The uniform field and pattern ERG in macaques with experimental glaucoma: removal of spiking activity.

PURPOSE: To determine whether the uniform field and pattern ERGs that are reduced in macaque eyes with experimental glaucoma have the same inner-retinal origins. METHODS: ERGs were recorded from 14 anesthetized adult macaques using DTL electrodes. Six monkeys had laser-induced experimental glaucoma, and two others received intravitreal injections of tetrodotoxin (TTX, 6 microM) to block spiking activity of inner-retinal neurons. The remaining 6 animals were normal. Uniform fields and grating patterns (0.1-3 cpd) were square-wave modulated at 1.7 Hz (transient) and 8 Hz (steady state). The test field (42 degrees x 32 degrees) had a mean luminance of 44 cd/m2 and a contrast of 10% to 82%. RESULTS: In normal eyes transient ERGs to uniform fields contained photopic negative responses (PhNR) after the b-wave and after the d-wave. Transient pattern electroretinograms (PERGs) at each contrast reversal showed positive (P50) potentials followed by negative (N95) potentials of time course similar to that of the PhNR. The PhNR and N95 were greatly reduced or eliminated by experimental glaucoma and by TTX. Summing responses to luminance increments and decrements of the uniform field could simulate the PERG to low spatial frequency stimuli. Further, the PERG responses to high spatial frequencies were similar to the simulation in shape but slightly delayed in time. Experimental glaucoma and TTX had similar effects on the N95 of the simulated PERG as to those on the actual PERG. However, P50 was more reduced by experimental glaucoma than by TTX, indicating a nonspiking contribution to P50. For the steady state condition, the uniform field ERG, the simulated PERG, and the actual PERG all were affected by experimental glaucoma and TTX, indicating that they contained contributions from the spiking activity of ganglion cells. CONCLUSIONS: The changes in the uniform field and PERG responses produced by experimental glaucoma are related and are largely a consequence of reduced spiking activity of ganglion cells and their axons. These findings raise the possibility that the uniform field ERG could serve as a useful alternative to the PERG in the assessment of clinical glaucomatous neuropathy.

The photopic negative response of the flash electroretinogram in primary open angle glaucoma.

PURPOSE: To determine whether the photopic negative response (PhNR) of the electroretinogram (ERG) is reduced in patients with primary open angle glaucoma (POAG). METHODS: ERGs were recorded with DTL electrodes from 62 normal subjects (16 to 82 years), 18 POAG patients (47 to 83 years) and 7 POAG suspects (46 to 73 years) to brief flashes (<6 ms), and also in a few subjects to long (200 ms) red, full-field ganzfeld flashes delivered on a rod-saturating blue background. At the time of ERG measurements, the intraocular pressures of most of the patients were controlled medically. Visual field sensitivities were measured with the Humphrey C24-2 threshold test and optic nerve head cup-to-disc ratio (C/D) was determined by binocular indirect ophthalmoscopy. RESULTS: ERGs of normal subjects contained a slow negative potential following the a- and b-waves, the PhNR, that increased slightly in latency with age. The a- and b-wave amplitudes and implicit times of POAG patients were similar to age-matched controls. In contrast, their PhNRs were small or virtually absent. PhNR amplitudes were reduced even when visual sensitivity losses were small, and were correlated significantly (P < 0.05) with mean deviation (MD), corrected pattern SD (CPSD), and C/D across the population of POAG patients whose MD losses ranged from 1 to 13 dB, CPSDs from 0 to 11 dB and C/Ds from 0.6 to 0.9. PhNRs of most POAG suspects also were small. CONCLUSIONS: PhNR amplitudes in POAG patients are smaller than those of normal subjects. PhNR amplitudes are reduced when visual field sensitivity losses are mild and become even smaller as sensitivity losses increase. There is a potential role for the PhNR in early detection and possibly in monitoring the progression of glaucomatous damage.

The photopic negative response of the macaque electroretinogram: reduction by experimental glaucoma.

