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ERG recordings of a primate pure cone retina (Tupaia glis)
Vision Res. Vol. 7, pp. SS3-563.
ERG PURE
Pcrgamon Press 1967.
Printed inGnatBrittrin.
RECORDINGS CONE RETINA
OF A PRIMATE (TUPAIA GLIS)
J. TIGGES*,BARBARAA. BR~~KS~AND M. R. KLEE Max-Planck-Institut fiir Himforschung, Frankfurt/Main, 46 (Neuro-anatomische Abteilung)
Deutschordenstr.
(Received 29 October 1966) INTRODUCTION AMONGmammals, the only animals previously known to have a pure cone retina have been limited to the family Sciuridae. Studies on the electroretinogram (ERG) are plentiful, and have concentrated upon the ground and tree squirrels. Recent anatomical investigations (ROHEN 1962,TIGGES1963,CASTENHOLZ1%5) have described a pure cone retina in Tupaia glis, or the tree shrew. Tupaia is at the base of the lower primates, sharing some features with the insectivores. Neither a fovea nor an area centralis is observable in the retina. In order to determine some of the neurophysiological correlates of its retinal morphology, the present study investigates the general properties of the Tupaia ERG, with special attention to spectral sensitivity. METHODS Subjects Ss were adult tree shrews (Tupaia glis Diard 1820) of both sexes which had been maintained in the institute colony from an early age on an ad lib. diet of fruit, vegetables and
chopped meat. Experimental 145 g.
weight varied from 120 to 190 g, with an average of about
Apparatus
All preparations were fixed in a specially constructed head holder and tested in a lighttight, electrically shielded box painted flat black on all surfaces. Body temperature was controlled with a heating pad. For flicker and paired stimulation a stroboscope (Knott, Model Strobotest II) with a continuously variable rate adjustment was used; each flash was monitored through an oscilloscope and permanently recorded on chart paper. Most single stimuli were provided by a xenon lamp (Leitz, Lampenhaus 250) located outside the test box. Filters (Schott) within the lamp housing eliminated infrared and ultraviolet wavelengths; exterior lenses focussed and collimated the emerging light. A mechanical, self-cocking camera shutter placed in the optic path regulated the duration of stimulation, and could be electrically monitored so that stimulus duration was recorded both on an oscilloscope and polygraph. Neutral density (Deutsche Balzers, Schott) and monochromatic interference titers were placed between the shutter and a small opening in the test box where the test light entered. l Present address: Yerkes Regional Primate Research Center, Emory University, Atlanta, Georgia 30322, U.S.A. t Present address: Abt. f. klinische Neurophysiologie der Universitat, Freiburg, Germany.
553
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J. TIGGES,BARBARAA. BROOKSAND M. R. KLEE
Each neutral density filter was calibrated in combination with all monochromatic filters using a spectrophotometer (Zeiss, Model PM QII); the same instrument was used to measure the density of the eye lens. The eleven color filters (Schott, Type double band) had their peak transmissions at 420,449,471,498, 508, 522, 540, 552, 570,604 and 655 nm. Inside the test box the light fell upon a circular translucent diffusion screen, mounted in front of the S’s eye and subtending a visual angle of approximately 100”. (Most spectral sensitivity experiments were done without the screen.) Electrodes for ERG recording consisted of glass pipettes approximately 8 cm long. The lower half of each pipette was 8lled with warm, fluid agar-agar solution containing 1 per cent NaCl. Upon cooling, a saturated ZnS04 solution was poured into the upper half of the pipette. Mercury amalgamated Zn rods which had been soldered to leads were then dropped into the sulfate solution. Electrode resistance was about 10,000 fi. The recording electrodes were connected to a direct coupled amplifier (input impedance : 3.9 MR) and an oscilloscope (Tijnnies) which was used for display of the signal. The output of the amplifier was filtered (half-amplitude high frequency responses at 0.3 kc) and then fed to a Honeywell Visicorder (Model 1508) for permanent recording. Signals recorded on the Honeywell chart paper by means of ultraviolet light could be rapidly developed by a few seconds exposure to any ordinary light source. Procedure Ss were anesthetized with urethane (20 per cent solution) administered intraperitoneally.
Their delicacy and individual susceptibility to anesthesia precluded a set routine. Usually an S was given 120 mg of urethane at the beginning of an experiment, followed by 80 and 60 mg at half-hourly intervals. Thereafter additional shots were given according to the condition of the animal. Tracheotomy was performed, 10 per cent neosynephrine was administered to dilate the pupils, and topical anesthesia of the eye was produced with 3 per cent cornecaine. The animal was then mounted in the head holder and dark adapted in the test box for 1 hr. (SCHULZE et al. have shown that despite its pure cone eye, dark adaptation of Tupaia may take up to 40 min when weak test stimuli are used.) The active electrode was placed on the cornea; the indifferent electrode rested upon the skull just above the eye through a slice in the upper lid. A hypodermic needle piercing the muscles of the foreleg grounded the animal. After the dark adaptation period, single brief stimuli were given once every 1 or 2 min until the termination of the experiment. Absolute intensity of white light at the position of the eye was measured with an S.I.E. exposure meter aimed at the diffusion screen before and after each experiment. Energy transmitted by the monochromatic filters was repeatedly monitored during experimentation by means of a thermocouple (Pyro-Werk, Type B) in connection with a galvanometer (Lange, Type MGOE). A pulley system allowed the thermocouple to interrupt the light path at a point just anterior to the eye within the closed test box, so that the animal was not exposed to extraneous light during these measurements. The Honeywell recorder was usually calibrated so that a 10 mm deflection equaled 30 PV of signal input. Chart speed was 25 mm+ for spectral sensitivity work and 200 mm/set for other investigations. AlI spectral sensitivity work used an “equal response” criterion-that is, the energy at each wavelength was varied until the amplitude of the ,y-wave, measured from the trough of the a-wave, was equal across all wavelengths. This
ERG Recordings of a Primate Pure Cone Retina (7upniu gfis)
555
method required continuous measurement of response amplitude directly from the chart paper throughout the experiment, and adjustment of test flash intensities. Each animal then received between 3-7 test flashes at each wavelength tested. The order of stimulation, whether involving spectral, duration, intensity or other variables, was balanced to compensate for preparation fatigue and possible adaptation effects due to repeated exposures. RESULTS General description
Curve E of Fig. 1 shows a tracing of the typical dark adapted ERG of T’aia, in response to relatively long stimulation. Wavelets superimposed upon this basic waveform have been recorded by other methods which avoid the heavy filtering used in the present procedure (!SCHULZE et al., unpublished results). The questions discussed in this paper, however, will be answered by reference to the gross features of the ERG. The initial rapid negative de5ection (a-wave) in curve E of Fig. 1 is succeeded by a fast rising positive x-wave. The average latency (42 msec) of the latter component is similar to that noted for pure cone squirrels (TANSLEY, 1957). The x-wave can be contrasted to the slower b-wave of the rod dominated guinea pig @‘DAY, 1947), shown in curve Fin response to the same intensity and duration stimulus. The x-wave is distinguished from the b-wave primarily by its short latency, but also by its faster rise time and shorter duration as first noted by MOTOKAWA and M~TA(1942) and ADRUN (1945).
