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Asymmetry in the brightness and darkness broca-sulzer effects
Virwa Rrwrrch 6 Pcqamon
Vol. 20. pp. 723 to 716 Press Lrd 1980. Printed m Great
Britain
RESEARCH NOTE
ASYMMETRY
IN THE BRIGHTNESS AND DARKNESS BROCA-SULZER EFFECT’S
THOMAS W.
WHITER,GREGG E. Irtvt~l and
MARY
C. WILLIAMS’
LDepartment of 3iobehavioral Sciences, Boston University Medical Center, Boston. MA 02118. U.S.A. ‘Visual Psychophysics Laboratories. Syracuse University, Syracuse, NY 13210. U.S.A. ‘Department of Psychology, SUNY Buffalo, Buffalo, NY 14226. U.S.A.
(Received 14 February
INTRODUCTION There
is increasing evidence that the bright and dark components of the retinal image are handled differently by the visual system (Glad and Magnussen, 1972; Magnussen and Glad, 197%. b, c; Burton et aI., 1977; DeValois, 1977; Remole, 1977; Walraven, 1977; Corwin, 1978). On and off channels have been suggested as possible neural bases for separate processing (Jung, 1973). In a comparison of the brightness and darkness Broca-Sulzer effects, we also found evidence consistent with differential processing.
1979)
100 possible pairs of flash durations. The subjects had no way of predicting which pair would be presented or in what order they would occur. After a pair had been presented, the subject pushed one of two buttons which told the computer which flash had been perceived as brighter (or darker) by the subject. Scales of brightness (and darkness) were derived from the resulting 10 x 10 dominance matrices with a rank scaling procedure (Dunn-Rankin and Wilcoxon, 1966; Dunn-Rankin and King, 1969). In the rank scaling method, rank sums are obtained for each duration by simply summing the frequency of times the duration
METHOD The stimuli were freely viewed in a ‘&channel, Iconix Tachistoscope. Achromatic light was provided either by tfuorescent sources or by Kodak slide pre jectors and rear projection screens (Sylvania 300 W, 120 V ELH). In any on‘e incremental or decremental condition, the test flash (TF) and background were MONOCULAR FREE-VIEWING from the same type of source, i.e. their color did not RIGHT EYE differ. A Uniblitz shutter shaped the flashes when projectors were used. Timing and data collection funcCONDmON A: tions were performed by a PDP-8f computer. The subjects viewed a 28’ TF with the fovea of the right eye. (See Fig. 1, top). Two different background configurations were employed. In one condition, the TF was added to or subtracted from the center of a 0 log mL, 3” diameter background. The background also contained a 10’ wide, 1” 2cT diameter, bIa& fixa- CONDITION B : tion ring. (See Fig. 1, condition A). In the other con0;m.l dition, the TF either incremented above a 0 log mL, 28’ diameter background or it decremented down to Iti + 3, IO OR IOOmL that 1eveL The 28’ background was surrounded by a 4,li OR 10lmLTO~mL 3” outer diameter, 28’ inner diameter, -1.0 log mL annulus which contained a fixation ring. (See Fig. 1, TEMPORAL SEQUENCE : condition B). 500 MSEC 500 MSEC RIOR R2+7OD The apparent brightness of increments and the MSEC apparent darkness of decrements were independently FI ws F2 scaled for each luminance and background condition using a paired-comparisons task (Torgerson, 1958). Fig. 1. Stimulus configuration. See text. TF = 1st flash; Bknd = background; FR = fixation ring: WS = waning (See Fig. 1, bottom). Ten TF durations were used. On sii@: fl = first IF; F2 = second TF; Rt = rgswnse 1 any trial, the computer randomly presented one of the (Fl > FZ); R2 = response 2 fF2 > Fl).
723
Research Not?
