INVESTIGATIVE OPHTHALMOLOGY. Rabbit visual potentials after laser photocoagulation

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1 September 1971 Volume 10, Number 9 INVESTIGATIVE OPHTHALMOLOGY Rabbit visual potentials after laser photocoagulation Franklin G. Hempel* Light and electrically evoked responses of the rabbit lateral geniculate nucleus and visual cortex were monitored before and after ruby laser photocoagulation of a 0.6 cm. wide retinal field. The electroretinogram was also recorded. Severe lesions, especially those inflicted by laser energy densities of 6.3 joules (J) per square centimeter, residt in a substantial reduction or even elimination of geniculate and cortical potentials evoked by photic stimuli while the electroretinogram b-wave is reduced maximally about 30 per cent. Less severe lesions produced by energy densities near 2 to 3 J per square centimeter cause a mean increase in geniculate and cortical response to low-intensity photic stimulation but not to high-intensity stimulation at a time that the electroretinogram is reduced in amplitude. The postsynaptic component of the response of the lateral geniculate nucleus and the visual cortex potential, both elicited by electrical shock of the optic nerve, are enhanced after irradiation with an energy density of 3.2 J per square centimeter or more. The data suggest that the laser insult acts to uncouple the tonic inhibition imposed on the higher visual centers by the retina, i.e., the lesion produces a disinhibition similar to that obtained when afferent visual impulses are interrupted by ocular ischemia or enucleation. T Key words: laser, evoked responses, visual cortex, lateral geniculate nucleus, photocoagulation, electroretinogram, rabbit, retinal burns.he morphology of retinal burns and the energy -Lh requirements for producing them have been described by many authors, including those writing recent reviews, 1 " 3 but few persons have dealt with the alterations in visual electrical activity which these lesions bring about. Depression of the electroretinogram (ERG) following From the Electrical Engineering Research Laboratories, The University of Texas at Austin, Austin, Texas. Supported in part by the Joint Services Electronics Program, The University of Texas. Manuscript submitted April 26, 1971; manuscript accepted July 14, *Present address: Department of Physiology and Pharmacology, Duke University Medical Center, Durham, N. C xenon photocoagulation has been reported in rabbit 4 and monkey 5 experiments as has reduction of the ERG and optic tract potentials after laser irradiation of the cat eye. G Microelectrode records have been made of unit responses in the rabbit retina following laser photocoagulation. 7 In order to examine the effects laser irradiation of the retina has on the central visual stations, I have monitored the control and postirradiation evoked potentials in the rabbit lateral geniculate nucleus (LGN) and striate cortex, as well as the ERG. Equipment and methods Fifty-two studies using photic excitation were carried out on 28 pigmented rabbits of mixed breed. The ERG was recorded with a chlorided

2 640 Hempel Investigative Ophthalmology September 1971 SCREEN FIRST SURFACE MIRROR /STEREOTAXIC 'APPARATUS E.G. a G. RADIOMETER Fig. 1. Equipment configuration for ruby laser coagulation and argon laser stimulation of the rabbit retina. The pupil is centered on the HeNe alignment beam. A converging lens (not shown) focuses the laser beams on the cornea. Subsequent divergence inside the eye produces the large retinal images. silver ring electrode on the cornea and a silver button clipped to the scalp. Responses from the geniculate nucleus and cortex contralateral to the stimulated eye were detected with coaxial electrodes consisting of an insulated center conductor with a 1 mm. long, 0.1 mm. wide bare tip threaded through a 25 gauge hypodermic needle. Electrodes were stereotaxically positioned in the dorsal LGN using the atlas of Fifkova and Marsala, 8 and about 2 mm. lateral to the splenial sulcus just through the dura above visual area I of the cortex. Occasionally the cortical electrode entered the substance of the cortex. The localization of the electrodes in the brain was secured by observing the evoked potentials as the electrodes were lowered into place. After the animal was killed this was verified by marking with a brief 100 milliampere (ma.)i heating current and inspection of the wet, fixed material. Signals were amplified on Brush EEG or Physiograph preamplifiers (bandpass 0.1 to 0.6 Hertz [Hz] to 1.0 KHz) and stored on an Ampex SP 300 tape recorder. The FM tape recorder limited the high-frequency response to 312 Hz. Evoked potentials were replayed through a TMC computer of average transients, averaged in ensembles of 50, and plotted on an X-Y recorder. A Hughes 3041H argon laser (pulse width = 50 /isec) was used as a stimulator, and a Korad K1Q ruby laser, operated in the conventional mode with a pulse duration of 800 /xsec, inflicted the retinal burn. The argon laser emits 40 per cent of its total output at nm., 25 per cent at nm., five per cent at nm., and ten per cent each at 476.4, 496.5, and nm. Since the peak of the rhodopsin absorption curve occurs near 500 nm., this laser provided adequate visual stimulation. The images of the ruby and argon beams were superimposed on the retina with mirrors and a beam splitter as shown in Fig. 1. Image diameters of 0.6 cm. for the ruby beam and an estimated 0.2 cm. for the argon beam (reflected from the first surface of the beam splitter) were possible with a single converging lens because of the difference in laser beam diameters. Outputs from both lasers were attenuated with neutral density filters and measured with a Model 580 EG & G radiometer. Perforation of the rabbit skull and a tracheostomy were performed under the deepest possible sodium thiopental anesthesia, supplemented with local infiltration with lidocaine. In addition, local anesthesia was used on wound margins and on the cornea (Pontocaine) before beginning the experimental session. While anesthetized, the rabbits were placed astride the sling of a stereotaxic apparatus modified to support the rabbit's head. Round ear bars were placed inside the auditory canal, but, for humane reasons, were not pushed into the meatus. No other restraining pins were used. In 13 experiments on 8 rabbits in pilot study using no paralytic agents, it was found that if allowed to recover from general anesthesia while in the stereotaxic device, the animals would tolerate their restraint quite well with no obvious signs of discomfort or distress. They would, however, react vigorously to light flashes or touch. Two hours or more were allowed for much of the effect of the general anesthesia to dissipate, since barbiturates are known to smooth out oscillations in the ERG b-wave, and in A and B waves of the geniculate potential, as well as secondary repetitive activity in the

3 Volume 10 Number 9 Rabbit visual potentials 641 cortical potential.** The presence of these oscillatory components, particularly in the ERG, was taken as evidence that the intravenous anesthetic had little influence on the visual responses when recording began. The animals were then paralyzed with d-tubocurarine 2 mg. per kilogram per half hour, started on artificial ventilation, and then dark adapted for 30 minutes prior to recording. A heating pad kept the rabbits warm. Mydriasis was maintained with ten per cent phenylephrine HC1, and corneal moistness with one per cent methylcellulose in normal saline, respectively. The intensity of the argon stimuli ranged from a relative value of 0 to 4 on a log scale, corresponding to an absolute energy value of to 90.0 A*J entering the cornea. Units of energy at the cornea could not easily be equated to units of retinal illumination, since the argon light was effective over an area greater than its 2 mm. image owing to entoptic scatter. Usually seven evenly spaced (log scale) intensities were used before and after the insult. These were presented in two schemes; either in ascending order of intensity in the prelesion series and in descending order afterwards, or in descending order in the preburn series and in ascending order afterwards. This stimulus program was chosen to keep the retina at the same level adaptation to the stimulus after the ruby laser was pulsed into the eye as it was immediately before, and to minimize any effects which the passage of time might have on the results. A flash repetition rate of 40 per minute was used when 50 responses were averaged. To elicit the ERG c-wave a rate of one flash every 4 seconds was used, and 15 responses, including the ERG b-wave, geniculate and cortical signals, were averaged. Approximately 20 minutes was required to go through the preburn stimulus series, produce the lesion, and repeat the series. The absence of eye movement artifacts in the ERG c-wave provided assurance that the position of the lesion did not change relative to the stimulus image during this interval. The retinal energy density of the ruby laser ranged from 0.9 to 6.3 J per