JOURNAL OF THE OPTICAL SOCIETY OF AMERICA
VOLUME 65, NUMBER 11
NOVEMBER 1975
Letters to the Editor Measurement of metacontrast Ronald Growney Department of Psychology, University of Connecticut, Storrs, Connecticut 06268
Naomi Weisstein Department of Psychology, State University of New York, Buffalo, New York 14226
Sue I. Cox Department of Psychology, San Francisco City College, San Francisco, California 94112 (Received 15 March 1975) Index Heading: Vision.
Metacontrast is a phenomenon in which the apparent brightness of a target stimulus is reduced or even oblit erated by a subsequent, masking, stimulus that has the same energy as the target, but does not overlap the target spatially. This reduction of perceptibility of the target is of interest with respect to theories of apparent brightness, contrast, and pattern recognition. Metacontrast can be measured in at least two ways, however, one of which poses theoretical difficulties of interpretation. In the first way, the response measure does not depend on manipulation of target luminance. To measure the reduction of apparent brightness of the t a r get, the luminance of a comparison stimulus can be varied to match the apparent brightness of the target, 1 or the apparent brightness of the target may be assigned a rating. 2 – 5 In either case, the target luminance remains unchanged in the assessment of the magnitude of masking. The second way involves manipulation of target lumi nance. The target luminance may be adjusted to match the brightness of a comparison stimulus. 6 ' 7 Whether or not target luminance is changed in the course of mea suring masking magnitude may be critical in the inter pretation of masking r e s u l t s . Changing target energy changes the state of the system that is being measured, so that additional factors, other than those that determine the masking of equal-energy stimuli, may influence the amount of increase of target luminance necessary to match the comparison stimulus. It is then difficult to deduce from the data the characteristics of visual p r o cessing that underlie the masking. This difficulty has been discussed previously 8 ; the shape of the masking curve (magnitude of masking with respect to temporal delay Δt, between target and mask) depends directly on ratio of the energy of the target to that of the mask. Changes of target luminance confound this energy ratio, so that the resulting masking curve represents a s a m pling of points from different energy ratios. Flaherty and Matteson 9 have suggested that, despite the difficulties of theoretical interpretation of masking results, variation of target luminance may be a p r o c e dure easier to apply in order to obtain complete masking functions. 10 Using a masking annulus at a retinal i l luminance of 11 600 td, they compared the masking func tions obtained by (1) varying target luminance to match the brightness of a 15.2 td comparison stimulus, to 1379
those obtained by (2) changing the luminance of a com parison stimulus to match the apparent brightness of the 72 000 td target stimulus. The masking functions ob tained in the first procedure were similar to typical metacontrast functions in that the curves were U shaped (nonmonotonic) and regular in appearance (only one peak). However, the functions that they obtained by varying the luminance of the comparison stimulus were highly i r r e g u l a r (multipeaked), quite different from typi cal metacontrast functions. The data of the two ob s e r v e r s , in addition, were very unreliable; what ap peared to be striking individual differences in amount of masking with respect to Δt (main effect of Δt, or equiv alenty, SOA using Flaherty and Matteson's terminology) were found to be not statistically significant. We have completed a study in which we varied the luminance of a comparison stimulus in order to measure masking magnitude; unlike Flaherty and Matteson, how ever, we obtained regular, U-shaped masking functions. The stimuli, 1° high, were narrow vertical lines either 1' or 3' wide, each of which was presented as both target and, in flanking pairs, as m a s k s . 1 1 The target and mask
FIG. 1. Data of observer SC for the condition in which target and mask were both 3' in width: amount of masking as a func tion of temporal delay, Δt, for selected (representative) tar get-mask spatial separations of 1', O; 5′, ▲; 10′, □; 30′, ●; and 1°, Δ . Amount of masking is indicated by density of filter needed to adjust the comparison stimulus to equal brightness with the target.
