Temporal integration at equiluminance and chromatic adaptation

Vol. 34, No. 8, pp. 1007-1018, 1994 Copyright © 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0042-6989/94 $6.00 + 0.00

Vision Res.

Pergamon

Temporal Integration at Equiluminance and Chromatic Adaptation YASUHIRO KAWABATA* Received 15 April 1993; in revised form 24 September 1993

The present study showed that at equiluminance the critical duration of temporal integration became shorter as the chromatic saturation of an adaptation field was increased. These results suggest that the chromatic coding system (which is assumed to posses poorer temporal resolution or larger temporal integration time than the luminance coding system) can change its temporal integrating organization with its own adaptation level, independently of the luminance system. Temporal integration Colorimetric purity

Equiluminance

INTRODUCTION

Chromatic system

Adaptation

Chromatic saturation

Smith et al. (1984) also reported that the isolated chromatic responses for different wavelengths were similar in their temporal integration properties. The purityduration functions for different wavelengths could be fitted by a fixed chromatic template displaced on the log purity axis. The present question is whether or not the temporal integration time for chromatic stimuli changes as the adaptation level of the chromatic system is varied. It is well known that for luminance stimuli, the critical duration of temporal integration is shortened as the intensity of an adaptation light is increased (Barlow, 1958; Sperling & Jolliffe, 1965; Krauskopf & MoUon, 1971; Saunders, 1975; Mitsuboshi, Kawabata & Aiba, 1987a; Kawabata & Aiba, 1990). This evidence may be supported by the physiological study (Baylor & Hodgkin, 1974). They showed, for turtle cone receptors, that the receptor responses quickened in response to light when they were light-adapted. How about the chromatic coding system? One possibility is that it is influenced by the chromaticity (here, chromatic saturation) of an adaptation field. The experiments described here measured temporal integration times at equiluminance, using the adaptation fields with different hues and chromatic saturations. We examined directly the prediction that the temporal integration times for the chromatic stimuli were shortened as the chromaticity of the adaptation field was increased.

The aim of this paper is to examine the temporal integration characteristics of human color vision. In the present study, we measured the chromatic temporal integration times, using the chromatic substitution technique (Regan & Tyler, 1971; Bowen, 1981; Smith, Bowen & Pokorny, 1984). Smith et al. (1984) obtained measures of the colorimetric purity threshold for a mixture of white and spectral lights (seven wavelengths between 430 and 630 nm) which was exchanged with an equiluminant steady white field. Equiluminant condition is intended to isolate the responses of chromatic systems (Cavanagh, 1991). Threshold purities necessary for detection were obtained as a function of test duration. The purity threshold-duration function showed decreasing purity as duration was increased to about 640 msec (Smith et al., 1984). For increment thresholds measured for white lights on a steady white field, the thresholdduration function showed no integration beyond 160 msec. The isolated chromatic systems have longer temporal integration than the achromatic (luminance) system. In the temporal contrast sensitivity paradigm, the degree of temporal integration is represented indirectly in the reduced sensitivity to high frequencies (highfrequency faU-off) in the contrast sensitivity function (CSF). The wavelength-modulated (red and green) temporal CSF shows low-pass frequency filtering characteristies compared to band-pass of a luminance-modulated one. In addition, the former has lower cut-off frequency METHODS (De Lange, 1958; Kelly & van Norren, 1977; Kelly, 1983). Both effects would be expected to prolong the Apparatus and stimuli critical duration of temporal integration. The stimuli were generated by a microcomputer (NEC, PC-9801FA) which had full color frame memory *Department of Behavioral Science, Faculty of Letters, Hokkaido on board, and were displayed on a RGB color monitor University, N-10, W-7, Kita-ku, Sapporo 060, Japan (NEC, PC-KD1511; vertical scanning frequency, 1007

