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AFOSR Grant No. AFOSR-89-0304 


The Effects of Luminance Boundaries on Color Perception 


Professor Richard E. Kronauer 
Harvard University 
Division of Applied Sciences 
Pierce Hall 324 
Cambridge, MA 02138 


April 30, 1992 


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DTIC 

ELECTE j 
MAY 18 1992 I 



Period Covered: March 15, 1991 to March 14, 1992 


ANNUAL TECHNICAL REPORT 


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The Effects of Luminance Boundaries on Color Perception (unclassified) _ 


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Kronauer, R.E., Stromeyer, C.F. Ill, Chaparro, A., and Eskew, R.T., Jr._ 


13a. TYPE OF REPORT 13b. TIME COVERED 14 DATE OF REPORT (Year, Month, Day) 15. PAGE COUNT 

Annual Technical Rpt from 3/15/91 TO 3/14/9 2 April 30, 1992 11 



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18. SUBJECT TERMS (Continue on reverse if necessary and identify by block number) 


19. ABSTRACT (Continue on reverse if necessary and identify by block number) 

Extensive measurements were made for detecting luminance and red-green flashes in the 
center of a bright yellow field. Thresholds, plotted in L and M-cone contrast coordinates, 
indicate that chromatic flashes are more visible than luminance flashes even at very small 
size (2' diameter). Over a wide range of flash diameters and durations the chromatic flashes 
are detected with considerably higher efficiency (in units of cone contrast energy) than the 
most detectable luminance stimuli (small drifting gratings). The higher gain of the chromatic 
mechanisms has important physiological implications and is potentially useful in display 
technology. Detailed studies with luminance and chromatic stimuli suggest that the chromatic 
mechanisms have a constant spectral tuning, even for spots as small as 2': the chromatic 
response is determined by a constant , equally weighted difference of L and M cone contrast. 

A suprathreshold luminance flash (a pedestal) facilitates detection of a coincident 
chromatic flash. Earlier studies suggested that the facilitation will grow strongly when 
the stimuli were decreased in size. In contrast, we find that facilitation is constant (2-3x) 
for stimuli from 2 ' to 2° diameter. 



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•AFOSR Grant No. 89-0304 
Annual Technical Report 1991/92 


1 


The Effects of Luminance Boundaries on Color Perception 
Period Covered: March 15, 1991 to March 14, 1992 



This report covers our activities since March 15, 1991. Our 
main accomplishments have been to: 1) Finish experiments on 
detection of small spots by luminance and red-green mechanisms, 
examining the role of a luminance pedestal in facilitating 
chromatic detection at small spot sizes. 2) Show that the red- 
green mechanism is more efficient than the luminance mechanism in 
contrast detection. 3) Consider further the roles of cone- 
selective adaptation and 'second-site' adaptation in controlling 
the sensitivity of the red-green detection mechanism. 4) Continue 
work on the spectral nature of the inputs to motion detection 
mechanism. 




On a large, bright yellow field we deliver a foveal flash 
consisting of simultaneous incremental and decremental red and 
green components. Detection thresholds are measured for many 
different red:green amplitude ratios, and thresholds are plotted 










■AFOSR Grant No. 89-0304 
Annual Technical Report 1991/92 


2 


Using a 200 ms test flash, we demonstrated the existence of 
the red-green mechanism by measuring detection contours for spots 
over a size range of 2.3' to 1“^ diameter. At the largest size the 
red-green mechanism is lOx more sensitive than the luminance 
mechanism, and this ratio decreases as the spot is made smaller, 
but the red-green mechanism is about 2x more sensitive even at the 
smallest size. 