_‘-I ! _
was judged brighter (or darker) than the other durations. These sums are transfo~ed into scale values using the following equation :
where R = calculated scale value; R, = the obtained rank sum; Rmin = the minimum possible rank total; and R,,, = the maximum possible total. This procedure results in a uni-dimensional scale which varies from 0 to 100 with each duration ~cupying a lccation on the scale. Critical ranges (CR) are also calculated for each scale. The CR is an expected value and it specifies how far apart two items must be on the scale in order to be considered as occupying separate locations on the scale. Selected data points were also scaled using the method of adjustment and viewing condition B. Here a variable duration TF was recycled successively with a 100msec comparison flash in a manner similar to that used in the paired-comparisons task. (See Fig. I. bottom). The subject had to adjust the amount of light in the comparison until it produced a sensation identical to that produced by the TF. RESULTS
Our sensation vs duration curves do not show the symmetry in response found for increments and decrements at Theshold (Blackwell. 1946: Cohn. 1974: Herrick, 1956; Pate1 and .?ones, 1968; Rashbass, 1970; Short, 1966). In Fig. 2, brightness (top curves) and darkness (bottom curves) are plotted as a function of log TF duration for luminance changes of 0.3. 0.5 and
1.0 log umt. In determmmg these curves, I&t w;lj either added to or subtracted from the center of the background field. The 0.3 log unit luminance change (left panel) produced brightness and darkness effects similar in time course but the darkness effect had 11 more pronounced overshoot. Increasing the magnitude of the luminance change to 0.5 (center panel) resulted in brightness and darkness effects which were more similar in shape but displaced in time. The maximum sensation occurred at a longer duration with decrements (100msec) than with increments (50 msec). Darkness enhancement was no longer cvident with a 1.0 (right panel) or a 2.0 (not shoun) log luminance change. This indicates saturation oi the darkness response as physical contrast approaches 1009,. A brightness effect was still present as Kould be expected given that increments are not contrast limited in the same way as decrements. Stimulus condition B allowed the scaling of darkness over a larger intensity range. Here the TF decremented down to a luminance level which possessed some finite brightness, i.e. it was brighter than its surround. Figure 3 plots brightness (top curves) and darkness (bottom curves) functions for luminance changes of 0.5, 1.0 and 2.0 log units. The 0.5 log increments (left panel) did not produce a Broca-Sulzer effect. The darkness effect was present with the 0.5 log decrement. The larger magnitude luminance changes caused both brightness and darkness effects. Again. the darkness effect appears to hal-e a slousr time course when compared with the brightness ctfect ( lM)-I 20 msec vs 50-70 msec). Selected data points from Fig. 3 were scaled using the method of adjustment. Figure 4 shows the amount of light necessary in a 100 msec comparison flash to
*OiR 40
I
.n X-Jr
I OLO IO
20
or
, 30
8Oi *7W.0 LOG
CWATION
( rnec
1
2.0
IO LOG
DURATION
(msec
I
LOG
DURATION
(msec
1
Fig. 2. Brightness (circles) and darkness (squares) curves for luminance changes of 0.3 [left), 0.5 (liddIe) and I.0 (right) log unit and stimulus condition A. In each panel, scale value (R) is plotted as a function of log TF duration (msec). Grouped data for two observers. 3000 trials each curve. CR = ?.57. 0.05 level of significance.
Research Note
eo-
BOI 1.0
3.0 2.0 LOG DURATION ( mfec t
I.0
2.0 LOG DURATION
1.0
2.0 LOG DURATION
3.0
( msec i
3.0
( rwec)
Fig. 3. Brightness (circles) and darkness (squares) curves for luminance changes of 0.5 (left), 1.0 (middle) and 2.0 fright) tog units and stimulus condition 3. In each paneI, scaic value (RI is plotted as a function of log TF duration (msec). Grouped data for 3 observers. &X0 trials each curve. CR = 2.33,O.M level.
match increments and decrements of 0.5, 1.0 and 2.0 units as a Function of their exposure duration. As in the other figures, brightness is plotted above darkness. The general features of Fig. 3 again appear, i.e. no brightness effect for the 0.5 increment and longer time-courses with decrements than with increments. In addition, this figure shows the darkness effect to grow in magnitude at a faster rate with increases in luminance than does the brightness effect. log
DISCUSSION
Our results are consistent with those which have been previously reported for the Briicke-Barthey e&t and its darkness anaiogue t.h&gmtssen and Glad, 1975a, b,c}. The only apparent disagreement is in terms of the timecourse of the brightness and darkness effects. Using square-wave flicker, brightness and darkness enhancement appear to occur at similar temporal frequencies whereas with individual light increments and decrements, maximum darkness occurred at longer durations than did maximum brightness. It could be argued that under stimuIus condition A the increments and decrements were not comparable since luminance changes matched according to iog magnitude result in di&rent values than those G&Xfated in terms of percent modulation around a mean luminance level. This procedural difference may be sufficient to account for the time-course differences found between brightness and darkness It is more difficult to use this argument to explain the same differences occurring under stimulus condition B since here the upward and downward luminance changes were symmetric,
1
1.0
2.0 LCE DURAT~~
I
30 tmsec
t
Fi& 4. Brightness (circles) and darkness (squares) scaled by the method of adjustment for luminance changes of 2.0, 1.0 and 0.5 log units. In each curve, the luminance necessary in a 100 msec comparison Rash to match a variable duration TF is plotted as a function of tog IF duration (msecb Grouped data for 3 subjects I5 total matches. SD < 0.2 tog unit.
