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The human rod ERG: Correlation with psychophysical responses in light and dark adaptation
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THE HUMAN ROD ERG: CORRELATION WITH PSYCHOPHYSICAL RESPONSES IN LIGHT AND DARK ADAPTATION ANNE 8. FULTON’ and WILLIAM 4. H. RUSHTON’ Institute for Molecular Biophysics, The Florida State University. Tallahassee, Florida 32306. U.S.A. (Receitted 18 August 1977) Abstract-The u- and b-waves over a large range of human scotopic ERG responses in various states of adaptation may be represented compactly by “H 2” curves. This representation allows correlation of b-wave and psychophysical thresholds obtained in the same conditions of adaptation and shows sensitivity and scaling of response size are two separable features of both the a- and b-waves in light and dark adaptation. The relation of these responses to the underlying retinal processes is discussed.
lNTRODUCXlON report compares human scotopic etectroretino~~ (ERG) responses in various conditions of tight and dark adaptation with psychophysical thresholds obtained in the same conditions of adag tation. In light adaptation a wide range of u-wave and b-wave responses from mudpuppy retina could be described in compact mathematical terms (F&on and Rushton, preceding paper) and so related to some underlying features of retinal physiology that have been studied especially with techniques using microelectrodes and pharmacological isolation of receptor responses. We not only wondered if human transretinal potentials in light adaptation followed the same pattern as the mudpuppy responses, but if the electrical response could be related to visual performance as measured psychophysically. Also, electrical and psychophysical responses in dark adaptation conditions, with known kinetics (Alpem, 1971) of rhodopsin regeneration and recovery of visua1 sensitivity, could be investigated in the human experiments. The present
METHODS Equipment The equipment was arranged so that ERG and psychophysical measurements could be made with the same apparatus and without the subject changing position or removing the ERG contact lens electrode. The optical components were arranged on a horizontal surface according to the plan shown in Fig. I. The integrating sphere was the same sized globe used for animal experiments. When the subject was seated for an cxpcrimenf his eye and lids were just inside a 3 cm dia opening at the south pole; he looked through the sphere to fixate on a smalI, dim light at the center of a 3 cm hole at the north pole. A brow rest and a deeply moulded chin rest sculptured from dental wax kept the subject’s head steady. Each beam of light which illuminated the inside of the sphere entered at the north pole and fell in a bright patch
’ Present address: Department of ~h~al~logy. The Children’s Hospital Medical Center. Boston. MA 02115. U.S.A. ’ Present address: Trinity College, Cambridge, England. 793
on the inside of rhe sphere near the eye but out of the view of the subject. Except for the opening at the north pole, the subject saw what looked like a flat, uniformly ilIuminat~ field of infinite extent. The opening at the north pole left a spot around the fovea free of direct stimulation from the Ganzfeld. This favored our desire to exclude cone responses. The Ganzfeld provided uniform and exactly known retinal illumination (Fulton and Rushton, preceding paper) so the precise relationship of light to electrical response could be made (Fry and Bartley, 1935; Boynton and Riggs, 195i: Brindiey, 1956; Asher, 1951). The psychophysical determinations, for comparison with electrical measurements. were also done with the Ganzfeld. In all experiments the subject’s pupil was dilated with Mydriacyl 1% or Cyclogyl 1%. Each of the four subjects who participated in the experiments had, except for minor refractive errors, normal eyes as determined by ophthalmological examination and previous participation in psychophysical
and retinal
densitometry
LiGHT
ADAFTATlON
experiments.
ERG experi~e~rs
Two different electrodes were available for the experiments. One was a custom contact lens electrode obtained through the kindness of Prof. L.A. Riggs; this electrode could be worn without topical anesthetic. The other e&&ode was of Burian-Alien design. Both electrodes gave identical results The voltages from the comeal lead were amplified by a Tektronix 122 pre-amplifier with a time constant of 1 set and a high frequency response (reduced 3 dBf at 10 KC and displayed by the vertical amplifier of a Tektronix 502 A cathode ray oscilloscope. The output of the vertical signal to cathode ray tube wti led dff from’ the’ back of the oscilloscope and fed to the input of a computer of average transients (CAT 1000). Twenty to eighty successive responses to each intensity of stimulating flash were summated and the results recorded on a Mosely 7035B X-Y plotter. The opening and closing of the stimulus shutters and the trig gering of the cathode ray and CAT sweeps were initiated by Tektronix pulse generators. The beam of 52 provided the Htmsec flash stimulus and, the beam of S, the steady background. The stimulating flashes were given in an order increasing by 0.3 or 0.6 log unit steps from the dimmest
794 of flash on steady backgrounds the oscilloscope. plotter.
averaged
were monitored on and recorded on the X-1’
Typical records are displayed in Fig. 2. On records such as these
the size of the u-wave was taken as the initial cornea1 negative slope; the h-wake amplitude was taken as the vertical distance from the maximum negative corneai volta_ee to the peak of the corneal positive wave. The maximum comeal positive waves obtained were 300 IO 400,uV. Ps_dophysics The subject, his eye at the south pole and contact lens electrode in place, determined psychophysical thresholds by adjustment of the neutra1 wedge in the beam of S,. Thresholds for seeing flashes illuminating the entire Ganzfeld could be determined. But in practice it was easier for the subject to find the threshold for seeing a 2” spot 13’ temporal to the fovea; this spot was put on the Ganzfeld by a fiber optics light pipe in the beam of S,. Thresholds for seeing the entire Ganzfeld flashing had the same absolute. dark Fig. t. Plan of optical equipment. The subject‘s eye was just inside the 3 cm opening at the South Pole of the I3 cm diameter integrating sphere. IS. S, = Si were 500 watt projection bulbs; the light beams from S, and Sz were condensed by Lens L. in each beam so the light passed through the opening in stop. St which was straddled by the blades of solenoid shutters. Sh. Lenses L: condensed the beams to pass through an appropriate area of the neutral density wedges. NW. in the beam of S, or through filter holders. FH in the beam of Sz. Lenses L3 further condensed the beams to pass through the 3 cm opening at the North Pole of the integrating sphere. For “pipe Rash” thresholds. the beam S, was masked to prevent any light from entering the integrating sphere except that which passed through the fiber optics light pipe. LP. to put on a 2’ test tlash. To make threshold settings for seeing flashes that illuminated the entire Ganzfeld, the fiber optics pipe and its mask were removed from the beam of S, and the light was allowed to pass through lens L, into the integrating sphere. Steady backgrounds were from source S,. a 6 V tungsten lamp. which was deflected into the integrating sphere by mirror. M, after passing through a condensing lens, L,, and fitter holder, F. which contained a deep red filter (Jena-Schott RG-2) and appropriate neutral density filters; the mirror was placed at a level such that the subject did not look directly into it. The lights from each beam fell near the subject’s eye but out of direct view of the subject. A micro lamp provided fixation point. FP.
