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The range of spatial frequency contingent color aftereffects
THE RANGE
J-ES
OF SPATIAL FREQUENCY COLOR AFTEREFFECTS
G. MAY*. GREGORY AGAMY*and wm
CONTINGENT H. MATTESON?
*University of New Orleans, Department of Psychology. New Orleans. Louisiana 70122 U.S.A. iTulane University. Department of Psychology New Orleans. Louisiana 70118. U.S.A. (Received 18 May 1976;
in
revised form 24 June 1977)
Abstract-Checkerboard stimuli contain twodimensional Fourier components oriented 45 degrees from edges of the individual checks. Adaptation to such stimuli and testing with rectilinear gratings showed that contingent color aftereffects are associated with the Fourier components rather than the edges of such patterned stimuli at high spatial frequencies. With low spatial frequency (0.8 c/d) contingent color aftereffects were aligned with the edges rather than the Fourier components. These data. obtained with a color cancellation procedure. are in agreement with previous reports of spatial frequency-contingent color aftereffects and indicate the range over which such effects can be obtained.
and Green, Corwin and Zemon (1976) showed that McCollough effects are associated with fundamental spatial frequency information rather than edges. In both studies, an achromatic square-wave test stimulus took on a hue complementary to the hue of a checkerboard adapting stimulus when the test stimulus was oriented in the same direction as one of the major Fourier components of the adapting stimulus (i.e. when the grating test was lined up with one of the diagonals of the checkerboard adapting stimulus, not when it was lined up with edges). So, in this situation. the McCollough effect is associated with major Fourier components rather than edges.
Kelly (1976) pointed out that the two-dimensional Fourier transform of a checkerboard pattern contains
four fundamental frequency components (rather than two, as is the case with a square-wave grating) oriented 45” from the horizontal and vertical edges of the checks. A checkerboard with horizontal and vertical check edges has no Fourier components on the horizontal or vertical axes, although the higher harmonics do lie arbitrarily close to the horizontal and vertical axes so that these harmonics could serve to define what appear phenomenologically as the edges of individual checks. When such a checkerboard is blurred (or viewed just above contrast threshold) it looks similar to two oblique sinusoidal gratings superimposed upon each other at 90” angles. Thus, the checkerboard, 1unlike a square-wave grating (which has its fundamental Fourier components and edges (i.e. higher harmonics) in the same orientation), is a stimulus which may be used to evaluate the relative importance of edges’ and the fundamental Fourier components in the mediation of perceptual phenomena. A number of studies tend to indicate that the human visual system processes pattern information by means of spatial frequency analysis (Campbell and Robson, 1964. 1968; Pollen, Lee and Taylor. 1971). and there are several studies which give fairly direct support to this point of view (Carter and Henning, 1971; Sullivan, Georgeson and Oatley. 1972; Weisstein and Bisaha, 1972). May and Matteson (1976)
‘While the edges of the individual elements of a well focused complex pattern seem intuitively to be most prep+ tent perceptual aspects of such a visual stimulus, they are oriented 45’ from the orientation of the fundamental frequency as revealed by two-dimensional Fourier analysis. It is logical to assume that these edges are defined by the higher harmonics of the fundamental frequency which tend to be aligned more or less on the horizontal and vertical axes. Thus, the checkerboard stimulus may prove to be a useful stimulus in partialling out the perceptual influence of the fundamental frequency as opposed to the higher harmonics (edges) in pattern vision.
