Evidence that a-wave latency of the electroretinogram is determined solely by photoreceptors

PURPOSE

To identify the retinal cells that determine the a-wave latency of rats.

METHODS

Electroretinograms (ERGs) were recorded from the rod-dominated (0.85% cones) retinas of Long-Evans rats following an intravitreal injection of 1 microL of 40 mM 2-amino-4-phosphonobutyric acid to block the activity of the ON pathway of the second order retinal neurons. ERGs were also recorded following an intraperitoneal injection of sodium iodate to destroy the retinal pigment epithelial (RPE) cells. Damage to a large area of the retina was produced by constant light exposure, and focal damage to the retina was induced by argon laser photocoagulation. The effects of age and anesthesia level on the a-wave latency were also determined.

RESULTS

Blocking the activity of the ON pathway of the second order retinal neurons did not alter the a-wave latency, and destroying the RPE cells also did not alter the a-wave latency. Damage to a large area of the retina resulted in prolonging the latency but focal retinal damage did not alter the a-wave latency. The a-wave latency was longer in young rat pups but was adult-like by 18 days. The level of anesthesia had no effect on the latency except at very deep stages.

CONCLUSIONS

The a-wave latency is determined solely by the activity of the photoreceptors. A prolonged latency would indicate that the photoreceptors are damaged over a large area of the retina.

The a-wave latency in control subjects and patients with retinal diseases

PURPOSE

To determine the a-wave latency of the electroretinograms (ERGs) recorded from control subjects and patients with retinal diseases.

METHODS

The a-wave latency and implicit time (IT) were measured retrospectively from the ERGs of 40 control subjects and 99 patients. The patients included 9 with complete congenital stationary night blindness (cCSNB), 13 with achromatopsia or cone dystrophy, 5 with supernormal and delayed rod ERG syndrome, and 72 with retinitis pigmentosa (RP). To assess whether latency measurements can be obtained reliably by different observers from patients with smaller a-wave amplitudes and noisier baselines, the a-wave latency and IT of the ERG of the right eye of 10 control subjects and 10 patients with RP were measured by three observers.

RESULTS

The mean a-wave latency measured for the same 10 control ERGs by three observers differed by less than 1 millisecond while the mean IT differed by 1.7 milliseconds. For 10 ERGs from RP patients, the mean for the a-wave latency measured by the three observers differed by less than 2.0 milliseconds and by 1.1 millisecond for the IT. The coefficient of variation varied from 24.8% to 36.7% for the latency and from 11.5% to 16.0% for the IT. The a-wave latencies elicited by the 0-dB stimulus under scotopic and photopic conditions from the 40 control subjects were not statistically different. The a-wave latency in patients with cCSNB did not differ significantly from that in control subjects. The longer a-wave latency in patients with achromatopsia suggested that the rods have a longer latency than cones. The scotopic and photopic a-wave latencies were significantly longer in RP patients. The longer latency in RP patients was not due to smaller a- or b-wave amplitudes.

CONCLUSIONS

The a-wave latency can be measured as reliably as the IT in control subjects but the reliability is not as good for the latency as for the IT in RP patients. The larger coefficients of variation in RP patients were most likely due to the measurements being made from RP patients at different stages of their disease. Our results suggest that the a-wave latency in control subjects is determined by cones under both scotopic and photopic conditions. The longer a-wave latency in RP patients suggests that the rods and cones are altered over a significant area of the retina.

Reliability and significance of measurements of a-wave latency in rats

PURPOSE

To determine whether measurements of the a-wave latency of the electroretinogram (ERG) can be made as reliably as that of the implicit time (IT) in rats. In addition, to determine the relationship between the potential level selected for the latency and the baseline potential level.

METHODS

ERGs, elicited by different stimulus intensities, were recorded from Long-Evans rats. The a-wave latency was determined by measuring the time between the stimulus onset and the beginning of the negative-going a-wave, and the IT was measured as the time between the stimulus onset and the peak of the a-wave. To test the reliability of the measurements of the latency, the a-wave latency and the IT were measured by three independent observers for the same 15 ERGs.

RESULTS

The mean a-wave latency was approximately 14 milliseconds, and the mean a-wave implicit time was approximately 36 milliseconds. The mean of the a-wave latency and the IT, as measured by the three observers, were within 1 millisecond of each other. The coefficient of variation was as good for the latency as for the IT of the a-wave. The potential level selected for the latency was lower than the mean baseline potential level by 1 to 2 standard deviations.

CONCLUSIONS

Selection of the a-wave latencies can be made as reliably as that for the IT. Because the a-wave latency is not affected by the activity of the second order neurons, the latency is a better measure than the IT of the time course of the a-wave.