PURPOSE: To investigate the photopic flash electroretinograms (ERGs) of macaque monkeys in which visual field defects developed as a consequence of experimental glaucoma. METHODS: Unilateral experimental glaucoma was induced in 10 monkeys by argon laser treatment of the trabecular meshwork. Visual field sensitivity was assessed behaviorally by static perimetry. Photopic ERGs were recorded to brief- (< or = 5 msec) and long-duration (200 msec) red ganzfeld flashes on a rod-suppressing blue-adapting background. Electroretinograms were recorded in four other monkeys, after intravitreal injection of tetrodotoxin (TTX; 3.8-8 p.M) to suppress action potentials of retinal ganglion and amacrine cells, and in six normal adult human subjects. RESULTS: Experimental glaucoma removed a cornea-negative response, the photopic-negative response (PhNR), from the ERG. The PhNR in control eyes was maximal approximately 60 msec after a brief flash, 100 msec after onset, and 115 msec after offset of the long-duration stimulus. The PhNR in experimental eyes was greatly reduced when the mean deviation of the visual field sensitivity was as little as -6 dB. As visual sensitivity declined further, the PhNR was reduced only slightly more. The a- and b-waves were unchanged, even when sensitivity decreased by more than 16 dB. Tetrodotoxin also selectively reduced the PhNR. The PhNR was observed in normal human ERGs. CONCLUSIONS: The cornea-negative PhNR of the photopic ERG depends on spiking activity and is reduced in experimental glaucoma when visual sensitivity losses are still mild. The PhNR most likely arises from retinal ganglion cells and their axons, but its slow timing raises the possibility that it could be mediated by glia. Regardless of the mechanism of its generation, the PhNR holds promise as an indicator of retinal function in early glaucomatous optic neuropathy.

The scotopic electroretinogram of macaque after retinal ganglion cell loss from experimental glaucoma.

PURPOSE: This study describes the dark-adapted electroretinograms (ERGs) of macaque monkeys with severe visual field defects and substantial retinal ganglion cell loss as a consequence of long-standing ocular hypertension. METHODS: Monocular experimental glaucoma was produced by argon laser trabeculoplasty, and visual fields were assessed with behavioral static perimetry. Electroretinographic responses to brief ganzfeld flashes under fully dark-adapted conditions were recorded using DTL fiber electrodes in anesthetized animals. The authors quantified retinal layer thickness and cell loss in 1-micron radial sections and inspected optic nervous under the light microscope. RESULTS: At the lowest intensities, a sensitive negative component of the scotopic ERG, which normally peaks approximately 200 msec after stimulus onset, was present in the control eyes but was reduced greatly or was virtually absent in the experimental eyes of monkeys with severe visual field loss. A previously unreported sensitive positive component of the scotopic ERG remained in both eyes. In the control eyes, the positive component gave rise to a sharp peak approximately 120 msec after stimulus onset, but in the experimental eyes, because of the absence of the more delayed sensitive negative potential, it was sustained, lasting as long as 700 msec. Scotopic a- and b-waves and oscillatory potentials in the experimental eyes were not consistently different from control eyes. Ganglion cell and optic nerve loss in the experimental eyes was substantial, and there was little other obvious retinal damage. CONCLUSIONS: A sensitive negative component is reduced or absent from the dark-adapted ERGs of macaque monkeys with severe visual field defects and substantial retinal ganglion cell loss as a consequence of long-standing ocular hypertension.

Summation of very close spatial frequencies: the importance of spatial probability summation.

In accounting for pattern thresholds it is necessary to consider probability summation (or equivalent nonlinear pooling) not only across detectors selective for different spatial frequencies but also across detectors in different spatial positions. Interestingly, calculation on this basis shows that the amount of summation between components of closely similar spatial frequency in a large grating is primarily determined by the variation in sensitivity of detectors at different spatial locations and is little affected by the spatial-frequency bandwidths of the detectors. To test this conclusion, we have measured the amount of summation between two components with spatial frequencies very close to 6 c/deg in two regions of the visual field: in the fovea (a region where sensitivity is very non-uniform) and in the periphery (where sensitivity is nearly uniform). As predicted, there was less summation between components of very closely similar frequencies in the nearly-uniform peripheral region than in the non-uniform foveal region. Measurements in the fovea of the summation of two components with spatial frequencies very near to either 1.5, 6 or 24 c/deg showed, as expected, that the amount of summation depends upon the ratio of the frequencies rather than their absolute difference, indicating that probability summation takes place over an area related to spatial frequency rather than over a fixed area.

In vivo studies of signaling in rod pathways of the mouse using the electroretinogram.

