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Temporal properties of brightness and color induction
0042-6989/86 s3.00 + 0.00 Copyright Q 1986 Pcrgamon Journals Ltd
1986 Vision RAT. Vol. 26, NO. 6, pp. 881-891, Printed in Great Britain. All rights reserved
TEMPORAL
PROPERTIES OF BRIGHTNESS COLOR INDUCTION
RUSSELLL. DE VALOLS,MICHAELA. Wnssrw
AND
and KAREN K. DE Vets
Departments of Psychology and Physiological Optics, University of California, Berkeley, CA 94120, U.S.A. and BERND LINGELBACH University of Marburg, F.R.G. (Received 8 August 1985; in revised form 14 January 1986)
Abstract-With a matching procedure, we studied the temporal properties of direct brightness (or lightness) and chromatic changes (produced by modulation of the region being matched) and induced brightness and chromatic changes (produced by modulation of the surround of the region being matched). The amount of direct brightness and color change was found to vary only slightly with temporal frequency over the 0.5-8 Hz range studied, whereas induced changes were found to occur only at low temporal frequencies, below about 2.5 Hz. With high temporal-frequency modulation of the surround, the induced patterns appeared to flicker but not to change in brightness or color. Despite the fact that chrominance and luminance temporal contrast sensitivity functions are very different, the temporal induction curves for color and brightness were very similar. However, brightness induction was found to increase approximately linearly with increasing surround modulation up to very high levels, whereas the amount of color induction was much less dependent on the modulation depth of the surround. Induction
Temporal frequency
Brightness contrast
Color contrast
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and color induction* effects were only apparent at very low temporal frequencies (a time-course for which we could find no counterpart, incidentally, in the responses of LGN cells to receptive field surround modulation). The main purpose of this study was to examine quantitatively this temporal dependence of brightness and color induction. For us to refer to all the earlier studies of brightness and color contrast would sorely try the patience of this journal’s type setters. But there appear in the vast literature to be few (e.g. Kinney, 1967; Boynton, 1983) direct studies of the temporal properties of the phenomena, and these are not directly relevant to our concerns here. This is perhaps not too surprising considering the technical difficulties there would have been in producing the requisite stimuli before the advent of computer-controlled oscilloscope displays.
INTRODUCTION
While looking at certain oscilloscope patterns we have used to study the responses of lateral geniculate nucleus (LGN) neurons, we have often noted some striking perceptual phenomena. Among these is the very powerful brightness change produced in a physically-unvarying center by the sinusoidal luminance modulation of an annular surround. This is of course the very familiar phenomenon of brightness contrast or induction, but the perceptual effect seems much more compelling in the temporallymodulated arrangement, perhaps because one can see it changing over time. We have also noted similar, although less striking, induction effects with chromatic modulation of a surround field. The presence of brightness and chromatic induction was of course not surprising, but what was unexpected was that these large brightness *Following traditional terminology, we refer to these phenomena as brightness and color induction or contrast. However, as we point out in the Discussion, the terms “lightness and chromatic induction” might be more appropriate for the perceptual changes seen under our experimental conditions.
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METHODS
This problem was first investigated with luminance modulation of two small black-white monitor oscilloscopes (Tektronix 602 with a P4
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phosphor), placed side-by-side with the centers 4 apart. This initial experiment with 3 observers was later replicated on another 4 observers and was extended to chromatic induction. In this later study we used a single large color monitor display under computer control. Since the results of the two studies are in excellent agreement, only the more recent data will be reported here. The patterns were displayed on a 12-inch Tektronix 650 color monitor, under the control of a Lexidata display system and a Nova 4x computer. The patterns were digitally produced, allowing us to compensate for the nonlinearities in the voltage-luminance function of the monitor-i.e. the voltages sent out to the monitor Z-axis were such as to produce an actual sinusoidal luminance output. The space-average chromaticity of the luminance-modulated patterns was that of illuminant C, and was produced by appropriate modulation of all three oscilloscope guns in phase. Red-green equiluminant color modulation was effected by modulating the R and G guns out of phase, the relative amplitudes of the two being adjusted for equal luminance. The yellow-blue modulation was similarly produced by modulating the B gun out of phase with the R + G guns. The CIE coordinates of the R, G, and B guns, respectively, were 0.59, 0.34; 0.28, 0.58; and 0.14, 0.07, as measured with a Pritchard spectrophotometer. The equiluminance values were determined for each subject individually by flicker photometry. Each observer set the redgreen and the blue-yellow balance for minimum flicker at 15 Hz; these values were then taken as the equiluminant settings for that observer. The space-average and time-average luminances of the displays were all maintained at 37 cd/m’. Two patterns, each consisting of a I’ square center patch in a 3” by 4’ surround, were presented side by side on the monitor. The 4” x 6” display region was itself surrounded by a screen maintained at approximately the same mean luminance. The spatial arrangement was thus much the same as in the classic Hess and Pretori (1894) experiment. In each of our experiments, however, the patterns were temporally modulated. For what we shall call the “matching pattern”, the center on one half of the display was sinusoidally modulated in luminance or color around the 37 cd/m’ mean level, the observer controlling the modulation depth with a potentiometer. The “inspection pattern” was presented on the other half of the monitor.
In some experiments (direct brightness or color change), the center of the inspection pattern was modulated, just as was the case for the matching pattern. In other experiments (induced brightness or color change), the surround was sinusoidally modulated while the center was kept constant at the mean level, see Fig. 1. The depth of modulation and the temporal modulation rate of each pattern could be independently varied under computer control to produce the appropriate conditions. The temporal frequencies studied were 0. 0.5, 0.75, 1.0, 1.5, 2, 3, 4, 6, and 8 Hz. The inspection patterns were modulated at each of two different depths, 60 and 15% contrast. For the luminance-varying patterns, contrast was defined as the Michelson contrast. L mm- LminiLmax For the RG color+ Lean. varying patterns, 100% contrast was arbitrarily defined as 100% modulation of the R gun, plus the modulation (out of phase) of the G gun required by that observer to produce equal luminance. For the BY patterns, 100% contrast was 100% modulation of the B gun plus the amount of modulation of the R + G guns out of phase required to produce equal luminance. Note that while the definitions of luminance and color contrast are not at all comparable, the actual contrast comparisons we make in this paper of luminance vs color are, for we are in each case comparing the refuriz~e modulation settings of the matching and the inspection patterns. The appearance of each of these patterns was strikingly similar, whether the center or the surround was being modulated. The central square in each case appeared to be oscillating between black and white or, in the case of the chromatic patterns, between red and green or blue and yellow. The task of the observer in each experiment was to adjust a potentiometer which controlled the modulation depth of the central square in the matching pattern until it appeared to be oscillating over the same blackwhite, red-green, or blue-yellow range as the apparent oscillations (direct or induced) of the center square in the inspection pattern. When the observer was satisfied with his judgment, he pressed a switch. The computer then recorded the setting and presented the next stimulus combination. In one experiment an additional potentiometer allowed the subject control over the temporal phase of the matching pattern as well. The observers were specifically instructed to ignore the appearance or nonappearance of
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Properties of brightness and color induction
of the intent or significance of the experiment, and one experimenter. Their data on the brightness induction were very similar to those obtained by three other observers tested in an earlier version of the experiment. In addition. tests were made on several other observers at selected points to verify the main conclusion.