Low-frequency damped electroretinographic wavelets in young asymptomatic patients with dominant retinitis pigmentosa: a new electroretinographic finding

OBJECTIVE

To describe a previously unreported electroretinographic (ERG) pattern in early retinitis pigmentosa (RP).

DESIGN

Two case reports.

PARTICIPANTS

Two unrelated young asymptomatic patients with autosomal-dominant retinitis pigmentosa were studied.

MAIN OUTCOME MEASURES

Clinical findings and ERG responses were assessed.

RESULTS

No ERG responses were detected scotopically with low-luminance stimuli. With increasingly brighter stimuli, a series of three to five low-frequency damped wavelets developed under both scotopic and photopic conditions. The period of the wavelets was 25 to 37 msec.

CONCLUSIONS

Low-frequency damped ERG wavelets occur in some young asymptomatic patients with autosomal-dominant RP. The ERG pattern suggests that these wavelets are predominantly cone-generated.

The c-wave of the electroretinogram recorded under clinical conditions from rabbits

To determine whether large and repeatable c-waves can be recorded from rabbits with equipment already in use in clinical electroretinographic laboratories, the Burian-Allen electrode, connected bipolarly or monopolarly, was used to record electroretinograms from pigmented rabbits. The Jet electrode was also used. The c-waves elicited by long-duration (4-second) stimuli were compared to those elicited by stroboscopic stimuli. In addition, the c-waves recorded with direct-coupled amplification were compared to those recorded with condenser-coupled amplification (one-half-amplitude bandpass=0.1 Hz). The b-wave amplitude was not altered by the amplifier coupling or by the two stimulus durations. The largest c-waves were elicited by 4-second-duration stimuli and recorded with direct-coupled amplification. Although the c-wave amplitude was reduced by stroboscopic stimuli and by condenser coupling, large and repeatable c-waves were elicited by stroboscopic stimuli and recorded with condenser-coupled amplification. A comparison of stimulus duration and amplifier coupling showed that the stimulus duration was more important in recording large-amplitude c-waves. Similar results were obtained with the Jet electrode. We conclude that repeatable and large c-waves can be elicited by a stroboscopic stimuli and can be recorded with condenser-coupled amplification with good low-frequency response from rabbits.

Effects of basic fibroblast growth factor on the retinal degeneration of the mi(vit)/mi(vit) (vitiligo) mouse: a morphologic and electrophysiologic study

Basic fibroblast growth factor (bFGF) has been shown to rescue dying photoreceptor cells in the RCS rat, a model with a genetic defect of the RPE that impairs outer segment phagocytosis. The purpose of the present study was to determine whether intravitreal injection of bFGF would have a similar effect on photoreceptor cell death in the vitiligo (C57BL/6-mi(vit)/mi(vit)) mouse. This mutant mouse loses photoreceptor cells slowly over many months. Experimental evidence suggests that the primary cellular site of the defect is the RPE. In the present study, bFGF was prepared with and without heparin in PBS and injected intravitreally into vitiligo mice at ages 2, 4, 6, 8 and 13 weeks, surrounding the onset of photoreceptor cell death. Non-injected, PBS-injected and heparin/PBS injected mice served as controls. Scotopic ERG's were performed on one group of mice prior to killing. Mice were killed 4, 6 or 10 weeks following the injection and the eyes were processed for histology and analysed. The amplitude of the b-wave was significantly smaller in mice injected with bFGF/PBS than in PBS-injected and non-injected eyes regardless of the time of injection or duration following injection. Histological examination revealed that the number of rows of photoreceptor cells did not differ significantly between bFGF-injected, vehicle- or non-injected mice. Although slight improvement in the attachment of outer segments to RPE was observed in 4 week mutants injected with bFGF/heparin/PBS, a similar result was obtained in heparin/PBS injected mutants. In mice injected with bFGF without heparin, detachment was severe and gross disruption of neural retina was observed. There were significantly more macrophages and photoreceptor cells in the subretinal space in bFGF injected mice. It appears that at the dosages and times administered, bFGF does not improve the electrophysiological functioning of the retina nor retard the degeneration of the retina in the vitiligo mouse as it does in the RCS rat.

