused by chloroquine and hydroxychloroquine is still controversial. With the multifocal pattern electroretinogram (mfPERG), a new electrophysiological technique has recently become available to detect early changes of ganglion cells. In this study this new technique is applied to a series of 10 patients seen consecutively receiving long-term chloroquine medication.

METHODS

In 10 patients receiving chloroquine medication, clinical examination, Amsler visual field testing and computerized color vision testing were performed. If toxicity was suspected, automated perimetry was carried out. In addition, in all patients conventional pattern electroretinogram (PERG) and mfPERG testing were performed.

RESULTS

On clinical examination 8 patients showed no chloroquine-associated maculopathy, while 2 patients did. Of these 2, only 1 reported abnormalities when viewing the Amsler chart, while automated perimetry showed typical, ring-like paracentral scotomas in both affected patients and color vision was significantly abnormal. In the normal patients, 4 of 8 had a mild color vision disturbance, which correlated to age-related macular changes. The amplitudes of the PERG and the central (approximately 10 degrees ) responses of the mfPERG were markedly reduced in chloroquine maculopathy, while the latencies were unchanged. The peripheral rings of mfPERG (ranging to 48 degrees ) were not affected by chloroquine toxicity. Both PERG and mfPERG were less affected by age-related macular changes.

CONCLUSIONS

The reduction of PERG and central mfPERG responses in chloroquine maculopathy may help with the early detection of toxicity.