FIG. 1. Sample tracings of the cornea1 ERG of Tupu~ (A-E) and guinea pig (F). A, evoked by white light; B, evoked by blue light (449 nm); C, evoked by green light (552 MI); D, evoked by yellow light (604 nm). A-D, energy was adjusted to produce equal x-wave amplitudes; stimulus duration was 8 msec. E and F were evoked by white light; stimulus duration was 1 sec. Light intensity (150 cd/mz) was the same in E and F. Lower traces are stimulus markers.
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J. TIGGES, BARBARA A. BRCOKS AND M. R. KLEE
A slow rising c-wave follows the x-wave of Tupaiu, and is particularly noticeable with long stimulation. Finally, the termination of a relatively long stimulus (about 40 msec or more) is followed by a substantial positive d-wave. Responses A through D of Fig. 1 were produced by stimuli of 8 msec duration; stimulus wavelength is different among these curves, but energy has been adjusted to give equal amplitude x-wave responses. In such conditions the latency and form of the x-wave and u-wave are basically unchanged across the spectrum and are similar to the white light response (curve A). Amp~it~e of the x- and a- wave as a function ~fst~rn~~~ intensity Dark adapted tree shrews were stimulated with a white test light of 8 msec duration and variable intensity. Amplitude of the x-wave, as measured from the trough of the a-wave, was then plotted as a function of log intensity of the test flash, as in Fig. 2 (@led circles). The resulting sigmoidal curve shows a linear’lncrease from about 35-1400 cdfmz, appro~~~ly 1.6 log units. When the animal is exposed to a white background light of 350 cd/m*, the threshold of the x-wave increases by about I log unit, and the linear portion of the function becomes somewhat more steep, covering a range of roughly 1-Olog unit (white circles). The weakest effective test flash intensity at this background level was approximately 15 cd/m?
FIG, 2. aptitude of the x- and a-waves as a function of stimulus intensity during dark (filled circles and triangles) and during exposure to background light of 350 cd/m2 (open circies and triangles). Each curve represents averaged results of the same 3 animals. Stimulus duration was 8 msec.
The straight line functions on the Iower part of Fig. 2 show the increase of the a-wave during the same conditions described above. The ma~mum growth of the wave was considerably less than the x-wave, but the change in threshold affected by light adaptation is similar. The a-wave appeared to be still increasing after the x-wave had ceased to grow. No change was noted in the amplitude of either wave when stimulus intensity was held constant and duration varied from 8-20-40 msec.
ERG Recordings of a Primate Pure Cone Retina (Tupaia@is)
557
Latency of the x-wave and a-wave as a fiction of stimulus intensity Within a range of weak test intensities the latency of both the x- and a-wave appears to decrease linearly with logarithmic increases of stimulus intensity. In Fig. 3 the longest latencies (about 73 msec for the x-wave, 26 for the a-wave) are noted in response to the weakest effective stimulus intensity; approximately l-4 cd/m2 to the dark adapted eye. Intensity increases exceeding 700 cd/m2 do not result in further remarkable latency decreases in either wave. Latency variability was greatest at low stimulus values, but this is partially an artifact due to the difficulty of accurately measuring the rather flat peaks of near threshold responses.
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Sthwtushtanslty red/m3 FIG. 3. Peak latency of the x- and u-waves as a function of stimulus intensity. Each curve is urements; each point is an averatp of values from basedonatotalofabout25Osinglemeas 5 animals. Brackets represent the standard deviation of -ts at each point. Stimulus duration was 8 msec.
The peak latency of the X- and a-waves appeared unchanged during stimulus durations of 8,20 and 40 msec, when other parameters were held constant. Flicker fusion and paired stimuli The flicker fusion frequency (FFF) was tested in four animals in darkness and during exposure to a medium intensity background light (350 cd/m2). A stroboscope was used on which the flash rate was continuously variable. The FFF appeared una5cted by background light or changes in the flash intensity; it averaged about 90 c/s, ranging between 81 and 101 c/s. The high FFF suggested that the time interval between two 5shes could be very short and still allow full responses to both stimuli. Using 10 psec paired stroboscope 5shes of high intensity (2200 cd/m2), this critical interval was tested during dark adaptation and found to be as short as 1 set, at which time the amplitude of the second x-wave would begin to decrease. In comparison the critical interval of the guinea pig b-wave, using similar flash intensities in darkness, was approximately 5 min. As the interval between paired stimuli in the dark was reduced below 1 set, the amplitude of the second x-wave, but not the second a-wave, was affected so that at an interval of approximately 100 msec the ERG had the appearance of Fig. 4. This picture was quite different when the eye was exposed to background illumination. Exposure to background light had the effect of greatly reducing the critical interval of the x-wave; at a background
J. TIGGES, BARBARAA. Bwo~s
558
AND
M. R. KLEE
FIG. 4.
Upper channel: response to paired stimuli approximately 100 msec apart; stimulus duration was about 10 w; stimulus intensity was approx. 2200 cd/m? Lower channel: stimulus and time marker (SOc/s). Photograph of original record.
light of 350 cd/m2 the second x-wave did not begin to decrease until only 70 msec separated the two stimuli. As the interval was shortened to 15 msec, the amplitude of the second x-wave dropped from 100 to 45 per cent of the value of the first x-wave. At 15 msec there was either a response of 45 per cent, or the second wave was impossible to identify. Between intervals of 70 and 100 msec the second x-wave often appeared somewhat enhanced, growing an additional 10 per cent of its original value. of interest is the fact that the x-wave critical interval can also be reduced during darkness in a manner simulating the effects of background exposure, if the intensity of the paired flashes is sufficiently lowered. In conditions of darkness the u-wave showed no statistically significant changes to high intensity paired flashes of variable interval. Measurement was confounded, however, at short intervals because the slope of the second a-wave was indistinguishable from the descending branch of the first x-wave. During background illumination, and after the stimulus interval was large enough for the second a-wave to be discriminated, it was seen that the latter was clearly larger than the first u-wave; amplitudes were sometimes as much as double the value of the first u-wave. This effect persisted until the stimulus interval reached about 200 msec, when the second a-wave was equal to the first and uninfluenced by further interval increases. A sufficient reduction in the intensity of the paired stimuli during darkness produced an enhancement of the second u-wave very similar to that observed during light adaptation. Spectral sensitivity Animals were tested in the dark with a stimulus of 8 msec duration. Energy was varied at each tested wavelength to produce a constant amplitude x-wave response of 60 PV.