The retinal dopamine network alters the adaptational properties of retinal ganglion cells in the cat

1. Single-unit extracellular recordings of optic tract fibers were used to study ganglion cell (GC) response properties of the intact cat eye before and after the intravitreal injection of haloperidol or SCH23390, dopamine-specific antagonists. Nearly all of the dopaminergic cells in the cat retina are amacrine cells (ACs); thus the dopamine antagonists are thought to primarily block the postsynaptic effects of these dopaminergic amacrine cells. All GCs encountered were subjected to a battery of receptive-field (RF) tests, including classification as X or Y, and as ON or OFF. 2. The effects of haloperidol were greatest in the light-adapted OFF-center pathways and especially in the OFF-center Y-cell. Within 30 min of haloperidol injection, both the spontaneous and light-evoked activity of the OFF-center Y-cell fell to zero, but when the same cell was exposed to lower levels of steady-state background illumination (scotopic levels), the response of the cell once again became robust. 3. OFF-Center Y-cells that had partially recovered from the drug effects and OFF-center X-cells recorded when the drug effect was maximal both possessed intensity-response curves that were shifted to the right of normal. 4. Recovery from the drug effects reflect supranormal responses after the initial response reductions and may be due to haloperidol's action on the dopamine autoreceptor. 5. Of the ON-center cells, only the Y-cells showed response alterations; possessing higher spontaneous activities and slightly reduced amplitudes to RF center (RFC) illumination. 6. The effects of SCH23390 paralleled those of haloperidol except that the onset was faster and the duration of the action of SCH23390 was much shorter, and no supranormal responses followed the initial effects. 7. Dark-adaptation functions of OFF-center GCs revealed a normal rod-cone shift; however, SCH23390 eliminated the rod-cone break, and threshold quickly fell to that of the rod mechanism. 8. The dopaminergic neurons of the cat retina appear to play an important role in regulating the activity of retinal OFF-center pathways in the photopically adapted eye, and one of its functions may be to control the relative contributions of the rod and cone systems to the response properties of light-adapted OFF-center GCs. 9. It is argued that dopamine is released in the light and enhances cone pathway activity, perhaps in the outer retina at bipolar and horizontal cells, and suppresses rod pathway activity, perhaps in the inner retina at amacrine cells.

Electroretinographic study of the C57BL/6-mivit/mivit mouse model of retinal degeneration

PURPOSE

The C57BL/6-mivit/mivit mouse model of retinal degeneration is characterized by slow progressive loss of photoreceptor cells, concomitant loss of rhodopsin, and uneven pigmentation of the retinal pigment epithelium. The purpose of this study was to determine how these alterations affected the electroretinogram (ERG).

METHODS

Scotopic ERGs were measured in two litters of mivit/mivit mice beginning at 4 weeks and continued in the same animals at 2-week intervals through 18 weeks.

RESULTS

The mean of the maximum b-wave amplitude (Vmax) at 4 weeks was 234 +/- 14 microV in mivit/mivit mice, which did not differ significantly from controls (266 +/- 26 microV). With increasing age, all components of the ERG decreased and by 12 weeks, the mean of the Vmax had decreased to 170 microV. At 18 weeks, the mean Vmax was 75 microV, and the b- to a-wave ratio was still > 1.0. Comparison of these physiologic data to previously reported morphologic and biochemical data showed a high correlation between the b-wave amplitude and the number of photoreceptor cell nuclei (r = 0.9772) as well as the b-wave amplitude and rhodopsin levels (r = 0.9019).

CONCLUSIONS

The loss of all components of the ERG and the lack of a negative-type ERG suggested that the primary cells altered in the mivit/mivit mouse were the photoreceptors. The high correlations between the ERG amplitude and the number of photoreceptor nuclei indicate that the Vmax of the ERG is a good measure of the degree of photoreceptor loss.

Correlation between the physiologic and morphologic changes in experimental autoimmune uveitis induced by peptide G of S-antigen

PURPOSE

The authors followed and correlated the physiologic and morphologic changes occurring in experimental autoimmune uveitis (EAU) induced by the peptide G of S-antigen.

METHODS

EAU was induced in Lewis rats by footpad inoculation of a 13-amino acid synthetic peptide (peptide G) in complete Freund's adjuvant. Electroretinography (ERG) was used to follow the physiologic changes, and light and electron microscopy were used to examine the morphologic changes.

RESULTS

Serial ERG recordings showed a progressive decrease in the b-wave amplitude and a depression of retinal sensitivity beginning on day 18-21 postinoculation (PI). By day 35 PI, the b-wave was decreased by 91%, and the sensitivity was depressed by 4.68 log units. Negative ERG were recorded during the intermediate and late stage. Light and electron microscopy of the retina showed better correlation of the pathologic changes with b-wave depression than with PI day.

CONCLUSIONS

ERG recordings were a good method to detect, follow, and quantify the severity of EAU. Their good correlation with the morphologic changes showed that this method can be used to assess the condition of the retina noninvasively.