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Experiment 1. Matching against a constant standard
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Position tdegreesl Fig. 1. A diagram of the appearance of the apparatus from the point of view of the observer, with the inspection pattern on the right and the adjustable matching pattern on the left. At top a front view of the monitor display; at bottom a cross section through the stimuli in a typical brightness induction test. A luminance modulation of the inspection surround induces an apparent luminance modulation in the physically unvarying inspection center. The observer matches this by adjusting the luminance modulation and (in some experiments) the phase of the matching square.
flicker in making their judgments. As we shall discuss later, there is often a clear dissociation between the perceptual appearance of flicker and that of a lightness or chromatic change in the induced pattern, particularly at high temporal frequencies. Some of the observers reported an asymmetry in the induction process which made it difficult to make a single setting on the matching square. That is, there might be a greater shift towards black than towards white with luminance modulation of the surround. The tests were repeated for these observers, having them make separate settings for each end point in turn, i.e. they made settings to match the black on one set of trials and to match the white on another. However, no consistent differences in the matching contrast settings emerged from these trials, so only the averaged data are reported. Data are presented for four subjects, three experienced visual observers who were ignorant
In this experiment the central oscillating square on the matching field was fixed at 2 Hz. Two different sets of inspection patterns were used. In one [direct brightness or color change, Experiment l(A)], the central squares of both the inspection and the matching fields were oscillated in luminance or color. In the other experiment [induced brightness or color change, Experiment l(B)], the surround alone oscillated in the inspection field. The intent of these experiments was to measure the brightness and color changes (both direct and induced) produced by oscillations at various temporal frequencies between 0.5 and 8 Hz. The matches in each case were made by varying the modulation depth on the matching field, which was oscillating at 2 Hz. Direct change. The results of the tests of direct brightness and color changes are shown in Figs 2 and 3. From Fig. 2 it can be seen that temporal frequencies over the range studied had relatively little effect on the apparent change in brightness at either of the two contrasts examined, although there was some attenuation of the apparent change at the lower temporal frequencies and higher contrasts. For instance, when the central square in the inspection pattern was oscillating at 0.5 Hz over a 60% contrast range, it was matched by the observers with a roughly 50% modulation of the 2 Hz matching pattern; that is, when the 2 Hz pattern was set at that level, these two patterns, flickering at different rates, were judged to be oscillating between the same black and white end points. A similar result was found for equiluminant red-green and blue-yellow color variations, but in the case of the color patterns there was also some decrease in the apparent change at the highest frequencies examined (Fig. 3). Thus the apparent brightness or color changes of a physically varying pattern do depend somewhat on its temporal frequency, but the effect of frequency is in fact very limited, and
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Fig. 2. Individual data for the direct brightness matches made by each of three observers. Plotted are the contrast settings of the 2 Hz matching pattern required to match the brightness changes in the inspection patterns, which were oscillating at the various temporal frequencies shown. Open symbols represent matches to inspection patterns modulated at 60% contrast, solid symbols to modulations of 15%. (Circles: M.W.; upward triangles: H.C.; downward triangles: J.F.) Standard errors for the contrast settings shown here and elsewhere averaged l-2%. Note that over the range studied temporal frequency has only a small effect on the perceived suprathreshold contrast.
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Fig. 3. The direct matches made to equiluminant red-green or blue-yellow color changes in the inspection square, averaged for all observers. Open symbols represent matches to inspection patterns modulated at 60% contrast, solid symbols to modulations of 15%. (Upward triangles, solid lines: red-green; downward triangles, dashed lines: blueyellow.) Here again, the temporal frequency has little effect on the perceived contrast.
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Fig. 4. Individual data on the contrast of the 2 Hz matching square required to match the induced brightness changes in the inspection square, as a function of the temporal frequency of the inducing surround. Unconnected symbols represent the matches made in the presence of static (bright or dark) surrounds. Open symbols represent matches to the inspection square in the presence of a 60% contrast modulation of the surround; solid symbols are for surround modulations of 15%. (Circles: M.W.; upward triangles: H.C.; downward triangles: J.F.) Note that the induction effect is greatly reduced for surround modulations above 2-3 Hz.
large brightness and color modulations are perceived over all of the 0.5-8 Hz range studied. Induced change. It can be seen in Fig. 4 that the temporal dependence of the induced brightness change is quite different from that seen in Experiment l(A) for direct brightness variations. There are two points of interest in these data. One is the very high absolute amount of induction found at low frequencies. Oscillating the surround of a center-surround pattern at low rates induces a third to a half as much brightness change in the (fixed luminance) center square as changing the center square luminance itself does. Also plotted in Fig. 4 are the data for the static case (0.0 Hz), in which the observer made matches at just the end points of the contrast range of the patterns. It can be seen that almost as much induction was produced in this static situation as at low temporal frequencies (note, of course, that the patterns were not stabilized
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Properties of brightness and color induction 70
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Fig. 5. A comparison of direct and induced brightness mat&es, averaged for all subjects. Circles represent direct matches of the 2 Hz standard to 60% (open) or 15%(solid) contrast modulations of the inspection center; triangles represent matches to the physically unvarying inspection center in the presence of 40% (open) or 15% (solid)contrast modulations of the inspection surround. Again, while the direct matches are roughly constant with temporal frequency, the induced matches fall sharply above 2 Hz.
on the retina, so eye movements effectively produced temporally-varying patterns). The main point of interest in the data in Fig. 4 is that-unlike the case with direct brightness matches-the amount of brightness induction falls drastically as temporal frequency is increased, the cut-off being at very low temporal rates. At low rates of temporal modulation the induced brightness changes in the center square are very large and perceptually totally compelling: it is impossible to perceive the central square as remaining constant. Above about 2.5 Hz, however, the perceptual experience is entirely different. The central pattern is seen to flicker; in fact, the flicker is more prominent than at lower frequencies, and not noticeably different from the flicker seen in the matching pattern. But the central square, at these only modestly high temporal frequencies, does not appear to change in brightness as it flickers, but rather remains a neutral grey throughout the cycle. At low frequencies, then, the central square in the induced pattern modulates gently
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Fig. 6. Matches to the induced red-green (upward triangies. solid lines) or blue-yellow (downward triangles, dashed lines) color changes in the inspection center due to 60% (open symbols) or 15% (solid) contrast modulations of the surround. Un~nn~ted symbols represent matches in the presence of static (red or green, blue or yellow) surrounds. As with the induced brightness changes, for both colors the induction effect falls off at higher temporal frequencies.
from black to white with little flicker but with a very pronounced brightness change; at mid to high frequencies the induced pattern stays a constant grey but nonetheless appears to flicker very strongly, particularly around the edges. In Fig. 5 are presented the averaged data for all four observers for the direct vs the induced brightness changes as a function of temporal frequency. Figure 6 presents the data for the tests of induced color change. It can be seen that the time-course for the induced color change is very similar to that for the induced brightness change, and very different from that of the direct color change. Color induction, like brightness induction, is high at low temporal frequencies, but falls drastically beyond 2-3 Hz. Experiment 2. Matching against a uariable standard Having established in Experiment 1 that the direct brightness and color variations of the central square remain fairly constant with temporal frequencies (Figs 2 and 31, we could measure the induced variations at different temporal frequencies in a better way. In this experiment, the temporal frequency of the matching square was set to the same frequency as the test surround in each case. So instead of attempting
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Fig. 7. Induced brightness changes matched by a matching square oscillating at the same rate as the inducing surround, instead of by the fixed 2 Hz standard shown in previous figures. Curves are averages for all observers for surround modulations of 60% (open triangles) or IS% (solid triangles). The fall off of the induction at frequencies above 2 Hz is once again apparent.