flash to produce an ERG detectable on a single sweep of she osciiloscope. The intervals between stimulating flashes were 2.5. 10 or 20 set for the brightest flashes; the interval was chosen on the basis of auxiliary experiments such that sufficient time was allowed between flashes so that subsequent responses were not depressed. The backgrounds were dark or a red Ganzfeld generated by a Jena-Schott RG-2 filter passing wavetengths greater than 630 nm in the beam of S,; the intensity of the red light was attenuated by neutral density filters in the beam of Sj. The range of iiiuminances used in the experiments are known to cause little or no bleaching of rhodopsin (Alpern. 1971). During the course of an experiment. backgrounds were changed from dark to increasingly bright red. The ERG responses to various intensities
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Fig. 2. Examples of ERG records: Records of responses such as these were made with the X-Y plotter. All flashes were white and of 50msec duration. On records such as these the a-wave slope and the b-wave trough to peak amplitude were measured. a. Responses to flashes on a dark background. 40 responses were averaged in this example. Numbers at the left of each trace indicate the strength of the stimulating flash. b. Responses to flashes on a red background which raised the absolute psychophysical threshold 1.0 log units. 20 responses were averaged in this example. Numbers at the left of each trace indicate the strength of the stimulating Rash.
795
The human rod ERG
adapted threshold, the same eigengrau and the same slope of Weber-Fechner line as thresholds for seeing the 2” &pipe flash”; therefore, thresholds for seeing the “pipe flash” were used to determine the subjective state of the sensitivity of the eye. Three types of psychophysical experiments were done. (1) The first of these was to determine the subjective change in sensitivity caused by the steady, red Ganzfeld backgrounds. The increment thresholds for seeing the 2” “pipe flash” flashed once a second. on various steady red backgrounds from dark to about 3 log scotopic trolands were determined by the subject setting the wedge in the beam of Sr. Steady lights of these intensities have been found by retinal densitometry to bleach little or no rhodopsin (Alpem. 1971). (2) Thresholds for seeing the “pipe flash” before and quickly after a series of the full field flashes, as used to stimulate ERG responses, were made by the subject during ERG runs. Thresholds of this sort indicated when the subject had become fully dark adapted at the start of the experiment, and also allowed evaluation of the state of sensitivity that the ERG stimulus itself left the eye in during an experiment. (3) The threshold for seeing the “pipe flash” interpolated in the midst of full field flashes assessed the effect of the ERG stimulus on visual function in the conditions under which individual ERG points on the curves given in Fig. 3 were obtained. The interpolated thresholds were determined as follows. Four full field flashes of the type used to elicit ERG responses were delivered at predetermined intervals; at the fifth count the full field flash was blocked and the “pipe flash” put on; the subject indicated that the pipe flash could be seen or could not be seen; the neutral wedge was adjusted by the experimenter and the ritual of four full field flashes and one “pipe flash” in the midst of full field flashes repeated until threshold for the “pipe flash” could be determined. Interpolated threshold settings were made for several levels of brightness of full field flashes.
DARK
ADAPTATION
ERG experiments
The contact lens electrode was put on the subject’s eye and the subject positioned at the integrating sphere. A Strobamatic flash gun, its face covered with a tracing paper diffuser, was used to deliver a series of flashes via the integrating sphere. The series of flashes was calculated to bleach 99% of rhodopsin; this was verified by retinal densitometry. Beginning immediately after the bleach, a range of 50 msec white test flashes, with interstimulus interval of two seconds, were put on at various times after the bleach. The intensity of the test flashes was varied by neutral density filters so that data for a stimulusresponse relationship was obtained for each of several times in the dark after the bleach. The u-wave slope and the b-wave trough to peak amplitude were measured. Ten or twenty ERG responses to these flashes were averaged and recorded on the X-Y plotter. In practice it took about 1 to 3 min to collect a set of responses from which the stimulus-response
relationships could be derived for each of several times after the bleach. Psychophysics The subject, with contact lens electrode in place. determined the threshold for seeing the ‘pipe flash” during intervals after the bleach when ERG data was not being collected. Also. in the midst of collecting ERG responses, psychophysical thresholds were determined from time to time to check the visual sensitivity. RESULTS I_ight adupration. The pattern of human scotopic c- and b-wave responses in tight adaptation is shown in Fig. 3. The pattern is the same as found for mudpuppy eye cup responses in similar experimental conditions of brief white test flashes on steady. red Ganzfeld backgrounds. The log a-wave slope as a function of log Stimufating flash follows an Hz curve for each steady background. The results of a single run. plotted as Hz curves (Fig. 3a) have apex points of the HZ curves on a straight line, slope - 1. The log stimulating Rash. which causes semisaturation of the responscl when the background is dark, is called log cru. The value of log un for many experiments ranged from 2.0 to 2.8 log td set with the average being 2.3 log td sec. This is in the range of semisaturation for receptors found by psychophysical methods (Aguilar and Stiles, 1954: Alpern, Rushton and Torii, 1970b; Sakitt, 1976). The log b-wave amplitude as a function of log stimulating flash may also be fit by an HZ curve. A separate H2 curve is obtained for each steady background. As the background is made brighter. the apex points move down and to the right (Fig. 3b). The apex points may be fit with a left facing Hz curve: the abscissa of the apex point of the left facing HZ curve is at log uu, that is the same log I value that caused semisaturation of the a-waves with the background dark. The absolute sensitivity of the b-wave is greater than the a-wave as is apparent from inspection of the apices of the a- and b-wave curves obtained with background dark. In Fig. 3 the abscissa of the apex point of the H2 curve for b-waves with background dark is about 2.5 log td set less than log cru for a-waves. The range of difference in sensitivity for a- and b-waves for these experiments was 2.2 to 3.0 log td sec. The a-wave sensitivity is much less affected by the backgrounds than the b-wave, not only as is apparent from comparison of the absolute sensitivities of each in the condition of a dark background, but the range of sensitivity change caused by steady. red backgrounds is less for the a-waves than the b-waves. This can be seen by examination of Fig 3; the horizontal change (indicating sensitivity change) in apex points for a-waves is less than the horizontal change for b-wave apices. On the other hand the range of scaling (vertical movement) caused by the backgrounds is the same for a-waves and b-waves. Psychophysical thresholds were raised by the steady backgrounds as is shown in Fig. 4. (See also Methods). For comparison with the psychophysical thresholds. a- and b-wave thresholds were determined as follows. The threshold in the dark was taken as
796
ANNE B. FCLTON and WILLIAM A. H. RLSHTON
Log
log td set
flash,
(b)
log flash,
log td set
Fig. 3. Plots of log response vs log stimulus energy for a single ERG run of a light adaptation experimenr. a. A-waves. The log n-wave dope is plotted against the log stimulating flash which produced the response. A template was used to draw an H, curve through the points obtained for each steady background; the backgrounds in log scotopic trolands are dark,: + -0.4 log td; x +0.2; l 0.8; 0 1.4: Cl 2.0: l 2.6: A 2.9: A 3.2. The apex point for each HZ curve is circled. A straight line with slope -1.0 has been drawn through the apex points. For clarity. alternate HZ curves have only the unique apex point displayed. b. B-waves. The log b-wave amplitude is plotted against the log stimulating Rash which produced the response. An H2 curve has been drawn through the points obtained for each steady background. The circled apex points lie on a left facing Hz curve. The symbols indicate the same backgrounds as in Fig. 3a. The thresholds for seeing the pipe flashes (see Methods) interpolated in the midst of the full field flashes to elicit the ERG responses are plotted by the inverted black triangles (V). The visual threshold was put up very little when there was a five second interval between Bashes. The line drawn through the inverted black triangles has a slope of less than +O.l.
