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Transient tritanopia after flicker adaptation
TRANSIENT
TRITANOPIA AFTER FLICKER ADAPTATION ADAM REEVES
lnstitut fir Arbeitsphysiologie an der Universitlt Dortmund. D-4600 Dortmund I, F. R. Germany (Rrcricud
7 June 1980)
Abstract-Transient tritanopia, the rise in blue test threshold following extinction of a yellow background. is “abolished” if the adapting background is flickered rectangularly at a low rate (0.25 Hz, duty cycle 0.2), and reduced at rates below 10 Hz. By “abolition”, is meant that the threshold falls to the same extent as after extinction of a I7 (-equated “opponently neutral” (494 nm) background. These results, like those of Loomis (1980), require modifications in Pugh and Mellon’s (1979) dynamic model for transient tritanopia.
sensitivity changes to the opponent site. To verify that sensitivity changes were so restricted, advantage was taken of Augenstein and Pugh’s (1977) finding that transient tritanopia was eliminated for blue-green (500 nm) backgrounds. If the yellow-blue opponent site does not respond to such backgrounds, as assumed in Pugh and Mellon’s (1979) model, the behaviour of test thresholds following extinction of a blue-green background (steady or flickered) can provide a baseline for interpreting opponent site effects with long-wavelength backgrounds.
INTRODUCTION Transient tritanopia, the large increase in the threshold of a 7t, detected short-wavelength test flash that occurs after a long-wavelength adaptation field has been extinguished (Mollon and Polden. 1977), may be greatly reduced if the background is flickered slowly (at 2.0 Hz) during adaptation* (Loomis, 1980). In this paper Loomis’s results, which were obtained with a 3.88 log td broad-band long-wavelength background, are extended over a range of intensity levels and of flicker rates, to characterize the phenomenon in more detail, and to test some features of Pugh and Mellon’s (1979) model for transient tritanopia. Intensity levels were chosen to cover the n, region of the threshold vs intensity curve where transient tritanopia occurs (Mellon and Polden, 1977). Flicker rates were chosen to encompass a region in which flicker is known to reduce chromatic adaptation (Loomis and Berger, 1979; Broekhuijsen et al., 1979), that is, between 5 Hz and about 50 Hz. Much slower flicker may give time for the receptors to adapt and recover in each cycle, and this may increase chromatic adaptation (Jameson et al., 1979): such rates were avoided here. Loomis (1980) turned off the background completely to measure dark adaptation over several minutes. This method, however, allows changes in sensitivity and in adaptive state to occur both at the receptors and at the opponent site held responsible for transient tritanopia by Pugh and Mollon (1979). In the present research the background was held at the same mean intensity throughout adaptation and testing, apart from brief interruptions during presentation of the test, in order (it was hoped) to maintain receptor adaptation at a constant level and restrict
*This research was undertaken independently of Loomis’s. 1 am grateful to Dr. Loomis for allowing me to see a prepublication version of his paper (Loomis, 1980), part of which was presented in the April 1979 meeting of ARVO, and for his helpful comments on an earlier draft of this paper.
METHOD
Apparatus
The three-channel Maxwellian view system described in Reeves (1981) was used. Channel 1 provided an 18” dia. circular background, channel 3 a 3” concentric circular test, and channel 2 was not used. The source was a tungsten halogen bulb rated at 100 W and 6.6 A, and slightly under-run at 14.8 V from an a.c. power supply. Light passed through 7.0 mm thick Edmund Scientific heat absorbing glass. A variable interference filter (Barr and Stroud type SS2), mounted close to the first filament image, could be positioned by hand to provide the background in channel 1 with any desired wavelength: full bandwidths at half-maximum were 13 nm or less. A Kodak Wratten 47B gelatin filter, measured in situ with &,,., = 435 nm, and full bandwidth at half-maximum 42 nm, provided the blue test. Lenses were Edmund Scientific coated achromat convex lenses. No final achromatizing lens was used. Observer AR’s myopia was corrected by moving the 3 mm aperture used to define the test forward, to bring the test into sharp focus. Timing was provided by shutters with rise and fall times of less than 2 msec, which covered or uncovered in each channel a 2 mm hole drilled in a copper sheet and placed near to the first filament image (thus providing a 2 mm dia. final filament image in the plane of the pupil). Stimulus intensity was con-
657
ADAM REEVES
658
CONSTANTSTEADY FLICKEREDSTEADY
(dc
m
fl
CONSTANTEXTINCTION
fi
FLICKEREO,E$~s”;lON
OTiSEC r 2 OR
ADAPTATION
I
I
3 MINS PHASE
E9
0.5)
TEST
PHASE
Fig. I. Temporal sequence in the four stimulus conditions: constant-steady, flickered-steady, constantextinction and flickered-extinction (the adaptation condition is referred to first). In constant adaptation, the background was always present. In flickered adaptation, it was rectangularly flickered at the same rate throughout an experimental run. Duty cycle (dc) was 0.5 as shown, or 0.2 (light on for 0.2 of the
cycle). The 2OOmsec.3” test (wavelength A) onset 200 msec after extinction of the 18” background (wavelength 11).The time from requesting a test, shown by arrows, and presentation of the test or extinction of the background was 500 msec.
trolled with accurately calibrated Lee neutral density filters, nominally 0.3 and 0.6 log units, supplemented by glass slides to provide 0.1 or 0.2 additional log units of density. The head was positioned with the aid of a plastic dental impression attached to a bite board moveable in three dimensions. Correct placement of test and background images on the pupil was checked before each run. The observer made the final small adjustments to the bite board necessary to ensure that the adaptation field appeared homogenous. Fovea1 fixation was aided by a tiny (2 min arc) black dot in the center of the adaptation field. Light intensity was calibrated by placing an EG & G type 55-2 multiprobe photodetector in the position of the observer’s eye. Radiometric measurements taken at each background wavelength agreed satisfactorily (within 0.2 log units) with measurements taken in “Vi mode” and adjusted with the manufacturer-provided calibrations of the photodetector. The latter measurements were preferred since the radiometric measurements may have been infIuenced by spectral side-bands.
indicated by arrows in the figure. In the condition, the background was continuously present and the test was an increment. In the extincriot1 condition, the background was turned off briefly-for 1 set-and the test was presented while the background was off. The observer could not request another test for at least 4 sec. (In pilot work, lengthening this time from 4 set to IO set had no effect on test thresholds.) Test duration was always 2OOmsec; and. unless otherwise noted, the test onset 2OOmsec after the background was turned off in the extinction condition. Referring to adaptation first, the four conditions shown in Fig. 1 are const~t-steady, flickeredsteady, constant-extinction, and flickered-extinction. The constant-steady condition is that used in the twocolor increment threshold method (Stiles, 1939). The constant-extinction condition is very similar to that used to demonstrate transient tritanopia (Mellon and Polden, 1977). The fixed spatial hyout is shown in the inset in Fig. 1, in which a 3” test is concentric with an I8” background. request,
steady
Observers
Procedure
author (A.R., aged 33) and D.R. (aged 26) served as observers. D.R. was previously inexperienced. Roth were color normal on the Dvorine Pseudo-Isochromatic Plates.