square centimeter. For each energy density, a ratio was obtained by dividing the amplitude of the ERG, geniculate, and cortical response after irradiation by their respective preburn amplitudes elicited with a stimulus of the same intensity. These ratios were plotted as a function of log stimulus intensity. Since the output of the unattenuated argon laser was not fixed from day to day, as is the case with a commerical photic stimulator, ratios are scattered along the intensity scale rather than being located at a precise point each time. In ten further experiments, a dual stimulus Unpublished observations. electrode with a 1 mm. tip separation was inserted into the optic chiasm. Electrically evoked signals from the LGN and visual cortex were amplified on a Tektronix 546 differential amplifier (bandpass 10 Hz to 10 KHz) and photographed. The 0.05 msec, electrical shock was produced by a Devices type 2533 stimulator. A voltage was chosen which elicited a consistent cortical or geniculate response which did not appreciably change characteristics on increasing the stimulus voltage. The voltage ranged from 25 to 50 volts in the ten experiments. Again, a retinal lesion was produced, and pre- and postburn responses were compared. After the animals were killed, the eyes were enucleated and fixed in Kolmer's fixative. Later the bulbs were opened, cleared of media, and photographed. Lesions were placed across the region of the visual streak. They were well within the retinal field sending impulses entirely contralaterally since only 15 per cent or so of the optic nerve fibers, arising from the extreme temporal retina, remain ipsilateral. 9 " 11 Results In Fig. 2 the usual ERG, geniculate, and cortical potentials evoked by the argon laser are shown. With stimulation at high energy levels, oscillatory wavelets are typically present on the ERG b-wave and on the LGN A and B waves. Low-energy stimuli result in simpler rounded waveshape for each of the 3 signals. In this work, a high-energy stimulus is defined as one of sufficient magnitude to evoke oscillatory ERG b-wave components. A lowenergy stimulus does not. High-energy stimuli occupied the band from 3 to 4 on a log scale and low-energy stimuli ranged from 0 to 1 on the same scale. The ratio of the postbuni response amplitude to the preburn amplitude is plotted as a function of log stimulus intensity in Fig. 3. In this example, six retinas have been injured with ruby pulses of 3.0 to 3.2 J per square centimeter. A distinct increase is seen in the ratio of postburn to preburn geniculate and cortical amplitudes when low-energy stimuli are compared to highenergy stimuli. The ERG b-wave is uniformly depressed at all stimulus energies. Within the low and high stimulus bands, an identical stimulus was used to compare the postburn to the preburn response.

4 642 Hempel Investigative Ophthalmology September 1971 A-Wave b-wove B-Wove ERG LGN CORTEX 25 msec Fig. 2. Top to bottom, a computer average of 50 ERG, lateral geniculate, and cortical responses to high-energy (upper trace) and low-energy (lower trace) stimulation. Arrows which indicate amplitudes also show how potentials in this study were measured. Traces begin at the onset of the stimulus flash. At particular combinations of lesionproducing ruby irradiances and stimulus intensities, the response of the LGN may be increased by an average of 18 per cent and the response of the cortex by an average of 20 per cent. At this time the ERG b-wave is reduced in mean amplitude by about 20 per cent. These relationships are illustrated in Fig. 4. The postirradiation enhancement of the geniculate and cortical signals is maximal when the retina is irradiated with a laser energy density near 3 J per square centimeter and lowenergy photic stimuli are employed. If the stimulus is made more intense, or the lesion more severe, or both, the geniculate and cortical potentials are reduced on the average from their control preirradiation values. Less severe injuries result in a mean increase in potentials from the LGN and cortex to low-energy but not to high-energy stimuli. The difference between the mean postburn/preburn ratio evoked by intense stimuli and the mean ratio evoked by lowenergy stimuli is significant at the five per cent significance level at 2.0, 3.1, and 5.1 J per square centimeter (LGN) and at 1.6 and 3.1 J per square centimeter (cortex). The comparison was made between the mean ratios with Student's t test for unpaired data. Whether it is evoked by a weak or by an intense flash, the ERB b-wave is reduced about the