EXPERIMENT I
One question about such spatial-frequency-contingent color aftereffects is the range of spatial frequencies over which this result can be observed. As Kelly (1976) pointed out, bars and checks become logically indistinguishable at very low spatial frequencies. With a circular field, both the checkerboard and the grating consist of one black semicircle and one white semicircle when grating bar width or check width equals or exceeds the radius of the circle. Exactly the same effect would be expected if one cycle of a test stimulus equalled or exceeded the size of some functional recep tive field. Thus, there should be no difference between the adaptational effects of gratings and checkerboards at some relatively low spatial frequency. The purpose of the present study was to delineate the range over which spatial-frequency contingent color aftereffects can be obtained and to determine the point at which the visual system begins to associate color with edge orientation rather than fundamental spatial frequency orientation. Method Observers. Thirty-six experimentally naive observers (I 8 women and 18 men) were recruited from introductory psychology classes. All observers had 20/20 acuity uncorrected or were corrected to 20/20 with eye lenses. The observers were tested for normal color vision with American H-R-R
917
Adapting
stimu11
Magenta
the observer to cancel out aitsr&ct mentar) colors added to the tuo A color-balancing filter. composed
Green
Test
stimuli
Fig. 1. Adaptation and test stimuli used in experiment one. The spatial frequency was varied from 0-7 c/d. pseudo-isochromatic plates. Their ages ranged from 19 to 36 years. Apparatus. All patterned stimuli were Thermofax copies of Kodalith negatives. The fundamental frequencies of checkerboard and grating stimuli were equated for each set of adapting and test stimuli (the width of a check edge was 1.4 times that of a grating stripe). The arbitrarily chosen spatial frequencies of test and adapting stimuli were always equal and the frequencies used were: 0.4. 0.8, 1.5, 3.0, 5.0 and 7.0 cycles per degree (c/d). The contrast of all stimuli was 65%. Checkerboard adapting stimuli were transilluminated in a two channel tachistoscope (Scientific Prototype model SOOF). The stimulus fields were squares, subtending 8 degrees of visual angle on a side. and they were viewed binocularlv at a distance of 48 cm. (see Fia 11 One field of the tachistoscope contained a checkerboard with horizontal and vertical edges (“squares”). This pattern was mounted together with a magenta filter (Kodak Wratten No. 32). The second field in the tachistoscope contained the same checkerboard pattern rotated 45’ (“diamonds”). This diamond pattern was superimposed on a green Wratten No. 53 filter. The space-averaged luminance of both adapting fields was 12.8 cd/m’. Adapting stimuli were presented in alternate 5 set periods for 15 min. Compound test stimuli were composed of adjacent panels of: (a) checkerboards, one containing checks with horizontal and vertical edges (“squares”) and one with oblique (45” and 135”) edges (“diamonds”), or (b) squarewave gratings, one with vertical edges and one with oblique (45’) edges. Each half of the test field was a rectangle 8’ (vertical) by 4’ (horizontal). Test stimuli were rearprojected on to a groundglass screen with a lantern slide projector (Beseler model 3620). Each test slide was composed of pairs of patterns (checkerboards or gratings) which were sandwiched with orthogonal polaroids. Test stimuli were projected through two daylight filters (Wratten 80A and 82A) which rendered the coloration of the test stimulus pair at its neutral position as close as possible to the coloration of the daylight fluoresdent lights used to illuminate the adapting patterns. The space-averaged luminance of all test stimuli was 10.2 cd/m’. The apparatus for projecting test stimuli was patterned closely after one described by White (1976) which allows
colors ,w~:h . ,mp;
magenta (Wrattrn CCZOM) and green lCC3OG) gel,ltin til. ters sandwiched with orthogonally polarized squarss m A checkerboard pattern. was positioned between the ?roJ one fundamental spatial frequency. and observers were randomly assigned to one of six groups. each group tested under a different spatial frequency condition. Each group included 3 women and 3 men. Each observer completed observations in a single 40-min session. Before adaptation. the observer viewed the test stimulus and adjusted the chromaticities of the two halves of the test lieid until they appeared to be the same bluish white by turning a knob which rotated the color-balancing filter described above. Ten pre-adaptation adjustments were made with both the checkerboard and grating test patterns. Half of the observers in each soup began with checkerboards; the other half began wtth gratings. Half of the adjustments were descending (starting with the color-balancing filter set above the match point), and the other half were ascending, and direction of initial adjustment was randomized. Observers were asked to fixate along the midline between the two panels while making their matches. Immediately after pre-adaptation determinations. all observers were adapted to 15 min of alternating 5 set periods of magenta squares and green diamonds. Observers were instructed to move their fixation around the middle two-thirds of the pattern and were cautioned not to fixate any one point for an extended period. After adaptation. observers were allowed approx two min to move from the tachistoscope headrest to the headrest of the test apparatus. Observers then made IO color matches with the grating test stimulus and 10 matches with the checkerboard test. Order of testing was the same as that used during adaptation determinations. A measure of color aftereffects was obtained by subtracting each observer’s mean pre-adaptation match from his (her) mean post-adaptation match. .4 positive score indicates that the observer had to add more magenta to the oblique stripes (or squares) and more green to the vertical stripes (or diamonds) to make a match after adaptation that was necessary before adaptation. Negative scores indicate the opposite; more magenta was added to verticals (or diamonds). and more green was added to oblique stripes (or squares) than before adaptation. After adaptation to magenta squares and green diamonds, a green aftereffect would be predicted with an oblique test grating (or squares), and a magenta aftereffect would be predicted with a vertical grating (or diamonds). since these test stimuli have major Fourier components in the same orientation as the major Fourier components of the adapting stimulus. In other words, the aftereffect color is complementary to the color of the adapting field with major Fourier components in the same orientation as the major Fourier components in the test stimulus. Addition of magenta to the oblique grating (or squares) test areas and green to vertical grating (or diamonds) test areas canc& out the aftereffect colors. The aftereffect measure is in arbitrary units directly proportional to the excitation purity of the colors necessary to cancel out the aftereffect colors (i.2.. the larger the absolute value of the measure. the more saturated was the after-effect color-see White. 19761
919
Range of spatial frequency contingent color aftereffects
t ___----
0 - Stripes 0 - Checks
b
1.0
2.0 Spatiol
3.0
4.0
frequency,
5.0
6.0
7.0
c/deg
Fig. 2. The mean strength of aftereffects as a function of spatial frequency for grating and checkerboard test stimuli. Each point represents the mean difference between pre- and post-adaptation settings of the color balancing filter for each group. Vertical bars indicate 1.0 SE.