a point on the H2 curve that most closely approaches the + 1 asymptote. which as follows from the mathematical properties of the H1 curve. is about two log units on the intensity axis to the left of the apex point. As the backgrounds were made brighter, the thresholds could be determined by inspection of the horizontal change in apex points caused by an increment of background. Another less exact method involves extending back the ascending arm of the experimental HZ curve until it approaches the + 1 asymptote. For this method. or if the experimental points themselves
produce an HZ curve with ascending arm of sufficient Iength to approach closely the + 1 asymptote, a horizontal tine drawn through the lower end of a family of Hz curves is equivalent to choosing a criterion voltage: the horizontal shift, or change in log 1. then approximates fairly closely the change in threshold. Log a-wave thresholds, plotted as a function of log backgrounds, followed a Weber-Fechner like curve, the linear portion having a to.4 slope as did the a-wave “increment threshold curve” obtained from the mudpuppy eye (Fulton and Rushton, preceding
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The human rod ERG
physical threshold. In the dark, the a-wave is affected by scaling processes (i.e. the curves move vertically),
log Bockqraund,
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fig. 4. Increment thresholds. The black, filled symbols plot the log b-wave thresholds as a function of log steady red background. Each symbol is for a separate ERG run. Fe results of s separate runs are displayed here). The method of determination of the b-wave thresholds is given in the text. The white, open symbols plot the psychophysical thresholds as given in the text. The white, open syt!tbols plot the psychophysical thresholds in log units above absolute threshold (vertical scale on the right) for seeing the 2’ pipe flash (see Methods) with retina 14” temporal to the fovea. These psychophysics thresholds were determined during the ERG runs with corresponding Symbols. The x’s plot the b-wave thresholds determined from the results given in Fig. 3B. The i’s plot the ~ychophysi~~ thresholds for seeing the pipe flash without the ERG contact lens in place.
paper) and typical x0 monochro~ts (Aipem, personal communication). The log b-wave increment thresholds follow a Weber-Fechner type curve (Fig. 4). The linear portion of the curve has a slope of + 1.0. Log $, the eigengrau is the background where the Fechner 45” line produced backwards meets the horizontal level of the absolute threshold. In these experiments this background ranged from - 15 to - 1.0 log td, somewhat greater than that obtained in the best psychophysical conditions. Saturation of the ERG thresholds occurred at about 2.5 log td which agrees with that found for the psychophysical thresholds found in these experiments and in other psychophysical experiments (Aguilar and Stiles, 1954; Alpern, Rushton and Torii, 1970; Sakitt, 1976). The ERG flashes themselves did not put up the visual threshold much. This was checked with interpolated flashes (see Methods). The multiple flashes at the repetition rates used in the experiments put the visual threshold up very little if at all as determined by. intermittent thresholds obtained during the ERG runs. Dark aaiaptation. After the 99% bleach ERG responses were collected as quickly as possible within the time intervals indicated in Fig. S(a). Curves (Fig. Sa) which plot log a-wave slope as a function of log stimulating Bash show the a-wave increases with time after the bleach. An Hz curve has been drawn through the experimental points obtained for each of the indicated intervals. The apices of the H2 curves rise along a vertical line passing through about 2.3 log td set, the flash energy found to semisaturate the a-wave in the dark and to cause semisaturation of the psycho-
but inspection of the apex points reveals no change in sensitivity (i.e. no horizontaf movement) during the time rhodopsin is known to be increasing. Figure 5(b) displays the behavior of the b-wave response to brief flashes as the eye recovers from the bleach. The log b-wave amplitude is plotted as a function of log stimulating flash at the indicated times after the bleach. An Hz curve is drawn through the experimental points obtained at each of the indicated times after the bleach. The apices of the Hz curves move up and to the left as the eye recovers in the dark. The psychophysical thresholds for seeing the 2” “pipe flash” (see Methods) during dark adaptation were determined by the ERG subjects with electrode in place. The psychophysical thresholds as a function of time in the dark are fit fairly closely by two exponential curves with a rod-cone break. The corresponding b-wave thresholds, chosen where the Hz curve approached the + 1.0 asymptote (see above in Light Adaptation Results), follow about the same course as the psychophysical thresholds. The curve drawn through the experimental points is an exponential with time constant 4OOsec, the same as found for the recovery of rhodopsin (Alpern, 1971). The apparent increase in a-wave sensitivity with time in the dark is also plotted in Fig. 6; these thresholds were obtained by the usual criterion voltage method (level indicated in Fig. 5a) and illustrate the error that may arise from the criterion method; the a-wave had no change in sensitivity (i.e. no horizontal change of apex points; see Fig. 5a). Sensitivity processing is quite different at the a- and b-wave levels, but scaling of signal size has similar characteristics for the a- and b-waves. In Fig. 5 the range of scaling (vertical movement) is the same for a-waves and b-waves. In addition the time course of scaling of a-waves and b-waves is the same; Fig. 7 shows the ordinates of the a-wave apex points are nearly coincident with the ordinates of the b-wave apex points as the ERG recovers in the dark after the bleach. In human dark adaptation scaling at the a-wave level is the same as at the b-wave level just as it was in light adaptation of the human and mudpuppy retina (Fulton and Rushton, Rod ERG of Mudpuppy). DlSCUSSlON The Hz log-log plots have allowed compact representation of a wide range of human ERG responses and permitted comparison of the electrical and psychophysical thresholds. In light adaptation the psychophysical and b-wave thresholds have the Same eigengrau, slope of increment threshold curve and level of saturation; in dark adaptation after a full bleach b-wave and psychophysical thresholds follow the same exponential recovery over the same range. These findings suggest the b-wave is a good objective measure of visual sensitivity in the conditions studied if, in choice of electrical threshold, the precautions mentioned above in the presentation of the results are taken. Our results contrast earlier correlative studies of electroretinal and psychophysical responses