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L E T T E R S TO THE
We found that the amount of masking changed in a regular manner with respect to At for stimuli of different widths at different spatial separations (except for the largest separations, where the amount of masking was very small). The greatest amount of masking was obtained for both observers when the target and mask were both 3′ wide; the data from this condition are shown 13 in Figs. 1 and 2. Each point is the mean of 6 replications; each curve represents a different spatial separation of target and mask. The masking curves for the data of both observers are U shaped (inverted U); their slopes increase or decrease regularly about the At at which maximum masking occurred. Furthermore, the amplitude of masking around the At, at which the greatest masking occurred, decreased with increasing spatial separation between target and mask. 1 4 These characteristics a r e representative of the masking that was obtained in the other t a r g e t - m a s k conditions ( 1 ' - 1 ' , 1' – 3', 3' –1'). Variation of comparison-stimulus luminance, therefore, yielded masking functions that were single peaked and had quite regular changes of slope.
Vol. 65
For a small set of optimal conditions, the target may be completely occluded by metacontrast, so that a floor effect occurs in the masking function. In order to obtain masking functions that avoided these floor effects, Flaherty and Matteson used quite high luminances, which, as they state, probably made judgments with respect to the comparison stimulus extremely difficult, owing to peripheral glare. On the basis of our data, we believe that the irregularity of the masking functions obtained by Flaherty and Matteson occurred because of extraneous effects, such as the effects of high values of luminance, rather than because of the method of varying the lumi nance of the comparison stimulus. 1 5 In addition, the ef fects of saturation, to the extent that they exist in meta contrast data, seem to be small in comparison with the amplitude and width of the over-all masking function. Complete occlusion of the target by the mask occurs only for a very narrow range of At, as evidenced by the narrowness of the peaks of the functions shown in Fig. 1, and only for a small set of optimal conditions (including stimulus width, 5 stimulus edge gradient, 1 6 stimulus en ergy, 8 and the spatial separation of the stimuli 3 * 6 ). The magnitude of any saturation of response measure is also indicated by the slope of the masking function as it ap proaches the At at which the greatest masking occurs. To the extent that the masking function describes a r e g ular, continuously increasing effect of the mask upon the target, any masking function is a complete masking function, in so far as the continuous change of slope of the masking function can be used to approximate the masking magnitude at each At. For these reasons, we suggest that the possibility of saturation of the response measure does not warrant the introduction of such high values of target and mask luminance, especially if the response measure is thereby rendered insensitive.
FIG. 2. Data of observer RG. See Fig. 1 for other informa tion.
were presented to the observer's left eye; the compari son stimulus was presented to the right eye after a delay of 2 s. The durations of all stimuli were 16 ms; they had a luminance of 55 mL on a 5 mL adaptation field. The method of limits was used with ascending and de scending o r d e r s of presentation counterbalanced across conditions. Neutral filters in density steps of 0.1 were inserted in the optical path of the comparison stimulus in one channel of a six-channel tachistoscope (Scientific Prototype, model G). The four width combinations of the target and mask (target-mask: l ' - l ' , l ' - 3 ' , 3 ' - l ' , 3 ' - 3 ' ) were presented at each of 16 spatial separations between target and mask (from 1' to 3°) at each of 15 temporal delays (Δt over the range - 100 ms, in which the mask precedes the target, to + 200 ms, in which the target precedes the mask). 1 2 Each t a r g e t - m a s k - s e p a r a tion condition was presented for the 15 randomized tem poral delays consecutively. Otherwise, all width and target-mask-separation conditions were presented in random order.
EDITOR
In summary, we conducted a metacontrast experiment in which we varied the luminance of a comparison stim ulus and obtained results quite different from those of Flaherty and Matteson. The results that we obtained were highly similar to the masking functions that have been obtained by the use of other methods. The method of varying comparison-stimulus luminance was easy to use; so are various rating methods. 1 – 5 Such methods have the added advantage of not changing the state of the s y s tem being measured to the same extent as does the meth od of changing test-stimulus luminance to match the brightness of a reference stimulus. We conclude that variation of comparison-stimulus luminance may be as easy a method to use as variation of test-stimulus lumi nance; and it does not involve the theoretical difficulties of interpretation posed by variation of test-stimulus luminance.