‘26
Research Note
A time-course difference is consistent with Jung’s (1973) two-channel model of brightness and darkness coding. In this model, darkness is slower due to the importance of surround activation in off-center channels as compared with the more direct central activation of on-center, brightness units. Of course, a single non-linear channel could also provide asymmetric responses to upward and downward luminance changes. Previous investigations of brightness enhancement have shown that a TF set in luminance near its increment threshold does not show enhancement (Rinalducci and Higgins, 1971; White er al., 1976). In the present study, brightness enhancement was found with the 0.5 log increment when viewed against the 3’, 0 log mL background but not when viewed against the 2s’. 0 log mL background. A 2s’ TF seen against a 28’ background would be expected to have a higher detection threshold than would a 28’ TF centered within a uniform 3” field. .4cknowledgements-This research was supported in part bv NIH erant No EY01571 and NRSA Biolonical Sciences
T;aining-Program
No MH-15189, NIMH. -The authors
would like to also thank Joseph F. Sturr for the use of his equipment and John (Jack) Wisowaty for his time spent as an observer. REFERENCES Blackwell H. R. (1946) Contrast thresholds of the human eye. J. opt. Sot. Am. 36, 624-643. Burton G. J., Nagshineh S. and Ruddock K. H. (1977) Processing by the human visual system of the light and dark contrast components of the retinal image. Biol. Cybernetics
27, 189-197.
Cohn T. E. (1974) A new hypothesis to explain why the inqem_
Suppl. 222.
DeValois K. K. ( 1977) Independence of black and w hate: phase specific adaptation. Vision RG. 17. X9-11 5. Dunn-Rankin P. and King F. J. (1969) Multiple comparisons in a simplified rank method of scaling. Educ. psychol. Meow. 29, 315-329. Dunn-Rankin P. and Wilcoxon F. (1966) The true distribution of the range of rank totals in the two-way class& cation. Psyhometrika 31. 573-580. Glad A. and .Ilagnussen S. (1972) Darkness enhancement in intermittent light: an experimental demonstration, I’lsion Res. 12, 11l-1 15. Herrick R. M. (1956) Fovea1 luminance discrimination as a function of the duration of the decrement or increment in luminance. J. camp. physiol. Psychol. 49, 437-443. Jung R. (1973) Visual perception and neurophysiology. In Handbook oj Sensory Physiology. Vol. VII/4, pp. L-152. Springer, Berlin. Magnussen S. and Glad A. (197ja) Brightness and darkness enhancement during flicker: perceptual correlates of neuronal B- and D-systems in human retina. Expl Brain Res. 22, 399313.
Magnussen S. and Glad A. (1975b) Temporal frequency characteristics of spatial interaction in human vision. Expl Brain Res. 23, 519-528.
Magnussen S. and Glad A. (1975c) Effects of steady surround illumination on the brightness and darkness enhancement of flickering lights. Vision Res. 15. 1113-1416.
Pate1 A. S. and Jones R. W. (1968) Increment and decrement visual thresholds. J. opt. Sot. Am. 58, 696-699. Rashbass C. (1970) The visibilitv of transient chances of luminance. J. Physiol. 210, 16clS6. Remole A. (1977) Brightness enhancement versus darkness enhancement at a border. Vision Res. 17, 1095-l 100. Rinalducci E. J. and Higgins K. E. (1971) An Incestigarion (II’the Efjecticr Intensity of Flashing Lights. Clearinghouse, Springfield. VA. Short A. fi. (7966) Decremental and incremental visual thresholds. J. Phvsiol. 185. 646-654. Torgerson W. S. (is%) Theory and Methods of Scaling. Wiley, New York. Walraven P. L. (1977) Color signals from incremental and decremental light stimuli. Vision Res. 17, 71-76. White T. W.. Collins S. B. and Rinalducci E. J. (1976) The Broca-Sulzer effect under scotopic viewing conditions. C’ision Res. 16, 1439-1443.
