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Separate chromatic thresholds for binary hue stimuli
Vision
RI,.
Vol.
16. pp. 321 to 322. Petpmon
Press 1976. Printed
LETTER SEPARATE
in Great Britain.
TO THE EDITORS
CHROMATIC BINARY HUE
THRESHOLDS STIMULI
(Received 10 March 1975; in recisedform
In the course of an extensive study of chromatic adaptation and after-images (Eichengreen, 1971), data were obtained indicating that there may be separate detection thresholds for the hue components of chromatic stimuli. Charpentier (1884). Monroe (1925). Purdy (1931), Dagher, Cruz, and Plaza (1958), Lie (1%3), Graham and Hsia (1%9), and Connors (1969), among others, have obtained estimates of the foveal achromatic (or photochromatic) interval for several observers as a function of spectral wavelength. The size of this interval (difference between energy levels for detecting the presence of a stimulus and perceiving hue) was found to vary with wavelength and different observers exhibited intervals of very different size as a function of wavelength. In fact, when the results of these experiments are viewed as a whole, there seems to be no common finding except that there is no achromatic interval at long wavelengths. There are two shortcomings in the previous studies: they did not allow for the possibility of more than one chromatic threshold at a given wavelength, and/or they did not intermix wavelengths as thresholds were determined. The earlier studies measured the interval between the detection threshold and a point where hue was first perceived. The criterion hue might have been a unitary (e.g. blue) or a binary (e.g. blue-green) hue. If there are two separate measurable chromatic thresholds, such variable criteria might have resulted in the determination of the higher threshold in some cases and the lower one in others. Randomization of wavelength in determinations of chromatic thresholds is critical to avoid the development of expectations of what stimulus, and therefore what hue would be seen on a given trial. In the present experiment the thresholds for individual hues were determined separately for stimuli that gave rise to binary hue perceptions at higher photopic luminance levels. In addition, the wavelengths of the stimuli the observer was exposed to in each session were varied from one flash to the next. Thus, all of the stimuli were presented in each session, and the observer could not develop expectations of what stimulus was to be presented on a given trial. Following preliminary determination of the energy range from slightly below the detection threshold to slightly above the chromatic threshold(s) for each wavelength, the method of constant stimuli was used to obtain precise estimates of the threshold values. Five narrow-band spectral distributions derived from Balzers B-40 interference filters were used as stimuli. The peaks 1%
I,,?
,.
FOR
28 Aptil 1975)
of these distributions occurred at 454,490,554,598 and 654 nm. These five are distributed across the visual spectrum and yield a reasonable sample of possible hue combinations. The preliminary range for each wavelength was divided into 10 equalenergy steps. The step sizes were O-30, 0.40, 0.45, 0.18 and 0.24 density units respectively for the series of wavelengths listed. All experimental sessions were preceded by 10 min of dark adaptation. Each session consisted of 110 presentations of 0.94 +-0.05set exposures of the 2”22’ Maxwellian-view stimulus. The inter-stimulus interval was 25 sec. Each of the 50 diierent test stimuli (five spectral distributions, 10 intensities of each) was presented twice in a session. The stimuli were presented in random order in blocks of 11 (10 test stimuli and one blank, or catch trial, per block). Ten sessions were conducted yielding a total of 20 judgments per test stimulus, A mechanical shutter provided aural cues as to the occurrence and duration of the stimulus. Following each presentation, the observer (a well practiced normal trichromat) responded “yes” or “no” depending on whether or not he saw the stimulus during the period the shutter was open. If the observer reported that he saw the stimulus, he was then asked to report its color. If it appeared achromatic, he responded “no hue” or “no color”. If it appeared chromatic, he reported the hue. The hue responses were limited to: red, green, yellow, blue and all combinations of these hue names. If he observed a combination of two hues, he was asked to report the stronger component first. Thresholds were calculated by fitting straight lines to the standard scores of the response frequencies corrected for “false-hue” reports. These are hues which were reported with low frequencies at low intensity levels, and were never reported on 50% of the trials at any intensity level. The occurrence of these “false hue” reports at particular energy levels, indicated that the observer was uncertain of the hue at that level, so the frequency of these reports was subtracted from the frequency of the opponent hue. Also, only the hue responses which occurred at levels that were not followed by a zero hue frequency were used in the threshold calculations. The calculated chromatic thresholds relative to the detection threshold (0.0 on the ordinate) are plotted in Fig. 1. These data indicate that the achromatic interval is about 1 log unit for the short wavelength stimulus distribution and widens until a maximum interval of about 3-O log units is reached in the mid-spectral region. The achromatic interval
321
Letter to the Editors
32
occur with a small interchromatic intcrbal. FCU example. with the 598-nm peak jtimulu:. th;
t, 400 ’
I 700
I
500
600
Peak wwelcngth of stlmuius.