Observers initially adapted to the dark for 3 min. They then adapted for 3 min to the lowest background intensity, and re-adapted for 2 min after each change of intensity, except that at the highest intensities they re-adapted for 3 min. Thresholds were obtained by a version of the descending method of limits. From preliminary work, the approximate threshold was known. The observer first saw the test about 0.6 log units above threshold. He gave the response “Yes” to indicate that he had seen the test. A 0.3 .ND filter was then added to the test channel Depending on the observer’s response, 0.1 ND filters were added until he reported “No”. The observer was
The
Stimulus
conditions
four temporal conditions shown in Fig. 1 were generated from two adaptation and two test conditions. In adaptation, shown on the left of the figure, the background was constant or Pickered. In the test phase, shown on the right, the background was kept constant at the same mean intensity, denoted I, as in adaptation. Tests were presented at the observer’s The
Transient tritanopia re-adapted (to the steady or flickered background) for 20-30sec. and the threshold taken once again. If the two measurements did not agree to within 0.1 log units, the procedure was repeated until two successive thresholds were obtained that did so agree. Usually, agreement was immediate, but in extinction afterimages of the background sometimes produced quite variable threshold settings. In these cases, observers were trained in practice sessions to distinguish between test and after-image by manually blocking the test channel on alternate presentations. A procedure in which the observer did not know whether the test was presented or not (it was so on half the trials) was sometimes used to ensure that threshold had been reached. In experimental. trials the observer was allowed at any time either to remove a 0.3 ND filter from the test channel, or to block it by hand, to help establish that threshold had been reached.
Sessions began with a standard condition, the steady threshold obtained after 3 min adaptation to a * The shape and position of the n, and nz branches were found from Wyszecki and Stiles (1967) Tables 7.47.6, with I = 435 nm. The branches are correctly positioned on the abscissa, except for a constant 0.08 log unit leftwards shift chosen by eye to improve the fit of both observers’ steady thresholds to n,. The same arbitrary vertical scaling was applied throughout, chosen by eye to fit the n, branch. (The n2 branch is positioned for Stiles’s average observer but is too variable across observers to be useful in fitting.) In review, it was suggested that the test might have been detected by the rods at low background intensities and in extinction. because the test was large (3”) and had a broad spectral characteristic. Observer A.R. therefore re-ran the 2 Hz, 0.2 dc condition with a I”, narrow band (12 nm) 435 nm test, and obtained results very similar to those in Fig. 2. Observer D.R. was unfortunately not available. Both observers in the main experiment noticed rod intrusions, in the shape of a “new moon”, or arc of white around the edge of the test furthest from the fovea, if they did not fixate centrally. It was not too difficult to avoid such intrusions. Both observers reported that test hue was blueish just above threshold, even at the lowest intensities where detection by n2 has been assumed. Nevertheless, some rod involvement may have occurred. t “Flicker-extinction” thresholds may not have reflected the full effects of flicker, because the background was always held constant during the testing phase (see Methods). Since it usually took between I5 set and 35 set to make a threshold determination, it was important to discover whether thresholds had in fact changed during the testing phase. To discover whether this was so, observers were asked to delay requesting the first test for some seconds, before determining the extinction thresholds. Extinction thresholds were also obtained between 7 set and 10sec after cessation of flicker, by a special procedure in which the observer made only one response before being briefly re-adapted to flicker. Results obtained at two rep resentative background intensities (- 1.24 and -2.14 log ergs/deg’/sec) showed that after exposure to 2Hz or 0.25 Hz flicker, with a duty cycle of 0.2, thresholds rose in the first 35 set by less than-O.iO log units (for A.R.) and 0.15 log units (for D.R.). Therefore flicker-extinction thresholds obtained in the usual procedure may underestimate the full effects of flicker, but only by 0. I log units or so on average. (After the initial 30sec, thresholds began to rise slowly, with an average slope of about 0.36 log units per min, to reach the level obtained after exposure to constant backgrounds after about 100 sec.)
659
background
of 580 nm and -2.24
to ensure that test threshold
log ergs/deg2/sec,
was within 0.2 log units
of that found in preliminary work. (If it was not, the observer readjusted the bite-board and the experimenter checked the proper functioning of the equipment.) Sessions took between 1.5 and 2.5 hr.
RESULTS Transient
tritanopia
after ,Jicker
obtained in the “steady” and “extinction” conditions, for the Wratten 47B test and a 580nm background, either after flickered adap tation (“flicker-steady” and “flicker-extinction”), or after constant adaptation (“constant-steady” and “constant-extinction”). The background was flickered at a chosen rate (or was constant) throughout an experimental run, in which background intensity was successively raised by 0.3 log unit steps. At each intensity level, extinction thresholds were obtained before steady thresholds. One descent to threshold in the extinction condition took between I8 and 35 sec. Thresholds of the 47B test are shown on an arbitrary log scale in Fig 2 (observer A.R.) and Fig. 3 (D.R.). Background wavelength was 580nm. Each data point is one threshold determination from a separate session. Different panels show thresholds obtained after adaptation to different flicker rates, or (at the bottom) after adaptation to a constant background. Crosses show “steady” thresholds, which cluster around the solid curves,* Stiles’s n2 (the lower branch) and n, (the upper). Open circles show thresholds obtained 2OOmsec after extinction of the background. Those in the lowest panels of Figs 2 and 3 show transient tritanopia after adaptation to a constant background, and were probably mediated by the green cones (Mollon and Polden, 1977). Open circles in the other panels are the “flicker-extinction” thresholds and show greatly reduced transient tritanopia. Dashed lines connecting the open circles are to aid the eye. The second and third panels confirm Loomis (1980) in showing that a smaller duty cycle (dc) decreases transient tritanopia at a 2.0 Hz flicker Thresholds
were
ratet. A “neutral”
background
Backgrounds with wavelengths in the vicinity of SOOnm (Augenstein and Pugh, 1977) which includes 494nm (Krauskopf and Reeves, 1980) may be “neutral” in that they do not desensitize the yellow-blue opponent site held responsible for transient tritanopia (Pugh and Mollon, 1979). If the 494 nm background indeed has no effect on the opponent site, and if the experimental method restricted sensitivity changes to the opponent site, then flickering the 494nm background during adaptation should have no effect on thresholds. In that case the 494nm background provides a control against which the effects of extinction of long-wavelength backgrounds may be measured.