The most efficiently detected stimulus on the bright yellow field 
These experiments were extended to determine "What the eye 
sees best?" measured in terms of contrast energy of the stimulus. 
Watson, Barlow and Robson (1983) attempted to answer this for 
stimuli in the luminance domain. They measured contrast energy 
thresholds (the square of contrast integrated over the spatial and 
temporal dimension of the test stimuli), using both incremental 
spots and small patches of a drifting grating. The best grating 
had an energy threshold 3x less than the best spot and identifies 
approximately the shape of the most sensitive luminance receptive 
field (one that matches the most efficient stimulus). We measured 
spot luminance thresholds over a wide range of flash sizes and 
durations. Our luminance spot energy thresholds agree very well 
with those of Watson ai- Our best-detected red or green 
chromatic spot has a cone contrast energy threshold 5-8x lower 
than our best luminance spot and about 3-8x lower than Watson ai. ' 
ai.'s optimal grating. The higher sensitivity for the chromatic 
stimulus is not solely due to better spatial and temporal 
integration in the chromatic pathways, for there is a clear 







AFOSR Grant No. 89-0304 
Annual Technical Report 1991/92 


3 


chromatic advantage even when the chromatic spot is matched in 
size and duration to the optimal luminance spot. 

The higher sensitivity to color is surprising, but may be 
consistent with recent findings of R.M. Shapley and colleagues 
(largely unpublished) on retinal ganglion cells. It is well know 
that the M ganglion cells have higher contrast gain than do the 
color-opponent P cells when tested with a luminance grating. 
However, when tested with a chromatic grating, matched in cone 
contrast to the luminance grating, then the P and M cell have 
similar contrast gains. The P cells are much more numerous, and 
their receptive fields often have largely overlapping L and M 
areas with similar weights but opposite signs, making them 
especially sensitive to color, and much less sensitive to 
luminance. 

Our work is described in the enclosed paper (Chaparro ££. al., 
1992), which we intend to submit to Nature . 

Chromatic facilitation by luminance pedestals at small spot size 

A suprathreshold luminance flash of 1° diameter presented 
simultaneously with a red or green equiluminant chromatic flash 
facilitates the latter's threshold by 2x. Earlier work by Hilz, 
Huppmann and Cavonius (1974) led us to suspect that the 
facilitation would grow much larger when the pedestal and 
chromatic flash were concomitantly reduced in size. However, we 
find that facilitation remains approximately constant at 2-3x, as 
measured by forced-choice methods, for flashes from 2.3' to 1° 


diameter. 









AFOSR Grant No. 89-0304 
Annual Technical Report 1991/92 


4 


The 'variable tuning' hypothesis advanced by Finkelstein and 
Hood (1984) postulates that the mechanism used to detect chromatic 
spots will change in its spectral sensitivity with variation of 
the test spot size. Our results would seem to be at variance with 
this view. Even for the smallest spots our red-green detection 
contours have a slope of +1.0, indicating that L and M cones 
contribute with equal and opposite weights (over the measured size 
range 2.3' to 1°). A luminance pedestal has a polar vector 
direction of +45° in the L',M' coordinates, approximately parallel 
to the red-green contour--thus stimulating the red-green mechanism 
very little. A 2.3' diameter luminance pedestal flash of even 25x 
threshold does not mask the chromatic flash, consistent with the 
view that the chromatic contour does have a constant slope of 
approximately +1.0—not of variable tuning. 

A surprising feature, remarked upon by Hood and Finkelstein 
(1984) is that small colored flashes with a strong luminance 
component (near the luminance axis in the L',M' plane), and 
presumably below the threshold of the red-green mechanism, appear 
colored reddish or greenish when only very slightly 
suprathreshold. We have confirmed this and shown that the results 
are quantitatively consistent with the view that the luminance 
pedestal starts to facilitate the red or green chromatic test when 
the pedestal reaches its own threshold. Thus there is a 
facilitating interaction between luminance and chromatic 
mechanisms that can explain the appearance of color of flashes 
that are near the luminance axis. We have shown that the threshold 
for identifying the hue of the test (red vs green) is well 







AFOSR Grant No. 89-0304 
Annual Technical Report 1991/92 


5 


predicted from the curve relating luminance pedestal amplitude and 
chromatic (facilitated) detection thresholds. 

We are presently writing up the results; the experiments are 
completed. 