to measure the induction produced by, say, a 0.5 Hz modulation by comparing it to a 2 Hz standard, the observer in Experiment 2 was given the simpler task of matching two patterns which always oscillated at the same rate. In this example, the matching pattern would also be oscillating at 0.5 Hz and the observer would adjust its depth of modulation to match the depth of the induced brightness or color changes in the inspection pattern. A second control potentiometer was now added, which allowed the observer to vary the temporal phase of the matching pattern as well. The two patterns could thus be set so that their centers not only oscillated
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the differences in the apparent modulation of the matching square at different temporal frequencies, so that the matches for the extreme frequencies should be slightly elevated to indicate the true induction effect. However, doing so in no way alters the principal conclusions, which are in fact simply reaffirmations of the main findings of Experiment 1: brightness induction is extremely large at low temporal frequencies but falls virtually to zero above 2-3 Hz. Color induction is again also found to have a similar time-course, very different from the direct color change. It is clear from this experiment, as it was from the other, that it is not the case that brightness and color cannot be seen to vary at high temporal frequencies. Center-modulated patterns appear to oscillate between white and black, or from one color to another, even at very high rates. Rather, it is induced brightness and color changes alone which cannot follow even up to the mid-frequency range. Modulation-response relation. In both Experiments 1 and 2 it can be seen that 60% surround modulation produces much more brightness induction than does 15% modulation. The same is not true for color, in which 60% modulation induces very little more (and occasionally less) color change than does 15%. To examine this difference more closely, we measured brightness and color induction at a
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Since the computer read in the phase as well as the modulation-depth setting when the observer indicated a match, any phase shift between direct and indirect brightness or color changes could also be measured. Induced change. In Figs 7 and 8 can be seen the brightness and color matches at various temporal frequencies for each of two modulation depths. These data are uncorrected for
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Fig. 8. Induced red-green (upward triangles, solid lines) or blue-yellow (downward triangles, dashed lines) color changes matched by a matching square oscillating at the same rate as the inducing surround. Curves are averages for all observers for surround modulations of 60% (open symbols) and 15% (solid). Both colors again show the fall off at higher frequencies.
Properties of brightness and color induction
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Fig. 9. The effect of depth of surround contrast on the amount of induction, averaged for observers M.W. and S.W. Both the inducing surround and the matching square oscillated at I Hz. Open circles represent matches to induced brightness changes; solid, upward triangles to induced redgreen changes; open, downward triangles and dashed lines to induced blue-yellow changes. In Fig. 9(a) is plotted the amount of induced brightness and color changes in relation to the amount of surround modulation, for various surround modulation depths. Note that surround modulation produces about 30% brightness induction, regardless of the depth of surround modulation. In the case of color induction. on the other hand. a small chromatic modulation of the surround produces as much as 80% induction, but the proportion drops drastically to less than 10% for large modulations of the surround. These same data are replotted in Fig. 9(b) as log induction vs log surround modulation. Note that while the induced brightness changes increase approximately linearly (slope of I on this log-log plot) with increasing surround contrast, the induced color changes reach a relatively high level at very low surround contrasts and then increase only slightly with higher levels of surround modulation.
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I Hz temporal modulation rate over a large range of contrasts. The average results for two subjects are shown in Fig. 9(a) and (b). Figure 9(a) shows the proportion of induction, relative to the modulation depth of the surround. It can be seen that, in the case of color induction, low contrast modulation of the surround produces almost as much (cu. 80%) change in color of the center as does modulating the chromaticity of the center itself. With increasing modulation depth, however, the amount of color change induced by modulation of the surround falls rapidly relative to the color change produced by direct modulation of the center. Thus a 100% modulation of the surround induces only about 10% as much color change in the center as would a 100% modulation of the center itself. It can also be seen in Fig. 9(a) that the situation for brightness induction is quite different: the amount of induction is about 30% of the direct effect, regardless of the modulation depth of the surround. These same data are replotted in Fig. 9(b) to show, on log-log axes, how the amount of induction varies with surround modulation depth. It can be seen that brightness induction increases fairly linearly (slope of almost one on a log-log plot) as surround modulation depth increases. For color induction, however, the amount of induction is high with small surround modulation, but it then goes up only slightly more as surround modulation increases up to even very high levels. Color induction is thus much more pronounced than brightness induction with small changes in the surround, but it is much less prominent than brightness induction with large surround changes. Phase. To a first approximation, there is a 180” phase shift between the brightness or color changes produced by center vs surround modulation. Increasing the luminance of the center has the same effect as decreasing the luminance of the surround: both make the center appear white. To a second approximation, however, the phase shift appears to be slightly greater than l80”, suggesting that the inducing effect takes some greater time to develop than does the direct change. We found, however, that this was very difficult to quantify, partly because of the impreciseness with which observers can make even direct phase matches (between 2 physically varying squares). Thus for the most part the data proved too variable to allow us to specify accurately the phase lag of the induction.
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Fig. 10. Phase matches for direct and induced brightness changes for observer M.W. The observer attempted to synchronize the oscillations of the matching and inspection squares, both of which were changing at the same rate between 0.5 and 2 Hz. Solid symbols and solid lines represent phase matches to direct oscillations of 60% (circles) or 15% (triangles) contrast in the inspection square. Open symbols and dashed lines are for matches to induced oscillations in the presence of 60% (circles) or 15% (triangles) contrast modulation of the surround. For the induced matches, 180” has been subtracted from the actual phase settings to obtain the relative settings shown here. Note that there is a roughly constant phase lag in the low contrast induction, while not in the high contrast induction.
However, for the one subject on whom we have complete sets of both direct and indirect phase matches, there was strong evidence of a roughly 10-15” phase lag (i.e. setting of 190-195”) in the brightness induction for a 15% surround modulation, see Fig. 10. At 60% surround modulation, though, a phase lag was not evident. The “larger” phase lag for the 15% surround and the fact that this lag is roughly constant over the 0.5-2 Hz frequency range examined are qualitatively consistent with the notion that the delay occurs because the surround must reach a fixed contrast before it triggers the induction. For instance, the average phase lag of 12.6” for the 15% surround would suggest that this threshold trigger contrast is 3.3% (the contrast of the surround at that phase). In turn, this contrast level would lead to the prediction that for the 60% surround a phase lag of only 3” should have been observed and, since our standard errors averaged 4.4”, such a slight delay is clearly not ruled out. However, this simple interpretation is inconsis-
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tent with the fact that induction was clearly apparent (though difficult for us to measure) for surround modulations at least as low as 2%. Pattern size. The experiments described so far were all done with I inspection and matching patterns. As a final question, we asked how critically our results depended on the stimulus size. For instance. if the spread of the induction across the inspection pattern were slow relative to the temporal frequency of the surround. then the high frequency fall off could be due to an averaging of several “waves” of induction across the pattern. In such a case the cut-off frequency might be expected to be higher for smaller areas. To test this we repeated Experiments 1 and 2 for two observers. with two different inspection pattern sizes: 15’ and 1 for observer M.W., and 14’, 46’. 1 , and 3 for observer R.G. However, no real differences in either the magnitude of the induction or in its temporal dependence were found for the other inspection patterns, relative to the standard Isize. DISCUSSION
The main finding of this paper is that the large induction effects observed fall virtually to zero with temporal modulation above about 2.5 Hz. The time-course for induction is thus very different from that for direct brightness changes produced by luminance variation of the area being judged: such direct brightness matches were found to be largely flat across the temporal range studied, from 0.5 to 8 Hz. The time course of induction is also quite different from that for the detection of flicker, which peaks at 4-6 Hz, extends to 30 Hz or so at this luminance level, and is actually attenuated over the low temporal frequency range at which induction is the strongest. The very slow temporal characteristics of induction suggest a cortical site-perhaps even a late cortical site-for the underlying interactions, rather than the retinal processes which are generally assumed to account for them. The fact that all of the cell types seen in monkey geniculate respond to receptive field (RF) surround modulation (or any other stimulus pattern we have examined) up to very high temporal frequencies reinforces this suspicion. Some time ago, we showed that the spatial distance over which brightness induction operates far exceeds the dimensions of LGN receptive fields in monkey (De Valois and