798
9. Fcrro?; and WILLIAstA. H.
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log td set
flash,
(b)
log
flesh
,
log
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Fig. 5. Dark adaptation after bleaching 99Oi of rhodopsin. The log stimulating energy is plotted against log response obtained at various times in the dark after the bleach. The symbols indicate the time after the bleach: l IO-13 minutes; n 13-16 minutes: ‘I 15-18 minutes; o 18-21 minutes; U 23-26 minutes: A Z-28 minutes; + 35-40 minutes. The data displayed were obtained in two separate runs on the same subject on the same day. (a) The points plot log a-wave slope against log stimulating flash at the various times after the bleach. A template was used to draw HL curves through the experimental points. For clarity the 15-18 minute curve (‘I) and the 25-28min curve (A) are represented by apex points only. The apex point for each HI curve is indicated by the circled symbols. The apex points move about vertically upward as the response recovers from the bleach. As the eye recovers sensitivity in the dark. the curves appear to move to the left if a low voltage criterion response is considered. (b) The points plot log b-wave amplitude against log stimulating flash at the various times after the bleach. A template was used to draw H, curves through the experimental points and locate the apex points obtained at the various times after the bleach. The apex poinrs move up and to the left as the eye recovers sensitivity in the dark.
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Fig. 6. Log thresholds during dark adaptation following a 999,; bleach of rhodopsin. The X’S plot the log psychophysical threshold (vertical scale on the left) for seeing the 2’ pipe flash. The other symbols plot the ERG log thresholds. The upper curve obtained during the rod portion of dark adaptation. displays the log a-wave “threshold” as determined by noting the log td set of flash (vertical scale on the right) required to give the log criterion responseindicated in Fig. 5(a). The log b-wave thresholds were determined as described in the text. The curve drawn through the rod segment of the results is an exponential with time constant 400 sec. The log b-wave thresholds follow fairly closely the log psychophysical thresholds in that they follow the same exponential and have the same 2 log unit range in the conditions of these experiments.
in light and dark adaptation (e.g. Johnson. 1949; Riggs and Johnson, 1949: Armington, 1952; Aipern and Faris, 1952; Biersdorf. 1972) which do not report exact parallels of the subjective and objective measures of adaptation. We suppose our methods which include signal averaging techniques, fixed and dilated pupil, a Ganzfeld produced by an integrating sphere fit fairly snugly about the eye, and extreme care in preventing stray fight of the apparatus from entering the eye are responsible for producing more exact relation of stimulus and mass electrical response and, thus, closer correlation of psychophysical and electrical scotopic responses. In light and dark adaptation. the scaling (vertical movement of H, apices) has the same range for a-
Minutes
in
dark
Fig. 7. Ordinate of apex points as a function of time. The black circles plot the ordinate of the apices (log u-wave slope scale on the left) of the a-wave curves (from Fig. 5a) as a function of time after the bleach. The white circles plot the ordinate (log b-wave amplitude scale on the right) of the b-wave curves (from Fig. 5b) vs time. The curve drawn through the points is an exponential with time constant 400 sec.
199
The human rod ERG and &-waves and in dark adaptation
foflows the same time course (Fig. 7). This regulation of maximum possible response size obtainable in each of the various conditions of adaptation examined appears to be set at the level of the a-wave and passeddirectly to the b-generators. Sensitivity adjustment. however. is quite different for a- and b-waves. In light and dark adaptation, the a-wave absolute sensitivity and sensitivity change (horizontal movement of Hz apices) caused by backgrounds is less than that measured by the b-wave. Previous investigations have pointed out the inadequacies of receptor desensitization in explaining adaptation (Rushton and Westheimer, 1962). If the sensitivity component of the u-wave. taken as a sign of receptor activity [perhaps more exactly receptor outer segment activity (Brown and Watanabe. 196211 becomes an input to the b-generators. some modi~cations [i.e. in light adaptation an increase of the Weber-Fechner slope from $0.4 to + 1.0; in dark adaptation development, proximal to the u-wave generators. of a sensitivity change parallel to the regeneration of rhodopsin] and amplification [by a factor of about 1001 would have to occur between the a-wave and b-wave. Although adaptive mechanisms in receptors may not influence more proximal retinal adaptation to dim backgrounds (Green et al., 1975), the growing body of evidence that indicates receptors usually do govern adaptation of the entire retina (Dowling and Ripps, 1977) along with the findings that the a-wave can carry information to more proximal retina (Penn and Hagins, 1969; Ernst and Arden, 1972) would be compatible with the hypothesis that the a-wave can be the input to the b-wave which is an indicator of visual performance. Our preIiminary work on a mathematical model with equations based on the experimental findings reported in this paper and the preceding paper (Fulton and Rushton, previous paper) also support the hypothesis. Another link of a- to b-wave is observed in the
light adaptation results of mudpuppy and human retina. The semisaturation level of flash, log Q~, for the a-waves with background dark is the same as the apex point of the left facing H, curve which is the locus of b-wave apices (Fig. 3). Also, b-wave ~nsitivity saturates at backgrounds of about 2.5 log td (Fig. 4). but scaling of the b-wave amplitude continues. At backgrounds of 2.5 log td or yeater, the a-wave Hz apices remain on a straight hne, stope - 1, as the processes of scaling and sensitivity adjustment continue. Although analysis of the stimulus-response data by the Hz curves shows sensitivity and scaling (or, in other words, adjustment of membrane potential) of response to be separable features, as did microelectrode studies of single receptors (Kleinschmidt and Dowling, 1975) and biochemical studies of outer segments (Brodie and Bownds, 1976; Woodruff et al., 1977). it is apparent that scaling and sensitivity of the u- and b-wave in retinal adaptation are closely associated; possibty this association can he more fully appreciated by development of a mathematical mode1 which encompasses the ex~~mentai results of this and the preceding paper (Fulton and Rushton, previous paper). The similar behavior of mudpuppy and human ERG’S in the presence of backgrounds suggests that
there are features of signal processing common to vertebrate retinas. The microelectrode and biochemical studies conducted on animal retinas have revealed some aspects of the physiology underlying the transretinal potential which can, with care, be correlated with psychophysical function. As animal studies continue, the behavior of the transretinal potential, even accessible clinically, may be understood more fully in terms of basic retinal processes. REFERENCES
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ANYE B. FULTON and WILLIAM A. H. RUSHTOY
Riggs L. A. and Johnson E. P. (1949) Electrical responses of the human retina. J. Exp. Psych. 39. 115-424. Rushton W. A. H. and Westheimer G. (1962) The Effect Upon the Rod Threshold of Bleaching Neighboring Rods. J. Physiol.. Land. 164. 318-329. Sakitt B. (1976). Psychophysical correlates of photoreceptar activity. Vision Res. 16. 129-140.