nm
Fig. 1. Achromatic intervals (distance from dotted line to first symbol) and interchromatic intervals (distance between symbols) for five narrow-band stimuli centered at 454,490,554,598 and 654nm. Vertical lines represent -1 S.D.
then decreases rapidly to near zero for stimuli from the long wavelength end of the spectrum. It should be pointed out that the threshold data on which the intervals are based exhibited quite high variability (see standard deviation ranges on Fig. 1) and the estimates might easily be as much as 0.5-1.0 log unit too small or too large. In addition, given the angular size of the stimuli the values for the detection thresholds may reflect the sensitivity of the rod as well as the cone system. The rod-free area of the fovea is usually estimated as less than 2” in humans, thus the 2Y2’ fovea1 area used in these experiments cannot be considered as rod-free. Thus, the achromatic intervals as specified may be a combination of the rod-cone interval and the achromatic interval of the cone system. More importantly, there does seem to be an interchromatic interval (between the first and second chromatic thresholds) for some of these chromatic stimuli, and its size tends to be independent of the size of the achromatic interval. That is, a small achromatic interval does not necessarily ‘The one report I know in the literature that considers is that of Yager and Taylor (1970). In their investigation of hue scaling as a function of luminance, they obtained separate thresholds for green and yellow at a s&e wavelength, naniely 350 nm for a 17’ test field. They found the green threshold for this stimulus to be 1.45 log units below the yellow threshold.
two separate hue thresholds
neutral, single hue, and binary hue percepts depending on the intensity of the stimulus. The finding of two separate chromatic thresholds, and the specification of the “interchromatic” interval. indicates that the conventional concept of the achromatic interval is inadequate. Instead of a threshold for “light” and a single higher threshold for “color”. a stimulus may appear chromatic, but “incompletely” so at low luminance levels since a second hue component will be seen at higher luminance levels. Thus, it is probably the case that at most spectral loci there are two thresholds for “color”. one for each hue of binary hue stimuli. JEFFREY M. EICHENGREEN The Colorado College, Colorado Springs, CO 80903. U.S.A. REFERESCES Eichengreen J. M. (1971) Time dependent chromatic adaptation. Unpublished doctoral dissertation, Univ. of Pennsylvania. Charpentier A. (1884) Nouvelles recherches analytique sur les fonctions visuelles. Archs Ophrul. 4, 291-323. Connors M. M. (1969) Luminance requirements for hue identification in small targets. J. opt. Sot. Am. 59, 91-97. Dagher M., Cruz A. and Plaza L. (1958) Colour thresholds with monochromatic stimuli in the spectral region 53@-630nm. In Visual Problems of Colour, pp. 388-398. HSO, London. Graham C. H. and Hsia Y. (1969) Saturation of the foveai achromatic interval. J. opt. Sot. Am. 59. 993-997. Lie I. (1963) Dark adaptation and the photochromatic interval. Documenta ophth. 17. 411-510. Monroe N. M. (1925) The energy values of the minimum visible chromatic for the different wavelengths of the spectrum. ‘Prychol. Monogr. No. 158. Purdy D. McL. (1931) On the saturation and chromatic thresholds of the spectral colours. Br. 1. Prychol, 21. 283-313. Yager D. and Taylor E. (1970) Experimental measures and theoretical account of hue scaling as a function of luminance. Percept. Psychophys. 7, 360-364.