Properties of brightness and color induction
Pease, 1971; Yund et al., 1977). Brightness induction can operate to 10” or more (Yund and Armington, 1975), whereas the maximum RF sizes in centrally-related LGN cells is at most a degree or two. We concluded on those grounds that brightness induction must reflect some later cortical, rather than a retinal, process. The temporal properties of induction, reported here, reinforce this view. The spatial-frequency specificity of induction effects seen psychophysically (McCourt, 1982) would also point in this direction, as does the work of Boynton (1983). In addition to examining the temporal properties of brightness variations, we also studied the time-course of direct and induced chromatic changes. As was the case for brightness in, duction, color induction produced by equiluminant chromatic modulation of the surround was found to have very different temporal properties from the direct change in color appearance produced by modulation of the center region itself. Again, color induction was found to fall almost to zero at temporal frequencies above about 2.5 Hz, whereas the direct color change was found to be comparatively flat from 0.5 to 8 Hz. While the time-courses for brightness and color induction were found to be very similar, their magnitudes were not. The amount of brightness induction increased approximately linearly with increased modulation of the surround, thus closely following Weber’s Law, confirming results first noted by Helmholtz (1924). Color induction, on the other hand, was quite pronounced even at very low levels of surround modulation, but then increased only slightly as the surround modulation depth was raised. This is consistent with the fact that color induction is easily demonstrated with very desaturated patterns, e.g. the classic color shadows situation. It is also consistent with the report of Kinney ( 1962). The induction produced by temporally modulating a pattern at some low frequency is very striking perceptually, much more so than the usual static demonstration of brightness contrast. However, the settings our observers made of the static situation did not bear out this impression: their matches of the static patterns were only slightly less than those made at low temporal frequencies. Thus it is perhaps simply the constantly changing nature of the temporally-modulated patterns which produces such a compelling impression. In the static
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situation both the inspection and matching patterns may well be subject to adaptation. This could perhaps reduce the salience of both of the patterns while not altering the match between them. The classical view of the way in which the brightness and color of an area are specified is that it is first of all a function of the luminance and wavelength variations of the region itself. These are then modified to some extent by lateral interactions with neighboring regions, in brightness and color induction. The presence of such very different time-courses for the direct and induced brightness and color changes we found in this experiment provide support for such a dual-stage model. It can perhaps be added to the long list of phenomena which cannot be accounted for by the Land (1959) model of a single process dependent on information about changes across edges. The induction effect we investigated here with luminance variation in the surround has classically been called “brightness induction”. However, the perceptual situation bears some similarity to surface color perception, with the variations seen being from black to white along a lightness scale. Indeed, we instructed our subjects to make their settings so that the induced changes in the central square appeared to be oscillating between the same black-white end points as the matching pattern. From the point of view of an opponent-color theory such as that of Hurvich and Jameson (1957), then, one might argue that we were really studying not brightness and color induction, but rather color induction along a black-white axis, as well as along the red-green and blue-yellow axes (brightness being determined by something else, such as the total visual activity in the region). If all of these situations involved color induction, rather than brightness in some and color in others, the similarity of the time-courses would not be unexpected. However, this would leave unexplained the very different modulationdepth vs induction curves (shown in Fig. 9) for luminance vs chromatic surround modulation. Numerous studies have shown that the spatial and temporal contrast sensitivity functions for luminance-varying and pure color-varying patterns are quite different from each other (e.g. van der Horst and Bouman, 1969; Kelly, 1975). Differences have also been reported in the temporal CSFs for red-green vs blue-yellow patterns. In general, luminance temporal contrast sensitivity shows considerable low-temporal fre-
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quency attenuation, with significant sensitivity up to very high temporal frequencies. With equiluminant red-green color variations, on the other hand, there is little or no low frequency attenuation and a low high-frequency cut. Yellow-blue sensiti~ty in turn is shifted to still lower temporal frequencies than red-green. Given this, it is all the more remarkable that the shapes of the induction curves in this study were so similar under all these conditions. Black-white, red-green, and yellow-blue induction settings all fell to almost zero at about 2.5 Hz. The differing CSFs for chromatic and luminance variations can be accounted for on the basis of the quite different RF properties of LGN opponent cells for luminance-varying vs chromatically-varying patterns (De Valois and De Valois, 1975; De Valois e? ai., 1977). The similarity of the temporal properties of color and brightness induction suggests a single, later, cortical process which operates similarly on both brightness and color information. The difference in the amount of induction as a function of the amount of surround contrast for the brightness vs color patterns, however, raises problems for such a simple explanation. Not only do the induction curves for both brightness and color matches differ drastically from their respective temporal contrast sensitivity functions, but the direct brightness, and perhaps color, matches do, too. While the threshold temporal CSF for luminance variations shows considerable low-frequency attenuation, we found the suprathreshold direct brightness matching curves to be relatively flat (see also Bowker, 1983). A similar discrepancy has been reported in the spatial domain, the suprathreshold spatial CSF for luminance variations being much flatter than the threshold curve (Georgeson and Sullivan, 1975). On the other hand, the low frequency attenuation we observed for the direct color matches is somewhat surprising, in that a low frequency falloff is generally not reported for the color CSF. Unfortunately, the threshold CSF could not be accurately measured for our stimulus conditions. Finally, we noted a very obvious perceptual dissociation between the impression of flicker and that of a change in brightness or lightness. At low temporal frequencies, the central square in the luminance induction pattern gradually changed from white to black, with little or no appearance of flicker. At high temporal frequencies, however, the pattern had a very pro-
nounced flicker, but did not appear to change at all in the black-white dimension, remaining a neutral grey throughout the cycle. A similar dissociation between (a color) flicker and a change in color appearance was seen at various temporal frequencies in the color-varying patterns. It may seem strange that a luminancevarying pattern can produce an appearance of flicker without the brightness or lightness actually changing at all, or of there being a color flicker without the color appearing to change. It is only one instance of many, however, in which logically incompatible percepts are clearly seen simultaneously. For instance, a field of resolvable red and blue dots can be seen as purple (the mixture of the red and the blue) while at the same time one can see the individual red and blue dots. Such a logically-incompatible judgment can be explained if one has simultaneously accessible the outputs from color-spatial channels tuned to quite different spatial frequency ranges. Another example, closely related to the findings in this paper, is the dissociation first reported by Wertheimer (1912) between the percept of movement and that of something having moved. Under conditions which produce “pure phi”, one sees movement between two alternated spots while simultaneously seeing both the spots as remaining stationary. So also in this induction situation with a surround luminance or chromatic oscillation at a high temporal frequency, one sees the square flickering without any change in its appearance along the black-white, red-green or yellow-blue axes. What this suggests is that there may be cells, or channels, involved in detecting flicker in a region that are separate from those channels involved in specifying the chromaticity or lightness of the region. The outputs of both of these types of channels SimultaneousIy contribute to our percept, despite sometimes carrying logically contradictory information. thank Helen Coromvli, Jennifer Franklin, and Shernaaz Webster for participating as subjects. This research was supported by NIH grant EY-00014 and NSF grant BNS 8242275. Acknowledgemenrs-We
REFERENCES Boynton R. M. (1983) Mechanisms of chromatic discrimination. In Colour Vision: Physiology and Psychaphvsics (Edited by Mellon J. D. and Sharpe L. T.), pp. 409-423. Academic Press, New York. Bowker D. 0. (1983) Suprathreshold spatiotemporal response characteristics of the human visual system. J. opt. Sot. Am. 73, 436-440.
Properties of brightness and color induction De Valois R. L. and De Valois K. K. (1975) Neural coding of color. In Handhook C$ Perception V (Edited by Carterette E. C. and Friedman M. P.). pp. 117-166. Academic Press, New York. De Valois R. L. and Pease P. L. (1971) Contours and contrast: responses of monkey lateral geniculate cells to luminance and color figures. Science 171, 694696. De Valois R. L., Snodderly D. M., Yund E. W. and Hepler N. (1977) Responses of macaque lateral geniculate cells to luminance and color figures. Sensory Proc. 1, 244-259. Georgeson M. A. and Sullivan G. D. (1975) Contrast constancy: deblurring in human visual system by spatial frequency channels. J. Physiol., Land. U2, 627-656. Helmholtz H. von (1924) Physiological Optics (Translated by Southall J.), Opt. Sot. Am., Rochester, N.Y. Hess C. and Pretori H. (1894) Messende Untersuchungen iiber die Gesetxmiissigkeit des simultanen Helligkeitscontrastes. Arch. Ophrhal. 40, l-24. Hurvich L. M. and Jameson D. (1957) An opponent-process theory of color vision. Psychol. Rev. 44, 384-404.