1
P. Schiller and M. Smith, J. Exp. Psychol. 71, 32 (1966). D. Kahneman, Psychol. Bull. 70, 404 (1968). N. Weisstein and R. Growney, Percept. Psychophys. 5, 321 (1969). 4 N. Weisstein, T. Jurkens, and T. Onderisin, J. Opt. Soc. Am. 60, 978 (1970). 5 R. Growney and N. Weisstein, J. Opt. Soc. Am. 62, 690 (1972). 6 M. Alpern, J. Opt. Soc. Am. 43, 648 (1953). 2
Nov. 1975 7
L E T T E R S TO T H E
H. H. Matteson, J. Opt. Soc. Am. 59, 1461 (1969). N. Weisstein, in Visual Psychophysics, edited by D. Jameson and L. M. Hurvich (Springer, Heidelberg, 1972). 9 T. Flaherty and H. H. Matteson, J. Opt. Soc. Am. 61, 828 (1971). 10 By "complete masking functions" we assume the authors in Ref. 9 meant functions that do not show saturation of the r e sponse m e a s u r e , because the target is never completely masked. 11 These rectangular (line) stimuli differ from the d i s k - r i n g configuration used by Flaherty and Matteson. However, we would not expect different results from Flaherty and Matte son on this basis for moderate values of luminance. Quite regular masking functions have been obtained both with abovethreshold, rectangular stimuli (Refs. 3 and 5) and with abovethreshold d i s k - r i n g stimuli (Refs. 1 and 4). At the high val ues of luminance used by Flaherty and Matteson, though, r e sponse m e a s u r e s obtained with respect to a disk–ring config uration may saturate more quickly than measures of a r e c 8
EDITOR
1381
tangular stimulus configuration [H. W. Horeman, Vision Res. 5, 331 (1965)]. Alpern (Ref. 6) obtained regular masking functions with rectangular stimuli that had high luminance. 2 Δt is measured from the onset of the first stimulus to the onset of the second stimulus. 3 Little forward masking (over the negative range of At) was obtained in any condition. 4 Slightly more masking was found for both observers in some instances at spatial separations of 2′ to 4′ and at a separation of 10' for observer RG for the 3' target-1′ mask condition. 5 The double-staircase method used by Flaherty and Matteson ought to be a more-sensitive measure of masking magnitude than was the method of limits that we used. However, the judgment difficulties described by Flaherty and Matteson, together with the over-all regularity of the masking functions that we obtained, suggest that the differences of sensitivity between the two methods would not account for the gross dif ferences between the results of the two studies. l6 R. Growney, P h . D . thesis (Loyola University, 1973).
VOLUME 65, NUMBER 11
NOVEMBER 1975
Letters to the Editor Measurement of metacontrast Ronald Growney Department of Psychology, University of Connecticut, Storrs, Connecticut 06268
Naomi Weisstein Department of Psychology, State University of New York, Buffalo, New York 14226
Sue I. Cox Department of Psychology, San Francisco City College, San Francisco, California 94112 (Received 15 March 1975) Index Heading: Vision.
Metacontrast is a phenomenon in which the apparent brightness of a target stimulus is reduced or even oblit erated by a subsequent, masking, stimulus that has the same energy as the target, but does not overlap the target spatially. This reduction of perceptibility of the target is of interest with respect to theories of apparent brightness, contrast, and pattern recognition. Metacontrast can be measured in at least two ways, however, one of which poses theoretical difficulties of interpretation. In the first way, the response measure does not depend on manipulation of target luminance. To measure the reduction of apparent brightness of the t a r get, the luminance of a comparison stimulus can be varied to match the apparent brightness of the target, 1 or the apparent brightness of the target may be assigned a rating. 2 – 5 In either case, the target luminance remains unchanged in the assessment of the magnitude of masking. The second way involves manipulation of target lumi nance. The target luminance may be adjusted to match the brightness of a comparison stimulus. 6 ' 7 Whether or not target luminance is changed in the course of mea suring masking magnitude may be critical in the inter pretation of masking r e s u l t s . Changing target energy changes the state of the system that is being measured, so that additional factors, other than those that determine the masking of equal-energy stimuli, may influence the amount of increase of target luminance necessary to match the comparison stimulus. It is then difficult to deduce from the data the characteristics of visual p r o cessing that underlie the masking. This difficulty has been discussed previously 8 ; the shape of the masking curve (magnitude of masking with respect to temporal delay Δt, between target and mask) depends directly on ratio of the energy of the target to that of the mask. Changes of target luminance confound this energy ratio, so that the resulting masking curve represents a s a m pling of points from different energy ratios. Flaherty and Matteson 9 have suggested that, despite the difficulties of theoretical interpretation of masking results, variation of target luminance may be a p r o c e dure easier to apply in order to obtain complete masking functions. 10 Using a masking annulus at a retinal i l luminance of 11 600 td, they compared the masking func tions obtained by (1) varying target luminance to match the brightness of a 15.2 td comparison stimulus, to 1379