Westheimer, G. (I%&. Bleached rhodopsin and retinal interaction. J. Physiol. Land. 195. 97-105. Woodruff M. L.. Bownds D.. Green S. H.. Morrisey J. L. and Shediovsky A. (I 977) Guanosine 3’s’-cyclic monophosphate and the in citro physiology of frog photoreceptor membranes. J. Gen. Pftysiol. 69, 667-79.
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THE HUMAN ROD ERG: CORRELATION WITH PSYCHOPHYSICAL RESPONSES IN LIGHT AND DARK ADAPTATION ANNE 8. FULTON’ and WILLIAM 4. H. RUSHTON’ Institute for Molecular Biophysics, The Florida State University. Tallahassee, Florida 32306. U.S.A. (Receitted 18 August 1977) Abstract-The u- and b-waves over a large range of human scotopic ERG responses in various states of adaptation may be represented compactly by “H 2” curves. This representation allows correlation of b-wave and psychophysical thresholds obtained in the same conditions of adaptation and shows sensitivity and scaling of response size are two separable features of both the a- and b-waves in light and dark adaptation. The relation of these responses to the underlying retinal processes is discussed.
lNTRODUCXlON report compares human scotopic etectroretino~~ (ERG) responses in various conditions of tight and dark adaptation with psychophysical thresholds obtained in the same conditions of adag tation. In light adaptation a wide range of u-wave and b-wave responses from mudpuppy retina could be described in compact mathematical terms (F&on and Rushton, preceding paper) and so related to some underlying features of retinal physiology that have been studied especially with techniques using microelectrodes and pharmacological isolation of receptor responses. We not only wondered if human transretinal potentials in light adaptation followed the same pattern as the mudpuppy responses, but if the electrical response could be related to visual performance as measured psychophysically. Also, electrical and psychophysical responses in dark adaptation conditions, with known kinetics (Alpem, 1971) of rhodopsin regeneration and recovery of visua1 sensitivity, could be investigated in the human experiments. The present
METHODS Equipment The equipment was arranged so that ERG and psychophysical measurements could be made with the same apparatus and without the subject changing position or removing the ERG contact lens electrode. The optical components were arranged on a horizontal surface according to the plan shown in Fig. I. The integrating sphere was the same sized globe used for animal experiments. When the subject was seated for an cxpcrimenf his eye and lids were just inside a 3 cm dia opening at the south pole; he looked through the sphere to fixate on a smalI, dim light at the center of a 3 cm hole at the north pole. A brow rest and a deeply moulded chin rest sculptured from dental wax kept the subject’s head steady. Each beam of light which illuminated the inside of the sphere entered at the north pole and fell in a bright patch
’ Present address: Department of ~h~al~logy. The Children’s Hospital Medical Center. Boston. MA 02115. U.S.A. ’ Present address: Trinity College, Cambridge, England. 793
on the inside of rhe sphere near the eye but out of the view of the subject. Except for the opening at the north pole, the subject saw what looked like a flat, uniformly ilIuminat~ field of infinite extent. The opening at the north pole left a spot around the fovea free of direct stimulation from the Ganzfeld. This favored our desire to exclude cone responses. The Ganzfeld provided uniform and exactly known retinal illumination (Fulton and Rushton, preceding paper) so the precise relationship of light to electrical response could be made (Fry and Bartley, 1935; Boynton and Riggs, 195i: Brindiey, 1956; Asher, 1951). The psychophysical determinations, for comparison with electrical measurements. were also done with the Ganzfeld. In all experiments the subject’s pupil was dilated with Mydriacyl 1% or Cyclogyl 1%. Each of the four subjects who participated in the experiments had, except for minor refractive errors, normal eyes as determined by ophthalmological examination and previous participation in psychophysical
and retinal
densitometry
LiGHT
ADAFTATlON
experiments.
ERG experi~e~rs
Two different electrodes were available for the experiments. One was a custom contact lens electrode obtained through the kindness of Prof. L.A. Riggs; this electrode could be worn without topical anesthetic. The other e&&ode was of Burian-Alien design. Both electrodes gave identical results The voltages from the comeal lead were amplified by a Tektronix 122 pre-amplifier with a time constant of 1 set and a high frequency response (reduced 3 dBf at 10 KC and displayed by the vertical amplifier of a Tektronix 502 A cathode ray oscilloscope. The output of the vertical signal to cathode ray tube wti led dff from’ the’ back of the oscilloscope and fed to the input of a computer of average transients (CAT 1000). Twenty to eighty successive responses to each intensity of stimulating flash were summated and the results recorded on a Mosely 7035B X-Y plotter. The opening and closing of the stimulus shutters and the trig gering of the cathode ray and CAT sweeps were initiated by Tektronix pulse generators. The beam of 52 provided the Htmsec flash stimulus and, the beam of S, the steady background. The stimulating flashes were given in an order increasing by 0.3 or 0.6 log unit steps from the dimmest
794 of flash on steady backgrounds the oscilloscope. plotter.
averaged
were monitored on and recorded on the X-1’
Typical records are displayed in Fig. 2. On records such as these
the size of the u-wave was taken as the initial cornea1 negative slope; the h-wake amplitude was taken as the vertical distance from the maximum negative corneai volta_ee to the peak of the corneal positive wave. The maximum comeal positive waves obtained were 300 IO 400,uV. Ps_dophysics The subject, his eye at the south pole and contact lens electrode in place, determined psychophysical thresholds by adjustment of the neutra1 wedge in the beam of S,. Thresholds for seeing flashes illuminating the entire Ganzfeld could be determined. But in practice it was easier for the subject to find the threshold for seeing a 2” spot 13’ temporal to the fovea; this spot was put on the Ganzfeld by a fiber optics light pipe in the beam of S,. Thresholds for seeing the entire Ganzfeld flashing had the same absolute. dark Fig. t. Plan of optical equipment. The subject‘s eye was just inside the 3 cm opening at the South Pole of the I3 cm diameter integrating sphere. IS. S, = Si were 500 watt projection bulbs; the light beams from S, and Sz were condensed by Lens L. in each beam so the light passed through the opening in stop. St which was straddled by the blades of solenoid shutters. Sh. Lenses L: condensed the beams to pass through an appropriate area of the neutral density wedges. NW. in the beam of S, or through filter holders. FH in the beam of Sz. Lenses L3 further condensed the beams to pass through the 3 cm opening at the North Pole of the integrating sphere. For “pipe Rash” thresholds. the beam S, was masked to prevent any light from entering the integrating sphere except that which passed through the fiber optics light pipe. LP. to put on a 2’ test tlash. To make threshold settings for seeing flashes that illuminated the entire Ganzfeld, the fiber optics pipe and its mask were removed from the beam of S, and the light was allowed to pass through lens L, into the integrating sphere. Steady backgrounds were from source S,. a 6 V tungsten lamp. which was deflected into the integrating sphere by mirror. M, after passing through a condensing lens, L,, and fitter holder, F. which contained a deep red filter (Jena-Schott RG-2) and appropriate neutral density filters; the mirror was placed at a level such that the subject did not look directly into it. The lights from each beam fell near the subject’s eye but out of direct view of the subject. A micro lamp provided fixation point. FP.