Bilateral electroretinographic changes induced by unilateral intra-visual cortex inoculation of herpes simplex virus type 1 in BALB/c mice

Intra-visual cortex inoculation of 10(2) plaque-forming units of herpes simplex virus type 1 (KOS-63) induced physiologic and morphologic retinal changes in 62.3% (33/53) of infected animals; of these, 91% were bilateral. In contrast, inoculation of the same viral titers into the frontal lobe induced retinal alterations in only 13.3% (2/15). Initially, there was a decrease of the b-wave amplitude and retinal sensitivity and necrotic changes of the ganglion cells and nuclei in the inner nuclear layer. Immunoperoxidase staining for virus-specific antigens showed positive staining of the same cell type. Over time, there was a progressive decrease in the electroretinogram until it was extinguished and the retina was replaced by gliotic tissue. Parallel viral recovery studies demonstrated detectable infectious virus in one of eight eyes on day 2 after inoculation and in three of eight eyes on day 4. Thereafter, there was an increase in the percentage of eyes with infectious virus and a concomitant increase in viral titers. Immunoperoxidase staining of brain sections obtained on days 6 through 8 demonstrated virus-specific antigens on cells in the lateral geniculate nuclei and the suprachiasmatic nuclei bilaterally.

Modulation of the receptive field center response of cat retinal ganglion cells by the shift response signal through amacrine cells

The effect of a stationary and an oscillating grating situated greater than 30 degrees from the RFC on the receptive field center response was examined. The transient firing rate of the center response was suppressed by the oscillating grating for all types of cells. The stationary grating also suppressed the transient firing but the degree of suppression was significantly less than that with the oscillating grating. There was also a significant elevation of the sustained firing rate in Y-cells. The on-going discharges were elevated in all types of units except on-center X-cells when the grating was oscillated. An increase in the area of a grating annulus did not increase the degree of suppression of the center response. This lack of spatial summation of the shift response was related to the properties of the on-off transient amacrine cells.

Physiologic and morphologic retinal changes induced by murine cytomegalovirus in BALB/c and severe combined immune deficient mice

Anterior chamber inoculation of murine cytomegalovirus (MCMV, 10(4) and 10(5) plaque-forming units) induced both physiologic and morphologic changes in the retinas of immunocompetent BALB/c and B- and T-cell-deficient severe combined immune deficient (SCID) mice. In BALB/c mice, the depression of the b-wave began on days 3-4 postinoculation (PI) and a further depression was recorded on day 7 PI. The electroretinograms (ERGs) remained depressed 1-2 weeks PI after which there was a recovery of the amplitude of the ERG 2-6 weeks later. The recovery was not complete; the maximum amplitude at 6 weeks was significantly lower than the preinoculation value. There was a greater loss in the amplitude than in the sensitivity of the ERG. Histologic examination of retinas with depressed ERGs showed swelling of the retinal pigment epithelium and distortion and shortening of the outer segment of the photoreceptors. With recovery of the ERG, there was normalization of the retinal histology. In SCID mice, the ERGs were extinguished, and there was no recovery. Histologically, there was a complete loss of the photoreceptors in the SCIDs, and electron microscopic examination showed viral particles in the retinal pigment epithelium and inner nuclear cells. These results demonstrate that MCMV can induce retinal pathology as reported in patients and show the importance of B- and T-lymphocytes in controlling the progression of this disease process.

HSV-2 alters retinal physiology and morphology bilaterally in mice

Anterior chamber inoculation of 10(4) PFU of the MS strain of HSV-2 resulted in physiologic and morphologic changes in the retina of the inoculated and the uninoculated eyes. In the inoculated eyes, electroretinogram (ERG) depression was first detected on day 3 and abolished ERGs on day 8 postinoculation (PI). The decrease in the ERGs was rapid and the time course was similar for all of the eyes. In spite of a 90% decrease in the amplitude of the b-wave, the retinal sensitivity did not change. Of 23 eyes tested on or after day 10 PI, none had normal, 4.3% had reduced, and 95.6% had abolished ERGs. In the uninoculated eyes, ERG depression was first detected on day 8 and abolished ERGs on day 12 PI. The course of the ERG depression was more variable, and some of the eyes showed a decrease in retinal sensitivity. Of the 22 eyes tested on or after day 17 PI, 18% had normal, 32% had reduced, and 50% had abolished ERGs. The majority (17/33) of the retinas of the inoculated eyes showed panretinal necrosis, although 7 of 33 retinas had pathology confined to the outer layers of the retina. In the uninoculated eyes, only 5 of 30 retinas were necrotic and 14 of 30 retinas had pathology limited to the outer layers of the retina. These observations suggested that the physiologic and morphologic changes progress through two stages: an early stage with reduced ERGs and pathology limited to the outer retinal layers, and a second stage in which the ERG is abolished and the pathologic changes extend into the inner retina. Not all retinas progress to the second stage.