Results The difference scores computed by subtracting the mean pre-adaptation match point from the mean post-adaptation match point for each subject were submitted to a two-way mixed analysis of variance (spatial frequency X test stimulus-checks or stripes). These data are presented in Fig. 2. The main effect for spatial frequency was statistically significant beyond the 0.01 level (F = 13.69, df = 5.30) but the main effect for test stimulus type (F = 0.27, df = 1.30) and she interaction (F = 1.23. df = 5.30) were nonsignificant {p > 0.05). It is readily apparent from Fig. 2 that appreciable spatial frequency contingent aftereffects were obtained with high spatial frequencies (3.0, 5.0, and 7.0 c/d) with both grating and checkerboard test stimuli. The checkerboard test stimuli resulted in slightly smaller aftereffects at 5.0 and 7.0 c/d. Checkerboard test stimuli resulted in no aftereffect colors at or below 1.5 cid. The negative scores obtained with the grating test at 0.8 c/d indicate a reversed aftereffect (or one resulting from edges rather than major Fourier components). In other words, green was added to the vertical grating test and magenta was added to the oblique grating test to cancel out a magenta aftereffect on the vertical stripes and a green aftereffect on the oblique stripes. EXPERIMEW 2
Although it was apparent from Experiment 1 that color aftereffect magnitude varies significantly with spatial frequency. the occurrence of negative scores could only be considered suggestive of edge-contingent aftereffects because of the high degree of variability in measures of aftereffect strength. The mean ‘This experiment was carried out by the first author whiie on sabbatical in the Visual Science Laboratories at
the Pennsylvania College of Optometry. We wish to acknowledge the invaluable aid of Dr. Kent E. Higgins.
aftereffect strength at 0.8 c/d did not differ significantly from zero, probably because of the effects of individual differences which were not controlled for by the between-subjects design empIoyed. For this reason, a second experiment, empioy~g a within-subject approach, was carried out at two spatial frequency points. Method Observers. Six experimentally naive observers (three mates and three remales), ranging in age from I7 to 36. participated in this experiment. All observers had normal color vision and. with the exception of one person (QD = 1.50 - 0.25 x 85: 20120 OS = 0.75 - 1.00 x 20: 20/120), possessed corrected or uncorrected 20!20 visual acuity. Apparatus. Since this experiment was carried out in a different laboratory.’ another apparatus was constructed. It was similar to the one used in experiment one with the following exceptions. The patterned stimuli were Koda-
lith negatives with a contrast of 96%. Checkerboard adapting stimuli were presented with a 35mm slide projector (Kodak-model 800) and a standard projection screen. The subjects were seated 12 ft from the screen and the adapting stimuli were presented at a space averaged luminance Ievel of 9.4cd/m2. Because a tungsten sour= was used during both adaptation and testing, the Wratten filters 80A and 88A were not used in the test apparatus. The color cancellation filter was constructed of a vectographic checkerboard print (American Polarizers Inc., Division of Smith, Kline and French) sandwiched together with squares of magenta (CC2OM) and green (CC3064 gelatin. filter. The squares in this composite were &inch on a side. Test stimuli were rear projected at a space-averaged luminance of
4.3 cd/m2.
Procedure. Half the observers began with the low spatial frequency condition and all subjects sewed in both spatial frequency conditions (0.8 and 3.0 c/d). As in Experiment I each subject made ten pre- and ten post-adaptation matches and the direction (ascending or descending) was alternated SO that the direction of initial adjustment was randomized. At least seven days were allowed to elapse between each subject’s participation in a high or low spatial frequency condition.
r
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.v_-_-_______
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-
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2
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7
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*fc Spatial Fig. 3. The
310 Preuuency,
c.fdeg
mean strength of aftere&cts for two seiected spatial frequencies for grating test stimuli.