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Kelly D. H. (1975) Luminous and chromatic flickering patterns have opposite effects. Science 188, 371-372. Kinney J. A. S. (1962) Factors affecting induced color. Vision Res. 2, 503-525.
Kinney J. A. S. (1967) Color induction using asynchronous Rashes. Vision Res. 7, 299-318. Land E. H. (1959) Color vision and the natural image. Proc. nafn. Acad. Sci. 45, I 15-129. McCourt M. E. (1982) A spatial frequency dependent grating-induction e&t. Vision Res. 22, I 19-134. van der Horst G. J. C. and Bouman M. A. (1969) Spatiotemporal chromaticity discrimination. J. opf. Sot. Am. 59, 1482-1488. Wertheimer M. (1912) Experimentelle Studien iiber das Sehen von Bewegung. Z. Psycho/. 61, 161-265. Yund E. W. and Armington J. C. (1975) Color and brightness contrast effects as a function of spatial variables. Vision Rex
15, 917-929.
Yund E. W., Snodderly D. M.. Hepler N. K. and De Valois R. L. (1977) Brightness contrast effects in monkey lateral geniculate nucleus. Sensory Proc. I, 260-271.
1986 Vision RAT. Vol. 26, NO. 6, pp. 881-891, Printed in Great Britain. All rights reserved
TEMPORAL
PROPERTIES OF BRIGHTNESS COLOR INDUCTION
RUSSELLL. DE VALOLS,MICHAELA. Wnssrw
AND
and KAREN K. DE Vets
Departments of Psychology and Physiological Optics, University of California, Berkeley, CA 94120, U.S.A. and BERND LINGELBACH University of Marburg, F.R.G. (Received 8 August 1985; in revised form 14 January 1986)
Abstract-With a matching procedure, we studied the temporal properties of direct brightness (or lightness) and chromatic changes (produced by modulation of the region being matched) and induced brightness and chromatic changes (produced by modulation of the surround of the region being matched). The amount of direct brightness and color change was found to vary only slightly with temporal frequency over the 0.5-8 Hz range studied, whereas induced changes were found to occur only at low temporal frequencies, below about 2.5 Hz. With high temporal-frequency modulation of the surround, the induced patterns appeared to flicker but not to change in brightness or color. Despite the fact that chrominance and luminance temporal contrast sensitivity functions are very different, the temporal induction curves for color and brightness were very similar. However, brightness induction was found to increase approximately linearly with increasing surround modulation up to very high levels, whereas the amount of color induction was much less dependent on the modulation depth of the surround. Induction
Temporal frequency
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and color induction* effects were only apparent at very low temporal frequencies (a time-course for which we could find no counterpart, incidentally, in the responses of LGN cells to receptive field surround modulation). The main purpose of this study was to examine quantitatively this temporal dependence of brightness and color induction. For us to refer to all the earlier studies of brightness and color contrast would sorely try the patience of this journal’s type setters. But there appear in the vast literature to be few (e.g. Kinney, 1967; Boynton, 1983) direct studies of the temporal properties of the phenomena, and these are not directly relevant to our concerns here. This is perhaps not too surprising considering the technical difficulties there would have been in producing the requisite stimuli before the advent of computer-controlled oscilloscope displays.
INTRODUCTION
While looking at certain oscilloscope patterns we have used to study the responses of lateral geniculate nucleus (LGN) neurons, we have often noted some striking perceptual phenomena. Among these is the very powerful brightness change produced in a physically-unvarying center by the sinusoidal luminance modulation of an annular surround. This is of course the very familiar phenomenon of brightness contrast or induction, but the perceptual effect seems much more compelling in the temporallymodulated arrangement, perhaps because one can see it changing over time. We have also noted similar, although less striking, induction effects with chromatic modulation of a surround field. The presence of brightness and chromatic induction was of course not surprising, but what was unexpected was that these large brightness *Following traditional terminology, we refer to these phenomena as brightness and color induction or contrast. However, as we point out in the Discussion, the terms “lightness and chromatic induction” might be more appropriate for the perceptual changes seen under our experimental conditions.
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METHODS
This problem was first investigated with luminance modulation of two small black-white monitor oscilloscopes (Tektronix 602 with a P4
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phosphor), placed side-by-side with the centers 4 apart. This initial experiment with 3 observers was later replicated on another 4 observers and was extended to chromatic induction. In this later study we used a single large color monitor display under computer control. Since the results of the two studies are in excellent agreement, only the more recent data will be reported here. The patterns were displayed on a 12-inch Tektronix 650 color monitor, under the control of a Lexidata display system and a Nova 4x computer. The patterns were digitally produced, allowing us to compensate for the nonlinearities in the voltage-luminance function of the monitor-i.e. the voltages sent out to the monitor Z-axis were such as to produce an actual sinusoidal luminance output. The space-average chromaticity of the luminance-modulated patterns was that of illuminant C, and was produced by appropriate modulation of all three oscilloscope guns in phase. Red-green equiluminant color modulation was effected by modulating the R and G guns out of phase, the relative amplitudes of the two being adjusted for equal luminance. The yellow-blue modulation was similarly produced by modulating the B gun out of phase with the R + G guns. The CIE coordinates of the R, G, and B guns, respectively, were 0.59, 0.34; 0.28, 0.58; and 0.14, 0.07, as measured with a Pritchard spectrophotometer. The equiluminance values were determined for each subject individually by flicker photometry. Each observer set the redgreen and the blue-yellow balance for minimum flicker at 15 Hz; these values were then taken as the equiluminant settings for that observer. The space-average and time-average luminances of the displays were all maintained at 37 cd/m’. Two patterns, each consisting of a I’ square center patch in a 3” by 4’ surround, were presented side by side on the monitor. The 4” x 6” display region was itself surrounded by a screen maintained at approximately the same mean luminance. The spatial arrangement was thus much the same as in the classic Hess and Pretori (1894) experiment. In each of our experiments, however, the patterns were temporally modulated. For what we shall call the “matching pattern”, the center on one half of the display was sinusoidally modulated in luminance or color around the 37 cd/m’ mean level, the observer controlling the modulation depth with a potentiometer. The “inspection pattern” was presented on the other half of the monitor.
In some experiments (direct brightness or color change), the center of the inspection pattern was modulated, just as was the case for the matching pattern. In other experiments (induced brightness or color change), the surround was sinusoidally modulated while the center was kept constant at the mean level, see Fig. 1. The depth of modulation and the temporal modulation rate of each pattern could be independently varied under computer control to produce the appropriate conditions. The temporal frequencies studied were 0. 0.5, 0.75, 1.0, 1.5, 2, 3, 4, 6, and 8 Hz. The inspection patterns were modulated at each of two different depths, 60 and 15% contrast. For the luminance-varying patterns, contrast was defined as the Michelson contrast. L mm- LminiLmax For the RG color+ Lean. varying patterns, 100% contrast was arbitrarily defined as 100% modulation of the R gun, plus the modulation (out of phase) of the G gun required by that observer to produce equal luminance. For the BY patterns, 100% contrast was 100% modulation of the B gun plus the amount of modulation of the R + G guns out of phase required to produce equal luminance. Note that while the definitions of luminance and color contrast are not at all comparable, the actual contrast comparisons we make in this paper of luminance vs color are, for we are in each case comparing the refuriz~e modulation settings of the matching and the inspection patterns. The appearance of each of these patterns was strikingly similar, whether the center or the surround was being modulated. The central square in each case appeared to be oscillating between black and white or, in the case of the chromatic patterns, between red and green or blue and yellow. The task of the observer in each experiment was to adjust a potentiometer which controlled the modulation depth of the central square in the matching pattern until it appeared to be oscillating over the same blackwhite, red-green, or blue-yellow range as the apparent oscillations (direct or induced) of the center square in the inspection pattern. When the observer was satisfied with his judgment, he pressed a switch. The computer then recorded the setting and presented the next stimulus combination. In one experiment an additional potentiometer allowed the subject control over the temporal phase of the matching pattern as well. The observers were specifically instructed to ignore the appearance or nonappearance of
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of the intent or significance of the experiment, and one experimenter. Their data on the brightness induction were very similar to those obtained by three other observers tested in an earlier version of the experiment. In addition. tests were made on several other observers at selected points to verify the main conclusion.