same amount by a particular ruby insult, down to an average of 70 per cent of its control value as an energy density of 6.3 J per square centimeter is applied to the retina. Geniculate and cortical responses may be completely extinguished, even when 50 are averaged, by ruby dosages that high. In all experiments, the ruby laser was triggered while stimulation was in progress. The ERB b- wave is always transiently reduced, but only for 5 or 6 seconds or less, by the bleaching effect of the deep red flash, regardless of its energy density. After that time, the ERG assumes a new amplitude which it maintains for the remainder of the recording session, i.e., at least ten minutes. The first few evoked responses after ruby irradiation were discarded during signal averaging. The a-wave of the ERG may appear unchanged after laser irradiation. Changes do occur in the negative excursions which

5 Volume 10 Number 9 Rabbit visual potentials 643 ERG 1.2 I.I <& o " I LGN I.I o.e I CORTEX I.I Log Relative Stimulus Intensity Fig. 3. Postburn/preburn response amplitude as a function of stimulus energy for six eyes exposed to 3.0 to 3.2 J per square centimeter ruby energy density. Each circle represents the comparison of responses to identical stimulus intensities before and after ruby laser irradiaton. ERG = a-wave to b-wave, peak to peak. follow the decay of the b- and c-wave (Fig. 5, A). These changes include shifts in the potential with respect to zero. It is common to find that the postburn longlatency ERG activity is a misshapen trace without the smooth features of the control. On two occasions the c-wave, measured from the trough following the b-wave, was enhanced after dosages of 1.6 J per square centimeter were administered. This trough is usually deeper when the b-wave is reduced. Dosages greater than 1.6 J per square centimeter usually reduced the amplitude of the c-wave. The effect that ruby laser radiation has on the electrically evoked LGN and cortical responses of the rabbit is illustrated by Fig. 6. The amplitude of the optic tract

6 644 Hempel Investigative Ophthalmology September I.I - U i If"- -J 1»^( t t= -^ rt ERG r LGN CORTEX Retinal Energy Density (J/cm J Fig. 4. Ratio of postbum/preburn amplitude comparing low-energy stimuli (filled circles and solid line) with high-energy stimuili (open circles and dashed line) from the argon laser versus energy density of the ruby laser which produced the burn. Circles represent the means of five to 11 values, arrows indicate the range of the postburn/preburn ratios. Asterisks identify those energy densities in which a difference between the means was significant (p <0.05). ERG = a-wave to b-wave, peak to peak. Three eyes received 2.8 J per square centimeter (cortical response not recorded) and four to seven eyes were exposed to each of the remaining energy densities. Five control eyes were shielded when the ruby laser fired. Results of four retinas exposed to 0.63, 2.4, 7.8, and 10.0 J per square centimeter are not reported.

7 Volume 10 Number 9 Rabbit visual potentials 645 b-wave c-wave b-wave a-wave B Fig. 5. A, Four second plot of 15 averaged ERGs before and after a 3.2 J per square centimeter radiation dose showing c-wave decrement, a downward shift in negativity following the b-wave, and loss of negative potential following the c-wave. B, First 250 msec, of ERGs shown in A. An expanded plot showing reduced b-wave amplitude but unchanged a-wave amplitude. (t) component of the LGN response is unchanged after a 3.2 J per square centimeter lesion is inflicted, but the amplitude of the postsynaptic optic radiation component (r) is increased about 90 per cent. In the cortex of a different rabbit, the same density burn results in an increase of about 70 per cent in signal amplitude. Ruby dosages greater than 3.2 J per square centimeter, up to 6.3 J per square centimeter, produce similar increases in the (r) component of the geniculate response and, again, act to increase the cortical response to electrical shock. Dosages less than 3.2 J per square centimeter were not evaluated. Gross examination of the open fixed bulb after irradiation of a 0.6 cm. wide area reveals small intersecting folds in the retinal tissue when dosages greater than 1.0 J per square centimeter are administered (Fig. 7). The margin of the injured field is easily seen as the energy density approaches 3 J per square centimeter. A smooth lesion characteristic of the therapeutic coagulation is seldom found. Discussion This research has shown that laser irradiation of a large retinal area will always result in a reduced ERG b-wave amplitude when sufficient energy levels are reached. At the same time the responses in the central visual system may be enhanced after the burn is inflicted. The ERG may have a normal amplitude when a small thermal lesion is present, 12 since the stimulus is not easily confined to the damaged area. For a lesion to reduce the ERG in situations where stray light is a significant stimulus, a large retinal area must be involved. To get an increase then in the geniculate and cortical responses after dosages of 2 or 3 J per square centimeter is surprising since these enhanced responses