Each bar represents the mean difference between pre- and pod-a~ptation scrtinp of the color batancing fitter for each condition. Symbols represent the difference scores for each subject under the two conditions. Verticat bars indicate I.0 SE.
Results The data were reduced in the same fashion as in experiment one. Aftereffect strengths (the difference score deriving from the average pm-post adaptation match points) for each subject, (symbols) as well as the mean aftereffect magnitude (bars), are depicted in Fig. 3. The vertical Iines represent f 1 S.E. for the condition mean. All scores for the 0.8 c/d condition were negative, indicating that after adaption to green diamonds and magenta squares, subjects had to add green fight to oblique gratings and magenta light to vertical gratings to achieve a null match. Thus, these color aftereffects were mediated by the higher harmonics components of the patterns. The 3.0 c/d condition resulted in positive difference scores for all subjects and revealed that color a~~~~ for this spatial frequency derived from the fundamental components of the patterns. Student’s I test revealed that the mean afterefgect strength for both conditions was signif&ntiy difftrent from zero. (0.8 c/d condition: t = 3,18. df = 5, P c 0.02% one tailed test; 3.0 c/d condition: r = 5.13, df = 5, P < 0.005, one taiied test}.
DISCUSSlO%
The results of the present study, which were obtained with a cancellation procedure, are in general agreement with previous reports of spatial frequency contingent color aftereffects which used color-naming (May and Matteson. 1976) and ranking (Green and Corwin, 1976) measures. The ~~~t~on procedure is more objective than ~o~or-n~i~g or ranking measures, and it is also more precise. May and Matteson found that only WA of their observers reported partial or complete color aibdkcts in the predicted orientations. All &servers in the present study tested with stimuli moot similar in spatial frequency (3.0 c/d) to those used previously (2.5 c/d) showed afteref%cts in the predicted direction. These results suggest that aheret%cts can probably be produced in most individuals. Although ahere@eet oolors might have been too desaturated to elicit color-naming responses from all individuals, there was always a measurable di@erence with the can&hation procedure. Cofor a%r&bcts ~nt~n~nt on the ~und~e~~~ Fourier components were obtained at high spatial frequencies (3.0,5.0, and 7.0 c/d) with grating test stimuli (i.e., the ~rnp~ent~ cofor occurred in the direc‘The distinction between spatial frequency- and edgetion of fundamental Fourier tomponents of the contingmt ai’teretrcaSis conccptuaiimd IWC with reference to the orientation of these aspects in the p8ktWrned stimuli checkerboard wiapting stimufus rather than in the as the edges of the individual and should not be taken to indicate tha i%Wcnce ofsvtpar- same orient&m checks). A&r adaptation to magenta squares and ate mechanisms within the human visual system. It may be the case that the visual system proc~~ aM the ekments green diamonda more maeta had to be adqeo to of a complex pattern in terms of @aW oblique bars and mrjte green had to be added to vertiW&S.The point at which the eonfingent cal bars than was neaosary prior to adaptation. tobealigncdwiththecdepsofdr~saJop~?o~ Edge-contmgent3 color ‘&er&IWs were observed major Fourier components may iudiuata t&at this visual when adaptation and test stimuli had a spatirif fremc&anhm proccsse3 chcdcsasifthaywerestripesina qutmey of O-8c,d. Under this condition, it was necesgrating at low fume spatial fm+cnM. The major sary to add more green to obhque bars and more Fourier components derive from the fiefs tpatial magenta to vertical bars after adaptation. Thus. color frequency of such patterns, while the edges may be defined aft~dec?s with this low fundamental spatial freby the higher harmonics.
Range of spatial frequency contingent color aftereffects
quency appeared to be associated with edges rather than major Fourier components. This result is at odds with the report of Green er al, (1976). Using a rating method. they were unable to demonstrate edgecontingent aftereffects at low fundamental spatial frequencies (0.4 and 0.85 c/d) and in fact observed weak spatial-frequency contingent aftereffects. This discrepancy may stem from methodological differences. The cancellation technique may be more sensitive than the ranking method in detecting these weak effects. Our data tend to indicate that a transition between edge and fundamental frequency effects occurs somewhere between 1.5 and 0.8 c/d. No significant aftereffects were found at 0.4 c/d. and this result agrees with previous studies of patterncontingent color aftereffects. Stromeyer (1972) investigated the low-frequency reduction of the strength of such effects. By increasing the number of bars in low frequency gratings, he has shown that the reduction in aftereffect
strength
cannot
be accounted
for on
the basis of fewer bars in the low-frequency gratings. Thus, the use of contingent color aftereffects as an approach to the study of pattern perception mechanisms seems to be limited to frequencies above 0.4 c/d. REFERENCES
Campbell F. W. and Robson J. G. (1968) Application of Fourier analysis to the visibility of gratings. J. Physiol. Lond. 197, 551-566.