ADAM REEVES
660
HZ=2 dc =0.2//
-1.2k
I
x 7-
dc -0.5 ‘.2
dc = 0.5
t
CONSTANT
-I
-l.SC -4.0
-2.0
-3.0 LOG MEAN
ERGS
- 1.0
* DEE’* SEf’
Fig. 2. Thresholds of a Wratten 47B (i.,,, = 435 nm) test on a 580 nm background. The abscissa is the log of the mean background intensity in ergs/deg’/sef (0 log ergs equals 11.5 log quanta at 580 nm). Crosses show steady thresholds and are joined by Stiles’s nl (lower branch) or n, (upper). Open circles. connected by dashed lines to aid the eye, are thresholds obtained 2OOmsec after extinction of the background. Each panel shows thresholds for a different combination of frequency and duty cycle (dc), as marked. Thresholds are expressed in the same arbitrary log units in every panel, and in Figs 2-6. Observer A.R.
Thresholds in Fig. 4 are for the Wratten 47B test and a 494nm background. Crosses show “steady” thresholds which cluster around Stiles’s T(, template. Triangles show extinction thresholds and demonstrate that there is indeed no effect of flickering the 494 nm background.
to high, or, in half the sessions, from high to low, through the values shown on the abscissa of Fig. 5. The high intensity background was run in the same way. Four sessions were run with A.R., and two with DR., on different days. Thresholds in Fig. 5 are for observer D.R., obtained in two sessions. Solid lines connect flicker-extinction Variation of Picker rate thresholds obtained after adaptation to a high intenSteady and extinction thresholds were found after sity (0) or low intensity (Cl) background whose flicker adaptation to several flicker rates, at two intensities of rate is on the abscissa. Duty cycle was 0.2. The extinca 580 nm background: “high” (- 1.24 log ergsJdeg2J tion thresholds fall sharply as the flicker rate is set), and “low” (-2.14). The observer adapted to the reduced. Symbols (0 and 0) on the far right show mean steady thresholds obtained on the same two low intensity background for four mins. After adaptation, he obtained extinction thresholds, and then backgrounds; these were about 0.1 and 0.4 log units steady thresholds, in the usual way. The flicker rate above the absolute threshold of K, shown by the during adaptation was progressively stepped from low arrow, for the low and the high intensity back-
661
Transient tritanopia
-3.0 LOG MEAN
-2.0 ERGS - DEi’.
SEC’
Fig. 3. Same as Fig. 2. Observer D.R.
grounds. (The absolute threshold of the blue test lies somewhat below the arrow, since this observer, like A.R., has a z2 branch.) Results shown in Fig. 6 are mean log thresholds obtained by A.R. in the same experiment; individual thresholds are not shown, but did not differ from the mean by more than 0.12 log units. As in Fig 5, solid lines connect flicker-extinction thresholds (0 and 0) for the high and low intensity backgrounds, flickered with a duty cycle of 0.2. They drop sharply as flicker rate is reduced, perhaps more so than for observer D.R. Solid lines connecting symbols (0 and n ) also show flicker-extinction thresholds, but for backgrounds flickered with a duty cycle of 0.5. These drop less sharply with the high intensity background, confirming the result shown in Fig. 2 that the effect of duty cycle is more pronounced at higher intensities. Symbols (0 and 0) on the far right show mean steady thresholds for the low intensity and high intensity backgrounds. (For neither observer did flicker have a measurable effect on the steady thresholds, which varied unsystematically over sessions and flicker rates
and did not depart from the mean by more than 0.1 log units. This was also the case in a separate session in which steady thresholds were measured as soon after the cessation of flicker as possible-about 15 set). Mollon and Polden (1977) and Reeves rt (11.(1978) showed that green cones may mediate detection in the constant-extinction condition, in which extinction of the yellow background has presumably made the “blue” pathway less sensitive to the test than the “green”. They might conceivably do so in flickerextinction, if the green pathway became more sensitive after adaptation to flicker. Hence extinction thresholds for a green 522 nm test (half-bandwidth I2 nm) were obtained with backgrounds of the same intensity as those used for the blue test. Resulting extinction thresholds are plotted with triangles in Fig. 6. They have been shifted vertically to coincide with the constant-extinction thresholds at the high background intensity, where blue and green tests were presumably detected by the same ‘green” pathway. The 522 nm thresholds are flat across flicker rates at both
ADAM REEVES
662
0.3 Q & s 0 2 2 -0.3 ;
-0.6 -0.9
LOG MEAN
ERGS.D&&.SEC-’
Fig. 4. Thresholds of a Wrattan 47B test on a 494 nm background. The abscissa is the log of the mean background inlensity in ergs:deg’/sec (0 log ergs equals Ii.4 log quanta at 494 nmf. Crosses show steady thresholds and are joined by Stile’s 8, branch. Triangles show extinction thresholds. Symbols ( x ) and (A) are for constant adaptation; symbols (+) and (A) are for adaptation to flicker (Hz = 2 for A.R., upper panel; Hz = 0.25 for DR.. lower panel). Duty cycle was 0.2.
/1 s5eo dc = 0.2
-1.5
1 0.25
I
0.5
ADAPTMG
1
1
3 3
FMXER
1
5 7 10 15 20 RATE
lN
HZ
EXfSTEADY CONSTARNT
Fig. 5. Thresholds of a Wrattan 478 test, in arbitrary log units, plotted against the flicker rate in Hz of a field, for observer D.R. Lines connect flicker-extinction thresholds for backgrounds of intensifies M, = - 1.24 and M, = - 2. I4 log ergs/deg2/sec (0 and CJ, respectively). Symbols (0) and (@on the far right show the mean steady thresholds on each of the two background intensities; they lie about 0.4 and 0.X log units above the absolute threshold of K%shown by the arrow. 580 nm adaptation
663
Transient tritanopia
0.25
05
I
AMPTING
23 FLICKER
5710152O~ADY RATE
IN
HZ
CONSTANT
Fig. 6. Symbols (0 and 0) are as in Fig. 5, but for observer A.R. Additional symbols (0 and w) show thresholds at the same two background intensities after flicker with a duty cycle of 0.5. Triangles show thresholds of a 522 nm test obtained after extinction of the 580 nm background, Bickered with duty cycle 0.5 at the same two intensities; the triangles have been shifted vertically to coincide with constant-extinction thresholds for the Wratten 47B test at the high intensity (M,, = - 1.24 log ergs/deg*/sec).
of the background. This result demonstrates that the “green” pathway did not detect the blue test at low flicker rates.
intensities
DISCUSSION Does Picker The
eliminate
transient
tritanopia?