8 Investigative Ophthalmology September Hempel CORTEX LGN PRE POST Fig. 6. Electrically evoked responses of the LGN (left) and cortex (right) before and after irradiation with a 3.2 J per square centimeter pulse. Two traces are superimposed. For the geniculate response, the calibration is 100 /iv x 0.5 msec, negativity is upwards, and * = optic tract, r optic radiation response. For the cortical potential, surface positivity is upwards, and the calibration is 500 /tv x 5 msec. are initiated by a retina, with a markedly reduced ERG output, indicating damage to a substantial retinal area. Studies have shown that tonic inhibition of the LGN principal cell (the neuron that sends axons to the cortex) is caused by retinal ganglion cells activating recurrent inhibitory pathways in the nucleus13-14 and that interruption of the inhibitory pathway by ocular ischemia15'ie or enucleation15'17 causes the LGN to be more responsive to stimulation of the optic tract. Likewise the cortical response to stimulation of the optic tract or the geniculocortical radiations is increased if the retinal output is blocked.17-1s Accentuation of photically evoked geniculate and cortical potentials following moderate laser burns could conceivably be a postsynaptic effect due to the removal of retinal inhibition. To examine this possibility, electrically evoked responses of the LGN and cortex were recorded. In the response of the geniculate nucleus to electrical shock of the chiasm, the preand postsynaptic activity can be distinguished from each other. This type of stim- Fig. 7. Gross appearance of the rabbit retina after irradiation with the ruby laser. Energy density 3.2 J per square centimeter. ulation shows that the postsynaptic discharge of the nucleus is much greater after retinal irradiation. Thus the increase in the amplitude of the geniculate response with low-energy photic stimulation suggests that after moderate irradiances, 2 to 3 J per square centimeter, geniculate neurons are disinhibited in a manner similar to sectioning the optic nerve. The electrically evoked signal in the cortex is also larger after the retina is damaged. It follows that the increase in the cortical response may be due to facilitation of the geniculate nucleus, although it is possible that the inhibitory neuron circuit suggested by Armstrong19 would function like that in the LGN. Therefore the facilitation of the cortical neurons leading to an enhanced response evoked by light or electrical shock may to some extent originate intracortically after tonic inhibition from the retina is lost. The milder lesion which produces enhanced light-evoked geniculate and cortical signals may have a number of surviving ganglion (and other) cells within and bordering the lesion. These surviving cells could be activated by stimuli falling on receptors peripheral to the lesion since large receptive fields,20 as wide as 2.4 mm.," activate rabbit ganglion cells. I can speculate that the mild lesion reduces

9 Volume 10 Number 9 Rabbit visual potentials 647 the excitation of the surviving ganglion cells, and this eventually leads to facilitation centrally by the disinhibition mechanism. Ganglion cells depend on intact photoreceptors for their maintained discharge." It is apparent that disinhibition, if it occurs as data from electrically evoked signals suggest, is operating whenever lesions are produced by 3.2 to 6.3 J per square centimeter, which were the only energy densities administered in electrical stimulation studies. The expression of the proposed disinhibition by photic stimulation, however, seems to depend on the adjustment of the magnitude of the stimulus to the depth of the retina involved by the thermal insult. Low-energy stimuli flashed onto a mild lesion produce increased geniculate and cortical potentials. A highenergy stimulus on the same lesion elicits signals depressed from controls. This could be evidence of interactions between scotopic and photopic systems in the retina, with the photopic system being emphasized by high-energy stimuli. An early component on the cat ERG b-wave has been suggested to be an expression of the photopic system. 