ERG Recordings of a Primate Pure Cone Retina (Tupaio gliu)
559
Movement of the preparation and disturbance of electrode position was a frequent cause of response variability. The reference wavelength, usually chosen at 449 nm, was used to monitor and control for such variability. With 449 nm as the initial stimulus in the experiment, energy was adjusted to produce a 60 FV response amplitude. Before and after all subsequent wavelength stimulations, the response to 449 nm was checked. Jf it had changed, previous measurements had to be discounted and repeated. Since several measures were taken and averaged at each wavelength, animals often did not survive the 4 hr experimental period required to go through all filters. Of 25 experiments, the results of only 12 animais were complete enough to include in the curves presented in Fig. 5.
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A
82 iodopsin
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FIG. 5. Upper frame: relative spaxral sensitivity of the intact eye (filled circles, average of 12 animals); the density of the lens (open circles, average. of 3 lenses). Lower frame: retinal sensitivity corrected for lens absorption (filled circies)). Triaq+s show the absorption of iodopsin taken from WILD efof. (19%).
The uppermost curve of Fig. 5 (filled circles) shows the log relative spectral sensitivity of the intact Tupaia eye; energy values used in calculating the average points were equated for an equal energy spectrum. Tbe density of the lens is shown in contrast (white circles), and it is evident that the influence of the lens upon sensitivity is negligible from about 450 nm through the red end of the spectrum. In order to determine the relative sensiti~ty of the retina, the relative density of the lens was subtracted from the whole eye measurements, resulting in the curve drawn in the lower half of Fig. 5 (filled circles). One peak of this curve is found at about 552 nm; another in the blue region of the spectrum. Seven Ss actually had individual peaks at 552, three at 570 and two at 540 nm. The function
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J. TIGGES, BARBARA A. BROOKSAND M. R. KLEE
described by the triangles shows the spectral absorption of iodopsin, isolated from the predominantly cone retina of the chicken by WALD et al. (1955). There was no detectable Purkinje shift in Tupaia. The c- and d-waves Neither the c-wave nor the d-wave was investigated systematically. The c-wave was not observed during light adaptation; given sufficient dark adaptation and the proper length stimulus it usually appeared, but amplitude and latency variability were great when stimulus duration was brief. The c-wave is definitely enhanced as stimulus duration grows longer, and may reach amplitude proportions and durations far exceeding any other component of the ERG (Fig. 1). The d-wave appears as a small positive deflection if stimulus duration is not less than 40 msec. The latency of this component in Tupaia is about 50 msec when fully developed (Fig. 1). The d-wave grows rapidly with increasing duration up to 200 msec, levels off, and is unchanged after 500 msec. This agrees with the results described by TANSLEYet al. (1961 b) for the pure cone ground squirrels whose d-wave was reported to be unaffected by duration increases above 250 msec. DISCUSSION Several characteristics of the Tupaia ERG are consistent with traditionally conceived photopic function. The absolute dark adapted threshold is high compared to the duplex or rod retina, and corresponds to ERG findings on other pure cone species (ARDEN and TANSLEY,1955 a; DODT, 1962). The threshold increase of 1 log unit resulting from light adaptation is small and the log intensity range over which amplitude variations occur is relatively restricted, compared to that of the duplex retina. Flicker fusion rate is high, and corresponds to that of pure cone members of Sciuridae (BORNSCHEIN,1961; BORNSCHEIN and SZEGVARI,1958), and that of the light adapted human eye (HECK, 1957). There is no evidence of a Purkinje shift. The x-wave, which has been elicited and tested in other species under conditions favoring photopic sensitivity (MOTOKAWAand MITA, 1942; ADRIAN, 1945; SCHUBERTand BORNSCHEIN,1952; JONES et al., 1964), is an outstanding feature of the Tupaia ERG; conversely, the slower b-component found in all animals having a duplex or rod retina is completely lacking. The fast recovery of the second response to paired stimuli as compared to the rod dominated eye, may be related to the generally rapid photochemical and electrical processes presumed to underlie cone vision. of particular interest is the spectral sensitivity curve (Fig. 5). All other pure cone mammals which have been tested by means of the ERG (ground and tree squirrels, chipmunk) have curves which, when compared to Tupaia, are far displaced toward the short wavelengths. The peaks of these curves vary between 490 and 535 nm (ARDEN and TANSLEY,1955 a, b; DODT, 1962; DOWLING, 1964; TANSLEY,COPENHAVER and GUNKEL, 1961, VAITER, 1966) while Tupuia shows a peak at about 552 nm, with a second indicated in the blue region. In contrast, the average maxima of human cone sensitivity and the general configuration of the light adapted spectral sensitivity curve (559 nm, WALD and BROWN, 1965) appear similar to that of Tupaiu. The agreement of the Tupaia curve with the iodopsin absorption function derived from the cone dominated chicken strongly suggests that one of its visual pigments is iodopsin. At the blue end of the spectrum the corrected curve for Tupaia (Fig. 5, lower left half) deviates upward from the usual gradual downward slope of the human psychophysical
ERG Recordings of a Primate Pure Cone Retina (Tu@u glis)
561
curve at this region, suggesting heightened blue sensitivity. While behavioral tests indicate that Tupaia has color vision (TIGGFS, 1961, 1963), no psychophysical determinations of relative spectral sensitivity have yet been published. WALD (1964), however, has reported psychophysical functions for a human subject (“R.H.“) showing unusually high sensitivity in the blue region, with normal response over the remainder of the spectrum. WALDjudged on the basis of selected criteria that the abnormal blue sensitivity was most likely due to a greater than average concentration of the blue receptor (or pigment, in his usage). When our lens-corrected average curve for Tupaia is superimposed over “R.H.‘s” fovea1 sensitivity curve, which has been corrected for ocular and macular absorptions, a satisfactory congruence results. This suggests that the concentration of pigments or receptors may be similar in Tupaia to that of the average human, with the exception that a larger amount of the blue receptor is present. It also suggests that the tree shrew eye may be subjected to adaptation experiments, similar to WALD’S on humans, in order to isolate the action spectra of the major component pigments. The evidence cited above underlines the potentiality of Tupaia as a tool for the understanding of cone function in the human retina. Acknowledgement-We would like to thank Prof. Dr. E. Dodt and Drs. C. Baumann, N. Seiler, J. Schulze and E. Rehse, and Mr. H. Feld, J. Kampe, W. Leber and J. Seyl for their kind advice and various technical assistance on this project. REFERENCES
ADRIAN,E. E. (1945). The electric response of the human eye. J. Physiol..,Lond. 104, 84-W. ARDEN, G. B. and TANSLEY,K. (1955 a). The spectral sensitivity of the pure-cone retina of the grey squirrel (Sciurus carolinensis leucotis). J. Physiol., Land. 127, 592-602. ARDEN, G. B. and TAN-, K. (1955 b). The spectral sensitivity of the pure-cone retina of the souslik (Citellus citellus). J. Physiol., Lot&. 130, 225-232. B~RNSCHEIN,H. (l%l). Zur Nctzhautfunktion dcs AlucnmurmclticrsMarmota marmota (L.). 2. veral. Physiol. &I, 262-261.