results show clearly that transient tritanopia is greatly reduced at most intensities of a slowly flickered 580 nm background. However, exact interpretation of the flicker-extinction thresholds is complicated, because it is not known how extinction thresholds would have behaved in the absence of the opponent site; and without this knowledge it is impossible to say whether flicker eliminated or merely reduced the effect of that site. However, if the extinction thresholds shown in Fig. 4 for the “neutral” (494nm) background are those to be expected in the absence of an opponent site effect, as argued above, then these thresholds provide a base-line for those obtained with other backgrounds. For example, the thresholds after extinction of the 494 nm background drop below 7crabout as far as they do after extinction of the 580nm background flickered at 0.25 Hz with dc = 0.2 (the top panels in Figs 2 and 3). On this basis, which implicitly detines transient tritanopia as the difference between steady-state and extinction thresholds, such flicker eliminates transient tritanopia in D.R. and leaves behind a residual effect of less than 0.2 log units in A.R. An alternative approach is to try to determine the amount of “opponent site effect” in both steady-state
and extinction conditions. If the rrr branch is determined by long-wavelength cone inputs (Pugh, 1976), it shows the opponent site effect on steady-state thresholds directly. Determining the opponent site effect on extinction thresholds at 580nm, however, requires comparison with thresholds for the neutral (494 nm) background. Assuming that the spectral sensitivity of the blue cones is given by Stiles’s l7,, blue cones are 2.80 log units less sensitive at 580 nm than at 494 nm. The total opponent site effect on A.R.‘s extinction thresholds at 580nm may be measured by subtracting the thresholds obtained in Fig. 4 after a lateral shift of 2.8 log units. This equates 580nm backgrounds of -0.74 log ergs/deg’/sec with 494 nm backgrounds of - 3.54 log ergs/deg2/sec, where extinction thresholds are unlikely to have risen above absolute threshold. Thus the unmodified extinction thresholds, as shown in Figs 2 and 3, reveal the entire opponent site effect on test threshold. This is a small effect in the 0.25 Hz, 0.2 dc condition, but one which increases with background intensity, with a slope of about 0.3 over the intensity range tested. Relation
to Pugh
and Mellon’s
(1979)
model
In this model, the n1 branch of the increment threshold curve results from adaptation of an opponently-coded site. During exposure to long-wavelength backgrounds, the opponent site is polarized in the yellow direction, loses sensitivity, and gives rise to the rrr branch. If the sensitivity of the opponent site recovered in dark adaptation, thresholds would fall after extinction, but since they do not, Pugh and MolIon (1979) postulated that a “restoring force” is built up during adaptation to long-wavelength backgrounds. This force partially compensates for the polarizing effects of the long-wavelength input to the opponent site, in the steady state; but after extinction of the background the restoring force is no longer opposed by an input, and it produces a “rebound” which temporarily perturbs sensitivity and gives rise to transient tritanopia. The mechanisms responsible for the rebound are not relevant here; what is important is the way in which the restoring force, whose magnitude determines the amount of transient tritanopia, is thought to build up during adaptation. In the “feed-forward” form of the model, the restoring force is a sluggish integral of the input to the opponent site. Because the input is smoothed out by a sluggish integration, flickering the input during adaptation should have no effect on thesholds. The present results, and those of Loomis (1980) clearly rule out this possibility: flicker in fact strongly decreases transient tritanopia. Loomis (1980) suggested that the input to the opponent site might be subject to a compressive non-linearity. If this were so, the on-phase in flicker would generate less input to the opponent site than would constant adaptation at the same mean intensity. The restoring force would thereby be less after flicker adaptation.
664
ADAM REEVES
Present results show that this hypothesis is insuffiThe bottom panels in Figs 2 and 3 show that in constant~xtin~ion, test thresholds rise very sharply with background intensity. The sharp rise, which may be underestimated if green cones took over detection (see above). reflects (in Pugh and Mellon’s model) only the increase in long-wavelength cone inputs to the opponent site. If compressive non-linearities occur in the input to that site, they must be mild enough For such sharp rises in threshold to be possible. Yet the flicker-extinction thresholds in the top paneis of Figs 2 and 3 increase only very slightly with background intensity. over a range of intensities considerably larger than the factor of five introduced in the 0.2 dc condition. To account for the results. the feed-forward model may be elaborated by multiplying the input to the restoring force by a function of the log adapting flicker rate, w. such as U&J + constant), which asymptotes to 1.0 at high flicker rates or steady backgrounds, and drops off proportionately to CL)at low Bicker rates, as in Figs 5 and 6. Such an elaboration can also account for the effect of duty cycle: a longer “off’ period allows greater recovery of the opponent site and a larger rebound away from the yellow direction. and hence reduces the restoring force even further. Alternatively. some version of Pugh and MolIon’s (1979) feed-back model. in which the restoring force is the integral of the output of the opponent site. may account for the results. It is clear that the mean output of the opponent site is less during flicker adaptation than during constant adaptation. because in the former case the output of the opponent site fluctuates between zero and a high positive value. while in the latter it remains high. Hence the feedback model does predict less transient tritanopia after flickered adaptation. However. it is not clear how that model cient.
could predict a nearly total absence of transient tritanopia after flicker, since some restoring force must build up during adaptation no matter what the flicker rate.
REFERENCES Augenstein E. J. and Pugh E. N. (1977) The dynamics of the rr, color mechanism: further evidence for two sites of adaptation. J. PQiof. 272, 247-281. Broekhuijsen M. L.. Uilenreef P. L. and Veringa F. (1979) Flicker and chromatic adaptation. Visim Rrs. 19. 565-.561. Jameson D.. Hurvich L. M. and Varner F. D. (1979) Receptoral and postreceptoral visual processes in recovery from chromatic adaptation. Proe. rratn. Acad. Sci. U.S.A. 76. (6). 30343038. Krauskopf J. and Reeves A. (1980) The measurement of the effect of photon noise on detection. L’isim Res. 20. l93- 196. Loomis J. M. (1980) Transient tritanopia: failure of timeintensity reciprocity in adaptation to longwave light. Vision Res. 2% 837-846. Loomis J. M. and Berger 7. (1979) Effects of chromatic adaptation on color discrimination and color appearance. Vision Res. 19, 89 I-90 I. Mollon J. D. and Polden P. G. (1977) An anomaly in the response of the eye to light of short wavelengths. P&l. Trans. R. Sm. 278, 207-240. Pugh E. N. (1976) The nature of the n, mechanism of W. S. Stiles. J. PIzysiol. 257, 713-747. Pugh E. N. and Mollon J. D. (1979) A theory of the R, and rr3 color mechanisms of Stiles. Vision Rex 19, 293-3 t2. Reeves A. (1981) Transient desensitization of a red-green opponent site. In preparation. Reeves A.. Krauskopf J. and Mellon J. D. (1978) Decline of transient tritanopia at high intensities. Inresr. Opltthal, risttai SC%. srq$. p. f 77. Stiles W. S. (1939) The directional sensitivity of the retina and the spectral sensitivities of the rods and cones. Prdc. R. SW. 127, 64 105. Wyszecki G. and Stiles W. S. (1967) Color Sciettcr. Wiley. New York.