RI,.
Vol.
16. pp. 321 to 322. Petpmon
Press 1976. Printed
LETTER SEPARATE
in Great Britain.
TO THE EDITORS
CHROMATIC BINARY HUE
THRESHOLDS STIMULI
(Received 10 March 1975; in recisedform
In the course of an extensive study of chromatic adaptation and after-images (Eichengreen, 1971), data were obtained indicating that there may be separate detection thresholds for the hue components of chromatic stimuli. Charpentier (1884). Monroe (1925). Purdy (1931), Dagher, Cruz, and Plaza (1958), Lie (1%3), Graham and Hsia (1%9), and Connors (1969), among others, have obtained estimates of the foveal achromatic (or photochromatic) interval for several observers as a function of spectral wavelength. The size of this interval (difference between energy levels for detecting the presence of a stimulus and perceiving hue) was found to vary with wavelength and different observers exhibited intervals of very different size as a function of wavelength. In fact, when the results of these experiments are viewed as a whole, there seems to be no common finding except that there is no achromatic interval at long wavelengths. There are two shortcomings in the previous studies: they did not allow for the possibility of more than one chromatic threshold at a given wavelength, and/or they did not intermix wavelengths as thresholds were determined. The earlier studies measured the interval between the detection threshold and a point where hue was first perceived. The criterion hue might have been a unitary (e.g. blue) or a binary (e.g. blue-green) hue. If there are two separate measurable chromatic thresholds, such variable criteria might have resulted in the determination of the higher threshold in some cases and the lower one in others. Randomization of wavelength in determinations of chromatic thresholds is critical to avoid the development of expectations of what stimulus, and therefore what hue would be seen on a given trial. In the present experiment the thresholds for individual hues were determined separately for stimuli that gave rise to binary hue perceptions at higher photopic luminance levels. In addition, the wavelengths of the stimuli the observer was exposed to in each session were varied from one flash to the next. Thus, all of the stimuli were presented in each session, and the observer could not develop expectations of what stimulus was to be presented on a given trial. Following preliminary determination of the energy range from slightly below the detection threshold to slightly above the chromatic threshold(s) for each wavelength, the method of constant stimuli was used to obtain precise estimates of the threshold values. Five narrow-band spectral distributions derived from Balzers B-40 interference filters were used as stimuli. The peaks 1%
I,,?
,.
FOR
28 Aptil 1975)
of these distributions occurred at 454,490,554,598 and 654 nm. These five are distributed across the visual spectrum and yield a reasonable sample of possible hue combinations. The preliminary range for each wavelength was divided into 10 equalenergy steps. The step sizes were O-30, 0.40, 0.45, 0.18 and 0.24 density units respectively for the series of wavelengths listed. All experimental sessions were preceded by 10 min of dark adaptation. Each session consisted of 110 presentations of 0.94 +-0.05set exposures of the 2”22’ Maxwellian-view stimulus. The inter-stimulus interval was 25 sec. Each of the 50 diierent test stimuli (five spectral distributions, 10 intensities of each) was presented twice in a session. The stimuli were presented in random order in blocks of 11 (10 test stimuli and one blank, or catch trial, per block). Ten sessions were conducted yielding a total of 20 judgments per test stimulus, A mechanical shutter provided aural cues as to the occurrence and duration of the stimulus. Following each presentation, the observer (a well practiced normal trichromat) responded “yes” or “no” depending on whether or not he saw the stimulus during the period the shutter was open. If the observer reported that he saw the stimulus, he was then asked to report its color. If it appeared achromatic, he responded “no hue” or “no color”. If it appeared chromatic, he reported the hue. The hue responses were limited to: red, green, yellow, blue and all combinations of these hue names. If he observed a combination of two hues, he was asked to report the stronger component first. Thresholds were calculated by fitting straight lines to the standard scores of the response frequencies corrected for “false-hue” reports. These are hues which were reported with low frequencies at low intensity levels, and were never reported on 50% of the trials at any intensity level. The occurrence of these “false hue” reports at particular energy levels, indicated that the observer was uncertain of the hue at that level, so the frequency of these reports was subtracted from the frequency of the opponent hue. Also, only the hue responses which occurred at levels that were not followed by a zero hue frequency were used in the threshold calculations. The calculated chromatic thresholds relative to the detection threshold (0.0 on the ordinate) are plotted in Fig. 1. These data indicate that the achromatic interval is about 1 log unit for the short wavelength stimulus distribution and widens until a maximum interval of about 3-O log units is reached in the mid-spectral region. The achromatic interval
321
Letter to the Editors
32
occur with a small interchromatic intcrbal. FCU example. with the 598-nm peak jtimulu:. th;
t, 400 ’
I 700
I
500
600
Peak wwelcngth of stlmuius.