flash to produce an ERG detectable on a single sweep of she osciiloscope. The intervals between stimulating flashes were 2.5. 10 or 20 set for the brightest flashes; the interval was chosen on the basis of auxiliary experiments such that sufficient time was allowed between flashes so that subsequent responses were not depressed. The backgrounds were dark or a red Ganzfeld generated by a Jena-Schott RG-2 filter passing wavetengths greater than 630 nm in the beam of S,; the intensity of the red light was attenuated by neutral density filters in the beam of Sj. The range of iiiuminances used in the experiments are known to cause little or no bleaching of rhodopsin (Alpern. 1971). During the course of an experiment. backgrounds were changed from dark to increasingly bright red. The ERG responses to various intensities
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Fig. 2. Examples of ERG records: Records of responses such as these were made with the X-Y plotter. All flashes were white and of 50msec duration. On records such as these the a-wave slope and the b-wave trough to peak amplitude were measured. a. Responses to flashes on a dark background. 40 responses were averaged in this example. Numbers at the left of each trace indicate the strength of the stimulating flash. b. Responses to flashes on a red background which raised the absolute psychophysical threshold 1.0 log units. 20 responses were averaged in this example. Numbers at the left of each trace indicate the strength of the stimulating Rash.
795
The human rod ERG
adapted threshold, the same eigengrau and the same slope of Weber-Fechner line as thresholds for seeing the 2” &pipe flash”; therefore, thresholds for seeing the “pipe flash” were used to determine the subjective state of the sensitivity of the eye. Three types of psychophysical experiments were done. (1) The first of these was to determine the subjective change in sensitivity caused by the steady, red Ganzfeld backgrounds. The increment thresholds for seeing the 2” “pipe flash” flashed once a second. on various steady red backgrounds from dark to about 3 log scotopic trolands were determined by the subject setting the wedge in the beam of Sr. Steady lights of these intensities have been found by retinal densitometry to bleach little or no rhodopsin (Alpem. 1971). (2) Thresholds for seeing the “pipe flash” before and quickly after a series of the full field flashes, as used to stimulate ERG responses, were made by the subject during ERG runs. Thresholds of this sort indicated when the subject had become fully dark adapted at the start of the experiment, and also allowed evaluation of the state of sensitivity that the ERG stimulus itself left the eye in during an experiment. (3) The threshold for seeing the “pipe flash” interpolated in the midst of full field flashes assessed the effect of the ERG stimulus on visual function in the conditions under which individual ERG points on the curves given in Fig. 3 were obtained. The interpolated thresholds were determined as follows. Four full field flashes of the type used to elicit ERG responses were delivered at predetermined intervals; at the fifth count the full field flash was blocked and the “pipe flash” put on; the subject indicated that the pipe flash could be seen or could not be seen; the neutral wedge was adjusted by the experimenter and the ritual of four full field flashes and one “pipe flash” in the midst of full field flashes repeated until threshold for the “pipe flash” could be determined. Interpolated threshold settings were made for several levels of brightness of full field flashes.
DARK
ADAPTATION
ERG experiments
The contact lens electrode was put on the subject’s eye and the subject positioned at the integrating sphere. A Strobamatic flash gun, its face covered with a tracing paper diffuser, was used to deliver a series of flashes via the integrating sphere. The series of flashes was calculated to bleach 99% of rhodopsin; this was verified by retinal densitometry. Beginning immediately after the bleach, a range of 50 msec white test flashes, with interstimulus interval of two seconds, were put on at various times after the bleach. The intensity of the test flashes was varied by neutral density filters so that data for a stimulusresponse relationship was obtained for each of several times in the dark after the bleach. The u-wave slope and the b-wave trough to peak amplitude were measured. Ten or twenty ERG responses to these flashes were averaged and recorded on the X-Y plotter. In practice it took about 1 to 3 min to collect a set of responses from which the stimulus-response
relationships could be derived for each of several times after the bleach. Psychophysics The subject, with contact lens electrode in place. determined the threshold for seeing the ‘pipe flash” during intervals after the bleach when ERG data was not being collected. Also. in the midst of collecting ERG responses, psychophysical thresholds were determined from time to time to check the visual sensitivity. RESULTS I_ight adupration. The pattern of human scotopic c- and b-wave responses in tight adaptation is shown in Fig. 3. The pattern is the same as found for mudpuppy eye cup responses in similar experimental conditions of brief white test flashes on steady. red Ganzfeld backgrounds. The log a-wave slope as a function of log Stimufating flash follows an Hz curve for each steady background. The results of a single run. plotted as Hz curves (Fig. 3a) have apex points of the HZ curves on a straight line, slope - 1. The log stimulating Rash. which causes semisaturation of the responscl when the background is dark, is called log cru. The value of log un for many experiments ranged from 2.0 to 2.8 log td set with the average being 2.3 log td sec. This is in the range of semisaturation for receptors found by psychophysical methods (Aguilar and Stiles, 1954: Alpern, Rushton and Torii, 1970b; Sakitt, 1976). The log b-wave amplitude as a function of log stimulating flash may also be fit by an HZ curve. A separate H2 curve is obtained for each steady background. As the background is made brighter. the apex points move down and to the right (Fig. 3b). The apex points may be fit with a left facing Hz curve: the abscissa of the apex point of the left facing HZ curve is at log uu, that is the same log I value that caused semisaturation of the a-waves with the background dark. The absolute sensitivity of the b-wave is greater than the a-wave as is apparent from inspection of the apices of the a- and b-wave curves obtained with background dark. In Fig. 3 the abscissa of the apex point of the H2 curve for b-waves with background dark is about 2.5 log td set less than log cru for a-waves. The range of difference in sensitivity for a- and b-waves for these experiments was 2.2 to 3.0 log td sec. The a-wave sensitivity is much less affected by the backgrounds than the b-wave, not only as is apparent from comparison of the absolute sensitivities of each in the condition of a dark background, but the range of sensitivity change caused by steady. red backgrounds is less for the a-waves than the b-waves. This can be seen by examination of Fig 3; the horizontal change (indicating sensitivity change) in apex points for a-waves is less than the horizontal change for b-wave apices. On the other hand the range of scaling (vertical movement) caused by the backgrounds is the same for a-waves and b-waves. Psychophysical thresholds were raised by the steady backgrounds as is shown in Fig. 4. (See also Methods). For comparison with the psychophysical thresholds. a- and b-wave thresholds were determined as follows. The threshold in the dark was taken as
796
ANNE B. FCLTON and WILLIAM A. H. RLSHTON
Log
log td set
flash,
(b)
log flash,
log td set
Fig. 3. Plots of log response vs log stimulus energy for a single ERG run of a light adaptation experimenr. a. A-waves. The log n-wave dope is plotted against the log stimulating flash which produced the response. A template was used to draw an H, curve through the points obtained for each steady background; the backgrounds in log scotopic trolands are dark,: + -0.4 log td; x +0.2; l 0.8; 0 1.4: Cl 2.0: l 2.6: A 2.9: A 3.2. The apex point for each HZ curve is circled. A straight line with slope -1.0 has been drawn through the apex points. For clarity. alternate HZ curves have only the unique apex point displayed. b. B-waves. The log b-wave amplitude is plotted against the log stimulating Rash which produced the response. An H2 curve has been drawn through the points obtained for each steady background. The circled apex points lie on a left facing Hz curve. The symbols indicate the same backgrounds as in Fig. 3a. The thresholds for seeing the pipe flashes (see Methods) interpolated in the midst of the full field flashes to elicit the ERG responses are plotted by the inverted black triangles (V). The visual threshold was put up very little when there was a five second interval between Bashes. The line drawn through the inverted black triangles has a slope of less than +O.l.