TRITANOPIA AFTER FLICKER ADAPTATION ADAM REEVES
lnstitut fir Arbeitsphysiologie an der Universitlt Dortmund. D-4600 Dortmund I, F. R. Germany (Rrcricud
7 June 1980)
Abstract-Transient tritanopia, the rise in blue test threshold following extinction of a yellow background. is “abolished” if the adapting background is flickered rectangularly at a low rate (0.25 Hz, duty cycle 0.2), and reduced at rates below 10 Hz. By “abolition”, is meant that the threshold falls to the same extent as after extinction of a I7 (-equated “opponently neutral” (494 nm) background. These results, like those of Loomis (1980), require modifications in Pugh and Mellon’s (1979) dynamic model for transient tritanopia.
sensitivity changes to the opponent site. To verify that sensitivity changes were so restricted, advantage was taken of Augenstein and Pugh’s (1977) finding that transient tritanopia was eliminated for blue-green (500 nm) backgrounds. If the yellow-blue opponent site does not respond to such backgrounds, as assumed in Pugh and Mellon’s (1979) model, the behaviour of test thresholds following extinction of a blue-green background (steady or flickered) can provide a baseline for interpreting opponent site effects with long-wavelength backgrounds.
INTRODUCTION Transient tritanopia, the large increase in the threshold of a 7t, detected short-wavelength test flash that occurs after a long-wavelength adaptation field has been extinguished (Mollon and Polden. 1977), may be greatly reduced if the background is flickered slowly (at 2.0 Hz) during adaptation* (Loomis, 1980). In this paper Loomis’s results, which were obtained with a 3.88 log td broad-band long-wavelength background, are extended over a range of intensity levels and of flicker rates, to characterize the phenomenon in more detail, and to test some features of Pugh and Mellon’s (1979) model for transient tritanopia. Intensity levels were chosen to cover the n, region of the threshold vs intensity curve where transient tritanopia occurs (Mellon and Polden, 1977). Flicker rates were chosen to encompass a region in which flicker is known to reduce chromatic adaptation (Loomis and Berger, 1979; Broekhuijsen et al., 1979), that is, between 5 Hz and about 50 Hz. Much slower flicker may give time for the receptors to adapt and recover in each cycle, and this may increase chromatic adaptation (Jameson et al., 1979): such rates were avoided here. Loomis (1980) turned off the background completely to measure dark adaptation over several minutes. This method, however, allows changes in sensitivity and in adaptive state to occur both at the receptors and at the opponent site held responsible for transient tritanopia by Pugh and Mollon (1979). In the present research the background was held at the same mean intensity throughout adaptation and testing, apart from brief interruptions during presentation of the test, in order (it was hoped) to maintain receptor adaptation at a constant level and restrict
*This research was undertaken independently of Loomis’s. 1 am grateful to Dr. Loomis for allowing me to see a prepublication version of his paper (Loomis, 1980), part of which was presented in the April 1979 meeting of ARVO, and for his helpful comments on an earlier draft of this paper.
METHOD
Apparatus
The three-channel Maxwellian view system described in Reeves (1981) was used. Channel 1 provided an 18” dia. circular background, channel 3 a 3” concentric circular test, and channel 2 was not used. The source was a tungsten halogen bulb rated at 100 W and 6.6 A, and slightly under-run at 14.8 V from an a.c. power supply. Light passed through 7.0 mm thick Edmund Scientific heat absorbing glass. A variable interference filter (Barr and Stroud type SS2), mounted close to the first filament image, could be positioned by hand to provide the background in channel 1 with any desired wavelength: full bandwidths at half-maximum were 13 nm or less. A Kodak Wratten 47B gelatin filter, measured in situ with &,,., = 435 nm, and full bandwidth at half-maximum 42 nm, provided the blue test. Lenses were Edmund Scientific coated achromat convex lenses. No final achromatizing lens was used. Observer AR’s myopia was corrected by moving the 3 mm aperture used to define the test forward, to bring the test into sharp focus. Timing was provided by shutters with rise and fall times of less than 2 msec, which covered or uncovered in each channel a 2 mm hole drilled in a copper sheet and placed near to the first filament image (thus providing a 2 mm dia. final filament image in the plane of the pupil). Stimulus intensity was con-
657
ADAM REEVES
658
CONSTANTSTEADY FLICKEREDSTEADY
(dc
m
fl
CONSTANTEXTINCTION
fi
FLICKEREO,E$~s”;lON
OTiSEC r 2 OR
ADAPTATION
I
I
3 MINS PHASE
E9
0.5)
TEST
PHASE
Fig. I. Temporal sequence in the four stimulus conditions: constant-steady, flickered-steady, constantextinction and flickered-extinction (the adaptation condition is referred to first). In constant adaptation, the background was always present. In flickered adaptation, it was rectangularly flickered at the same rate throughout an experimental run. Duty cycle (dc) was 0.5 as shown, or 0.2 (light on for 0.2 of the
cycle). The 2OOmsec.3” test (wavelength A) onset 200 msec after extinction of the 18” background (wavelength 11).The time from requesting a test, shown by arrows, and presentation of the test or extinction of the background was 500 msec.
trolled with accurately calibrated Lee neutral density filters, nominally 0.3 and 0.6 log units, supplemented by glass slides to provide 0.1 or 0.2 additional log units of density. The head was positioned with the aid of a plastic dental impression attached to a bite board moveable in three dimensions. Correct placement of test and background images on the pupil was checked before each run. The observer made the final small adjustments to the bite board necessary to ensure that the adaptation field appeared homogenous. Fovea1 fixation was aided by a tiny (2 min arc) black dot in the center of the adaptation field. Light intensity was calibrated by placing an EG & G type 55-2 multiprobe photodetector in the position of the observer’s eye. Radiometric measurements taken at each background wavelength agreed satisfactorily (within 0.2 log units) with measurements taken in “Vi mode” and adjusted with the manufacturer-provided calibrations of the photodetector. The latter measurements were preferred since the radiometric measurements may have been infIuenced by spectral side-bands.
indicated by arrows in the figure. In the condition, the background was continuously present and the test was an increment. In the extincriot1 condition, the background was turned off briefly-for 1 set-and the test was presented while the background was off. The observer could not request another test for at least 4 sec. (In pilot work, lengthening this time from 4 set to IO set had no effect on test thresholds.) Test duration was always 2OOmsec; and. unless otherwise noted, the test onset 2OOmsec after the background was turned off in the extinction condition. Referring to adaptation first, the four conditions shown in Fig. 1 are const~t-steady, flickeredsteady, constant-extinction, and flickered-extinction. The constant-steady condition is that used in the twocolor increment threshold method (Stiles, 1939). The constant-extinction condition is very similar to that used to demonstrate transient tritanopia (Mellon and Polden, 1977). The fixed spatial hyout is shown in the inset in Fig. 1, in which a 3” test is concentric with an I8” background. request,
steady
Observers
Procedure
author (A.R., aged 33) and D.R. (aged 26) served as observers. D.R. was previously inexperienced. Roth were color normal on the Dvorine Pseudo-Isochromatic Plates.