those obtained by (2) changing the luminance of a com parison stimulus to match the apparent brightness of the 72 000 td target stimulus. The masking functions ob tained in the first procedure were similar to typical metacontrast functions in that the curves were U shaped (nonmonotonic) and regular in appearance (only one peak). However, the functions that they obtained by varying the luminance of the comparison stimulus were highly i r r e g u l a r (multipeaked), quite different from typi cal metacontrast functions. The data of the two ob s e r v e r s , in addition, were very unreliable; what ap peared to be striking individual differences in amount of masking with respect to Δt (main effect of Δt, or equiv alenty, SOA using Flaherty and Matteson's terminology) were found to be not statistically significant. We have completed a study in which we varied the luminance of a comparison stimulus in order to measure masking magnitude; unlike Flaherty and Matteson, how ever, we obtained regular, U-shaped masking functions. The stimuli, 1° high, were narrow vertical lines either 1' or 3' wide, each of which was presented as both target and, in flanking pairs, as m a s k s . 1 1 The target and mask
FIG. 1. Data of observer SC for the condition in which target and mask were both 3' in width: amount of masking as a func tion of temporal delay, Δt, for selected (representative) tar get-mask spatial separations of 1', O; 5′, ▲; 10′, □; 30′, ●; and 1°, Δ . Amount of masking is indicated by density of filter needed to adjust the comparison stimulus to equal brightness with the target.
1380
L E T T E R S TO THE
We found that the amount of masking changed in a regular manner with respect to At for stimuli of different widths at different spatial separations (except for the largest separations, where the amount of masking was very small). The greatest amount of masking was obtained for both observers when the target and mask were both 3′ wide; the data from this condition are shown 13 in Figs. 1 and 2. Each point is the mean of 6 replications; each curve represents a different spatial separation of target and mask. The masking curves for the data of both observers are U shaped (inverted U); their slopes increase or decrease regularly about the At at which maximum masking occurred. Furthermore, the amplitude of masking around the At, at which the greatest masking occurred, decreased with increasing spatial separation between target and mask. 1 4 These characteristics a r e representative of the masking that was obtained in the other t a r g e t - m a s k conditions ( 1 ' - 1 ' , 1' – 3', 3' –1'). Variation of comparison-stimulus luminance, therefore, yielded masking functions that were single peaked and had quite regular changes of slope.
Vol. 65
For a small set of optimal conditions, the target may be completely occluded by metacontrast, so that a floor effect occurs in the masking function. In order to obtain masking functions that avoided these floor effects, Flaherty and Matteson used quite high luminances, which, as they state, probably made judgments with respect to the comparison stimulus extremely difficult, owing to peripheral glare. On the basis of our data, we believe that the irregularity of the masking functions obtained by Flaherty and Matteson occurred because of extraneous effects, such as the effects of high values of luminance, rather than because of the method of varying the lumi nance of the comparison stimulus. 1 5 In addition, the ef fects of saturation, to the extent that they exist in meta contrast data, seem to be small in comparison with the amplitude and width of the over-all masking function. Complete occlusion of the target by the mask occurs only for a very narrow range of At, as evidenced by the narrowness of the peaks of the functions shown in Fig. 1, and only for a small set of optimal conditions (including stimulus width, 5 stimulus edge gradient, 1 6 stimulus en ergy, 8 and the spatial separation of the stimuli 3 * 6 ). The magnitude of any saturation of response measure is also indicated by the slope of the masking function as it ap proaches the At at which the greatest masking occurs. To the extent that the masking function describes a r e g ular, continuously increasing effect of the mask upon the target, any masking function is a complete masking function, in so far as the continuous change of slope of the masking function can be used to approximate the masking magnitude at each At. For these reasons, we suggest that the possibility of saturation of the response measure does not warrant the introduction of such high values of target and mask luminance, especially if the response measure is thereby rendered insensitive.