nm
Fig. 1. Achromatic intervals (distance from dotted line to first symbol) and interchromatic intervals (distance between symbols) for five narrow-band stimuli centered at 454,490,554,598 and 654nm. Vertical lines represent -1 S.D.
then decreases rapidly to near zero for stimuli from the long wavelength end of the spectrum. It should be pointed out that the threshold data on which the intervals are based exhibited quite high variability (see standard deviation ranges on Fig. 1) and the estimates might easily be as much as 0.5-1.0 log unit too small or too large. In addition, given the angular size of the stimuli the values for the detection thresholds may reflect the sensitivity of the rod as well as the cone system. The rod-free area of the fovea is usually estimated as less than 2” in humans, thus the 2Y2’ fovea1 area used in these experiments cannot be considered as rod-free. Thus, the achromatic intervals as specified may be a combination of the rod-cone interval and the achromatic interval of the cone system. More importantly, there does seem to be an interchromatic interval (between the first and second chromatic thresholds) for some of these chromatic stimuli, and its size tends to be independent of the size of the achromatic interval. That is, a small achromatic interval does not necessarily ‘The one report I know in the literature that considers is that of Yager and Taylor (1970). In their investigation of hue scaling as a function of luminance, they obtained separate thresholds for green and yellow at a s&e wavelength, naniely 350 nm for a 17’ test field. They found the green threshold for this stimulus to be 1.45 log units below the yellow threshold.
two separate hue thresholds
neutral, single hue, and binary hue percepts depending on the intensity of the stimulus. The finding of two separate chromatic thresholds, and the specification of the “interchromatic” interval. indicates that the conventional concept of the achromatic interval is inadequate. Instead of a threshold for “light” and a single higher threshold for “color”. a stimulus may appear chromatic, but “incompletely” so at low luminance levels since a second hue component will be seen at higher luminance levels. Thus, it is probably the case that at most spectral loci there are two thresholds for “color”. one for each hue of binary hue stimuli. JEFFREY M. EICHENGREEN The Colorado College, Colorado Springs, CO 80903. U.S.A. REFERESCES Eichengreen J. M. (1971) Time dependent chromatic adaptation. Unpublished doctoral dissertation, Univ. of Pennsylvania. Charpentier A. (1884) Nouvelles recherches analytique sur les fonctions visuelles. Archs Ophrul. 4, 291-323. Connors M. M. (1969) Luminance requirements for hue identification in small targets. J. opt. Sot. Am. 59, 91-97. Dagher M., Cruz A. and Plaza L. (1958) Colour thresholds with monochromatic stimuli in the spectral region 53@-630nm. In Visual Problems of Colour, pp. 388-398. HSO, London. Graham C. H. and Hsia Y. (1969) Saturation of the foveai achromatic interval. J. opt. Sot. Am. 59. 993-997. Lie I. (1963) Dark adaptation and the photochromatic interval. Documenta ophth. 17. 411-510. Monroe N. M. (1925) The energy values of the minimum visible chromatic for the different wavelengths of the spectrum. ‘Prychol. Monogr. No. 158. Purdy D. McL. (1931) On the saturation and chromatic thresholds of the spectral colours. Br. 1. Prychol, 21. 283-313. Yager D. and Taylor E. (1970) Experimental measures and theoretical account of hue scaling as a function of luminance. Percept. Psychophys. 7, 360-364.