Observers initially adapted to the dark for 3 min. They then adapted for 3 min to the lowest background intensity, and re-adapted for 2 min after each change of intensity, except that at the highest intensities they re-adapted for 3 min. Thresholds were obtained by a version of the descending method of limits. From preliminary work, the approximate threshold was known. The observer first saw the test about 0.6 log units above threshold. He gave the response “Yes” to indicate that he had seen the test. A 0.3 .ND filter was then added to the test channel Depending on the observer’s response, 0.1 ND filters were added until he reported “No”. The observer was
The
Stimulus
conditions
four temporal conditions shown in Fig. 1 were generated from two adaptation and two test conditions. In adaptation, shown on the left of the figure, the background was constant or Pickered. In the test phase, shown on the right, the background was kept constant at the same mean intensity, denoted I, as in adaptation. Tests were presented at the observer’s The
Transient tritanopia re-adapted (to the steady or flickered background) for 20-30sec. and the threshold taken once again. If the two measurements did not agree to within 0.1 log units, the procedure was repeated until two successive thresholds were obtained that did so agree. Usually, agreement was immediate, but in extinction afterimages of the background sometimes produced quite variable threshold settings. In these cases, observers were trained in practice sessions to distinguish between test and after-image by manually blocking the test channel on alternate presentations. A procedure in which the observer did not know whether the test was presented or not (it was so on half the trials) was sometimes used to ensure that threshold had been reached. In experimental. trials the observer was allowed at any time either to remove a 0.3 ND filter from the test channel, or to block it by hand, to help establish that threshold had been reached.
Sessions began with a standard condition, the steady threshold obtained after 3 min adaptation to a * The shape and position of the n, and nz branches were found from Wyszecki and Stiles (1967) Tables 7.47.6, with I = 435 nm. The branches are correctly positioned on the abscissa, except for a constant 0.08 log unit leftwards shift chosen by eye to improve the fit of both observers’ steady thresholds to n,. The same arbitrary vertical scaling was applied throughout, chosen by eye to fit the n, branch. (The n2 branch is positioned for Stiles’s average observer but is too variable across observers to be useful in fitting.) In review, it was suggested that the test might have been detected by the rods at low background intensities and in extinction. because the test was large (3”) and had a broad spectral characteristic. Observer A.R. therefore re-ran the 2 Hz, 0.2 dc condition with a I”, narrow band (12 nm) 435 nm test, and obtained results very similar to those in Fig. 2. Observer D.R. was unfortunately not available. Both observers in the main experiment noticed rod intrusions, in the shape of a “new moon”, or arc of white around the edge of the test furthest from the fovea, if they did not fixate centrally. It was not too difficult to avoid such intrusions. Both observers reported that test hue was blueish just above threshold, even at the lowest intensities where detection by n2 has been assumed. Nevertheless, some rod involvement may have occurred. t “Flicker-extinction” thresholds may not have reflected the full effects of flicker, because the background was always held constant during the testing phase (see Methods). Since it usually took between I5 set and 35 set to make a threshold determination, it was important to discover whether thresholds had in fact changed during the testing phase. To discover whether this was so, observers were asked to delay requesting the first test for some seconds, before determining the extinction thresholds. Extinction thresholds were also obtained between 7 set and 10sec after cessation of flicker, by a special procedure in which the observer made only one response before being briefly re-adapted to flicker. Results obtained at two rep resentative background intensities (- 1.24 and -2.14 log ergs/deg’/sec) showed that after exposure to 2Hz or 0.25 Hz flicker, with a duty cycle of 0.2, thresholds rose in the first 35 set by less than-O.iO log units (for A.R.) and 0.15 log units (for D.R.). Therefore flicker-extinction thresholds obtained in the usual procedure may underestimate the full effects of flicker, but only by 0. I log units or so on average. (After the initial 30sec, thresholds began to rise slowly, with an average slope of about 0.36 log units per min, to reach the level obtained after exposure to constant backgrounds after about 100 sec.)
659
background
of 580 nm and -2.24
to ensure that test threshold
log ergs/deg2/sec,
was within 0.2 log units
of that found in preliminary work. (If it was not, the observer readjusted the bite-board and the experimenter checked the proper functioning of the equipment.) Sessions took between 1.5 and 2.5 hr.
RESULTS Transient
tritanopia
after ,Jicker
obtained in the “steady” and “extinction” conditions, for the Wratten 47B test and a 580nm background, either after flickered adap tation (“flicker-steady” and “flicker-extinction”), or after constant adaptation (“constant-steady” and “constant-extinction”). The background was flickered at a chosen rate (or was constant) throughout an experimental run, in which background intensity was successively raised by 0.3 log unit steps. At each intensity level, extinction thresholds were obtained before steady thresholds. One descent to threshold in the extinction condition took between I8 and 35 sec. Thresholds of the 47B test are shown on an arbitrary log scale in Fig 2 (observer A.R.) and Fig. 3 (D.R.). Background wavelength was 580nm. Each data point is one threshold determination from a separate session. Different panels show thresholds obtained after adaptation to different flicker rates, or (at the bottom) after adaptation to a constant background. Crosses show “steady” thresholds, which cluster around the solid curves,* Stiles’s n2 (the lower branch) and n, (the upper). Open circles show thresholds obtained 2OOmsec after extinction of the background. Those in the lowest panels of Figs 2 and 3 show transient tritanopia after adaptation to a constant background, and were probably mediated by the green cones (Mollon and Polden, 1977). Open circles in the other panels are the “flicker-extinction” thresholds and show greatly reduced transient tritanopia. Dashed lines connecting the open circles are to aid the eye. The second and third panels confirm Loomis (1980) in showing that a smaller duty cycle (dc) decreases transient tritanopia at a 2.0 Hz flicker Thresholds
were
ratet. A “neutral”
background
Backgrounds with wavelengths in the vicinity of SOOnm (Augenstein and Pugh, 1977) which includes 494nm (Krauskopf and Reeves, 1980) may be “neutral” in that they do not desensitize the yellow-blue opponent site held responsible for transient tritanopia (Pugh and Mollon, 1979). If the 494 nm background indeed has no effect on the opponent site, and if the experimental method restricted sensitivity changes to the opponent site, then flickering the 494nm background during adaptation should have no effect on thresholds. In that case the 494nm background provides a control against which the effects of extinction of long-wavelength backgrounds may be measured.