921
Campbell F. W. and Robson J. G. (1974) Application of Fourier Analysis to the modulation response of the eye. J. opt. Sot. Am. 54, 581A. Carter B. E. and Henning G. B. (1971) The detection of gratings in narrow-band visual noise. J. Physiol. Land. 219. 355-365. Green M., Corwin T. R. and Zemon V. (1976) A comparison of Fourier analysis and feature analysis in patternspecific color aftereffects. Science 192, 147-148. Kelly D. H. (1976) Pattern detection and the two-dimensional Fourier transform: flickering checkerboards and chromatic mechanisms. Vision Res. 16. 277-289. May J. G. and Matteson H. H. (1976) Spatial frequency contingent color aftereffects. Science 194. 145-147. Pollen D. A. Jr.. Lee J. H. and Taylor J. H. (1971) How does the visual cortex begin the reconstruction of the visual world? Science 173. 74-77. Stromeyer C. F. (1972) Edge-contingent color aftereffects: spatial frequency specificity, Vision Res. 12. 717-734. Sullivan G. D., Georgeson M. A. and Oatley, K. (1972) Channels for spatial frequency selection and the detection of single bars by the human visual system. Vision Res. 12, 383-394. Weisstein N. and Bisaha J. (1972) Gratings mask bars and
bars mask gratings: Visual frequency response to a periodic stimuli. Science, 176, 1047-1049. White K. D. (1976) Luminance as a parameter in establishment and testing of the McCollough effect. Yision Res. 16, 297-302.
J-ES
OF SPATIAL FREQUENCY COLOR AFTEREFFECTS
G. MAY*. GREGORY AGAMY*and wm
CONTINGENT H. MATTESON?
*University of New Orleans, Department of Psychology. New Orleans. Louisiana 70122 U.S.A. iTulane University. Department of Psychology New Orleans. Louisiana 70118. U.S.A. (Received 18 May 1976;
in
revised form 24 June 1977)
Abstract-Checkerboard stimuli contain twodimensional Fourier components oriented 45 degrees from edges of the individual checks. Adaptation to such stimuli and testing with rectilinear gratings showed that contingent color aftereffects are associated with the Fourier components rather than the edges of such patterned stimuli at high spatial frequencies. With low spatial frequency (0.8 c/d) contingent color aftereffects were aligned with the edges rather than the Fourier components. These data. obtained with a color cancellation procedure. are in agreement with previous reports of spatial frequency-contingent color aftereffects and indicate the range over which such effects can be obtained.
and Green, Corwin and Zemon (1976) showed that McCollough effects are associated with fundamental spatial frequency information rather than edges. In both studies, an achromatic square-wave test stimulus took on a hue complementary to the hue of a checkerboard adapting stimulus when the test stimulus was oriented in the same direction as one of the major Fourier components of the adapting stimulus (i.e. when the grating test was lined up with one of the diagonals of the checkerboard adapting stimulus, not when it was lined up with edges). So, in this situation. the McCollough effect is associated with major Fourier components rather than edges.