a point on the H2 curve that most closely approaches the + 1 asymptote. which as follows from the mathematical properties of the H1 curve. is about two log units on the intensity axis to the left of the apex point. As the backgrounds were made brighter, the thresholds could be determined by inspection of the horizontal change in apex points caused by an increment of background. Another less exact method involves extending back the ascending arm of the experimental HZ curve until it approaches the + 1 asymptote. For this method. or if the experimental points themselves
produce an HZ curve with ascending arm of sufficient Iength to approach closely the + 1 asymptote, a horizontal tine drawn through the lower end of a family of Hz curves is equivalent to choosing a criterion voltage: the horizontal shift, or change in log 1. then approximates fairly closely the change in threshold. Log a-wave thresholds, plotted as a function of log backgrounds, followed a Weber-Fechner like curve, the linear portion having a to.4 slope as did the a-wave “increment threshold curve” obtained from the mudpuppy eye (Fulton and Rushton, preceding
797
The human rod ERG
physical threshold. In the dark, the a-wave is affected by scaling processes (i.e. the curves move vertically),
log Bockqraund,
lop scotoDictd
fig. 4. Increment thresholds. The black, filled symbols plot the log b-wave thresholds as a function of log steady red background. Each symbol is for a separate ERG run. Fe results of s separate runs are displayed here). The method of determination of the b-wave thresholds is given in the text. The white, open symbols plot the psychophysical thresholds as given in the text. The white, open syt!tbols plot the psychophysical thresholds in log units above absolute threshold (vertical scale on the right) for seeing the 2’ pipe flash (see Methods) with retina 14” temporal to the fovea. These psychophysics thresholds were determined during the ERG runs with corresponding Symbols. The x’s plot the b-wave thresholds determined from the results given in Fig. 3B. The i’s plot the ~ychophysi~~ thresholds for seeing the pipe flash without the ERG contact lens in place.
paper) and typical x0 monochro~ts (Aipem, personal communication). The log b-wave increment thresholds follow a Weber-Fechner type curve (Fig. 4). The linear portion of the curve has a slope of + 1.0. Log $, the eigengrau is the background where the Fechner 45” line produced backwards meets the horizontal level of the absolute threshold. In these experiments this background ranged from - 15 to - 1.0 log td, somewhat greater than that obtained in the best psychophysical conditions. Saturation of the ERG thresholds occurred at about 2.5 log td which agrees with that found for the psychophysical thresholds found in these experiments and in other psychophysical experiments (Aguilar and Stiles, 1954; Alpern, Rushton and Torii, 1970; Sakitt, 1976). The ERG flashes themselves did not put up the visual threshold much. This was checked with interpolated flashes (see Methods). The multiple flashes at the repetition rates used in the experiments put the visual threshold up very little if at all as determined by. intermittent thresholds obtained during the ERG runs. Dark aaiaptation. After the 99% bleach ERG responses were collected as quickly as possible within the time intervals indicated in Fig. S(a). Curves (Fig. Sa) which plot log a-wave slope as a function of log stimulating Bash show the a-wave increases with time after the bleach. An Hz curve has been drawn through the experimental points obtained for each of the indicated intervals. The apices of the H2 curves rise along a vertical line passing through about 2.3 log td set, the flash energy found to semisaturate the a-wave in the dark and to cause semisaturation of the psycho-
but inspection of the apex points reveals no change in sensitivity (i.e. no horizontaf movement) during the time rhodopsin is known to be increasing. Figure 5(b) displays the behavior of the b-wave response to brief flashes as the eye recovers from the bleach. The log b-wave amplitude is plotted as a function of log stimulating flash at the indicated times after the bleach. An Hz curve is drawn through the experimental points obtained at each of the indicated times after the bleach. The apices of the Hz curves move up and to the left as the eye recovers in the dark. The psychophysical thresholds for seeing the 2” “pipe flash” (see Methods) during dark adaptation were determined by the ERG subjects with electrode in place. The psychophysical thresholds as a function of time in the dark are fit fairly closely by two exponential curves with a rod-cone break. The corresponding b-wave thresholds, chosen where the Hz curve approached the + 1.0 asymptote (see above in Light Adaptation Results), follow about the same course as the psychophysical thresholds. The curve drawn through the experimental points is an exponential with time constant 4OOsec, the same as found for the recovery of rhodopsin (Alpern, 1971). The apparent increase in a-wave sensitivity with time in the dark is also plotted in Fig. 6; these thresholds were obtained by the usual criterion voltage method (level indicated in Fig. 5a) and illustrate the error that may arise from the criterion method; the a-wave had no change in sensitivity (i.e. no horizontal change of apex points; see Fig. 5a). Sensitivity processing is quite different at the a- and b-wave levels, but scaling of signal size has similar characteristics for the a- and b-waves. In Fig. 5 the range of scaling (vertical movement) is the same for a-waves and b-waves. In addition the time course of scaling of a-waves and b-waves is the same; Fig. 7 shows the ordinates of the a-wave apex points are nearly coincident with the ordinates of the b-wave apex points as the ERG recovers in the dark after the bleach. In human dark adaptation scaling at the a-wave level is the same as at the b-wave level just as it was in light adaptation of the human and mudpuppy retina (Fulton and Rushton, Rod ERG of Mudpuppy). DlSCUSSlON The Hz log-log plots have allowed compact representation of a wide range of human ERG responses and permitted comparison of the electrical and psychophysical thresholds. In light adaptation the psychophysical and b-wave thresholds have the Same eigengrau, slope of increment threshold curve and level of saturation; in dark adaptation after a full bleach b-wave and psychophysical thresholds follow the same exponential recovery over the same range. These findings suggest the b-wave is a good objective measure of visual sensitivity in the conditions studied if, in choice of electrical threshold, the precautions mentioned above in the presentation of the results are taken. Our results contrast earlier correlative studies of electroretinal and psychophysical responses