23 ' 24 Responses characteristic of photopic elements have been seen in the rabbit electrophysiologically, 25 " 27 and cone-like receptor terminals have been seen histologically, 28 although this retina is not considered to have color-discriminating cones. 29 In any case, the retinal output affecting the LGN and cortex will be shifting from scotopic to photopic in nature as the stimulus becomes more intense. It is unlikely that the growing stimulus area which accompanies the increase in intensity plays a big role in the depressant effect of higher stimulus energies since the per cent decrease in the ERG is essentially independent of stimulus intensity. Ordinarily, one would expect the effect of injury to be greater at low stimulus levels where more of the stimulus fell within the boundaries of the lesion. The severest injury results only in a decrement in light-evoked signals from the LGN and cortex. Tonic inhibition is probably disengaged, but this is not visible in light-evoked potentials because, I suspect, the full thickness of the retina in the irradiated area is nonfunctional. An electrical stimulus exterior to the retina in the chiasm easily stimulates more optic nerve fibers than the light stimulus and consequently evokes an augmented response. Again, the ERG b-wave is reduced maximally (30 per cent) beyond the 5.1 J per square centimeter dosage, implying that, from this dosage on, the retina at the irradiated site will be totally refractory to light stimulation. Near 3 J per square centimeter light-evoked brain potentials may be potentiated by the lesion, but the destruction of the retina within the lesion is not complete, because the ERG has been reduced by only 20 per cent or so. The source of the c-wave of the ERG has been identified as the pigment epithelium. 30 This wave is a prominent feature of the ERG from the rod-dominant retina of the dark-adapted rabbit. Negative troughs occurring before and after the c- wave have an unknown origin. The first trough may be remnant negativity of the late receptor potential (late RP) since this trough agrees in timing with the late RP after the b-wave is eliminated (see Brown, 31 Figs. 12 and 15) and since the c-wave is thought to mask or hasten the return to zero potential of the rod late RP. 31 The laser insult appears to delay the timing of the c-wave or reduce its amplitude and this may deepen the negative trough after the b-potential. Perhaps a loss in b-wave amplitude also deepens the trough. The b-wave is obviously depressed by a laser lesion. With high stimulus intensities, when a large ERG with a prominent a-wave is present, the per cent reduction of the postburn ERG is virtually the same as when a low-intensity stimulus is used and only a small b-wave, with no a-wave, is evoked. It is not possible to state how generation of the b-wave, now closely associated with MiiUer cells, 32 can be af-

10 648 Hevipel Investigative Ophthalmology September 1971 fected without visible deterioration of the a-wave, formed by the leading edge of the late RP generated distally. 31 It is likely that the a-wave is modified, but this escapes notice in cases in which it is restored by concomitant loss of the b-wave. I am tempted to speculate that the brilliant argon stimulus can evoke a long-lasting late RP which is still visible after the c-wave decays, and that this second trough, when flattened, reflects a reduced late receptor potential. The reasons for an increased c-wave after irradiation with a low-energy density are obscure. A local heating effect can be hypothesized to play a role. Further work should determine whether a- and c-wave changes are the earliest detectable measures of retinal injury since the c- wave is generated in the primary site of laser light absorption, the pigment epithelium, and since the a-wave is generated by receptors nearby. I thank Dr. A. J. Welch of the University of Texas for financial aid and review of the manuscript, and Dr. Myron Wolbarsht of Duke University for suggestions concerning the manuscript. I am indebted to Dr. James Larimer for consistently good advice. Dr. Kenneth Castleman, now of Jet Propulsion Laboratories, Pasadena, California, provided technical assistance. REFERENCES 1. Campbell, C. J., and Rittler, M. C: The effects of lasers on the eye, Ann. N. Y. Acad. Sci. 168: 627, Clarke, A. M.: Ocular hazards from lasers and other optical sources. Chemical Rubber Co. critical review, Environ. Control 1: 307, Vassiliadis, A., Zweng, H. C, Peppers, N. A., Peabody, R. R., and Honey, R. C: Thresholds of laser eye hazards, Arch. Environ. Health 20: 161, McNeer, K., Ghosh, M., Geeraets, W. J., and Guerry, D.: Electroretinography after light coagulation, Acta Ophthalmol. 