Kelly (1976) pointed out that the two-dimensional Fourier transform of a checkerboard pattern contains
four fundamental frequency components (rather than two, as is the case with a square-wave grating) oriented 45” from the horizontal and vertical edges of the checks. A checkerboard with horizontal and vertical check edges has no Fourier components on the horizontal or vertical axes, although the higher harmonics do lie arbitrarily close to the horizontal and vertical axes so that these harmonics could serve to define what appear phenomenologically as the edges of individual checks. When such a checkerboard is blurred (or viewed just above contrast threshold) it looks similar to two oblique sinusoidal gratings superimposed upon each other at 90” angles. Thus, the checkerboard, 1unlike a square-wave grating (which has its fundamental Fourier components and edges (i.e. higher harmonics) in the same orientation), is a stimulus which may be used to evaluate the relative importance of edges’ and the fundamental Fourier components in the mediation of perceptual phenomena. A number of studies tend to indicate that the human visual system processes pattern information by means of spatial frequency analysis (Campbell and Robson, 1964. 1968; Pollen, Lee and Taylor. 1971). and there are several studies which give fairly direct support to this point of view (Carter and Henning, 1971; Sullivan, Georgeson and Oatley. 1972; Weisstein and Bisaha, 1972). May and Matteson (1976)
‘While the edges of the individual elements of a well focused complex pattern seem intuitively to be most prep+ tent perceptual aspects of such a visual stimulus, they are oriented 45’ from the orientation of the fundamental frequency as revealed by two-dimensional Fourier analysis. It is logical to assume that these edges are defined by the higher harmonics of the fundamental frequency which tend to be aligned more or less on the horizontal and vertical axes. Thus, the checkerboard stimulus may prove to be a useful stimulus in partialling out the perceptual influence of the fundamental frequency as opposed to the higher harmonics (edges) in pattern vision.
EXPERIMENT I
One question about such spatial-frequency-contingent color aftereffects is the range of spatial frequencies over which this result can be observed. As Kelly (1976) pointed out, bars and checks become logically indistinguishable at very low spatial frequencies. With a circular field, both the checkerboard and the grating consist of one black semicircle and one white semicircle when grating bar width or check width equals or exceeds the radius of the circle. Exactly the same effect would be expected if one cycle of a test stimulus equalled or exceeded the size of some functional recep tive field. Thus, there should be no difference between the adaptational effects of gratings and checkerboards at some relatively low spatial frequency. The purpose of the present study was to delineate the range over which spatial-frequency contingent color aftereffects can be obtained and to determine the point at which the visual system begins to associate color with edge orientation rather than fundamental spatial frequency orientation. Method Observers. Thirty-six experimentally naive observers (I 8 women and 18 men) were recruited from introductory psychology classes. All observers had 20/20 acuity uncorrected or were corrected to 20/20 with eye lenses. The observers were tested for normal color vision with American H-R-R
917
Adapting
stimu11
Magenta
the observer to cancel out aitsr&ct mentar) colors added to the tuo A color-balancing filter. composed
Green
Test
stimuli
Fig. 1. Adaptation and test stimuli used in experiment one. The spatial frequency was varied from 0-7 c/d. pseudo-isochromatic plates. Their ages ranged from 19 to 36 years. Apparatus. All patterned stimuli were Thermofax copies of Kodalith negatives. The fundamental frequencies of checkerboard and grating stimuli were equated for each set of adapting and test stimuli (the width of a check edge was 1.4 times that of a grating stripe). The arbitrarily chosen spatial frequencies of test and adapting stimuli were always equal and the frequencies used were: 0.4. 0.8, 1.5, 3.0, 5.0 and 7.0 cycles per degree (c/d). The contrast of all stimuli was 65%. Checkerboard adapting stimuli were transilluminated in a two channel tachistoscope (Scientific Prototype model SOOF). The stimulus fields were squares, subtending 8 degrees of visual angle on a side. and they were viewed binocularlv at a distance of 48 cm. (see Fia 11 One field of the tachistoscope contained a checkerboard with horizontal and vertical edges (“squares”). This pattern was mounted together with a magenta filter (Kodak Wratten No. 32). The second field in the tachistoscope contained the same checkerboard pattern rotated 45’ (“diamonds”). This diamond pattern was superimposed on a green Wratten No. 53 filter. The space-averaged luminance of both adapting fields was 12.8 cd/m’. Adapting stimuli were presented in alternate 5 set periods for 15 min. Compound test stimuli were composed of adjacent panels of: (a) checkerboards, one containing checks with horizontal and vertical edges (“squares”) and one with oblique (45” and 135”) edges (“diamonds”), or (b) squarewave gratings, one with vertical edges and one with oblique (45’) edges. Each half of the test field was a rectangle 8’ (vertical) by 4’ (horizontal). Test stimuli were rearprojected on to a groundglass screen with a lantern slide projector (Beseler model 3620). Each test slide was composed of pairs of patterns (checkerboards or gratings) which were sandwiched with orthogonal polaroids. Test stimuli were projected through two daylight filters (Wratten 80A and 82A) which rendered the coloration of the test stimulus pair at its neutral position as close as possible to the coloration of the daylight fluoresdent lights used to illuminate the adapting patterns. The space-averaged luminance of all test stimuli was 10.2 cd/m’. The apparatus for projecting test stimuli was patterned closely after one described by White (1976) which allows
colors ,w~:h . ,mp;
magenta (Wrattrn CCZOM) and green lCC3OG) gel,ltin til. ters sandwiched with orthogonally polarized squarss m A checkerboard pattern. was positioned between the ?roJ
919
Range of spatial frequency contingent color aftereffects
t ___----
0 - Stripes 0 - Checks
b
1.0
2.0 Spatiol
3.0
4.0
frequency,
5.0
6.0
7.0
c/deg
Fig. 2. The mean strength of aftereffects as a function of spatial frequency for grating and checkerboard test stimuli. Each point represents the mean difference between pre- and post-adaptation settings of the color balancing filter for each group. Vertical bars indicate 1.0 SE.