Mechanisms of adaptation in the red-green pathways 

Previously we (Stromeyer, et al., 1985) observed that on 
bright chromatic adapting fields, the sensitivity of the red-green 
detection mechanism is controlled by two adaptation mechanisms. 
First, the L and M cones differentially adapt (or the L and M 
cone-selective pathways differentially adapt) following Weber's 
Law, so that the red-green detection contour maintains a slope of 
about +1.0 in the L',M' coordinates for different colored adapting 
fields. Second, there is also second-site adaptation, which 
reduces sensitivity at an opponent site via response saturation 
when the adapting field is strongly chromatic (producing an 
extreme in the ratio L/M for mean adaptation). The second-site 
adaptation cause the red-green detection contour to move outward 
from the origin in the L',M' coordinates, reflecting reduced 
sensitivity. 

Kraus)copf and Gegenfurtner (1991) measured red-green 
detection thresholds on different colored fields with test stimuli 
restricted the equiluminant plane . They found surprisingly 
little influence of field color on threshold and argued that 
first-site adaptation plays little role in determining 
equiluminant red-green sensitivity. We have shown, however, 

(Es)cew, Stromeyer and Kronauer, 1992) that these results are 
consistent with both first- and second-site adaptation. The 








•AFOSR Grant No. 89-0304 
Annual Technical Report 1991/92 


6 


difficulty associated with stimuli restricted to the equiluminant 
plane has to do with the fact that, although this plane is 2 
dimensional, one dimension is assigned to S cone stimulation (the 
so-called blue-yellow axis) so that only one dimension remains for 
the L/M cone system. Therefore only a highly limited view of L/M 
cone interaction is seen in the isoluminant plane. Rather one 
should measure the full red-green detection contour in the L',M' 
cone contrast coordinates, since the effects of adaptation are 
particularly evident near the unique L' or M' axis. When we 
restrict our detection data to the isoluminant conditions of 
Krauskopf and Gegenfurtner we find results very similar to theirs. 
Since Krauskopf and Gegenfurtner's data were obtained at lower 
adapting levels than we used, we intend this summer to make 
thorough measurements of the red-green contour, in the L',M' 
coordinates, for a range of adapting colors and mean levels, to 
assess the role of the two postulated adaptation mechanisms. 

Cone inputs for motion detection 

A large part of our recent effort has been devoted to 
carefully assessing the spectral nature of mechanisms detecting 
motion. Thus far we have been examining the L and M cone signals. 
The observer views a 1 cpd vertical red-plus-green grating that 
moves left or right on a 3500 td yellow field. We typically 
measure contrast thresholds for discriminating motion (left vs 
right). As described in our previous progress report, we have 
identified two spectral motion mechanisms, which are distinct from 
the red-green hue mechanism. The latter hue mechanism has a 
constant slope of +1.0 in the L',M' cone contrast coordinates 






•AFOSR Grant No. 89-0304 
Annual Technical Report 1991/92 


7 


(measured by simple detection or hue identification). The two 
motion mechanisms are a luminance type mechanism and a spectrally- 
opponent mechanism. The latter responds to the differences of L 
and M cone contrast, but the M cone contribution drops rapidly as 
velocity is increased. The luminance mechanism responds to the sum 
of L and M cone contrast signals: at high velocities the two cone 
types contribute with similar weights, while at low velocities the 
mechanism becomes L-cone dominated. Similar temporally-dependent 
properties of L and M inputs have been observed in macaque phasic 
ganglion cells by Lee, Martin and Valberg (1989). 

We have shown that the outputs of the luminance and 
spectrally-opponent motion mechanisms eventually summate within 
opponent motion mechanisms that are sensitive to the differences 
of right and left motion components. 

We have obtained very extensive measurements on three 
observers, but still hsvs considersble data to collect, “specially 
pertaining to the summation of signals from the different spectral 
mechanisms. 

References: 

Chaparro, A., Stromeyer, C.F. Ill, Huang, E.P., Kronauer, R.E., 

and Eskew, R.T., Jr. (1992). What the eye sees best: Colour. 
To be submitted to Nature (enclosed). 