H. and SZEGVhU, G. (1958). Fl’nnmerekktroretinographische Studie bei einem SHuger mit reiner Zapfennetzhaut (Citellus citellus L.). Z. Biol. 110,285-290. CASTENHOLZ, E. (1965). &er die Strukm dcr Netzhautmitte bei Primaten. Z. Zellfosch. mikrosk. Anar. &RNStXEIN,
66,646-661. DODT,
E. (1962). Vergleichende Untersuchungen
fiber das adaptive Verhalten reiner ZapfennetzhHute.
Pfltiers Arch. Res. Physiol. 275. 561-573. Do&&, J. E. (1964). _ Structu~ and function in the all-cone retina of the ground squirrel.
In the Symposium on: The Physiological Basis for Form Discrimination. Brown University, Providence, R. I., pp. 17-23. HECK, J. (1957). The flicker electroretinogram of the human eye. Acta Physiol. stand. 39, 158-166. JONES,A. E., POWN, M. C. AND DE VALOR,R. L. (1964). Mangabey x and b wave electroretinogram components: their dark-adapted luminosity functions. Science, N. Y. 146, 14861487. M~KAWA, K. and MITA, T. (1942). oba eine einfachere Untersuchungsmethode und Eigenschaften der Aktionsstriime der Netzhaut. Tohoku 1. exp. Med. 42, 114-133. O’DAY, K. (1947). Visual cells of the guinea pig. Nature, Lond. 168, 648. ROHEN,J. (1962). Sehoqan. In: Primatologla 11/l/6, pp. 210. Edited by H. Ho=, A. H. SCHULTZand D. STARCK. Base1 and New York. SCHUBERT, G. and &XUWXEIN, H. (1952). Beitrag zur Analyse des menschlichen Elektroretinogramms. Ophthalmologica, Base1 123, 396413. SCHULZE,J., REHSE,E. and TIGOES,J. Der Verlauf der Dunkeladaptation bei Tupuia glis. (In preparation.) TANSLEY,K. (1957). Some observations on mammalian cone electrorctinograms. Biblrhca Ophrhul. 48,7-14. TANSLPI,K. COPENHAVER, R. M. and GUNKEL,R. D. (1961 a). Spectral sensitivity curves of diurnal squirrels. Vision Res. 1, 154-165. TANSLEY,K., COPENHAVER, R. M. and GUNKEL, R. D. (1961 b). Some observations on the off-effect of the mammalian cone electroretinogram. J. opt. See. Am. ST, 207-213. TIGGES,J. (1961). Sind alle Halbaffen farbcnblind? Narurwissenschrrften48, 677.
TIGGES,J. (1963). Untersuchungcn ilber den FarbensEntt von Tupaiugli$ (DKARD, 1820). Z, ~~ovph.Anrhrop. 53. 109-123. V&R, 0. (1966). I.&s ph~trscftev~derte ~~~~~~~~~ und die opt&&e Sensitivitat des Pfatanenhz%mhena (scrii&.s n&#f&r)* P&&o Ref. 6,6f-81* W#&& G. (X%4). The receptors of hr.&man%-&orvision, &%vice*iV.Y*135, ~~~-~~~6. WAL.D,G. and &tow~, P. K. (X965). Human c&or vision and color blindness. In: Sensory Receptors. Cold Spring Harbor Symposium. Vol. 30, 345-359. WALD, G., BRCWN,P. K. and Shnrru, P. H. (1955). Iodopsin. J. gen. Physbl. 38, 623-681, AI8stmt--The gross con&uration
of the pure cone T.vpu& ERG is characterized by wellde&ed @, X-, 0 and &va?!e& The short ktteney, fast rkinS x-wave contmsts to the slower &wave found in aB duptcJr rvKi rod &tXts heretofore test&. The c-wav& not p~vi~s~~ reported for pure cone retinae, can usual& be avok& during dark adaptation. The et%et%ct or &wave appears predictably when stimtuus durations are tong enough. Several featums are eon&tent with the exchtsive presence of cones and their ftmction, includinp: (1) high FFF, (2) lack of Purkinje shit?, (3) high absolute threshold in the dark and only a small increase in the threshold as a function of light adaptation, (4) restricted range of intensity over whii amplitude variations are elicited, and (2) pesk of spectrai sensitivity at 592 mu. The spectrai sensitivity Curve, in contmst to alI the other pure cone mammah, shows canSruetu~ with the fovea! psychophysical curve of the human and with the absorption curve of iodopsin, with the exception of mom marked sensitivity in the blue region. R&tm&-La con6guration &t&ale de 1’ERG de Twaiu (r&no h tines) est caract&is& par des on&s bien d&&ties a, x, c et rf. L’onde x 8 courte latence et mot&% rapide contmste avec i’onde ir phrs tcntn: trouvee &is louses tea &tines doubk!s ou a b%onnets exp&hnentees jus@ici. L%r& c, que Pornn’avait paa jusqu%i d&&e dana ies r&tinesa cones seu& apparait dhabitude pendant ~a~~t~on B l’obscurit& L’eftet ~#(onde d) sembk prOvisible quand les du& de stimulation sum assez tongues. Diversas caract&stiqtm+ sont en accord avec la presence exchtsive de canm et leurs fonctianaI a savoir: (1) frequence de fusion Ilev&e, (2) abserrcet d’effet Purkinje, (3) seuil absolu elev6 dans l’obscurit6 et qui n’augmente que peu en fonction de l’adaptation a la huni&m~(4) intetv&e Otroit d’intenai~ qui pro&tit dcp variations ~arnp~~d~ et (5) maximum de abet speetrahz B 562 nm. La courbe de ~~~~ spectt&_ foci B tous lea atttrei ~rn~f~~ ii pars c&es, concorde avec ie courbe psychophysique fovealc humaine et avec Ia wurbe d’absorption de l’iodopsine, sauf une sensibilite plus marquee dans le bleu, ZBDas ERG der reinen Zapfenretina von Tupah ist charakterisiert durch wohlausttepragte a-, X-, c-, und d-Wellen. Die kutze Gipfelhttenz und der schnelle Anstieg der #-We% stehen im s&a&en C&renm& zu der -merea f+Wehe der bisher ~~~~~ ~~~te~ und S~~~~. Die e-We&, die bii bei keinem anderen S&@er mit einer reinen Zapfenrethut acfunden wurde, kann bei Tuparh im a&emeinen im dunke3adaptierten Zustand au&&t werden. Der o&Efpzkt bzw. die CWeHe wird mit entsprechend langen Reizen au&&t. Fin@ Charakteristika stehen im EinklanS mit dem Be& einer reinen Zapfenretina, wie: (1) die hohe FFF. (2) das Fehlen des PurkimtPhilnomens. (3) die hohe absolute Dunkels&we& und die .&r&e &m&me der SchGelle withren~ ~~~ta~n, (4) tire ~nt~~~~~~~~ in dem verschiedene ~p~~tu~~~~ evoziert werden k&men, (5) das Amax der spektraten ~v~~~t iii bei 552 nm. Tm Getrensatz zu ahen anderen ssittgetie%n mit reiner Zapfenmtina deckt sich die spektrate Sensitivitlitskurve von Tapala mit der mit psychophysischen Methoden gewonnenen Kurve der fovealen Receptoren des Menschen und mit der Iodopsinabsorptionskurvcr. Etwas abweichend bei Tupaia ist die grossere Blauempiindlichkeit.