Results The difference scores computed by subtracting the mean pre-adaptation match point from the mean post-adaptation match point for each subject were submitted to a two-way mixed analysis of variance (spatial frequency X test stimulus-checks or stripes). These data are presented in Fig. 2. The main effect for spatial frequency was statistically significant beyond the 0.01 level (F = 13.69, df = 5.30) but the main effect for test stimulus type (F = 0.27, df = 1.30) and she interaction (F = 1.23. df = 5.30) were nonsignificant {p > 0.05). It is readily apparent from Fig. 2 that appreciable spatial frequency contingent aftereffects were obtained with high spatial frequencies (3.0, 5.0, and 7.0 c/d) with both grating and checkerboard test stimuli. The checkerboard test stimuli resulted in slightly smaller aftereffects at 5.0 and 7.0 c/d. Checkerboard test stimuli resulted in no aftereffect colors at or below 1.5 cid. The negative scores obtained with the grating test at 0.8 c/d indicate a reversed aftereffect (or one resulting from edges rather than major Fourier components). In other words, green was added to the vertical grating test and magenta was added to the oblique grating test to cancel out a magenta aftereffect on the vertical stripes and a green aftereffect on the oblique stripes. EXPERIMEW 2
Although it was apparent from Experiment 1 that color aftereffect magnitude varies significantly with spatial frequency. the occurrence of negative scores could only be considered suggestive of edge-contingent aftereffects because of the high degree of variability in measures of aftereffect strength. The mean ‘This experiment was carried out by the first author whiie on sabbatical in the Visual Science Laboratories at
the Pennsylvania College of Optometry. We wish to acknowledge the invaluable aid of Dr. Kent E. Higgins.
aftereffect strength at 0.8 c/d did not differ significantly from zero, probably because of the effects of individual differences which were not controlled for by the between-subjects design empIoyed. For this reason, a second experiment, empioy~g a within-subject approach, was carried out at two spatial frequency points. Method Observers. Six experimentally naive observers (three mates and three remales), ranging in age from I7 to 36. participated in this experiment. All observers had normal color vision and. with the exception of one person (QD = 1.50 - 0.25 x 85: 20120 OS = 0.75 - 1.00 x 20: 20/120), possessed corrected or uncorrected 20!20 visual acuity. Apparatus. Since this experiment was carried out in a different laboratory.’ another apparatus was constructed. It was similar to the one used in experiment one with the following exceptions. The patterned stimuli were Koda-
lith negatives with a contrast of 96%. Checkerboard adapting stimuli were presented with a 35mm slide projector (Kodak-model 800) and a standard projection screen. The subjects were seated 12 ft from the screen and the adapting stimuli were presented at a space averaged luminance Ievel of 9.4cd/m2. Because a tungsten sour= was used during both adaptation and testing, the Wratten filters 80A and 88A were not used in the test apparatus. The color cancellation filter was constructed of a vectographic checkerboard print (American Polarizers Inc., Division of Smith, Kline and French) sandwiched together with squares of magenta (CC2OM) and green (CC3064 gelatin. filter. The squares in this composite were &inch on a side. Test stimuli were rear projected at a space-averaged luminance of
4.3 cd/m2.
Procedure. Half the observers began with the low spatial frequency condition and all subjects sewed in both spatial frequency conditions (0.8 and 3.0 c/d). As in Experiment I each subject made ten pre- and ten post-adaptation matches and the direction (ascending or descending) was alternated SO that the direction of initial adjustment was randomized. At least seven days were allowed to elapse between each subject’s participation in a high or low spatial frequency condition.
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mean strength of aftere&cts for two seiected spatial frequencies for grating test stimuli.
Each bar represents the mean difference between pre- and pod-a~ptation scrtinp of the color batancing fitter for each condition. Symbols represent the difference scores for each subject under the two conditions. Verticat bars indicate I.0 SE.