Alterations of the cat's electroretinogram induced by the lesioning of the indoleamine-accumulating amacrine cells

The neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) was used to destroy the indoleamine-accumulating amacrine cells located in the cat's retina. With 100 micrograms of 5,7-DHT, the alterations in the electroretinogram (ERG) were present in all of the treated eyes and the morphological changes were confined to some of the processes in the inner plexiform layer. The ERGs recorded from treated eyes consisted of negative waves at low intensities and depressed b wave amplitude at higher intensities. The duration of the b wave was not altered but the oscillatory potentials were strongly depressed. The changes were probably permanent. The differences in the ERG changes in cats and rabbits were suggested to arise from the differences in signal processing in the inner retina of rabbits and cats.

Bilateral alterations of the ERG and retinal histology following unilateral HSV-1 inoculation

The physiological condition of the retinas of BALB/c mice inoculated unilaterally in the anterior chamber with the KOS strain of herpes simplex virus type 1 (HSV-1) was monitored by ERG recordings. After the ERG recordings, the retinas were examined for histopathological changes. In the inoculated eye, depressed ERGs were recorded on day 2 PI and abolished ERGs on day 4 PI. The changes in the ERGs were complete by day 5-6 PI. Of the 53 inoculated eyes followed for longer than day 6 PI, four (7.5%) remained normal, 30 (56.6%) had reduced ERGs and 19 (35.8%) had abolished ERGs. In the contralateral eyes, the first changes were noted on day 8 PI, and abolished ERGs were recorded on day 9 PI. Of the 55 contralateral eyes followed for longer than 10 days, 15 (27.3%) remained normal, four (7.2%) had reduced ERGs and 36 (65.4%) had abolished ERGs. The percentage of eyes with depressed ERGs was significantly higher in the inoculated than in the uninoculated eyes, and the percentage of eyes with abolished ERGs was significantly higher in the uninoculated eyes than in the inoculated eyes. The histopathological alterations were different for the two eyes. In the inoculated eyes, the changes were mainly in the outer retina, with characteristic folds in the photoreceptor and outer nuclear layer interspersed with normal appearing retina. The pigment epithelium was also abnormal. In the uninoculated eyes, the changes began in the inner retina but rapidly spread to all layers of the retina. This panretinal necrosis accounted for the higher percentage of abolished ERGs in the uninoculated eyes. The differences in the alterations of the ERG and the histopathological changes may be related to the underlying mechanism of action of the HSV-1 during the evolution of the experimental retinopathy.

Light ON depresses and light OFF enhances the release of dopamine from the cat's retina

The release of [3H]dopamine from the cat's retina was determined by measuring the level of radioactivity in a series of vials in which the retina was incubated. At light ON; there was a sustained decrease and at light OFF there was an enhanced release of dopamine. The results also demonstrate that dopamine is released continuously in the dark.

Intranuclear rodlets in the rabbit retina following treatment with MPTP

Retinas from pigmented rabbits treated with N-methyl-4-phenyl-1,2,3,6- tetrahydropyridine (MPTP, a chemical inducer of Parkinsonism in man and monkeys) were studied using light- and electron microscopy. The nuclei of many cells in the inner nuclear layer and the ganglion cell layer of the treated retinas contained crystalloids (intranuclear rodlets) of varying length (0.5-8 microns) which were composed of bundles of 12 nm filaments and which were seen only rarely in untreated retinas. The induced rodlets are oval to round cylinders, 15-20 filaments across (although they are smaller in untreated retinas). Similar intranuclear inclusions have been described under varying conditions in neurons and glia in the central nervous system of several animal species. In rabbits injected acutely with MPTP, most of the affected cells are in the inner nuclear layer in the position of bipolar cells, while in the chronically injected animals, clearly identifiable amacrine cells, and the nuclei of some cells in the ganglion cell layer also contained the inclusions. Evidence is presented that the rodlet-containing cells in the ganglion cell layer include both ganglion cells and displaced amacrine cells. These anatomic findings are relevant to understanding the physiological and biochemical effects of the drug on the retina which we reported previously (Wong, Ishibashi, Tucker and Hamasaki, 1985).