FIG. 2. Data of observer RG. See Fig. 1 for other informa tion.
were presented to the observer's left eye; the compari son stimulus was presented to the right eye after a delay of 2 s. The durations of all stimuli were 16 ms; they had a luminance of 55 mL on a 5 mL adaptation field. The method of limits was used with ascending and de scending o r d e r s of presentation counterbalanced across conditions. Neutral filters in density steps of 0.1 were inserted in the optical path of the comparison stimulus in one channel of a six-channel tachistoscope (Scientific Prototype, model G). The four width combinations of the target and mask (target-mask: l ' - l ' , l ' - 3 ' , 3 ' - l ' , 3 ' - 3 ' ) were presented at each of 16 spatial separations between target and mask (from 1' to 3°) at each of 15 temporal delays (Δt over the range - 100 ms, in which the mask precedes the target, to + 200 ms, in which the target precedes the mask). 1 2 Each t a r g e t - m a s k - s e p a r a tion condition was presented for the 15 randomized tem poral delays consecutively. Otherwise, all width and target-mask-separation conditions were presented in random order.
EDITOR
In summary, we conducted a metacontrast experiment in which we varied the luminance of a comparison stim ulus and obtained results quite different from those of Flaherty and Matteson. The results that we obtained were highly similar to the masking functions that have been obtained by the use of other methods. The method of varying comparison-stimulus luminance was easy to use; so are various rating methods. 1 – 5 Such methods have the added advantage of not changing the state of the s y s tem being measured to the same extent as does the meth od of changing test-stimulus luminance to match the brightness of a reference stimulus. We conclude that variation of comparison-stimulus luminance may be as easy a method to use as variation of test-stimulus lumi nance; and it does not involve the theoretical difficulties of interpretation posed by variation of test-stimulus luminance.
1
P. Schiller and M. Smith, J. Exp. Psychol. 71, 32 (1966). D. Kahneman, Psychol. Bull. 70, 404 (1968). N. Weisstein and R. Growney, Percept. Psychophys. 5, 321 (1969). 4 N. Weisstein, T. Jurkens, and T. Onderisin, J. Opt. Soc. Am. 60, 978 (1970). 5 R. Growney and N. Weisstein, J. Opt. Soc. Am. 62, 690 (1972). 6 M. Alpern, J. Opt. Soc. Am. 43, 648 (1953). 2
Nov. 1975 7
L E T T E R S TO T H E
H. H. Matteson, J. Opt. Soc. Am. 59, 1461 (1969). N. Weisstein, in Visual Psychophysics, edited by D. Jameson and L. M. Hurvich (Springer, Heidelberg, 1972). 9 T. Flaherty and H. H. Matteson, J. Opt. Soc. Am. 61, 828 (1971). 10 By "complete masking functions" we assume the authors in Ref. 9 meant functions that do not show saturation of the r e sponse m e a s u r e , because the target is never completely masked. 11 These rectangular (line) stimuli differ from the d i s k - r i n g configuration used by Flaherty and Matteson. However, we would not expect different results from Flaherty and Matte son on this basis for moderate values of luminance. Quite regular masking functions have been obtained both with abovethreshold, rectangular stimuli (Refs. 3 and 5) and with abovethreshold d i s k - r i n g stimuli (Refs. 1 and 4). At the high val ues of luminance used by Flaherty and Matteson, though, r e sponse m e a s u r e s obtained with respect to a disk–ring config uration may saturate more quickly than measures of a r e c 8
EDITOR
1381
tangular stimulus configuration [H. W. Horeman, Vision Res. 5, 331 (1965)]. Alpern (Ref. 6) obtained regular masking functions with rectangular stimuli that had high luminance. 2 Δt is measured from the onset of the first stimulus to the onset of the second stimulus. 3 Little forward masking (over the negative range of At) was obtained in any condition. 4 Slightly more masking was found for both observers in some instances at spatial separations of 2′ to 4′ and at a separation of 10' for observer RG for the 3' target-1′ mask condition. 5 The double-staircase method used by Flaherty and Matteson ought to be a more-sensitive measure of masking magnitude than was the method of limits that we used. However, the judgment difficulties described by Flaherty and Matteson, together with the over-all regularity of the masking functions that we obtained, suggest that the differences of sensitivity between the two methods would not account for the gross dif ferences between the results of the two studies. l6 R. Growney, P h . D . thesis (Loyola University, 1973).