Results The data were reduced in the same fashion as in experiment one. Aftereffect strengths (the difference score deriving from the average pm-post adaptation match points) for each subject, (symbols) as well as the mean aftereffect magnitude (bars), are depicted in Fig. 3. The vertical Iines represent f 1 S.E. for the condition mean. All scores for the 0.8 c/d condition were negative, indicating that after adaption to green diamonds and magenta squares, subjects had to add green fight to oblique gratings and magenta light to vertical gratings to achieve a null match. Thus, these color aftereffects were mediated by the higher harmonics components of the patterns. The 3.0 c/d condition resulted in positive difference scores for all subjects and revealed that color a~~~~ for this spatial frequency derived from the fundamental components of the patterns. Student’s I test revealed that the mean afterefgect strength for both conditions was signif&ntiy difftrent from zero. (0.8 c/d condition: t = 3,18. df = 5, P c 0.02% one tailed test; 3.0 c/d condition: r = 5.13, df = 5, P < 0.005, one taiied test}.
DISCUSSlO%
The results of the present study, which were obtained with a cancellation procedure, are in general agreement with previous reports of spatial frequency contingent color aftereffects which used color-naming (May and Matteson. 1976) and ranking (Green and Corwin, 1976) measures. The ~~~t~on procedure is more objective than ~o~or-n~i~g or ranking measures, and it is also more precise. May and Matteson found that only WA of their observers reported partial or complete color aibdkcts in the predicted orientations. All &servers in the present study tested with stimuli moot similar in spatial frequency (3.0 c/d) to those used previously (2.5 c/d) showed afteref%cts in the predicted direction. These results suggest that aheret%cts can probably be produced in most individuals. Although ahere@eet oolors might have been too desaturated to elicit color-naming responses from all individuals, there was always a measurable di@erence with the can&hation procedure. Cofor a%r&bcts ~nt~n~nt on the ~und~e~~~ Fourier components were obtained at high spatial frequencies (3.0,5.0, and 7.0 c/d) with grating test stimuli (i.e., the ~rnp~ent~ cofor occurred in the direc‘The distinction between spatial frequency- and edgetion of fundamental Fourier tomponents of the contingmt ai’teretrcaSis conccptuaiimd IWC with reference to the orientation of these aspects in the p8ktWrned stimuli checkerboard wiapting stimufus rather than in the as the edges of the individual and should not be taken to indicate tha i%Wcnce ofsvtpar- same orient&m checks). A&r adaptation to magenta squares and ate mechanisms within the human visual system. It may be the case that the visual system proc~~ aM the ekments green diamonda more maeta had to be adqeo to of a complex pattern in terms of @aW oblique bars and mrjte green had to be added to vertiW&S.The point at which the eonfingent cal bars than was neaosary prior to adaptation. tobealigncdwiththecdepsofdr~saJop~?o~ Edge-contmgent3 color ‘&er&IWs were observed major Fourier components may iudiuata t&at this visual when adaptation and test stimuli had a spatirif fremc&anhm proccsse3 chcdcsasifthaywerestripesina qutmey of O-8c,d. Under this condition, it was necesgrating at low fume spatial fm+cnM. The major sary to add more green to obhque bars and more Fourier components derive from the fiefs tpatial magenta to vertical bars after adaptation. Thus. color frequency of such patterns, while the edges may be defined aft~dec?s with this low fundamental spatial freby the higher harmonics.
Range of spatial frequency contingent color aftereffects
quency appeared to be associated with edges rather than major Fourier components. This result is at odds with the report of Green er al, (1976). Using a rating method. they were unable to demonstrate edgecontingent aftereffects at low fundamental spatial frequencies (0.4 and 0.85 c/d) and in fact observed weak spatial-frequency contingent aftereffects. This discrepancy may stem from methodological differences. The cancellation technique may be more sensitive than the ranking method in detecting these weak effects. Our data tend to indicate that a transition between edge and fundamental frequency effects occurs somewhere between 1.5 and 0.8 c/d. No significant aftereffects were found at 0.4 c/d. and this result agrees with previous studies of patterncontingent color aftereffects. Stromeyer (1972) investigated the low-frequency reduction of the strength of such effects. By increasing the number of bars in low frequency gratings, he has shown that the reduction in aftereffect
strength
cannot
be accounted
for on
the basis of fewer bars in the low-frequency gratings. Thus, the use of contingent color aftereffects as an approach to the study of pattern perception mechanisms seems to be limited to frequencies above 0.4 c/d. REFERENCES
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921
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