Vol. 20. pp. 723 to 716 Press Lrd 1980. Printed m Great
Britain
RESEARCH NOTE
ASYMMETRY
IN THE BRIGHTNESS AND DARKNESS BROCA-SULZER EFFECT’S
THOMAS W.
WHITER,GREGG E. Irtvt~l and
MARY
C. WILLIAMS’
LDepartment of 3iobehavioral Sciences, Boston University Medical Center, Boston. MA 02118. U.S.A. ‘Visual Psychophysics Laboratories. Syracuse University, Syracuse, NY 13210. U.S.A. ‘Department of Psychology, SUNY Buffalo, Buffalo, NY 14226. U.S.A.
(Received 14 February
INTRODUCTION There
is increasing evidence that the bright and dark components of the retinal image are handled differently by the visual system (Glad and Magnussen, 1972; Magnussen and Glad, 197%. b, c; Burton et aI., 1977; DeValois, 1977; Remole, 1977; Walraven, 1977; Corwin, 1978). On and off channels have been suggested as possible neural bases for separate processing (Jung, 1973). In a comparison of the brightness and darkness Broca-Sulzer effects, we also found evidence consistent with differential processing.
1979)
100 possible pairs of flash durations. The subjects had no way of predicting which pair would be presented or in what order they would occur. After a pair had been presented, the subject pushed one of two buttons which told the computer which flash had been perceived as brighter (or darker) by the subject. Scales of brightness (and darkness) were derived from the resulting 10 x 10 dominance matrices with a rank scaling procedure (Dunn-Rankin and Wilcoxon, 1966; Dunn-Rankin and King, 1969). In the rank scaling method, rank sums are obtained for each duration by simply summing the frequency of times the duration
METHOD The stimuli were freely viewed in a ‘&channel, Iconix Tachistoscope. Achromatic light was provided either by tfuorescent sources or by Kodak slide pre jectors and rear projection screens (Sylvania 300 W, 120 V ELH). In any on‘e incremental or decremental condition, the test flash (TF) and background were MONOCULAR FREE-VIEWING from the same type of source, i.e. their color did not RIGHT EYE differ. A Uniblitz shutter shaped the flashes when projectors were used. Timing and data collection funcCONDmON A: tions were performed by a PDP-8f computer. The subjects viewed a 28’ TF with the fovea of the right eye. (See Fig. 1, top). Two different background configurations were employed. In one condition, the TF was added to or subtracted from the center of a 0 log mL, 3” diameter background. The background also contained a 10’ wide, 1” 2cT diameter, bIa& fixa- CONDITION B : tion ring. (See Fig. 1, condition A). In the other con0;m.l dition, the TF either incremented above a 0 log mL, 28’ diameter background or it decremented down to Iti + 3, IO OR IOOmL that 1eveL The 28’ background was surrounded by a 4,li OR 10lmLTO~mL 3” outer diameter, 28’ inner diameter, -1.0 log mL annulus which contained a fixation ring. (See Fig. 1, TEMPORAL SEQUENCE : condition B). 500 MSEC 500 MSEC RIOR R2+7OD The apparent brightness of increments and the MSEC apparent darkness of decrements were independently FI ws F2 scaled for each luminance and background condition using a paired-comparisons task (Torgerson, 1958). Fig. 1. Stimulus configuration. See text. TF = 1st flash; Bknd = background; FR = fixation ring: WS = waning (See Fig. 1, bottom). Ten TF durations were used. On sii@: fl = first IF; F2 = second TF; Rt = rgswnse 1 any trial, the computer randomly presented one of the (Fl > FZ); R2 = response 2 fF2 > Fl).
723
Research Not?