1008

YASUHIRO KAWABATA

56.4 Hz) with a palette of 256 intensity levels for each of the three colors (red, green and blue). Look-up tables in the computer compensated for the nonlinearities of each phosphor. Figure 1 shows a schematic view of the stimuli. A circular test stimulus (T) of total diameter 1.2 deg was presented to the subject at the center of a background field (B) whose sides measured 4 deg visual angle. The central area (0.5 deg dia) of T was the most saturate (the ascending series), or the most desaturated (the descending series). At the periphery of T, the colorimetric purity was ramped to that of B. This design was used to blur the border between T and B. Four white lines (15 cd/m 2) for fixation were placed in B. The exposure duration of T was varied from 17 to 2560 msec in nine steps. The interstimulus interval was about 4 sec. A principal feature of the present study is the use of a method of presenting chromatic stimuli called chromatic substitution. This is a form of pure chromaticity modulation in which a chromatic test stimulus briefly substitutes for a portion of a background field which has a equivalent luminance, but a different chromaticity. B was not always achromatic in the present study. The dominant wavelengths and colorimetric purities of B were varied systematically. Table 1 lists their CIE

4 dea

~.

T

0.5 deg

Ascending

\/

Dlli¢ll1~llng

TABLE 1. CIE chromaticity coordinates and colorimetric purities of B Dominant Wavelength (nM) 607

548

468

571

Background field BI B2 B3 B4 B5 B1 B2 B3 B4 B5 B1 B2 B3 B4 B5 BI B2 B3 B4 B5

CIE chromaticity coordinate (0.616, (0.541, (0.465, (0.386, (0.310, (0.298, (0.301, (0.304, (0.307, (0.310, (0.144, (0.161, (0.204, (0.235, (0.310, (0.419, (0.397, (0.370, (0.340, (0.310,

0.344) 0.341) 0.338) 0.334) 0.330) 0.595) 0.529) 0.468) 0.396) 0.330) 0.066) 0.094) 0.162) 0.212) 0.330) 0.502) 0.467) 0.425) 0.376) 0.330)

Colorimetric purity 0.894 0.681 0.461 0.229 0.0 0.843 0.712 0.558 0.315 0.0 0.698 0.438 0.197 0.097 0.0 0.859 0.735 0.560 0.307 0.0

chromaticity coordinates and Fig. 2 shows their positions on the CIE chromaticity diagram. The four hues of B were red (open circles), green (solid circles), blue (solid triangles) and yellow (open triangles); their dominant wavelengths were 607, 548, 468 and 571 nm respectively. Five colorimetric purities were used for each dominant wavelength. The chromaticity coordinates of the white background field (B5) were (0.31, 0.33), and the luminance is 7 cd/m 2. The chromaticity and luminance of each background field were calibrated by the Pritchard Speetrophotometer (Model 01980A) and the Minolta CRT Color Analyzer (CA-100). In order to define equiluminance conditions between B5 and the other chromatic background fields (BI-4), the method of heterochromatic flicker photometry was used on each subject. A circular target flickered between white light and one of chromatic lights (those chromaticity coordinates were equal to those of the chromatic background fields, and its luminance was variable) at 14.1 Hz. The target was the same size as T employed in the main experiment, and the luminance of the white light was fixed at 7 cd/m 2. The subject adjusted the luminance of the chromatic lights until perceived flicker was minimal. The equiluminance value was taken as the mean of eight setting. Because of the relatively low intensity of the flicker field used, this flicker photometry was probably within the linear range (de Vries, 1948). A chin rest was used for head stabilization of the observer. The stimuli were observed foveally through a 2 mm artificial pupil by the right eye of the subject, with his left eye being occluded.

1.2 deg

Subjects FIGURE 1. Schematic view of stimuli. A circular test stimulus (T) of total diameter 1.2 deg was presented at the center of a background field (B) of 4 deg visual angle in a side. Four white lines for fixation were placed in B.

Two well-trained subjects YK (the author) and RS participated in the present study. The subjects had normal visual acuity and color vision as tested by the

EQUILUMINANCE

1009

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0.8

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Y 0.4

0.2

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=

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FIGURE 2. The positions of Bs on CIE chromaticity diagram. The four dominant wavelength conditions of B were 607 nm (open circles), 571 nm (open triangles), 548 nm (solid circles) and 468 nm (solid triangles).