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flicker in making their judgments. As we shall discuss later, there is often a clear dissociation between the perceptual appearance of flicker and that of a lightness or chromatic change in the induced pattern, particularly at high temporal frequencies. Some of the observers reported an asymmetry in the induction process which made it difficult to make a single setting on the matching square. That is, there might be a greater shift towards black than towards white with luminance modulation of the surround. The tests were repeated for these observers, having them make separate settings for each end point in turn, i.e. they made settings to match the black on one set of trials and to match the white on another. However, no consistent differences in the matching contrast settings emerged from these trials, so only the averaged data are reported. Data are presented for four subjects, three experienced visual observers who were ignorant
In this experiment the central oscillating square on the matching field was fixed at 2 Hz. Two different sets of inspection patterns were used. In one [direct brightness or color change, Experiment l(A)], the central squares of both the inspection and the matching fields were oscillated in luminance or color. In the other experiment [induced brightness or color change, Experiment l(B)], the surround alone oscillated in the inspection field. The intent of these experiments was to measure the brightness and color changes (both direct and induced) produced by oscillations at various temporal frequencies between 0.5 and 8 Hz. The matches in each case were made by varying the modulation depth on the matching field, which was oscillating at 2 Hz. Direct change. The results of the tests of direct brightness and color changes are shown in Figs 2 and 3. From Fig. 2 it can be seen that temporal frequencies over the range studied had relatively little effect on the apparent change in brightness at either of the two contrasts examined, although there was some attenuation of the apparent change at the lower temporal frequencies and higher contrasts. For instance, when the central square in the inspection pattern was oscillating at 0.5 Hz over a 60% contrast range, it was matched by the observers with a roughly 50% modulation of the 2 Hz matching pattern; that is, when the 2 Hz pattern was set at that level, these two patterns, flickering at different rates, were judged to be oscillating between the same black and white end points. A similar result was found for equiluminant red-green and blue-yellow color variations, but in the case of the color patterns there was also some decrease in the apparent change at the highest frequencies examined (Fig. 3). Thus the apparent brightness or color changes of a physically varying pattern do depend somewhat on its temporal frequency, but the effect of frequency is in fact very limited, and
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Fig. 2. Individual data for the direct brightness matches made by each of three observers. Plotted are the contrast settings of the 2 Hz matching pattern required to match the brightness changes in the inspection patterns, which were oscillating at the various temporal frequencies shown. Open symbols represent matches to inspection patterns modulated at 60% contrast, solid symbols to modulations of 15%. (Circles: M.W.; upward triangles: H.C.; downward triangles: J.F.) Standard errors for the contrast settings shown here and elsewhere averaged l-2%. Note that over the range studied temporal frequency has only a small effect on the perceived suprathreshold contrast.
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Fig. 3. The direct matches made to equiluminant red-green or blue-yellow color changes in the inspection square, averaged for all observers. Open symbols represent matches to inspection patterns modulated at 60% contrast, solid symbols to modulations of 15%. (Upward triangles, solid lines: red-green; downward triangles, dashed lines: blueyellow.) Here again, the temporal frequency has little effect on the perceived contrast.
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Fig. 4. Individual data on the contrast of the 2 Hz matching square required to match the induced brightness changes in the inspection square, as a function of the temporal frequency of the inducing surround. Unconnected symbols represent the matches made in the presence of static (bright or dark) surrounds. Open symbols represent matches to the inspection square in the presence of a 60% contrast modulation of the surround; solid symbols are for surround modulations of 15%. (Circles: M.W.; upward triangles: H.C.; downward triangles: J.F.) Note that the induction effect is greatly reduced for surround modulations above 2-3 Hz.
large brightness and color modulations are perceived over all of the 0.5-8 Hz range studied. Induced change. It can be seen in Fig. 4 that the temporal dependence of the induced brightness change is quite different from that seen in Experiment l(A) for direct brightness variations. There are two points of interest in these data. One is the very high absolute amount of induction found at low frequencies. Oscillating the surround of a center-surround pattern at low rates induces a third to a half as much brightness change in the (fixed luminance) center square as changing the center square luminance itself does. Also plotted in Fig. 4 are the data for the static case (0.0 Hz), in which the observer made matches at just the end points of the contrast range of the patterns. It can be seen that almost as much induction was produced in this static situation as at low temporal frequencies (note, of course, that the patterns were not stabilized
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Properties of brightness and color induction 70
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Fig. 5. A comparison of direct and induced brightness mat&es, averaged for all subjects. Circles represent direct matches of the 2 Hz standard to 60% (open) or 15%(solid) contrast modulations of the inspection center; triangles represent matches to the physically unvarying inspection center in the presence of 40% (open) or 15% (solid)contrast modulations of the inspection surround. Again, while the direct matches are roughly constant with temporal frequency, the induced matches fall sharply above 2 Hz.
on the retina, so eye movements effectively produced temporally-varying patterns). The main point of interest in the data in Fig. 4 is that-unlike the case with direct brightness matches-the amount of brightness induction falls drastically as temporal frequency is increased, the cut-off being at very low temporal rates. At low rates of temporal modulation the induced brightness changes in the center square are very large and perceptually totally compelling: it is impossible to perceive the central square as remaining constant. Above about 2.5 Hz, however, the perceptual experience is entirely different. The central pattern is seen to flicker; in fact, the flicker is more prominent than at lower frequencies, and not noticeably different from the flicker seen in the matching pattern. But the central square, at these only modestly high temporal frequencies, does not appear to change in brightness as it flickers, but rather remains a neutral grey throughout the cycle. At low frequencies, then, the central square in the induced pattern modulates gently
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Fig. 6. Matches to the induced red-green (upward triangies. solid lines) or blue-yellow (downward triangles, dashed lines) color changes in the inspection center due to 60% (open symbols) or 15% (solid) contrast modulations of the surround. Un~nn~ted symbols represent matches in the presence of static (red or green, blue or yellow) surrounds. As with the induced brightness changes, for both colors the induction effect falls off at higher temporal frequencies.
from black to white with little flicker but with a very pronounced brightness change; at mid to high frequencies the induced pattern stays a constant grey but nonetheless appears to flicker very strongly, particularly around the edges. In Fig. 5 are presented the averaged data for all four observers for the direct vs the induced brightness changes as a function of temporal frequency. Figure 6 presents the data for the tests of induced color change. It can be seen that the time-course for the induced color change is very similar to that for the induced brightness change, and very different from that of the direct color change. Color induction, like brightness induction, is high at low temporal frequencies, but falls drastically beyond 2-3 Hz. Experiment 2. Matching against a uariable standard Having established in Experiment 1 that the direct brightness and color variations of the central square remain fairly constant with temporal frequencies (Figs 2 and 31, we could measure the induced variations at different temporal frequencies in a better way. In this experiment, the temporal frequency of the matching square was set to the same frequency as the test surround in each case. So instead of attempting
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Fig. 7. Induced brightness changes matched by a matching square oscillating at the same rate as the inducing surround, instead of by the fixed 2 Hz standard shown in previous figures. Curves are averages for all observers for surround modulations of 60% (open triangles) or IS% (solid triangles). The fall off of the induction at frequencies above 2 Hz is once again apparent.