_‘-I ! _
was judged brighter (or darker) than the other durations. These sums are transfo~ed into scale values using the following equation :
where R = calculated scale value; R, = the obtained rank sum; Rmin = the minimum possible rank total; and R,,, = the maximum possible total. This procedure results in a uni-dimensional scale which varies from 0 to 100 with each duration ~cupying a lccation on the scale. Critical ranges (CR) are also calculated for each scale. The CR is an expected value and it specifies how far apart two items must be on the scale in order to be considered as occupying separate locations on the scale. Selected data points were also scaled using the method of adjustment and viewing condition B. Here a variable duration TF was recycled successively with a 100msec comparison flash in a manner similar to that used in the paired-comparisons task. (See Fig. I. bottom). The subject had to adjust the amount of light in the comparison until it produced a sensation identical to that produced by the TF. RESULTS
Our sensation vs duration curves do not show the symmetry in response found for increments and decrements at Theshold (Blackwell. 1946: Cohn. 1974: Herrick, 1956; Pate1 and .?ones, 1968; Rashbass, 1970; Short, 1966). In Fig. 2, brightness (top curves) and darkness (bottom curves) are plotted as a function of log TF duration for luminance changes of 0.3. 0.5 and
1.0 log umt. In determmmg these curves, I&t w;lj either added to or subtracted from the center of the background field. The 0.3 log unit luminance change (left panel) produced brightness and darkness effects similar in time course but the darkness effect had 11 more pronounced overshoot. Increasing the magnitude of the luminance change to 0.5 (center panel) resulted in brightness and darkness effects which were more similar in shape but displaced in time. The maximum sensation occurred at a longer duration with decrements (100msec) than with increments (50 msec). Darkness enhancement was no longer cvident with a 1.0 (right panel) or a 2.0 (not shoun) log luminance change. This indicates saturation oi the darkness response as physical contrast approaches 1009,. A brightness effect was still present as Kould be expected given that increments are not contrast limited in the same way as decrements. Stimulus condition B allowed the scaling of darkness over a larger intensity range. Here the TF decremented down to a luminance level which possessed some finite brightness, i.e. it was brighter than its surround. Figure 3 plots brightness (top curves) and darkness (bottom curves) functions for luminance changes of 0.5, 1.0 and 2.0 log units. The 0.5 log increments (left panel) did not produce a Broca-Sulzer effect. The darkness effect was present with the 0.5 log decrement. The larger magnitude luminance changes caused both brightness and darkness effects. Again. the darkness effect appears to hal-e a slousr time course when compared with the brightness ctfect ( lM)-I 20 msec vs 50-70 msec). Selected data points from Fig. 3 were scaled using the method of adjustment. Figure 4 shows the amount of light necessary in a 100 msec comparison flash to
*OiR 40
I
.n X-Jr
I OLO IO
20
or
, 30
8Oi *7W.0 LOG
CWATION
( rnec
1
2.0
IO LOG
DURATION
(msec
I
LOG
DURATION
(msec
1
Fig. 2. Brightness (circles) and darkness (squares) curves for luminance changes of 0.3 [left), 0.5 (liddIe) and I.0 (right) log unit and stimulus condition A. In each panel, scale value (R) is plotted as a function of log TF duration (msec). Grouped data for two observers. 3000 trials each curve. CR = ?.57. 0.05 level of significance.
Research Note
eo-
BOI 1.0
3.0 2.0 LOG DURATION ( mfec t
I.0
2.0 LOG DURATION
1.0
2.0 LOG DURATION
3.0
( msec i
3.0
( rwec)
Fig. 3. Brightness (circles) and darkness (squares) curves for luminance changes of 0.5 (left), 1.0 (middle) and 2.0 fright) tog units and stimulus condition 3. In each paneI, scaic value (RI is plotted as a function of log TF duration (msec). Grouped data for 3 observers. &X0 trials each curve. CR = 2.33,O.M level.
match increments and decrements of 0.5, 1.0 and 2.0 units as a Function of their exposure duration. As in the other figures, brightness is plotted above darkness. The general features of Fig. 3 again appear, i.e. no brightness effect for the 0.5 increment and longer time-courses with decrements than with increments. In addition, this figure shows the darkness effect to grow in magnitude at a faster rate with increases in luminance than does the brightness effect. log
DISCUSSION
Our results are consistent with those which have been previously reported for the Briicke-Barthey e&t and its darkness anaiogue t.h&gmtssen and Glad, 1975a, b,c}. The only apparent disagreement is in terms of the timecourse of the brightness and darkness effects. Using square-wave flicker, brightness and darkness enhancement appear to occur at similar temporal frequencies whereas with individual light increments and decrements, maximum darkness occurred at longer durations than did maximum brightness. It could be argued that under stimuIus condition A the increments and decrements were not comparable since luminance changes matched according to iog magnitude result in di&rent values than those G&Xfated in terms of percent modulation around a mean luminance level. This procedural difference may be sufficient to account for the time-course differences found between brightness and darkness It is more difficult to use this argument to explain the same differences occurring under stimulus condition B since here the upward and downward luminance changes were symmetric,
1
1.0
2.0 LCE DURAT~~
I
30 tmsec
t
Fi& 4. Brightness (circles) and darkness (squares) scaled by the method of adjustment for luminance changes of 2.0, 1.0 and 0.5 log units. In each curve, the luminance necessary in a 100 msec comparison Rash to match a variable duration TF is plotted as a function of tog IF duration (msecb Grouped data for 3 subjects I5 total matches. SD < 0.2 tog unit.