Landolt ring, the Ishihara pseudoisochromatic plates, the FM-100 hue test and the Nagel-type anomaloscope (Hioki version, Handaya, Japan). The subject RS was not aware of the aims of this study.

Procedure The subjects sat at a distance of 200 cm from the display in a darkened room. At the beginning of each session, the subjects were dark adapted for 10 min. They were adapted to all background fields (B) for additional 3 min prior to any measurements. Chromatic thresholds were then obtained for nine exposure durations of T on each B, using the method of limits. Just-noticeable differences from B were determined by changing the colorimetric purity of T in either ascending or descending series of measurements of the chromatic thresholds. In the ascending series, the subjects increased the colorimetric purity of T by pushing a first button, until they could discriminate T from B. In the descending series, the subjects decreased the colorimetric purity of T by pushing a second button until they could discriminate T from B. In both series, a third button was used for signaling the completion of the adjustment to the computer. Eight adjustments were carried out for each series of measurement. When the background field was the most saturated color that our apparatus could produce (B 1), the ascending series of measurements were not used since the colorimetric purity of T could not be increased above the B purity. The reverse held when B was completely achromatic (B5). Table 2 shows the series of measurements examined for each B. Each session consisted of four blocks, each of which was for a particular colorimetric purity of B. Within a

block the nine test durations were scrambled. The four dominant wavelengths of B and the two series of measurements were not changed in a session and each session was done on different days, for each of four dominant wavelengths and two series of measurements.

Measurement of equiluminance In each hue condition, only the colorimetric purity of T was changed, since the chromaticity coordinate of T moved on lines connecting the five chromaticity coordinates of B1-5 (see Fig. 2). The luminance and dominant wavelength were almost constant in each condition. The detail procedure is as follows. For each of 20 background conditions (four dominant wavelengths.five colorimetric purities), we executed the minimum flicker photometry to establish the isoluminant points between B and the various T. Take the 607 dominant wavelength, the most desaturated background (B5) and the ascending series, for example. At the beginning, the subject was dark adapted for 10min. A target was set against a background (whose luminance and chromaticity coordinates were equal to those of B1). The target and background were the same size as T and backgrounds (B1-5) employed in the main experiment. The subject TABLE 2. The series of measurement and background field conditions examined in the present study Method of measurement: Background field BI B2 B3 134 B5

Ascending series

Descending series

x O O O O

O O O O x

1010

Y A S U H I R O KAWABATA

was adapted to B for 3 min. The target flickered between the first light (L1, which had the same luminance and chromaticity as B1) and the second lights (L2, the candidates of T), with flicker rate 14.1 Hz. First, the candidates of T (2) were selected, more or less automatically, by the spectrophotometer (which was connected to the computer by IEEE-488). In principle, the chromaticity coordinates of the candidates (2) were selected on lines connecting the five chromaticity coordinates of B1-5 (see Fig. 2). The variation of the CIE x , y value from the lines was < 3%. The luminance of the candidates ranged from 1 to 15 cd/m 2. The subject adjusted the luminance of the candidates. If there was a minimum flicker point which perceived flicker was least detectable, the candidate was adopted as the test stimulus of the main experiment. If stimuli are highly saturated, the dip in performance, if any, near equiluminance is generally quite broad (e.g. Cavanagh, Tyler & Favreau, 1984). This implies that equiluminance point is not precisely determined. For example, minimum flicker setting between monochromatic red and green may be most difficult task for the subjects. However, the present two flicker components have always the same dominant wavelength and only a low chromatic saturation contrast. At least, their flicker setting tasks were easier than those between a white light and each monochromatic light (which was the typical condition to measure V~). The two subjects of the present study had had experience in this type of psychophysical experiments before participating in this study. The other point to notice is that the minimum flicker was set on each background field (which was the same dominant wavelength as flickering lights). We think that any minimum flicker setting were made after the subject was in a stable of chromatic adaptation. When the luminance of the candidates was adjusted, the chromaticity of the candidates changed to some degree, gradually along the lines connecting the chromaticity coordinates of B1-5. As the change became larger and larger, less test stimuli could be used. The change of the chromaticity was different in each dominant wavelength condition. For 607, 548, 468 and 571 nm dominant wavelengths of the chromatic axes, 212, 125, 219 and 111 steps test stimuli which had different colorimetric purities were available, respectively. The transitions created by this sampling were below threshold for all chromatic backgrounds. In order to insure that equiluminance conditions had actually been established in the main experiment, the heterochromatic flicker photometry was executed at intervals of 10 steps of T on each subject, prior to any measurements of chromatic threshold, after he was adapted to each B. The notion of the equiluminance presumes that there is only one luminance pathway and that it is characterized by a single null However, in truth, there may be many luminance pathways. Image information passes through separate units in the visual system. Each of these could be considered a separate spatial luminance pathway with a different equiluminance point. For example,