ADAM REEVES
660
HZ=2 dc =0.2//
-1.2k
I
x 7-
dc -0.5 ‘.2
dc = 0.5
t
CONSTANT
-I
-l.SC -4.0
-2.0
-3.0 LOG MEAN
ERGS
- 1.0
* DEE’* SEf’
Fig. 2. Thresholds of a Wratten 47B (i.,,, = 435 nm) test on a 580 nm background. The abscissa is the log of the mean background intensity in ergs/deg’/sef (0 log ergs equals 11.5 log quanta at 580 nm). Crosses show steady thresholds and are joined by Stiles’s nl (lower branch) or n, (upper). Open circles. connected by dashed lines to aid the eye, are thresholds obtained 2OOmsec after extinction of the background. Each panel shows thresholds for a different combination of frequency and duty cycle (dc), as marked. Thresholds are expressed in the same arbitrary log units in every panel, and in Figs 2-6. Observer A.R.
Thresholds in Fig. 4 are for the Wratten 47B test and a 494nm background. Crosses show “steady” thresholds which cluster around Stiles’s T(, template. Triangles show extinction thresholds and demonstrate that there is indeed no effect of flickering the 494 nm background.
to high, or, in half the sessions, from high to low, through the values shown on the abscissa of Fig. 5. The high intensity background was run in the same way. Four sessions were run with A.R., and two with DR., on different days. Thresholds in Fig. 5 are for observer D.R., obtained in two sessions. Solid lines connect flicker-extinction Variation of Picker rate thresholds obtained after adaptation to a high intenSteady and extinction thresholds were found after sity (0) or low intensity (Cl) background whose flicker adaptation to several flicker rates, at two intensities of rate is on the abscissa. Duty cycle was 0.2. The extinca 580 nm background: “high” (- 1.24 log ergsJdeg2J tion thresholds fall sharply as the flicker rate is set), and “low” (-2.14). The observer adapted to the reduced. Symbols (0 and 0) on the far right show mean steady thresholds obtained on the same two low intensity background for four mins. After adaptation, he obtained extinction thresholds, and then backgrounds; these were about 0.1 and 0.4 log units steady thresholds, in the usual way. The flicker rate above the absolute threshold of K, shown by the during adaptation was progressively stepped from low arrow, for the low and the high intensity back-
661
Transient tritanopia
-3.0 LOG MEAN
-2.0 ERGS - DEi’.
SEC’
Fig. 3. Same as Fig. 2. Observer D.R.
grounds. (The absolute threshold of the blue test lies somewhat below the arrow, since this observer, like A.R., has a z2 branch.) Results shown in Fig. 6 are mean log thresholds obtained by A.R. in the same experiment; individual thresholds are not shown, but did not differ from the mean by more than 0.12 log units. As in Fig 5, solid lines connect flicker-extinction thresholds (0 and 0) for the high and low intensity backgrounds, flickered with a duty cycle of 0.2. They drop sharply as flicker rate is reduced, perhaps more so than for observer D.R. Solid lines connecting symbols (0 and n ) also show flicker-extinction thresholds, but for backgrounds flickered with a duty cycle of 0.5. These drop less sharply with the high intensity background, confirming the result shown in Fig. 2 that the effect of duty cycle is more pronounced at higher intensities. Symbols (0 and 0) on the far right show mean steady thresholds for the low intensity and high intensity backgrounds. (For neither observer did flicker have a measurable effect on the steady thresholds, which varied unsystematically over sessions and flicker rates
and did not depart from the mean by more than 0.1 log units. This was also the case in a separate session in which steady thresholds were measured as soon after the cessation of flicker as possible-about 15 set). Mollon and Polden (1977) and Reeves rt (11.(1978) showed that green cones may mediate detection in the constant-extinction condition, in which extinction of the yellow background has presumably made the “blue” pathway less sensitive to the test than the “green”. They might conceivably do so in flickerextinction, if the green pathway became more sensitive after adaptation to flicker. Hence extinction thresholds for a green 522 nm test (half-bandwidth I2 nm) were obtained with backgrounds of the same intensity as those used for the blue test. Resulting extinction thresholds are plotted with triangles in Fig. 6. They have been shifted vertically to coincide with the constant-extinction thresholds at the high background intensity, where blue and green tests were presumably detected by the same ‘green” pathway. The 522 nm thresholds are flat across flicker rates at both
ADAM REEVES
662
0.3 Q & s 0 2 2 -0.3 ;
-0.6 -0.9
LOG MEAN
ERGS.D&&.SEC-’
Fig. 4. Thresholds of a Wrattan 47B test on a 494 nm background. The abscissa is the log of the mean background inlensity in ergs:deg’/sec (0 log ergs equals Ii.4 log quanta at 494 nmf. Crosses show steady thresholds and are joined by Stile’s 8, branch. Triangles show extinction thresholds. Symbols ( x ) and (A) are for constant adaptation; symbols (+) and (A) are for adaptation to flicker (Hz = 2 for A.R., upper panel; Hz = 0.25 for DR.. lower panel). Duty cycle was 0.2.
/1 s5eo dc = 0.2
-1.5
1 0.25
I
0.5
ADAPTMG
1
1
3 3
FMXER
1
5 7 10 15 20 RATE
lN
HZ
EXfSTEADY CONSTARNT
Fig. 5. Thresholds of a Wrattan 478 test, in arbitrary log units, plotted against the flicker rate in Hz of a field, for observer D.R. Lines connect flicker-extinction thresholds for backgrounds of intensifies M, = - 1.24 and M, = - 2. I4 log ergs/deg2/sec (0 and CJ, respectively). Symbols (0) and (@on the far right show the mean steady thresholds on each of the two background intensities; they lie about 0.4 and 0.X log units above the absolute threshold of K%shown by the arrow. 580 nm adaptation
663
Transient tritanopia
0.25
05
I
AMPTING
23 FLICKER
5710152O~ADY RATE
IN
HZ
CONSTANT
Fig. 6. Symbols (0 and 0) are as in Fig. 5, but for observer A.R. Additional symbols (0 and w) show thresholds at the same two background intensities after flicker with a duty cycle of 0.5. Triangles show thresholds of a 522 nm test obtained after extinction of the 580 nm background, Bickered with duty cycle 0.5 at the same two intensities; the triangles have been shifted vertically to coincide with constant-extinction thresholds for the Wratten 47B test at the high intensity (M,, = - 1.24 log ergs/deg*/sec).
of the background. This result demonstrates that the “green” pathway did not detect the blue test at low flicker rates.
intensities
DISCUSSION Does Picker The
eliminate
transient
tritanopia?