ERG Recordings of a Primate Pure Cone Retina (Tupaia glis) Peamae -
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563
ERG PURE
Pcrgamon Press 1967.
Printed inGnatBrittrin.
RECORDINGS CONE RETINA
OF A PRIMATE (TUPAIA GLIS)
J. TIGGES*,BARBARAA. BR~~KS~AND M. R. KLEE Max-Planck-Institut fiir Himforschung, Frankfurt/Main, 46 (Neuro-anatomische Abteilung)
Deutschordenstr.
(Received 29 October 1966) INTRODUCTION AMONGmammals, the only animals previously known to have a pure cone retina have been limited to the family Sciuridae. Studies on the electroretinogram (ERG) are plentiful, and have concentrated upon the ground and tree squirrels. Recent anatomical investigations (ROHEN 1962,TIGGES1963,CASTENHOLZ1%5) have described a pure cone retina in Tupaia glis, or the tree shrew. Tupaia is at the base of the lower primates, sharing some features with the insectivores. Neither a fovea nor an area centralis is observable in the retina. In order to determine some of the neurophysiological correlates of its retinal morphology, the present study investigates the general properties of the Tupaia ERG, with special attention to spectral sensitivity. METHODS Subjects Ss were adult tree shrews (Tupaia glis Diard 1820) of both sexes which had been maintained in the institute colony from an early age on an ad lib. diet of fruit, vegetables and
chopped meat. Experimental 145 g.
weight varied from 120 to 190 g, with an average of about
Apparatus
All preparations were fixed in a specially constructed head holder and tested in a lighttight, electrically shielded box painted flat black on all surfaces. Body temperature was controlled with a heating pad. For flicker and paired stimulation a stroboscope (Knott, Model Strobotest II) with a continuously variable rate adjustment was used; each flash was monitored through an oscilloscope and permanently recorded on chart paper. Most single stimuli were provided by a xenon lamp (Leitz, Lampenhaus 250) located outside the test box. Filters (Schott) within the lamp housing eliminated infrared and ultraviolet wavelengths; exterior lenses focussed and collimated the emerging light. A mechanical, self-cocking camera shutter placed in the optic path regulated the duration of stimulation, and could be electrically monitored so that stimulus duration was recorded both on an oscilloscope and polygraph. Neutral density (Deutsche Balzers, Schott) and monochromatic interference titers were placed between the shutter and a small opening in the test box where the test light entered. l Present address: Yerkes Regional Primate Research Center, Emory University, Atlanta, Georgia 30322, U.S.A. t Present address: Abt. f. klinische Neurophysiologie der Universitat, Freiburg, Germany.
553
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J. TIGGES,BARBARAA. BROOKSAND M. R. KLEE
Each neutral density filter was calibrated in combination with all monochromatic filters using a spectrophotometer (Zeiss, Model PM QII); the same instrument was used to measure the density of the eye lens. The eleven color filters (Schott, Type double band) had their peak transmissions at 420,449,471,498, 508, 522, 540, 552, 570,604 and 655 nm. Inside the test box the light fell upon a circular translucent diffusion screen, mounted in front of the S’s eye and subtending a visual angle of approximately 100”. (Most spectral sensitivity experiments were done without the screen.) Electrodes for ERG recording consisted of glass pipettes approximately 8 cm long. The lower half of each pipette was 8lled with warm, fluid agar-agar solution containing 1 per cent NaCl. Upon cooling, a saturated ZnS04 solution was poured into the upper half of the pipette. Mercury amalgamated Zn rods which had been soldered to leads were then dropped into the sulfate solution. Electrode resistance was about 10,000 fi. The recording electrodes were connected to a direct coupled amplifier (input impedance : 3.9 MR) and an oscilloscope (Tijnnies) which was used for display of the signal. The output of the amplifier was filtered (half-amplitude high frequency responses at 0.3 kc) and then fed to a Honeywell Visicorder (Model 1508) for permanent recording. Signals recorded on the Honeywell chart paper by means of ultraviolet light could be rapidly developed by a few seconds exposure to any ordinary light source. Procedure Ss were anesthetized with urethane (20 per cent solution) administered intraperitoneally.