Destruction of the indoleamine-accumulating amacrine cells alters the ERG of rabbits

The indoleamine-accumulating amacrine cells in the rabbit's retina were destroyed by intravitreal injections of 5,7-DHT according to the technique of Ehinger and Florén. One week after the injections, histofluoresence microscopy failed to show the IA-cells in the retina. At this time, the b-wave of the treated eyes was significantly smaller, the OPs were more prominent, and the duration of the b-wave was longer. The a- and c-waves and the off-response were not affected. The loss of the IA-cells resulted in a reduction in the range of dark-adaptation but did not affect the rate of recovery of the threshold. The loss of the IA-cells also had no effect on the response of the retina to flickering light. In a double-flash experiment, the suppression of the response to the second flash was significantly weaker in the treated eyes. These results can be explained by the loss of a negative feedback circuit that has been proposed for the IA-cells from morphologic studies.

Abnormal Y/X ratio in the area centralis of the Siamese cat

The responses of X- and Y-type retinal ganglion cells were recorded in the optic tract of Siamese cats in order to investigate the encounter rate of Y-cells with respect to retinal eccentricity. The percentage of Y-cells in Siamese cats was highest in or near the area centralis and it decreased with eccentricity. This is in contrast to the proportion of Y-cells in normally pigmented cats, which was lowest in the area centralis and increased with eccentricity. Thus, the Y/X ratio in Siamese cats is higher in the area centralis, but significantly lower in the peripheral retina compared to those in normal controls. The lower percentage of Y-cells in the periphery parallels an additional finding that Y-cells exhibiting very high conduction velocities were missing from the Siamese cat optic tract. Finally, the receptive-field center (RFC) size of X-cells in the area centralis was larger in Siamese cats, and the correlation between the RFC size and conduction velocity was weak in these animals. The results are discussed in terms of behavioral deficits in Siamese cat vision.

A neural pathway for the shift response in the cat

The shift response (McIlwain effect) was elicited by moving a large grating situated greater than 15 degrees from the conventional receptive field center (RFC). We examined the change in the amplitude of the shift response induced by placing a steady target on the RFC or the RF surround. We found that appropriate stimulation of the RFC or the RF surround will increase and inappropriate stimulation will decrease the amplitude of the shift response in a graded manner. The amplitude of the shift response was not correlated with the maintained activity but was correlated with the transient peak firing rate which is evoked by flashing the enhancing stimulus. A shift stimulus which elicits a strong shift response can be blocked by inappropriate stimulation. The results suggest that the shift signal is modulating a tonic signal which is present in the RF. Because the shift response is a transient excitation, we suggest that the shift response results from a disinhibitory process. A possible neural pathway for the shift response is presented.

Physiological development of the kitten's retina: an ERG study

The physiological development of the retina was followed by recording ERGs and OPs from kittens of different ages. We found that different properties of the retina attain adult values at different ages: the b-wave elicited by higher stimulus intensities became adultlike by five to seven weeks while the b-waves elicited by lower stimulus intensities required another 3-5 weeks; the implicit time for the b-waves elicited by the full intensity stimulus attained adult values by 10 weeks of age; and the oscillatory potentials did not become adultlike until 18 weeks of age. The physiological development of the ERG was related to the development of the photoreceptors and the ganglion cells as well as to the morphological development of the second order neurons. It was concluded that the physiological development, as the morphological development, proceeds in three stages: an initial slow phase during which the late receptor potential and the b-waves are first recorded; a second rapid phase, during which the amplitude of the b-waves and OPs increase rapidly; and a third slow differentiation phase during which the final development of the properties of the retina are attained.

A comparison of the shift response of X- and Y-cells in the cat's retina

The shift response (McIlwain or peripheral effect) was elicited by either flashing or shifting a grating while the receptive field (RF) was covered by a 30 degree mask in the cat. The responses elicited by shifting the grating was comparable to that elicited by flashing the grating. In 10% of the units, the on- and off-responses elicited by flashing the grating were unequal in amplitude. The larger response corresponded with the light phase which leads to excitation of the surround mechanism of the RF. The maximum firing rates of the shift response did not differ in the different types of units, but the amplitude of the shift response (maximum - maintained firing rates) was significantly larger in Y-cells. For all types of cells, the amplitude of the shift response increased with greater eccentricity of the RF. A strong inhibitory period was found in on-center Y-cells but not in the other types of cells. The latency of the shift response was significantly shorter in Y-cells. The differences in the responses of X- and Y-cells suggest that the lateral pathways used are different for the X- and Y-cells.