PURPOSE: (a) To examine the possibility that there is a threshold in the synaptic mechanism linking rods to rod bipolar cells that can reduce the transmission of continuous noise from the rods without blocking the transmission of any significant proportion of single-photon responses. (b) To estimate the level of this threshold and the amplitude of the continuous noise which it can serve to reduce. (c) To identify the location of the threshold mechanism in the rod to rod bipolar cell pathway. METHODS: Corneal electroretinogram recordings were made from dark-adapted mice anesthetized with ketamine/xylazine after inner-retinal components had been suppressed to isolate PII, the response of depolarizing bipolar cells. Suppression was achieved by intravitreal injections of GABA, TTX, or in Cx36 KO animals by crushing the optic nerve and waiting for ganglion cells to degenerate. RESULTS: All energy-scaled records of isolated PII obtained with ganzfeld stimuli that gave rise to much less than one photoisomerization (R*) per rod (0.01-0.2 R*/rod), had an essentially identical waveform. Stronger stimuli caused a reduction in the peak amplitude of energy-scaled records (saturation) and stimuli strong enough to produce multiple isomerizations in individual rods resulted in a shortening of the response latency and an increase of the energy-scaled amplitude at early times (supralinearity). The shape of the rising edge of isolated PII changed with flash energy in a way that was consistent with the existence of a synaptic threshold whose level was less than one tenth of the amplitude of single-photon signals and a continuous noise whose rms amplitude was even less than this. However, when measured at the time of the peak, the amplitude of PII increased linearly in proportion to stimulus energy from the very lowest levels up to the point where there was, on average, 0.2 R*/rod. CONCLUSIONS: There is a threshold nonlinearity operating at the output of the rod to rod bipolar cell synapse that can usefully reduce the transmission of continuous rod noise without significantly affecting the transmission of single-photon signals. This nonlinearity does not affect the overall linear function of the rod pathway at levels at which it is effectively operating in a photon-counting mode.

Identifying inner retinal contributions to the human multifocal ERG.

Contributions to the multifocal electroretinogram (ERG) from the inner retina (i.e. ganglion and amacrine cells) were identified by recording from monkeys before and after intravitreal injections of n-methyl DL aspartate (NMDLA) and/or tetrodotoxin (TTX). Components similar in waveform to those removed by the drugs were identified in the human multifocal ERG if the stimulus contrast was set at 50% rather than the typically employed 100% contrast. These components were found to be missing or diminished in the records from some patients with glaucoma and diabetes, diseases which affect the inner retina.

Steady discharges of X and Y retinal ganglion cells of cat under photopic illuminance.

The discharges of ON- and OFF-center X and Y retinal ganglion cells in the presence of stationary patterns or of a uniform field of photopic luminance were recorded from urethane-anesthetized adult cats. The interval statistics and power spectra of these discharges were determined from these discharge records. The patterned stimuli were selected and positioned with respect to a cell's receptive field so as to generate steady discharges that were different in mean discharge rate from that cell's discharge for the diffuse field. The interval statistics of discharges recorded for diffuse or patterned illumination for all cell types can be modeled, approximately, as coming from renewal processes with gamma-distributed intervals. The gamma order of the interval distributions was found to be nearly proportional to the mean discharge rate for X cells, but not for Y cells. Typical values for the gamma orders and their dependence on mean rate for different cell types are given. The same model of a renewal process with gamma-distributed intervals is used to model the measured power spectra and performs well. When the gamma order is proportional to mean rate, the power spectral density at low temporal frequencies is independent of discharge rate. Gamma order was proportional to mean rate for X cells but not for Y cells. Nonetheless, the power spectral densities of both cell types at low frequencies were approximately independent of discharge rate. Hence, noise in this band of frequencies can be considered additive. The consequences of departures from the renewal process and of the gamma order not being proportional to mean rate are considered. The significance of different rates of discharge for signaling is discussed.

Response linearity and kinetics of the cat retina: the bipolar cell component of the dark-adapted electroretinogram.

The electroretinogram (ERG) of the dark-adapted cat eye in response to brief ganzfeld flashes of a wide range of intensities was recorded after intravitreal injection of n-methyl DL aspartate (NMDLA, cumulative intravitreal concentration of 1.3-3.9 mM) to suppress inner-retinal components, and after intravitreal DL or L-2-amino-4-phosphonobutyric acid (DL-APB, 1-3 mM; L-APB, 1.2 mM) and 6-cyano-7-nitroquinoxaline-2,3 dione (CNQX, 40-60 microM), to suppress all post-receptoral neuronal responses. Rod PII, the ERG component arising from rod bipolar cells, was derived by subtracting records obtained after APB and CNQX from post-NMDLA records. When we measured the derived response at fixed times after the stimulus, we found that PII initially increased in proportion to stimulus intensity without any sign of a threshold. The leading edge of PII at early times after the stimulus, when the response was still small, was well described by V(t) = kI(t-td)5 where k is a constant, I is the intensity of the stimulus, and td is a brief delay of about 3 ms. Correspondingly, the time for the response to rise to an arbitrary small criterion voltage Vcrit was adequately fitted by tcrit = td + (Vcrit/kI)1/5. The time course of the leading edge of the PII response can be interpreted to indicate that the mechanism generating PII introduces three stages of temporal integration in addition to the three stages that are provided by the mechanism of the rod photoreceptors. This finding is consistent with the operation within the rod bipolar cell of a G-protein cascade similar to that in the rods.

Dissecting the dark-adapted electroretinogram.