the contribution of this interunit variability to the effectiveness of equiluminous stimuli has been tested in a motion paradigm (Cavanagh & Anstis, 1991). The results showed that although the effect of interunit variability in equiluminance was undoubtedly present, it accounted for only a small portion of the overall contribution of color to motion (Cavanagh, 1991). Livingstone and Hubel (1987) have reported considerable variation in the equiluminance settings across tasks, suggesting that the different tasks might be evaluating different luminance pathways. Given that multiple luminance pathways may be carrying luminance information each potentially with different equiluminance points, a possible achromatic contribution should be considered; i.e. the present experimental stimuli might not provide information only to chromatic pathway. The equiluminance setting are known to vary as a function of spatial and temporal frequency (Cavanagh, Anstis & MacLeod, 1987). In the present study, there is no difference in spatial configuration between the flicker stimuli and the stimuli of main experiment. The equiluminance setting maybe corresponds to low spatial frequency content. However the temporal frequency content of the two stimuli is more or less different. Therefore this experimental design may not perfectly eliminate the achromatic contribution. Although the previous study (Smith et al., 1984) used the same design as this study did, the results principally reflect the properties of chromatic pathway. Similar data could be obtained in this study when the white background (B5) was used (which is the same background condition as Smith et al.). Therefore, the achromatic contribution caused by the present study is less expected and probably negligible. The aim of this study is to examine temporal integration time on the different chromatic backgrounds. At least, this type of achromatic contribution will not depend on the chromaticity of the background field.

Optical factors Because of chromatic aberration (longitudinal chromatic aberration), the accommodation system generally creates a luminance contrast at an equiluminous color border by bringing one of the two colors into focus and slightly blurring the other. However in the present experiment, the effect caused by the chromatic difference of focus is less expected, because all color borders between T and B are blurred and made only by the low chromatic saturation contrast of just-noticeable difference. Moreover the dominant wavelength of T is always the same as that of B in a session. The 2 mm artificial pupil, as was used in the present study, increases the depth of focus enough to largely overcome the chromatic difference of focus. It seems unlikely that there would have been significant residual axial chromatic aberration for this stimulus viewed through a small pupil. The lateral chromatic aberration related to the chromatic difference of magnification is potentially a more serious problem, and it is not overcome by the use of an artificial pupil. However, for a small stimulus presented relatively

EQUILUMINANCE

near the optical axis, the lateral aberration was also probably negligible. DEFINITION OF TEMPORAL INTEGRATION