798
9. Fcrro?; and WILLIAstA. H.
ANXE
Rar~onsc
loq
16
I-
log td set
flash,
(b)
log
flesh
,
log
td set
Fig. 5. Dark adaptation after bleaching 99Oi of rhodopsin. The log stimulating energy is plotted against log response obtained at various times in the dark after the bleach. The symbols indicate the time after the bleach: l IO-13 minutes; n 13-16 minutes: ‘I 15-18 minutes; o 18-21 minutes; U 23-26 minutes: A Z-28 minutes; + 35-40 minutes. The data displayed were obtained in two separate runs on the same subject on the same day. (a) The points plot log a-wave slope against log stimulating flash at the various times after the bleach. A template was used to draw HL curves through the experimental points. For clarity the 15-18 minute curve (‘I) and the 25-28min curve (A) are represented by apex points only. The apex point for each HI curve is indicated by the circled symbols. The apex points move about vertically upward as the response recovers from the bleach. As the eye recovers sensitivity in the dark. the curves appear to move to the left if a low voltage criterion response is considered. (b) The points plot log b-wave amplitude against log stimulating flash at the various times after the bleach. A template was used to draw H, curves through the experimental points and locate the apex points obtained at the various times after the bleach. The apex poinrs move up and to the left as the eye recovers sensitivity in the dark.
RLSHTO-.
Fig. 6. Log thresholds during dark adaptation following a 999,; bleach of rhodopsin. The X’S plot the log psychophysical threshold (vertical scale on the left) for seeing the 2’ pipe flash. The other symbols plot the ERG log thresholds. The upper curve obtained during the rod portion of dark adaptation. displays the log a-wave “threshold” as determined by noting the log td set of flash (vertical scale on the right) required to give the log criterion responseindicated in Fig. 5(a). The log b-wave thresholds were determined as described in the text. The curve drawn through the rod segment of the results is an exponential with time constant 400 sec. The log b-wave thresholds follow fairly closely the log psychophysical thresholds in that they follow the same exponential and have the same 2 log unit range in the conditions of these experiments.
in light and dark adaptation (e.g. Johnson. 1949; Riggs and Johnson, 1949: Armington, 1952; Aipern and Faris, 1952; Biersdorf. 1972) which do not report exact parallels of the subjective and objective measures of adaptation. We suppose our methods which include signal averaging techniques, fixed and dilated pupil, a Ganzfeld produced by an integrating sphere fit fairly snugly about the eye, and extreme care in preventing stray fight of the apparatus from entering the eye are responsible for producing more exact relation of stimulus and mass electrical response and, thus, closer correlation of psychophysical and electrical scotopic responses. In light and dark adaptation. the scaling (vertical movement of H, apices) has the same range for a-
Minutes
in
dark
Fig. 7. Ordinate of apex points as a function of time. The black circles plot the ordinate of the apices (log u-wave slope scale on the left) of the a-wave curves (from Fig. 5a) as a function of time after the bleach. The white circles plot the ordinate (log b-wave amplitude scale on the right) of the b-wave curves (from Fig. 5b) vs time. The curve drawn through the points is an exponential with time constant 400 sec.
199
The human rod ERG and &-waves and in dark adaptation
foflows the same time course (Fig. 7). This regulation of maximum possible response size obtainable in each of the various conditions of adaptation examined appears to be set at the level of the a-wave and passeddirectly to the b-generators. Sensitivity adjustment. however. is quite different for a- and b-waves. In light and dark adaptation, the a-wave absolute sensitivity and sensitivity change (horizontal movement of Hz apices) caused by backgrounds is less than that measured by the b-wave. Previous investigations have pointed out the inadequacies of receptor desensitization in explaining adaptation (Rushton and Westheimer, 1962). If the sensitivity component of the u-wave. taken as a sign of receptor activity [perhaps more exactly receptor outer segment activity (Brown and Watanabe. 196211 becomes an input to the b-generators. some modi~cations [i.e. in light adaptation an increase of the Weber-Fechner slope from $0.4 to + 1.0; in dark adaptation development, proximal to the u-wave generators. of a sensitivity change parallel to the regeneration of rhodopsin] and amplification [by a factor of about 1001 would have to occur between the a-wave and b-wave. Although adaptive mechanisms in receptors may not influence more proximal retinal adaptation to dim backgrounds (Green et al., 1975), the growing body of evidence that indicates receptors usually do govern adaptation of the entire retina (Dowling and Ripps, 1977) along with the findings that the a-wave can carry information to more proximal retina (Penn and Hagins, 1969; Ernst and Arden, 1972) would be compatible with the hypothesis that the a-wave can be the input to the b-wave which is an indicator of visual performance. Our preIiminary work on a mathematical model with equations based on the experimental findings reported in this paper and the preceding paper (Fulton and Rushton, previous paper) also support the hypothesis. Another link of a- to b-wave is observed in the
light adaptation results of mudpuppy and human retina. The semisaturation level of flash, log Q~, for the a-waves with background dark is the same as the apex point of the left facing H, curve which is the locus of b-wave apices (Fig. 3). Also, b-wave ~nsitivity saturates at backgrounds of about 2.5 log td (Fig. 4). but scaling of the b-wave amplitude continues. At backgrounds of 2.5 log td or yeater, the a-wave Hz apices remain on a straight hne, stope - 1, as the processes of scaling and sensitivity adjustment continue. Although analysis of the stimulus-response data by the Hz curves shows sensitivity and scaling (or, in other words, adjustment of membrane potential) of response to be separable features, as did microelectrode studies of single receptors (Kleinschmidt and Dowling, 1975) and biochemical studies of outer segments (Brodie and Bownds, 1976; Woodruff et al., 1977). it is apparent that scaling and sensitivity of the u- and b-wave in retinal adaptation are closely associated; possibty this association can he more fully appreciated by development of a mathematical mode1 which encompasses the ex~~mentai results of this and the preceding paper (Fulton and Rushton, previous paper). The similar behavior of mudpuppy and human ERG’S in the presence of backgrounds suggests that
there are features of signal processing common to vertebrate retinas. The microelectrode and biochemical studies conducted on animal retinas have revealed some aspects of the physiology underlying the transretinal potential which can, with care, be correlated with psychophysical function. As animal studies continue, the behavior of the transretinal potential, even accessible clinically, may be understood more fully in terms of basic retinal processes. REFERENCES
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