Their delicacy and individual susceptibility to anesthesia precluded a set routine. Usually an S was given 120 mg of urethane at the beginning of an experiment, followed by 80 and 60 mg at half-hourly intervals. Thereafter additional shots were given according to the condition of the animal. Tracheotomy was performed, 10 per cent neosynephrine was administered to dilate the pupils, and topical anesthesia of the eye was produced with 3 per cent cornecaine. The animal was then mounted in the head holder and dark adapted in the test box for 1 hr. (SCHULZE et al. have shown that despite its pure cone eye, dark adaptation of Tupaia may take up to 40 min when weak test stimuli are used.) The active electrode was placed on the cornea; the indifferent electrode rested upon the skull just above the eye through a slice in the upper lid. A hypodermic needle piercing the muscles of the foreleg grounded the animal. After the dark adaptation period, single brief stimuli were given once every 1 or 2 min until the termination of the experiment. Absolute intensity of white light at the position of the eye was measured with an S.I.E. exposure meter aimed at the diffusion screen before and after each experiment. Energy transmitted by the monochromatic filters was repeatedly monitored during experimentation by means of a thermocouple (Pyro-Werk, Type B) in connection with a galvanometer (Lange, Type MGOE). A pulley system allowed the thermocouple to interrupt the light path at a point just anterior to the eye within the closed test box, so that the animal was not exposed to extraneous light during these measurements. The Honeywell recorder was usually calibrated so that a 10 mm deflection equaled 30 PV of signal input. Chart speed was 25 mm+ for spectral sensitivity work and 200 mm/set for other investigations. AlI spectral sensitivity work used an “equal response” criterion-that is, the energy at each wavelength was varied until the amplitude of the ,y-wave, measured from the trough of the a-wave, was equal across all wavelengths. This
ERG Recordings of a Primate Pure Cone Retina (7upniu gfis)
555
method required continuous measurement of response amplitude directly from the chart paper throughout the experiment, and adjustment of test flash intensities. Each animal then received between 3-7 test flashes at each wavelength tested. The order of stimulation, whether involving spectral, duration, intensity or other variables, was balanced to compensate for preparation fatigue and possible adaptation effects due to repeated exposures. RESULTS General description
Curve E of Fig. 1 shows a tracing of the typical dark adapted ERG of T’aia, in response to relatively long stimulation. Wavelets superimposed upon this basic waveform have been recorded by other methods which avoid the heavy filtering used in the present procedure (!SCHULZE et al., unpublished results). The questions discussed in this paper, however, will be answered by reference to the gross features of the ERG. The initial rapid negative de5ection (a-wave) in curve E of Fig. 1 is succeeded by a fast rising positive x-wave. The average latency (42 msec) of the latter component is similar to that noted for pure cone squirrels (TANSLEY, 1957). The x-wave can be contrasted to the slower b-wave of the rod dominated guinea pig @‘DAY, 1947), shown in curve Fin response to the same intensity and duration stimulus. The x-wave is distinguished from the b-wave primarily by its short latency, but also by its faster rise time and shorter duration as first noted by MOTOKAWA and M~TA(1942) and ADRUN (1945).
FIG. 1. Sample tracings of the cornea1 ERG of Tupu~ (A-E) and guinea pig (F). A, evoked by white light; B, evoked by blue light (449 nm); C, evoked by green light (552 MI); D, evoked by yellow light (604 nm). A-D, energy was adjusted to produce equal x-wave amplitudes; stimulus duration was 8 msec. E and F were evoked by white light; stimulus duration was 1 sec. Light intensity (150 cd/mz) was the same in E and F. Lower traces are stimulus markers.
556
J. TIGGES, BARBARA A. BRCOKS AND M. R. KLEE
A slow rising c-wave follows the x-wave of Tupaiu, and is particularly noticeable with long stimulation. Finally, the termination of a relatively long stimulus (about 40 msec or more) is followed by a substantial positive d-wave. Responses A through D of Fig. 1 were produced by stimuli of 8 msec duration; stimulus wavelength is different among these curves, but energy has been adjusted to give equal amplitude x-wave responses. In such conditions the latency and form of the x-wave and u-wave are basically unchanged across the spectrum and are similar to the white light response (curve A). Amp~it~e of the x- and a- wave as a function ~fst~rn~~~ intensity Dark adapted tree shrews were stimulated with a white test light of 8 msec duration and variable intensity. Amplitude of the x-wave, as measured from the trough of the a-wave, was then plotted as a function of log intensity of the test flash, as in Fig. 2 (@led circles). The resulting sigmoidal curve shows a linear’lncrease from about 35-1400 cdfmz, appro~~~ly 1.6 log units. When the animal is exposed to a white background light of 350 cd/m*, the threshold of the x-wave increases by about I log unit, and the linear portion of the function becomes somewhat more steep, covering a range of roughly 1-Olog unit (white circles). The weakest effective test flash intensity at this background level was approximately 15 cd/m?
FIG, 2. aptitude of the x- and a-waves as a function of stimulus intensity during dark (filled circles and triangles) and during exposure to background light of 350 cd/m2 (open circies and triangles). Each curve represents averaged results of the same 3 animals. Stimulus duration was 8 msec.
The straight line functions on the Iower part of Fig. 2 show the increase of the a-wave during the same conditions described above. The ma~mum growth of the wave was considerably less than the x-wave, but the change in threshold affected by light adaptation is similar. The a-wave appeared to be still increasing after the x-wave had ceased to grow. No change was noted in the amplitude of either wave when stimulus intensity was held constant and duration varied from 8-20-40 msec.
ERG Recordings of a Primate Pure Cone Retina (Tupaia@is)
557
Latency of the x-wave and a-wave as a fiction of stimulus intensity Within a range of weak test intensities the latency of both the x- and a-wave appears to decrease linearly with logarithmic increases of stimulus intensity. In Fig. 3 the longest latencies (about 73 msec for the x-wave, 26 for the a-wave) are noted in response to the weakest effective stimulus intensity; approximately l-4 cd/m2 to the dark adapted eye. Intensity increases exceeding 700 cd/m2 do not result in further remarkable latency decreases in either wave. Latency variability was greatest at low stimulus values, but this is partially an artifact due to the difficulty of accurately measuring the rather flat peaks of near threshold responses.
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The peak latency of the X- and a-waves appeared unchanged during stimulus durations of 8,20 and 40 msec, when other parameters were held constant. Flicker fusion and paired stimuli The flicker fusion frequency (FFF) was tested in four animals in darkness and during exposure to a medium intensity background light (350 cd/m2). A stroboscope was used on which the flash rate was continuously variable. The FFF appeared una5cted by background light or changes in the flash intensity; it averaged about 90 c/s, ranging between 81 and 101 c/s. The high FFF suggested that the time interval between two 5shes could be very short and still allow full responses to both stimuli. Using 10 psec paired stroboscope 5shes of high intensity (2200 cd/m2), this critical interval was tested during dark adaptation and found to be as short as 1 set, at which time the amplitude of the second x-wave would begin to decrease. In comparison the critical interval of the guinea pig b-wave, using similar flash intensities in darkness, was approximately 5 min. As the interval between paired stimuli in the dark was reduced below 1 set, the amplitude of the second x-wave, but not the second a-wave, was affected so that at an interval of approximately 100 msec the ERG had the appearance of Fig. 4. This picture was quite different when the eye was exposed to background illumination. Exposure to background light had the effect of greatly reducing the critical interval of the x-wave; at a background
J. TIGGES, BARBARAA. Bwo~s
558
AND
M. R. KLEE
FIG. 4.