In the present study, threshold colorimetric purity (Pc) was defined by a difference between colorimetric purity of T and that of each B. In many previous studies, the critical duration of temporal integration was defined by the duration, in a log I • t-log t plot (where I = threshold intensity, t = test duration), at which straight lines of the slope 0 (complete temporal integration formulated as Block's law) and of the slope 1 (complete absence of temporal integration) intersect. Though this two-line fitting procedure may be arbitrary, the advantage is that the critical durations of temporal integrations for different conditions are easily comparable. First we used this procedure. For the detailed procedure see our previous studies (Mitsuboshi, Funakawa, Kawabata & Aiba, 1987b; Kawabata & Aiba, 1990; Kawabata, 1990). On the other hand, the previous study at equiluminance suggested that the slopes of the functions show a gradual change with duration increase (Smith et al., 1984). At the shortest durations, the purity threshold and the duration showed approximate reciprocity. As the duration was increased, there was a more gradual change in the purity until the asymptote was approached. Moreover, the functions for the different wavelengths were of similar shape but were displaced on the purity axis, i.e. all the data could be described by a common template. They assumed that chromatic temporal processing can be described as a linear low-pass filter. They based their choice of filter on the data of Wisowaty (1981) representing the temporal response of an isolated red-green chromatic pathway. High frequency responses for an isolated blue mechanism also showed similar slope to the red-green chromatic responses (Wisowaty & Boynton, 1980). We also used their theoretical prediction, but only one exception which we assumed was that the chromatic temporal filter was changed with chromatic adaptation. According to Smith et al. (1984), an impulse-response for a minimum phase linear filter, which corresponds to a linear low-pass filter, is described by the following equation I ( t ) = (t"-'e-E~c')/(n -- 1)!

where n is the number of stages, c is corner frequency (Hz) and t is in msec. This is normalized by dividing by its maximal value which occurs at time (n - 1)/2nc. To model a threshold response, Smith et al. (1984) assumed that the observer acted as a peak detector (Roufs, 1972), and used a discrete convolution procedure to obtain the response R at discrete times J for different duration stimuli using the following equation 2560 K = J

R=

~

~ A(K)IN(J-K)

J=O K=O

where J, K were discrete times sampled at 5 msec intervals for J varying from 0 to 2560 msec and K from 0 to

1011

J; .4 (K) was the stimulus step defined as unity for K less than the stimulus duration and zero elsewhere; and IN (J - K ) was the normalized impulse-response function given by equation evaluated at time J - K. They then assumed that threshold would be proportional to the reciprocal of the peak response (R) at each duration. Smith et al. (1984) used a five-stage number/SHz corner frequency filter which was based on the Wisowaty (1981) and Wisowaty and Boynton (1980) data. In this procedure, c determines the range of the temporal integration. As c becomes smaller and smaller, the temporal integration period is shorter. In the present study, c is varied systematically from 2 (0.301 log) to 30 (1.48 log) Hz by 0.01 log step. The chromatic response templates totaled 119. Each template was fitted to the data allowing only scaling on the vertical axis. The sums of squares of the differences between the data points on the templates and the actual data points are made minimal in log value. Thus the best combination of c and the template level that minimized the differences was chosen. RESULTS Figure 3 shows log Pc" t-log t functions obtained with the ascending and descending series for the 607 nm dominant wavelength. Figures 4, 5 and 6 show log Pc" t-log t functions for the 548, 468 and 571 nm dominant wavelengths, respectively. These figures show that the chromatic response template is fitted to each set of the data points, apparently better than the two lines with slopes 0 and 1. The average residuals of the former were always less than those of the latter. In some Bs it was impossible to obtain thresholds for short test durations, because of the limits of chromatic contrasts of the present color monitor. Therefore in these conditions the estimation of temporal integration may be less reliable. These figures show that as the colorimetric purity of B increases, the temporal integration time becomes shorter, i.e. c becomes larger. However, a problem to be considered is that saturation coefficient varies with the dominant wavelength for each hue. For example, if the monochromatic light of 440nm has 100% saturation, those of 607, 548, 468 and 571 nm (the dominant wavelengths of the present stimuli) have about 58%, 51%, 82% and 35% saturations respectively (Hurvich, 1981). The present definition of the background Pc does not take the saturation coefficients into account; the monochromatic light of each dominant wavelength is always defined as 100% colorimetric purity despite its wavelength. In Fig. 7, critical duration of temporal integration is plotted as a function of the Pc of B, divided by the saturation purity threshold (T) for a long duration stimulus on the white background (which was derived from the upper templates in Figs 3-6). It seems unequivocal that the critical duration is reduced as the chromatic saturation of B (Pc~T) is increased. It should be noted