76: (Suppl.) 94, 1963; 5. Aronson, S. B., and Garoutte, B.: The effect of retinal lesions on the ERG, INVEST. OPHTHALMOL. 1: 416, (abst.) 6. Nicholson, A. N., and Allwood, M.: Laser lesions: Changes in retinal excitability, Nature 210: 637, Banyard, R. D., Noyori, K. S., Campbell, C. J., and Marg, E.: Retinal effects of ruby laser photocoagulation, Arch. Ophthalmol. 85: 107, Fifkova, E., and Marsala, J.: Stereotaxic atlases for the cat, rabbit and rat, in Bures, J., Petran, M., and Zachar, J., editors: Electrophysiological methods in biological research, New York, I960, Academic Press, Inc. 9. Brouwer, B.: Anatomical, phylogenetical, and clinical studies on the central nervous system, Baltimore, 1927, The Williams & Wilkins, Co. 10. Thompson, J. M., Woolsey, C. N., and Talbot, S. A.: Visual areas I and II of cerebral cortex of rabbit, J. Neurophysiol. 13: 277, Brouwer, B., and Zeeman, W. P.: The projection of the retina in the primary optic neuron in monkeys, Brain 49: 1, Jacobson, J. H., Najac, H. T., Stephens, G., Kara, G. B., and Gestring, G. F.: The role of the macula in the electroretinogram of monkey and man, Am. J. Ophthalmol. 50: 889, Burke, W., and Sefton, A. J.: Recovery of responsiveness of cells of lateral geniculate nucleus of rat, J. Physiol. 187: 213, Burke, W., and Sefton, A. J.: Discharge patterns of principal cells and interneurones in lateral geniculate nucleus of rat, J. Physiol. 187: 201, Hansen, S. M., Bruce, I. S. C, and Burke, W.: The effect of retinal illumination and retinal blockade on synaptic transmission in the lateral geniculate nucleus of the cat, Vision Res. 7: 401, Suzuki, H.: Effect of reversible retinal blockade on population response of the lateral geniculate nucleus, Jap. J. Physiol. 17: 335, Nakai, Y., and Domino, E. F.: Recicular facilitation of visually evoked responses by optic tract stimulation before and after enucleation, Exp. Neurol. 22: 532, Posternak, J. M., Fleming, T. C, and Evarts, E. V.: Effects of interruption of the visual pathway on the response to geniculate stimulation, Science 129: 39, Armstrong, C. M.: The inhibitory path from the lateral geniculate body to the optic cortex in the cat, Exp. Neurol. 21: 429, Barlow, H. B., Hill, R. M., and Levick, W. R.: Retinal ganglion cells responding selectively to direction and speed of image motion in the rabbit, J. Physiol. 173: 377, Ames, A., Ill, and Pollen, D. A.: Neurotransmission in central nervous tissue: A study of isolated rabbit retina, J. Neurophysiol. 32: 424, 1969.

11 Volume 10 Number 9 Rabbit visual potentials Rodieck, R. W.: Maintained activity of cat retinal ganglion cells, J. Neurophysiol. 30: 1043, Gouras, P., and Hoff, M.: Retinal function in an isolated, perfused mammalian eye, INVEST. OPHTHALMOL. 9: 388, Jacobson, J. H., and Masuda, Y.: Oscillatory components of the cat B-wave, in Burian, H. M., and Jacobson, J. H., editors: Clinical electroretinography, New York, 1966, Pergamon Press, Inc. 25. Dodt, E., and Walther, J. B.: Photopic sensitivity mediated by visual purple, Experientia 14: 142, Vatter, O., Koller, T., and Monnier, M.: Die spektrale Sensitivitat der Retina und des optischen Cortex beim Kaninchen (Einflus der Farben auf die Antwort-Komponenten), Vision Res. 4: 329, Dodt, E., and Elenius, V.: Spektrale Sensitivitat eingelner Elemente der Kaninchennetzhaut, Pfluegers Arch. 262: 301, DeRobertis, E., and Franchi, C: Electron microscopic observations on synaptic vesicles in synapses of retinal rods and cones, J. Biophys. Biochem. Cytol. 2: 307, Sjostrand, F. S.: The outer plexiform layer and the neural organization of the retina, in Straatsma, B. R., Hall, M. O., Allen, R. A., and Crescitelli, F., editors: The Retina, Berkeley, Calif., 1969, University of California Press. 30. Steinberg, R. H., Schmidt, R., and Brown, K. T.: Intracellular responses to light from cat pigment epithelium: Origin of the electroretinogram c-wave, Nature 227: 728, Brown, K. T.: The electroretinogram: Its components and their origins, Vision Res. 8: 633, Miller, R. F., and Dowling, J. E.: Intracellular responses of the Miiller (glial) cells of mudpuppy retina: Their relation to b- wave of the electroretinogram, J. Neurophysiol. 33: 323, 1970.