Physiologic and anatomic development of the photoreceptors of normally-reared and dark-reared rabbits

The postnatal development of the rod photoreceptor of normally-reared and dark-reared pigmented rabbits was studied using physiological and anatomical methods. The late receptor potential (LRP) was recorded in vitro and the threshold and maximum amplitude determined. The same specimens used in the electrophysiological studies were then prepared for microscopy, and rod cell outer and inner segment dimensions and photoreceptor spacing were determined. In the light-reared animals a small LRP was first recorded at 5 days, but only at very high stimulus intensities. Thereafter, there was a rapid decrease in the threshold and an increase in the amplitude of the LRP. The threshold and amplitude of the LRP reached adult values at 14 days. Of the anatomical parameters, maturation of inter-receptor spacing (14 days) is clearly associated with the attainment of the adult LRP amplitude levels. Outer segment length was also adultlike by 14 days of age and thus paralleled physiological maturity of the photoreceptor. Changes over time in the mean diameter and length of rod cell inner segments (adultlike at 21 days) follows the pattern of ontogenetic maturation of the LRP. These findings imply a close relationship during ontogeny between the LRP and development of the outer segment. In dark-reared pups there is a delay in the maturation of the photoreceptor mosaic, but by 4 weeks of age, physiological characteristics of the photoreceptor are adultlike.

Amblyopic eyes have longer reaction times

The RT (reaction time) to a 0.25 degree spot of light was measured monocularly on 25 normal subjects and 36 strabismic patients. The overall mean RT and the delta RT (difference in RT between the right and left eyes) for the 19 strabismic patients without amblyopia did not differ significantly from that of the normal subjects. The overall mean RT of the dominant eyes of the 17 amblyopic patients did not differ significantly from that of either the faster of slower eyes of the normal subjects. The overall mean RT of the amblyopic eyes was significantly longer than that of the normal subjects. The delta RT was significantly larger in the amblyopic patients. A regression analysis of the visual acuity of the amblyopic eye and the delta RT showed that the data was best fit by a linear equation. The coefficient of correlation was +0.82. Additional experiments showed that the longer Rt of the amblyopic eyes was not caused by eccentric fixation or by the reduced visual acuity.

Abnormal responses of retinal ganglion cells in Siamese cats to moving stimuli

The responses of X and Y-Type retinal ganglion cells in Siamese cats to moving slits were compared to those in common cats, in order to assess the center/surround interactions in Siamese cat receptive fields. Responses in the latter animal were quantitatively lower than those in common cats, the RF centers were larger, and the encounter rate for Y-type optic tract fibers was significantly lower than in common cats. Whereas response enhancement of common cat units was predictable following masking of the surround, such an effect could only be observed in Siamese cats when the contrast between the stimulus and background was increased considerably. These data suggest anomalous center/surround interactions in Siamese cats, probably due to the presence of weaker surround influences in that animal.

Development of receptive field properties of retinal ganglion cells in kittens raised with a convergent squint

The effects on retinal ganglion cell receptive fields of rearing kittens with convergent squint, surgically induced on the 12th post-natal day, were investigated by utilizing the extracellular single unit recording technique. The data revealed that responses of cross-eyed cat ganglion cells to contrast reversal stimuli were severely depressed and the retinal region exhibiting the best responses varied according to the degree of convergent misalignment of the eyes displayed by each animal. Receptive field sizes of X-type (but not Y-type) units located within 10 degrees of the area centralis of cross-eyed cats were significantly larger than those in normally reared cats. Finally, the encounter rate for units exhibiting non-linearity of spatial summation (Y-type) were much lower in cross-eyed cats. The results suggest developmental alterations in the retinal neurophysiology of common cats reared with a large convergent squint.

Response of X and Y cat retinal ganglion cells to moving stimuli

Single optic tract fibers in the cat were classified as X or Y cells by a contrast reversal stimulus. A slit of light was then moved across the receptive field at velocities from 10--1000 degrees/s. The preferred velocity was that velocity which elicited the strongest response from the cell. The maximum velocity was the highest velocity target to which a cell could respond. Y cells as a group both preferred and could follow faster targets than X cells, and were more broadly tuned to the preferrred velocity.