1012

YASUHIRO KAWABATA

2.0

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LOG TEST DURATION (msec)

FIGURE 3. The results obtained with the dominant wavelength of 607 nm. Log Pc" t is plotted against log t, where Pc and t are the threshold colorimetric purity for a test stimulus and its duration (msec) respectively. The open circles show the ascending series and the solid circles show the descending series. Left and right columns are obtained for subject RS and YK respectively. All data points were the means of eight adjustments of each subject. The vertical error bars indicate _+1 SD. Each panel has been obtained for the different colorimetric purity of the field. The solid and dashed curves represent a selected chromatic template. The estimated corner frequency and standard error from the template is written in each panel. All other functions of the ascending series except for the 0% B colorimetric purity are displaced upwards by steps of 0.5 log units for clarity.

EQUILUMINANCE here that there was some scatter, which might be use to intersubject and interday differences. Spearman rank-order correlation coefficients between the critical duration and Pc/T are -0.81397 and -0.92119 for the subject RS and YK, respectively.

2.5

DISCUSSION Smith et al. (1984) showed that there were no effects of wavelength on the temporal integration of the isolated chromatic systems, and the identical template could be fitted, i.e. it was merely scaled vertically along the

2.5

(a) 548 nm, Pc=0.0 cf(a.s.)=3.89Hz, Se=0.025

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FIGURE4. The resultsobtainedwiththe dominantwavelengthof 548nm.

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3.0

(mNc)

1014

YASUHIRO KAWABATA 1.7

2.0

(a) 4 6 8 n m , P c = 0 . 0 cf(a.s.)=3.31Hz, Se=0.018

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threshold purity axis according to trichromatic purity discrimination. Hence, they chose to model their data of the hue substitution mode with only one low-pass filter whose corner frequency was 5 Hz (another low-pass filter was used to model the data of the luminance mode). The choice is reasonable for their experimental conditions, i.e. they used only modulation from white. In the

present study, the temporal integration times obtained with the modulation from white approximately seem to correspond to Smith et al.'s data of hue substitution mode. As noted in the Introduction, it is suggested that the chromatic systems have longer temporal integration times than the luminance (achromatic) system. These results can be predicted by the previous studies of

EQUILUMINANCE

determined jointly by the chromatic and achromatic (luminance) systems. As they point out, it seems that the temporal integration functions under such condition are explained by vector combination of an achromatic function (which has short critical duration) and an appropriate chromatic function (which has long critical duration) (King-Smith & Carden, 1976; Smith et al., 1984;

chromatically modulated temporal CSF. In the present study, it must be emphasized that modulations from different chromaticities were used, i.e. it is necessary that the effects of chromatic adaptation are taken into consideration. Smith et al. also concluded that the temporal integration for luminance-incremental chromatic stimuli was

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LOG Pc / T FIGURE 7. Critical duration of temporal integration is plotted as a function of Pc of B, divided by T for a long duration stimulus on white background (which was derived from the upper templates in Figs 3--6). If the chromatic threshold on the white background (B5) is depended only on the chromaticity of each dominant wavelength, it is the reciprocal of sensitivity in the visual system, i.e. they may reflect the saturation coefficient of each dominant wavelength. Each subject's data is normalized by his own threshold. The critical duration was estimated by the pair of lines with the slopes of 0 and 1 (the two-line fitting procedure). The open symbols show the results of the ascending series and the solid symbols the descending series. Circles, squares, triangles and diamonds show the results of the dominant wavelength of 607, 548, 468 and 571 nm respectively. The error bars indicate the standard error of the critical durations.