‘26
Research Note
A time-course difference is consistent with Jung’s (1973) two-channel model of brightness and darkness coding. In this model, darkness is slower due to the importance of surround activation in off-center channels as compared with the more direct central activation of on-center, brightness units. Of course, a single non-linear channel could also provide asymmetric responses to upward and downward luminance changes. Previous investigations of brightness enhancement have shown that a TF set in luminance near its increment threshold does not show enhancement (Rinalducci and Higgins, 1971; White er al., 1976). In the present study, brightness enhancement was found with the 0.5 log increment when viewed against the 3’, 0 log mL background but not when viewed against the 2s’. 0 log mL background. A 2s’ TF seen against a 28’ background would be expected to have a higher detection threshold than would a 28’ TF centered within a uniform 3” field. .4cknowledgements-This research was supported in part bv NIH erant No EY01571 and NRSA Biolonical Sciences
T;aining-Program
No MH-15189, NIMH. -The authors
would like to also thank Joseph F. Sturr for the use of his equipment and John (Jack) Wisowaty for his time spent as an observer. REFERENCES Blackwell H. R. (1946) Contrast thresholds of the human eye. J. opt. Sot. Am. 36, 624-643. Burton G. J., Nagshineh S. and Ruddock K. H. (1977) Processing by the human visual system of the light and dark contrast components of the retinal image. Biol. Cybernetics
27, 189-197.
Cohn T. E. (1974) A new hypothesis to explain why the inqem_
Suppl. 222.
DeValois K. K. ( 1977) Independence of black and w hate: phase specific adaptation. Vision RG. 17. X9-11 5. Dunn-Rankin P. and King F. J. (1969) Multiple comparisons in a simplified rank method of scaling. Educ. psychol. Meow. 29, 315-329. Dunn-Rankin P. and Wilcoxon F. (1966) The true distribution of the range of rank totals in the two-way class& cation. Psyhometrika 31. 573-580. Glad A. and .Ilagnussen S. (1972) Darkness enhancement in intermittent light: an experimental demonstration, I’lsion Res. 12, 11l-1 15. Herrick R. M. (1956) Fovea1 luminance discrimination as a function of the duration of the decrement or increment in luminance. J. camp. physiol. Psychol. 49, 437-443. Jung R. (1973) Visual perception and neurophysiology. In Handbook oj Sensory Physiology. Vol. VII/4, pp. L-152. Springer, Berlin. Magnussen S. and Glad A. (197ja) Brightness and darkness enhancement during flicker: perceptual correlates of neuronal B- and D-systems in human retina. Expl Brain Res. 22, 399313.
Magnussen S. and Glad A. (1975b) Temporal frequency characteristics of spatial interaction in human vision. Expl Brain Res. 23, 519-528.
Magnussen S. and Glad A. (1975c) Effects of steady surround illumination on the brightness and darkness enhancement of flickering lights. Vision Res. 15. 1113-1416.
Pate1 A. S. and Jones R. W. (1968) Increment and decrement visual thresholds. J. opt. Sot. Am. 58, 696-699. Rashbass C. (1970) The visibilitv of transient chances of luminance. J. Physiol. 210, 16clS6. Remole A. (1977) Brightness enhancement versus darkness enhancement at a border. Vision Res. 17, 1095-l 100. Rinalducci E. J. and Higgins K. E. (1971) An Incestigarion (II’the Efjecticr Intensity of Flashing Lights. Clearinghouse, Springfield. VA. Short A. fi. (7966) Decremental and incremental visual thresholds. J. Phvsiol. 185. 646-654. Torgerson W. S. (is%) Theory and Methods of Scaling. Wiley, New York. Walraven P. L. (1977) Color signals from incremental and decremental light stimuli. Vision Res. 17, 71-76. White T. W.. Collins S. B. and Rinalducci E. J. (1976) The Broca-Sulzer effect under scotopic viewing conditions. C’ision Res. 16, 1439-1443.