Anatomtic and physiologic development of the photoreceptor of the kitten

The postnatal development of the photoreceptor of the cat was studied using physiological and anatomical methods. The late receptor potential (LRP) was recorded in vitro and the threshold and maximum amplitude determined. The same specimens used in the electrophysiological studies were then prepared for microscopy, and rod cell outer and inner segment length and diameter, photoreceptor density, and inter-receptor distance were determined. A small LRP was first recorded at 9--10 days, but only at very high stimulus intensities. Thereafter, there was a rapid decrease in the threshold and an increase in the amplitude of the LRP. The threshold reached adult values at 17--18 days, while the amplitude of the LRP was adultlike at 23--26 days. Of the anatomical parameters examined, inter-receptor spacing and rod cell diameters seem to be most clearly associated, respectively, with the attainment of adult LRP threshold and amplitude. Outer segment length was adult-like at 35--43 days of age and thus postdated physiological maturity of the photoreceptor. These observations suggest that the surface area of the rod cell outer segment tips is more critical in the development of the adult LRP than is the number of discs in the outer segment. In addition, changes over time in the mean diameter and length of rod cell inner segments follows the pattern of ontogenetic changes in LRP amplitude. These findings imply a close relationship during ontogeny between the metabolic functions of the inner segment and phototransduction at the outer segment disc.

Classification of cat retinal ganglion cells into X- and Y-cells with a contrast reversal stimulus

A contrast reversal (alternating phase) stimulus was used to study the responses of 150 retinal ganglion cells from 15 adult cats. Because the majority of the cells did not show perfect linear spatial summation, a ratio of the firing rates at two time periods was used to express the degree of nonlinearity. Y-cells showed a high degree of nonlinearity, and their mean null ratio was significantly lower than that of X-cells. With the stimulus at the null position, X-cells had an unmodulated discharge rate which was significantly higher than maintained activity, while the firing rate of Y-cells was lower than maintained activity. With the stimulus placed at an eccentric position in the receptive field, X-cells responded in a sustained manner, while Y-cells respond transiently. Because of these observations, we conclude that X-cells correspond to the sustained cells, while Y-cells correspond to the transient cells.

Development of X- and Y-cells in kittens

A contrast reversal stimulus was used to classify the retinal ganglion cells of kittens 3--12 weeks of age. At 3 and 4 weeks of age, the majority of the units were classified as Y-cells. The percentage of X-cells was similar to that of the adult at 5--6 weeks of age although other response properties were not adult-like. The angular size of the receptive field center was significantly larger than that of the adult through 5--6 weeks of age. An estimate of the position of the vertical meridian was obtained from the positions of the receptive fields in the visual field which permitted the calculation of the posterior nodal distance in kittens of various ages. From the posterior nodal distance, the linear extent of the receptive field center was determined. This showed that there is only a very slight increase in the linear size of the receptive field center from 3 weeks to adulthood.

Properties of X- and Y-cells in the rabbit retina

Bevelled glass microelectrodes were used to record spike potentials extracellularly from the ganglion cells of the rabbit retina. Good responses were obtained from the isolated retina or eye-cup preparation for at least 12 hr. Using a contrast reversal stimulus, 63.8% (67/105) of the units showed linear spatial summation (X-cells), and 21.0% (22/105) showed nonlinear spatial summation (Y-cells). The X- and Y-cells in the rabbit retina had physiological properties which were similar to those in cat retina. Directionally selective cells (15.2%) were found to respond poorly, if at all, to the contrast reversal stimulus.

Properties of X- and Y-type retinal ganglion cells in Siamese cats

There is ample evidence that the visual system of the Siamese cat is different from common cats. These abnormalities suggest possible retinal origins, although no documentation exists. In the present study, single unit recordings were made from 91 misrouted and 209 normally-routed optic tract fibers in Siamese cats. Electrophysiological responses of the misrouted fibers did not differ from those found in the normally-routed fibers of the Siamese cat with the exception of depressed responses to contrast reversal stimuli. X/Y classification of units and experiments on receptive field center sizes, intensity-response functions, and responses to flicker failed to demonstrate significant differences between the misrouted and normally-routed fibers in Siamese cats. These results were not affected by different degrees of interocular misalignment exhibited by the Siamese cat studied. Response properties of retinal ganglion cells in Siamese cats, however, were found to be quite abnormal when compared with common cats. Only 14% (42/300) of all units studied were Y-cells in Siamese cats in comparison to 35% (60/170) in common cats. The percentage of Y-units also was correlated with the severity of interocular misalignment in Siamese cats, i.e. the greater the misalignment of the eyes, the lower the percentage of Y-cells. Experiments on response to contrast reversal stimuli, intensity-response functions and responses to flicker revealed that the ganglion cells in Siamese cats are not as responsive as those in common cats.

Siamese cats: abnormal responses of retinal ganglion cells

Comparison of optic tract recordings in Siamese and ordinary cats reveals that Siamese cats have a significantly lower percentage of Y-cells than of X-cells. In addition, Siamese cats show depressed responses to a contrast-reversal stimulus, a result that supports the lower spatial contrast sensitivity demonstrated behaviorally by these animals. Both physiological findings suggest neurophysiological anomalies in the Siamese retina.