to measure the induction produced by, say, a 0.5 Hz modulation by comparing it to a 2 Hz standard, the observer in Experiment 2 was given the simpler task of matching two patterns which always oscillated at the same rate. In this example, the matching pattern would also be oscillating at 0.5 Hz and the observer would adjust its depth of modulation to match the depth of the induced brightness or color changes in the inspection pattern. A second control potentiometer was now added, which allowed the observer to vary the temporal phase of the matching pattern as well. The two patterns could thus be set so that their centers not only oscillated
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the differences in the apparent modulation of the matching square at different temporal frequencies, so that the matches for the extreme frequencies should be slightly elevated to indicate the true induction effect. However, doing so in no way alters the principal conclusions, which are in fact simply reaffirmations of the main findings of Experiment 1: brightness induction is extremely large at low temporal frequencies but falls virtually to zero above 2-3 Hz. Color induction is again also found to have a similar time-course, very different from the direct color change. It is clear from this experiment, as it was from the other, that it is not the case that brightness and color cannot be seen to vary at high temporal frequencies. Center-modulated patterns appear to oscillate between white and black, or from one color to another, even at very high rates. Rather, it is induced brightness and color changes alone which cannot follow even up to the mid-frequency range. Modulation-response relation. In both Experiments 1 and 2 it can be seen that 60% surround modulation produces much more brightness induction than does 15% modulation. The same is not true for color, in which 60% modulation induces very little more (and occasionally less) color change than does 15%. To examine this difference more closely, we measured brightness and color induction at a
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Since the computer read in the phase as well as the modulation-depth setting when the observer indicated a match, any phase shift between direct and indirect brightness or color changes could also be measured. Induced change. In Figs 7 and 8 can be seen the brightness and color matches at various temporal frequencies for each of two modulation depths. These data are uncorrected for
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Fig. 8. Induced red-green (upward triangles, solid lines) or blue-yellow (downward triangles, dashed lines) color changes matched by a matching square oscillating at the same rate as the inducing surround. Curves are averages for all observers for surround modulations of 60% (open symbols) and 15% (solid). Both colors again show the fall off at higher frequencies.
Properties of brightness and color induction
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Fig. 9. The effect of depth of surround contrast on the amount of induction, averaged for observers M.W. and S.W. Both the inducing surround and the matching square oscillated at I Hz. Open circles represent matches to induced brightness changes; solid, upward triangles to induced redgreen changes; open, downward triangles and dashed lines to induced blue-yellow changes. In Fig. 9(a) is plotted the amount of induced brightness and color changes in relation to the amount of surround modulation, for various surround modulation depths. Note that surround modulation produces about 30% brightness induction, regardless of the depth of surround modulation. In the case of color induction. on the other hand. a small chromatic modulation of the surround produces as much as 80% induction, but the proportion drops drastically to less than 10% for large modulations of the surround. These same data are replotted in Fig. 9(b) as log induction vs log surround modulation. Note that while the induced brightness changes increase approximately linearly (slope of I on this log-log plot) with increasing surround contrast, the induced color changes reach a relatively high level at very low surround contrasts and then increase only slightly with higher levels of surround modulation.
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I Hz temporal modulation rate over a large range of contrasts. The average results for two subjects are shown in Fig. 9(a) and (b). Figure 9(a) shows the proportion of induction, relative to the modulation depth of the surround. It can be seen that, in the case of color induction, low contrast modulation of the surround produces almost as much (cu. 80%) change in color of the center as does modulating the chromaticity of the center itself. With increasing modulation depth, however, the amount of color change induced by modulation of the surround falls rapidly relative to the color change produced by direct modulation of the center. Thus a 100% modulation of the surround induces only about 10% as much color change in the center as would a 100% modulation of the center itself. It can also be seen in Fig. 9(a) that the situation for brightness induction is quite different: the amount of induction is about 30% of the direct effect, regardless of the modulation depth of the surround. These same data are replotted in Fig. 9(b) to show, on log-log axes, how the amount of induction varies with surround modulation depth. It can be seen that brightness induction increases fairly linearly (slope of almost one on a log-log plot) as surround modulation depth increases. For color induction, however, the amount of induction is high with small surround modulation, but it then goes up only slightly more as surround modulation increases up to even very high levels. Color induction is thus much more pronounced than brightness induction with small changes in the surround, but it is much less prominent than brightness induction with large surround changes. Phase. To a first approximation, there is a 180” phase shift between the brightness or color changes produced by center vs surround modulation. Increasing the luminance of the center has the same effect as decreasing the luminance of the surround: both make the center appear white. To a second approximation, however, the phase shift appears to be slightly greater than l80”, suggesting that the inducing effect takes some greater time to develop than does the direct change. We found, however, that this was very difficult to quantify, partly because of the impreciseness with which observers can make even direct phase matches (between 2 physically varying squares). Thus for the most part the data proved too variable to allow us to specify accurately the phase lag of the induction.
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Fig. 10. Phase matches for direct and induced brightness changes for observer M.W. The observer attempted to synchronize the oscillations of the matching and inspection squares, both of which were changing at the same rate between 0.5 and 2 Hz. Solid symbols and solid lines represent phase matches to direct oscillations of 60% (circles) or 15% (triangles) contrast in the inspection square. Open symbols and dashed lines are for matches to induced oscillations in the presence of 60% (circles) or 15% (triangles) contrast modulation of the surround. For the induced matches, 180” has been subtracted from the actual phase settings to obtain the relative settings shown here. Note that there is a roughly constant phase lag in the low contrast induction, while not in the high contrast induction.