Mitsuboshi et al., 1987a; Ejima & Takahashi, 1988; Kawabata & Aiba, 1990). Vector summation has been used successfully to predict brightness of spectral lights (Guth, Donley & Marroco, 1969). On the other hand,

our previous data with bichromatically-mixed adaptation field (Mitsuboshi et aL, t987b) was interpreted by the two sites of adaptation model of Wandell and Pugh (1980a, b). It was demonstrated that the temporal

EQUILUMINANCE

1017

integration time in n 5 pathway was increased by adding opponent system is polarized by a red background field, a background light (e.g. green) that was color opponent it would mean that the chromatic sensitivity of the to a initial background color (e.g. red). As the intensity system is lower in the direction of red chromaticity of the adding background is increased, the first (recep- increment or decrement than the other directions. If it is tor) sites become more adapted, while the second sites assumed, as Wandell and Pugh (1980a, b) have (chromatically-opponent systems) become released from suggested, that the polarization of the opponent system adaptation (i.e. less polarized). It is reasonable that the should be followed by shortening of its integration time, prolongation of critical durations is caused by the it would be possible to predict that thresholds are reduction of adaptation in the chromatic coding system. relatively higher and thus integration time is shorter for However, there is another way to interpret the results, red chromaticity-increment (decrement) test stimulus. i.e. a shifting Of detection from the achromatic (lumi- However, on the white background, thresholds are lower nance) to chromatic system. It is unequivocal that the and so integration time is longer, because the red/green present data at equiluminance can not be explained by opponent system is depolarized. This is what was found the shifting hypothesis. These stimuli are used to provide in the present study. information to the chromatic pathway but not the Several differences between the processing of the luminance pathway, and so to test the capacities of the luminance and chromatic information have also been chromatic pathways in isolation. The present results may reported, principally differences in spatial and temporal suggest that the chromatic coding system changes its resolution. Minimum flicker photometry is based on the own temporal integrating organization with adaptation hypothesis that the luminance pathway has high spatial level, independently of the luminance coding system. and temporal resolution, while the chromatic pathways At least, it would be noted that the chromatic adap- do not. When the chromatic pathways have the same tation level has some influence on the temporal inte- high temporal resolution (short temporal integration gration time. In the luminance coding system, it is well period) as the luminance pathway, the hypothesis is not known that the temporal characteristics for luminance valid. This implies that the method is not always adeincremental stimuli closely depend on luminance adap- quate to isolate the chromatic pathways. However, in the tation level. As the adaptation light or mean luminance present study the variable range of temporal integration level is increased, the sensitivity and the temporal inte- obtained at equiluminance, in principle, covers longer gration are reduced gradually (Barlow, 1958; Sperling & temporal region than those obtained for luminanceJolliffe, 1965; Krauskopf & Mollon, 1971; Uetsuki & incremental stimuli (both achromatic stimuli and chroIkeda, 1971; Saunders, 1975; Mitsuboshi et al., 1987a; matic stimuli) in the previous data (Barlow, 1958; Kawabata & Aiba, 1990). The flicker fusion frequency King-Smith & Carden, 1974; Smith et al., 1984; Mitsu(de Lange, 1958; Brown, 1965; Truss, 1957; Kelly & van boshi et al., 1987a, b; Kawabata & Aiba, 1990). ExcepNorren, 1977; Kelly, 1983; Boynton & Baron, 1975) and tions are the B1 condition of the 607 and 468 nm, i.e. the the peak of the temporal contrast sensitivity function estimated critical durations are below 100 msec. In these (Green, 1969; Kelly, 1974; Cavonious & Estevez, 1975; conditions, the minimum flicker method may be inCicerone & Green, 1978) shift to high temporal fre- adequate to isolate the chromatic pathways, and the quency region. Thus, if luminance adaptation can be obtained data may be less reliable. 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Acknowledgements--The author would like to express their sincere gratitude to Professor Thomas S. Aiba of Hokkaido University and Professor Muneo Mitsuboshi of Kanagawa University who gave helpful comments and suggestions, and critically read the manuscript for improving the English.