Upper channel: response to paired stimuli approximately 100 msec apart; stimulus duration was about 10 w; stimulus intensity was approx. 2200 cd/m? Lower channel: stimulus and time marker (SOc/s). Photograph of original record.
light of 350 cd/m2 the second x-wave did not begin to decrease until only 70 msec separated the two stimuli. As the interval was shortened to 15 msec, the amplitude of the second x-wave dropped from 100 to 45 per cent of the value of the first x-wave. At 15 msec there was either a response of 45 per cent, or the second wave was impossible to identify. Between intervals of 70 and 100 msec the second x-wave often appeared somewhat enhanced, growing an additional 10 per cent of its original value. of interest is the fact that the x-wave critical interval can also be reduced during darkness in a manner simulating the effects of background exposure, if the intensity of the paired flashes is sufficiently lowered. In conditions of darkness the u-wave showed no statistically significant changes to high intensity paired flashes of variable interval. Measurement was confounded, however, at short intervals because the slope of the second a-wave was indistinguishable from the descending branch of the first x-wave. During background illumination, and after the stimulus interval was large enough for the second a-wave to be discriminated, it was seen that the latter was clearly larger than the first u-wave; amplitudes were sometimes as much as double the value of the first u-wave. This effect persisted until the stimulus interval reached about 200 msec, when the second a-wave was equal to the first and uninfluenced by further interval increases. A sufficient reduction in the intensity of the paired stimuli during darkness produced an enhancement of the second u-wave very similar to that observed during light adaptation. Spectral sensitivity Animals were tested in the dark with a stimulus of 8 msec duration. Energy was varied at each tested wavelength to produce a constant amplitude x-wave response of 60 PV.
ERG Recordings of a Primate Pure Cone Retina (Tupaio gliu)
559
Movement of the preparation and disturbance of electrode position was a frequent cause of response variability. The reference wavelength, usually chosen at 449 nm, was used to monitor and control for such variability. With 449 nm as the initial stimulus in the experiment, energy was adjusted to produce a 60 FV response amplitude. Before and after all subsequent wavelength stimulations, the response to 449 nm was checked. Jf it had changed, previous measurements had to be discounted and repeated. Since several measures were taken and averaged at each wavelength, animals often did not survive the 4 hr experimental period required to go through all filters. Of 25 experiments, the results of only 12 animais were complete enough to include in the curves presented in Fig. 5.
*
retina
A
82 iodopsin
Wuvelength
fnmJ
FIG. 5. Upper frame: relative spaxral sensitivity of the intact eye (filled circles, average of 12 animals); the density of the lens (open circles, average. of 3 lenses). Lower frame: retinal sensitivity corrected for lens absorption (filled circies)). Triaq+s show the absorption of iodopsin taken from WILD efof. (19%).
The uppermost curve of Fig. 5 (filled circles) shows the log relative spectral sensitivity of the intact Tupaia eye; energy values used in calculating the average points were equated for an equal energy spectrum. Tbe density of the lens is shown in contrast (white circles), and it is evident that the influence of the lens upon sensitivity is negligible from about 450 nm through the red end of the spectrum. In order to determine the relative sensiti~ty of the retina, the relative density of the lens was subtracted from the whole eye measurements, resulting in the curve drawn in the lower half of Fig. 5 (filled circles). One peak of this curve is found at about 552 nm; another in the blue region of the spectrum. Seven Ss actually had individual peaks at 552, three at 570 and two at 540 nm. The function
560
J. TIGGES, BARBARA A. BROOKSAND M. R. KLEE
described by the triangles shows the spectral absorption of iodopsin, isolated from the predominantly cone retina of the chicken by WALD et al. (1955). There was no detectable Purkinje shift in Tupaia. The c- and d-waves Neither the c-wave nor the d-wave was investigated systematically. The c-wave was not observed during light adaptation; given sufficient dark adaptation and the proper length stimulus it usually appeared, but amplitude and latency variability were great when stimulus duration was brief. The c-wave is definitely enhanced as stimulus duration grows longer, and may reach amplitude proportions and durations far exceeding any other component of the ERG (Fig. 1). The d-wave appears as a small positive deflection if stimulus duration is not less than 40 msec. The latency of this component in Tupaia is about 50 msec when fully developed (Fig. 1). The d-wave grows rapidly with increasing duration up to 200 msec, levels off, and is unchanged after 500 msec. This agrees with the results described by TANSLEYet al. (1961 b) for the pure cone ground squirrels whose d-wave was reported to be unaffected by duration increases above 250 msec. DISCUSSION Several characteristics of the Tupaia ERG are consistent with traditionally conceived photopic function. The absolute dark adapted threshold is high compared to the duplex or rod retina, and corresponds to ERG findings on other pure cone species (ARDEN and TANSLEY,1955 a; DODT, 1962). The threshold increase of 1 log unit resulting from light adaptation is small and the log intensity range over which amplitude variations occur is relatively restricted, compared to that of the duplex retina. Flicker fusion rate is high, and corresponds to that of pure cone members of Sciuridae (BORNSCHEIN,1961; BORNSCHEIN and SZEGVARI,1958), and that of the light adapted human eye (HECK, 1957). There is no evidence of a Purkinje shift. The x-wave, which has been elicited and tested in other species under conditions favoring photopic sensitivity (MOTOKAWAand MITA, 1942; ADRIAN, 1945; SCHUBERTand BORNSCHEIN,1952; JONES et al., 1964), is an outstanding feature of the Tupaia ERG; conversely, the slower b-component found in all animals having a duplex or rod retina is completely lacking. The fast recovery of the second response to paired stimuli as compared to the rod dominated eye, may be related to the generally rapid photochemical and electrical processes presumed to underlie cone vision. of particular interest is the spectral sensitivity curve (Fig. 5). All other pure cone mammals which have been tested by means of the ERG (ground and tree squirrels, chipmunk) have curves which, when compared to Tupaia, are far displaced toward the short wavelengths. The peaks of these curves vary between 490 and 535 nm (ARDEN and TANSLEY,1955 a, b; DODT, 1962; DOWLING, 1964; TANSLEY,COPENHAVER and GUNKEL, 1961, VAITER, 1966) while Tupuia shows a peak at about 552 nm, with a second indicated in the blue region. In contrast, the average maxima of human cone sensitivity and the general configuration of the light adapted spectral sensitivity curve (559 nm, WALD and BROWN, 1965) appear similar to that of Tupaiu. The agreement of the Tupaia curve with the iodopsin absorption function derived from the cone dominated chicken strongly suggests that one of its visual pigments is iodopsin. At the blue end of the spectrum the corrected curve for Tupaia (Fig. 5, lower left half) deviates upward from the usual gradual downward slope of the human psychophysical