However, for the one subject on whom we have complete sets of both direct and indirect phase matches, there was strong evidence of a roughly 10-15” phase lag (i.e. setting of 190-195”) in the brightness induction for a 15% surround modulation, see Fig. 10. At 60% surround modulation, though, a phase lag was not evident. The “larger” phase lag for the 15% surround and the fact that this lag is roughly constant over the 0.5-2 Hz frequency range examined are qualitatively consistent with the notion that the delay occurs because the surround must reach a fixed contrast before it triggers the induction. For instance, the average phase lag of 12.6” for the 15% surround would suggest that this threshold trigger contrast is 3.3% (the contrast of the surround at that phase). In turn, this contrast level would lead to the prediction that for the 60% surround a phase lag of only 3” should have been observed and, since our standard errors averaged 4.4”, such a slight delay is clearly not ruled out. However, this simple interpretation is inconsis-
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tent with the fact that induction was clearly apparent (though difficult for us to measure) for surround modulations at least as low as 2%. Pattern size. The experiments described so far were all done with I inspection and matching patterns. As a final question, we asked how critically our results depended on the stimulus size. For instance. if the spread of the induction across the inspection pattern were slow relative to the temporal frequency of the surround. then the high frequency fall off could be due to an averaging of several “waves” of induction across the pattern. In such a case the cut-off frequency might be expected to be higher for smaller areas. To test this we repeated Experiments 1 and 2 for two observers. with two different inspection pattern sizes: 15’ and 1 for observer M.W., and 14’, 46’. 1 , and 3 for observer R.G. However, no real differences in either the magnitude of the induction or in its temporal dependence were found for the other inspection patterns, relative to the standard Isize. DISCUSSION
The main finding of this paper is that the large induction effects observed fall virtually to zero with temporal modulation above about 2.5 Hz. The time-course for induction is thus very different from that for direct brightness changes produced by luminance variation of the area being judged: such direct brightness matches were found to be largely flat across the temporal range studied, from 0.5 to 8 Hz. The time course of induction is also quite different from that for the detection of flicker, which peaks at 4-6 Hz, extends to 30 Hz or so at this luminance level, and is actually attenuated over the low temporal frequency range at which induction is the strongest. The very slow temporal characteristics of induction suggest a cortical site-perhaps even a late cortical site-for the underlying interactions, rather than the retinal processes which are generally assumed to account for them. The fact that all of the cell types seen in monkey geniculate respond to receptive field (RF) surround modulation (or any other stimulus pattern we have examined) up to very high temporal frequencies reinforces this suspicion. Some time ago, we showed that the spatial distance over which brightness induction operates far exceeds the dimensions of LGN receptive fields in monkey (De Valois and
Properties of brightness and color induction
Pease, 1971; Yund et al., 1977). Brightness induction can operate to 10” or more (Yund and Armington, 1975), whereas the maximum RF sizes in centrally-related LGN cells is at most a degree or two. We concluded on those grounds that brightness induction must reflect some later cortical, rather than a retinal, process. The temporal properties of induction, reported here, reinforce this view. The spatial-frequency specificity of induction effects seen psychophysically (McCourt, 1982) would also point in this direction, as does the work of Boynton (1983). In addition to examining the temporal properties of brightness variations, we also studied the time-course of direct and induced chromatic changes. As was the case for brightness in, duction, color induction produced by equiluminant chromatic modulation of the surround was found to have very different temporal properties from the direct change in color appearance produced by modulation of the center region itself. Again, color induction was found to fall almost to zero at temporal frequencies above about 2.5 Hz, whereas the direct color change was found to be comparatively flat from 0.5 to 8 Hz. While the time-courses for brightness and color induction were found to be very similar, their magnitudes were not. The amount of brightness induction increased approximately linearly with increased modulation of the surround, thus closely following Weber’s Law, confirming results first noted by Helmholtz (1924). Color induction, on the other hand, was quite pronounced even at very low levels of surround modulation, but then increased only slightly as the surround modulation depth was raised. This is consistent with the fact that color induction is easily demonstrated with very desaturated patterns, e.g. the classic color shadows situation. It is also consistent with the report of Kinney ( 1962). The induction produced by temporally modulating a pattern at some low frequency is very striking perceptually, much more so than the usual static demonstration of brightness contrast. However, the settings our observers made of the static situation did not bear out this impression: their matches of the static patterns were only slightly less than those made at low temporal frequencies. Thus it is perhaps simply the constantly changing nature of the temporally-modulated patterns which produces such a compelling impression. In the static
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situation both the inspection and matching patterns may well be subject to adaptation. This could perhaps reduce the salience of both of the patterns while not altering the match between them. The classical view of the way in which the brightness and color of an area are specified is that it is first of all a function of the luminance and wavelength variations of the region itself. These are then modified to some extent by lateral interactions with neighboring regions, in brightness and color induction. The presence of such very different time-courses for the direct and induced brightness and color changes we found in this experiment provide support for such a dual-stage model. It can perhaps be added to the long list of phenomena which cannot be accounted for by the Land (1959) model of a single process dependent on information about changes across edges. The induction effect we investigated here with luminance variation in the surround has classically been called “brightness induction”. However, the perceptual situation bears some similarity to surface color perception, with the variations seen being from black to white along a lightness scale. Indeed, we instructed our subjects to make their settings so that the induced changes in the central square appeared to be oscillating between the same black-white end points as the matching pattern. From the point of view of an opponent-color theory such as that of Hurvich and Jameson (1957), then, one might argue that we were really studying not brightness and color induction, but rather color induction along a black-white axis, as well as along the red-green and blue-yellow axes (brightness being determined by something else, such as the total visual activity in the region). If all of these situations involved color induction, rather than brightness in some and color in others, the similarity of the time-courses would not be unexpected. However, this would leave unexplained the very different modulationdepth vs induction curves (shown in Fig. 9) for luminance vs chromatic surround modulation. Numerous studies have shown that the spatial and temporal contrast sensitivity functions for luminance-varying and pure color-varying patterns are quite different from each other (e.g. van der Horst and Bouman, 1969; Kelly, 1975). Differences have also been reported in the temporal CSFs for red-green vs blue-yellow patterns. In general, luminance temporal contrast sensitivity shows considerable low-temporal fre-
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quency attenuation, with significant sensitivity up to very high temporal frequencies. With equiluminant red-green color variations, on the other hand, there is little or no low frequency attenuation and a low high-frequency cut. Yellow-blue sensiti~ty in turn is shifted to still lower temporal frequencies than red-green. Given this, it is all the more remarkable that the shapes of the induction curves in this study were so similar under all these conditions. Black-white, red-green, and yellow-blue induction settings all fell to almost zero at about 2.5 Hz. The differing CSFs for chromatic and luminance variations can be accounted for on the basis of the quite different RF properties of LGN opponent cells for luminance-varying vs chromatically-varying patterns (De Valois and De Valois, 1975; De Valois e? ai., 1977). The similarity of the temporal properties of color and brightness induction suggests a single, later, cortical process which operates similarly on both brightness and color information. The difference in the amount of induction as a function of the amount of surround contrast for the brightness vs color patterns, however, raises problems for such a simple explanation. Not only do the induction curves for both brightness and color matches differ drastically from their respective temporal contrast sensitivity functions, but the direct brightness, and perhaps color, matches do, too. While the threshold temporal CSF for luminance variations shows considerable low-frequency attenuation, we found the suprathreshold direct brightness matching curves to be relatively flat (see also Bowker, 1983). A similar discrepancy has been reported in the spatial domain, the suprathreshold spatial CSF for luminance variations being much flatter than the threshold curve (Georgeson and Sullivan, 1975). On the other hand, the low frequency attenuation we observed for the direct color matches is somewhat surprising, in that a low frequency falloff is generally not reported for the color CSF. Unfortunately, the threshold CSF could not be accurately measured for our stimulus conditions. Finally, we noted a very obvious perceptual dissociation between the impression of flicker and that of a change in brightness or lightness. At low temporal frequencies, the central square in the luminance induction pattern gradually changed from white to black, with little or no appearance of flicker. At high temporal frequencies, however, the pattern had a very pro-
nounced flicker, but did not appear to change at all in the black-white dimension, remaining a neutral grey throughout the cycle. A similar dissociation between (a color) flicker and a change in color appearance was seen at various temporal frequencies in the color-varying patterns. It may seem strange that a luminancevarying pattern can produce an appearance of flicker without the brightness or lightness actually changing at all, or of there being a color flicker without the color appearing to change. It is only one instance of many, however, in which logically incompatible percepts are clearly seen simultaneously. For instance, a field of resolvable red and blue dots can be seen as purple (the mixture of the red and the blue) while at the same time one can see the individual red and blue dots. Such a logically-incompatible judgment can be explained if one has simultaneously accessible the outputs from color-spatial channels tuned to quite different spatial frequency ranges. Another example, closely related to the findings in this paper, is the dissociation first reported by Wertheimer (1912) between the percept of movement and that of something having moved. Under conditions which produce “pure phi”, one sees movement between two alternated spots while simultaneously seeing both the spots as remaining stationary. So also in this induction situation with a surround luminance or chromatic oscillation at a high temporal frequency, one sees the square flickering without any change in its appearance along the black-white, red-green or yellow-blue axes. What this suggests is that there may be cells, or channels, involved in detecting flicker in a region that are separate from those channels involved in specifying the chromaticity or lightness of the region. The outputs of both of these types of channels SimultaneousIy contribute to our percept, despite sometimes carrying logically contradictory information. thank Helen Coromvli, Jennifer Franklin, and Shernaaz Webster for participating as subjects. This research was supported by NIH grant EY-00014 and NSF grant BNS 8242275. Acknowledgemenrs-We
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