results show clearly that transient tritanopia is greatly reduced at most intensities of a slowly flickered 580 nm background. However, exact interpretation of the flicker-extinction thresholds is complicated, because it is not known how extinction thresholds would have behaved in the absence of the opponent site; and without this knowledge it is impossible to say whether flicker eliminated or merely reduced the effect of that site. However, if the extinction thresholds shown in Fig. 4 for the “neutral” (494nm) background are those to be expected in the absence of an opponent site effect, as argued above, then these thresholds provide a base-line for those obtained with other backgrounds. For example, the thresholds after extinction of the 494 nm background drop below 7crabout as far as they do after extinction of the 580nm background flickered at 0.25 Hz with dc = 0.2 (the top panels in Figs 2 and 3). On this basis, which implicitly detines transient tritanopia as the difference between steady-state and extinction thresholds, such flicker eliminates transient tritanopia in D.R. and leaves behind a residual effect of less than 0.2 log units in A.R. An alternative approach is to try to determine the amount of “opponent site effect” in both steady-state
and extinction conditions. If the rrr branch is determined by long-wavelength cone inputs (Pugh, 1976), it shows the opponent site effect on steady-state thresholds directly. Determining the opponent site effect on extinction thresholds at 580nm, however, requires comparison with thresholds for the neutral (494 nm) background. Assuming that the spectral sensitivity of the blue cones is given by Stiles’s l7,, blue cones are 2.80 log units less sensitive at 580 nm than at 494 nm. The total opponent site effect on A.R.‘s extinction thresholds at 580nm may be measured by subtracting the thresholds obtained in Fig. 4 after a lateral shift of 2.8 log units. This equates 580nm backgrounds of -0.74 log ergs/deg’/sec with 494 nm backgrounds of - 3.54 log ergs/deg2/sec, where extinction thresholds are unlikely to have risen above absolute threshold. Thus the unmodified extinction thresholds, as shown in Figs 2 and 3, reveal the entire opponent site effect on test threshold. This is a small effect in the 0.25 Hz, 0.2 dc condition, but one which increases with background intensity, with a slope of about 0.3 over the intensity range tested. Relation
to Pugh
and Mellon’s
(1979)
model
In this model, the n1 branch of the increment threshold curve results from adaptation of an opponently-coded site. During exposure to long-wavelength backgrounds, the opponent site is polarized in the yellow direction, loses sensitivity, and gives rise to the rrr branch. If the sensitivity of the opponent site recovered in dark adaptation, thresholds would fall after extinction, but since they do not, Pugh and MolIon (1979) postulated that a “restoring force” is built up during adaptation to long-wavelength backgrounds. This force partially compensates for the polarizing effects of the long-wavelength input to the opponent site, in the steady state; but after extinction of the background the restoring force is no longer opposed by an input, and it produces a “rebound” which temporarily perturbs sensitivity and gives rise to transient tritanopia. The mechanisms responsible for the rebound are not relevant here; what is important is the way in which the restoring force, whose magnitude determines the amount of transient tritanopia, is thought to build up during adaptation. In the “feed-forward” form of the model, the restoring force is a sluggish integral of the input to the opponent site. Because the input is smoothed out by a sluggish integration, flickering the input during adaptation should have no effect on thesholds. The present results, and those of Loomis (1980) clearly rule out this possibility: flicker in fact strongly decreases transient tritanopia. Loomis (1980) suggested that the input to the opponent site might be subject to a compressive non-linearity. If this were so, the on-phase in flicker would generate less input to the opponent site than would constant adaptation at the same mean intensity. The restoring force would thereby be less after flicker adaptation.
664
ADAM REEVES
Present results show that this hypothesis is insuffiThe bottom panels in Figs 2 and 3 show that in constant~xtin~ion, test thresholds rise very sharply with background intensity. The sharp rise, which may be underestimated if green cones took over detection (see above). reflects (in Pugh and Mellon’s model) only the increase in long-wavelength cone inputs to the opponent site. If compressive non-linearities occur in the input to that site, they must be mild enough For such sharp rises in threshold to be possible. Yet the flicker-extinction thresholds in the top paneis of Figs 2 and 3 increase only very slightly with background intensity. over a range of intensities considerably larger than the factor of five introduced in the 0.2 dc condition. To account for the results. the feed-forward model may be elaborated by multiplying the input to the restoring force by a function of the log adapting flicker rate, w. such as U&J + constant), which asymptotes to 1.0 at high flicker rates or steady backgrounds, and drops off proportionately to CL)at low Bicker rates, as in Figs 5 and 6. Such an elaboration can also account for the effect of duty cycle: a longer “off’ period allows greater recovery of the opponent site and a larger rebound away from the yellow direction. and hence reduces the restoring force even further. Alternatively. some version of Pugh and MolIon’s (1979) feed-back model. in which the restoring force is the integral of the output of the opponent site. may account for the results. It is clear that the mean output of the opponent site is less during flicker adaptation than during constant adaptation. because in the former case the output of the opponent site fluctuates between zero and a high positive value. while in the latter it remains high. Hence the feedback model does predict less transient tritanopia after flickered adaptation. However. it is not clear how that model cient.
could predict a nearly total absence of transient tritanopia after flicker, since some restoring force must build up during adaptation no matter what the flicker rate.
REFERENCES Augenstein E. J. and Pugh E. N. (1977) The dynamics of the rr, color mechanism: further evidence for two sites of adaptation. J. PQiof. 272, 247-281. Broekhuijsen M. L.. Uilenreef P. L. and Veringa F. (1979) Flicker and chromatic adaptation. Visim Rrs. 19. 565-.561. Jameson D.. Hurvich L. M. and Varner F. D. (1979) Receptoral and postreceptoral visual processes in recovery from chromatic adaptation. Proe. rratn. Acad. Sci. U.S.A. 76. (6). 30343038. Krauskopf J. and Reeves A. (1980) The measurement of the effect of photon noise on detection. L’isim Res. 20. l93- 196. Loomis J. M. (1980) Transient tritanopia: failure of timeintensity reciprocity in adaptation to longwave light. Vision Res. 2% 837-846. Loomis J. M. and Berger 7. (1979) Effects of chromatic adaptation on color discrimination and color appearance. Vision Res. 19, 89 I-90 I. Mollon J. D. and Polden P. G. (1977) An anomaly in the response of the eye to light of short wavelengths. P&l. Trans. R. Sm. 278, 207-240. Pugh E. N. (1976) The nature of the n, mechanism of W. S. Stiles. J. PIzysiol. 257, 713-747. Pugh E. N. and Mollon J. D. (1979) A theory of the R, and rr3 color mechanisms of Stiles. Vision Rex 19, 293-3 t2. Reeves A. (1981) Transient desensitization of a red-green opponent site. In preparation. Reeves A.. Krauskopf J. and Mellon J. D. (1978) Decline of transient tritanopia at high intensities. Inresr. Opltthal, risttai SC%. srq$. p. f 77. Stiles W. S. (1939) The directional sensitivity of the retina and the spectral sensitivities of the rods and cones. Prdc. R. SW. 127, 64 105. Wyszecki G. and Stiles W. S. (1967) Color Sciettcr. Wiley. New York.