ERG Recordings of a Primate Pure Cone Retina (Tu@u glis)
561
curve at this region, suggesting heightened blue sensitivity. While behavioral tests indicate that Tupaia has color vision (TIGGFS, 1961, 1963), no psychophysical determinations of relative spectral sensitivity have yet been published. WALD (1964), however, has reported psychophysical functions for a human subject (“R.H.“) showing unusually high sensitivity in the blue region, with normal response over the remainder of the spectrum. WALDjudged on the basis of selected criteria that the abnormal blue sensitivity was most likely due to a greater than average concentration of the blue receptor (or pigment, in his usage). When our lens-corrected average curve for Tupaia is superimposed over “R.H.‘s” fovea1 sensitivity curve, which has been corrected for ocular and macular absorptions, a satisfactory congruence results. This suggests that the concentration of pigments or receptors may be similar in Tupaia to that of the average human, with the exception that a larger amount of the blue receptor is present. It also suggests that the tree shrew eye may be subjected to adaptation experiments, similar to WALD’S on humans, in order to isolate the action spectra of the major component pigments. The evidence cited above underlines the potentiality of Tupaia as a tool for the understanding of cone function in the human retina. Acknowledgement-We would like to thank Prof. Dr. E. Dodt and Drs. C. Baumann, N. Seiler, J. Schulze and E. Rehse, and Mr. H. Feld, J. Kampe, W. Leber and J. Seyl for their kind advice and various technical assistance on this project. REFERENCES
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TIGGES,J. (1963). Untersuchungcn ilber den FarbensEntt von Tupaiugli$ (DKARD, 1820). Z, ~~ovph.Anrhrop. 53. 109-123. V&R, 0. (1966). I.&s ph~trscftev~derte ~~~~~~~~~ und die opt&&e Sensitivitat des Pfatanenhz%mhena (scrii&.s n&#f&r)* P&&o Ref. 6,6f-81* W#&& G. (X%4). The receptors of hr.&man%-&orvision, &%vice*iV.Y*135, ~~~-~~~6. WAL.D,G. and &tow~, P. K. (X965). Human c&or vision and color blindness. In: Sensory Receptors. Cold Spring Harbor Symposium. Vol. 30, 345-359. WALD, G., BRCWN,P. K. and Shnrru, P. H. (1955). Iodopsin. J. gen. Physbl. 38, 623-681, AI8stmt--The gross con&uration
of the pure cone T.vpu& ERG is characterized by wellde&ed @, X-, 0 and &va?!e& The short ktteney, fast rkinS x-wave contmsts to the slower &wave found in aB duptcJr rvKi rod &tXts heretofore test&. The c-wav& not p~vi~s~~ reported for pure cone retinae, can usual& be avok& during dark adaptation. The et%et%ct or &wave appears predictably when stimtuus durations are tong enough. Several featums are eon&tent with the exchtsive presence of cones and their ftmction, includinp: (1) high FFF, (2) lack of Purkinje shit?, (3) high absolute threshold in the dark and only a small increase in the threshold as a function of light adaptation, (4) restricted range of intensity over whii amplitude variations are elicited, and (2) pesk of spectrai sensitivity at 592 mu. The spectrai sensitivity Curve, in contmst to alI the other pure cone mammah, shows canSruetu~ with the fovea! psychophysical curve of the human and with the absorption curve of iodopsin, with the exception of mom marked sensitivity in the blue region. R&tm&-La con6guration &t&ale de 1’ERG de Twaiu (r&no h tines) est caract&is& par des on&s bien d&&ties a, x, c et rf. L’onde x 8 courte latence et mot&% rapide contmste avec i’onde ir phrs tcntn: trouvee &is louses tea &tines doubk!s ou a b%onnets exp&hnentees jus@ici. L%r& c, que Pornn’avait paa jusqu%i d&&e dana ies r&tinesa cones seu& apparait dhabitude pendant ~a~~t~on B l’obscurit& L’eftet ~#(onde d) sembk prOvisible quand les du& de stimulation sum assez tongues. Diversas caract&stiqtm+ sont en accord avec la presence exchtsive de canm et leurs fonctianaI a savoir: (1) frequence de fusion Ilev&e, (2) abserrcet d’effet Purkinje, (3) seuil absolu elev6 dans l’obscurit6 et qui n’augmente que peu en fonction de l’adaptation a la huni&m~(4) intetv&e Otroit d’intenai~ qui pro&tit dcp variations ~arnp~~d~ et (5) maximum de abet speetrahz B 562 nm. La courbe de ~~~~ spectt&_ foci B tous lea atttrei ~rn~f~~ ii pars c&es, concorde avec ie courbe psychophysique fovealc humaine et avec Ia wurbe d’absorption de l’iodopsine, sauf une sensibilite plus marquee dans le bleu, ZBDas ERG der reinen Zapfenretina von Tupah ist charakterisiert durch wohlausttepragte a-, X-, c-, und d-Wellen. Die kutze Gipfelhttenz und der schnelle Anstieg der #-We% stehen im s&a&en C&renm& zu der -merea f+Wehe der bisher ~~~~~ ~~~te~ und S~~~~. Die e-We&, die bii bei keinem anderen S&@er mit einer reinen Zapfenrethut acfunden wurde, kann bei Tuparh im a&emeinen im dunke3adaptierten Zustand au&&t werden. Der o&Efpzkt bzw. die CWeHe wird mit entsprechend langen Reizen au&&t. Fin@ Charakteristika stehen im EinklanS mit dem Be& einer reinen Zapfenretina, wie: (1) die hohe FFF. (2) das Fehlen des PurkimtPhilnomens. (3) die hohe absolute Dunkels&we& und die .&r&e &m&me der SchGelle withren~ ~~~ta~n, (4) tire ~nt~~~~~~~~ in dem verschiedene ~p~~tu~~~~ evoziert werden k&men, (5) das Amax der spektraten ~v~~~t iii bei 552 nm. Tm Getrensatz zu ahen anderen ssittgetie%n mit reiner Zapfenmtina deckt sich die spektrate Sensitivitlitskurve von Tapala mit der mit psychophysischen Methoden gewonnenen Kurve der fovealen Receptoren des Menschen und mit der Iodopsinabsorptionskurvcr. Etwas abweichend bei Tupaia ist die grossere Blauempiindlichkeit.
ERG Recordings of a Primate Pure Cone Retina (Tupaia glis) Peamae -
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563