Although gross recordings of the ganzfeld flash-evoked electroretinogram (ERG) can potentially provide information about the activity of many, if not all, retinal cell types, it is necessary to dissect the ERG into its components to realize this potential fully. Here we describe various procedures that have been used in intact mammalian eyes to identify and characterize the contributions to the dark-adapted ERG of different cells in the retinal rod pathway. These include (1) examination of the very early part of the response to a flash (believed to reflect directly the photocurrent of rods), (2) application of high-energy probe flashes to provide information about the underlying rod photoreceptor response even when this component is obscured by the responses of other cells, (3) pharmacological suppression of responses of amacrine and ganglion cells to identify the contribution of these cells and to reveal the weaker responses of bipolar cells, (4) use of pharmacological agents that block transmission of signals from rods to more proximal neurons to separate responses of rods from those of later neurons, (5) examination of the ERG changes produced by ganglion-cell degeneration or pharmacological block of nerve-spike generation to identify the contribution of spiking neurons, (6) modeling measured amplitude-energy functions and timecourse of flash responses and (7) using steady backgrounds to obtain differential reductions in sensitivity of different cell types. While some of these procedures can be applied to humans, the results described here have all been obtained in studies of the ERG of anaesthetized cats, or macaque monkeys whose retinas are very similar to those of humans.

Photoreceptor and bipolar cell contributions to the cat electroretinogram: a kinetic model for the early part of the flash response.

The time course of the initial negative wave of the flash electroretinogram of the dark-adapted cat has been found to be critically dependent of contributions from cells of the inner retina, not only for very low-intensity flashes for which the negative scotopic threshold response is dominant but also when the stimulus is sufficiently intense for the rods themselves to contribute directly to the electroretinogram. However, if the inner-retinal responses are blocked pharmacologically or are suppressed by a steady adapting background, the initial negative wave of the remaining electroretinogram (the alpha wave) can be explained as the sum of photoreceptor and bipolar-cell components that can be modeled as described by Lamb and Pugh [J. Physiol. (London) 449, 717 (1992)] and Robson and Frishman [Vis. Neurosci. 12, 837 (1995)], respectively.

The yellow colour of the lens of man and other primates.

1. Measurements have been made of the absorption spectra of the lenses of human, baboon (Papio), rhesus monkey (Macaca), squirrel monkey (Saimiri sciureus) and bush baby (Galago crassicaudatus).2. In all these species an absorption maximum was found between 365 and 368 nm.3. The pigment responsible for this absorption was water-soluble and aqueous extracts were examined. Protein-free aqueous extracts showed an additional maximum at 260 nm which could be only partially accounted for by the presence of ascorbic acid.4. Chromatography of the protein-free solution from human lenses yielded a fast-moving yellow component with a blue fluorescence. Its absorption spectrum was very similar to that of the original protein-free solution. A fast-moving yellow component from the baboon lens had a yellow fluorescence.5. The human lenses appeared to contain more of the yellow, water-soluble pigment at birth than in adult life. The concentration remains constant during adult life.6. There is evidence for the appearance of another pigment in the human lens in later adult life. It is not water-soluble and has an absorption maximum at about 330 nm.

The yellow colour of the lens of the grey squirrel (sciurus carolinensis leucotis).

1. The absorption spectrum of the lens of the grey squirrel (Sciurus carolinensis leucotis) has been measured, revealing an absorption maximum at 370 nm. Peak densities in the intact lens ranged from 12 to 20.2. This peak is not present in the lenses of horse, cow, pig, dog, cat, ferret (Mustelo furo), or guinea-pig (Cavia porcellus).3. The pigment responsible for this absorption is water-soluble and aqueous extracts have been examined. Protein-free aqueous extracts show an additional maximum at 265 nm, which can only partially be accounted for by the presence of ascorbic acid.4. The absorption spectrum of extracts of lens material from the ground squirrel (Citellus mexicanus) also had maxima at 265 and 370 nm.5. Chromatography of the protein-free solution separated two yellow components, both of which had a yellow fluorescence. The faster component had a very similar absorption spectrum to the original protein-free solution.6. Possible functions of the yellow pigment are discussed.

Application of Fourier analysis to the visibility of gratings.

1. The contrast thresholds of a variety of grating patterns have been measured over a wide range of spatial frequencies.2. Contrast thresholds for the detection of gratings whose luminance profiles are sine, square, rectangular or saw-tooth waves can be simply related using Fourier theory.3. Over a wide range of spatial frequencies the contrast threshold of a grating is determined only by the amplitude of the fundamental Fourier component of its wave form.4. Gratings of complex wave form cannot be distinguished from sine-wave gratings until their contrast has been raised to a level at which the higher harmonic components reach their independent threshold.5. These findings can be explained by the existence within the nervous system of linearly operating independent mechanisms selectively sensitive to limited ranges of spatial frequencies.