Finkelstein, M.A. and Hood, D.C. (1984). Detection and 

discrimination of small, brief lights: variable tuning of 
opponent channels. Vision Res . 24 , 175-181. 




AFOSR Grant No. 89-0304 
Annual Technical Report 1991/92 


8 


Hilz, R.L., Huppma m, G. and Cavonius, C.R. (1974). Influence of 
luminance contrast on hue discrimination. Journal of the 
Optical Society of America ^ M/ 763-766. 

Krauskopf, J. and Gegenfurtner, K. (1991). Adaptation and color 

discrimination. In Valberg, A. and Lee, B.B. From Pigments to 
Perception . N.Y., Plenum. 

Lee, A.B., Martin, P.R. and Valberg, A. (1989). Sensitivity of 
macaque retinal gar.glion cells to chromatic and luminance 
flicker. J. Phvsiol . 414 . 223-243. 

Stromeyer, C.F. Ill, Cole, G.R., and Kronauer, R.E. (1985). 

Second-site adaptation in the red-green mechanism. Vision 
Eai. 2^, 219-327. 

Watson, A.B., Barlow, H.B. and Robson, J.G. (1983). What does the 
eye see best? Nature 302 , 419-422. 

Participating Professionals 

Professor Richard E. Kronauer 
Dr. Charles F. Stromeyer III 
Dr. Rhea T. Eskew, Jr. 

Dr. Alex Chaparro 

Euill icatlPns and Publications in Progress 

Chaparro, A., Stromeyer, C.F. Ill, Huang, E.P., Kronauer, R.E., 
and Eskew, R.T., Jr. (1992). What the eye sees "best": 
Colour. To be submitted shortly to Nature (manuscript 
enclosed). 











. AFOSR Grant No. 89-0304 
Annual Technical Report :991/92 


9 


Eskew, R.T., Jr., Stromeyer, C.F. Ill, and Kronauer, R.E. (1992). 
The co.istancy of equiluminant red-green thresholds examined 
j 1 two color spaces. In Proceedings of QSA Meeting : Advances 
in Color Vision. Technical Digest (Optical Society of 
America) 4., 195-197 (enclosed) . 

Eskew, R.T., Jr., Stromeyer, C.F. Ill and Kronauer, R.E. (1992a). 

The time course of the facilitation of chromatic detection by 
luminance contours. Paper in progress (manuscript available 
upon request). 

Eskew, R.T., Jr., Stromeyer, C.F. Ill ana Kronauer, R.E. (1992b). 

On the temporal chromatic impulse response function. Paper in 
progress (manuscript available upon request). 

Stromeyer, C.F. Ill, Lee, J. and Eskew, R.T., Jir. (1992). 

Peripheral chromatic sensitivity for flashes: a post- 
receptoral red-green asymmetry. Vision Research (In press) 
(manuscript enclosed). 

Papfixs bei ng delivered at recent professional meetings 

(Abstracts enclosed). 

Chaparro, A., Stromeyer, C.F. Ill, Kronauer, R.E., and Eskew, 

R.T., Jr. (1992). The detection efficiency of chromatic 
stimuli. Annual meeting of Association for Research in Vision 
and Ophthalmology. Invest. Qphthal. & Visual Sci. 33/4 . 755. 

Eskew, R.T., Jr., Chaparro, A., Stromeyer, C.F. Ill and Kronauer, 
R.E. (1992). Facilitation of red-green detection by 
luminance pedestals at small spot size. Annual Meeting of 
Association for Research in Vision and Ophthalmology. Invest . 
Qphthal. _ & Visual Science 33/4 , 702. 





AFOSR Grant No. 89-0304 
Annual Technical Report 1991/92 


10 


Eskew, R.T., Jr., Stromeyer, C.F. Ill, Chaparro, A., and Kronauer, 
R.E. (1992). Why is chromatic sensitivity greater than 
luminance sensitivity? Annual Meeting, Optical Society of 
America, Albuquerque, NM. 

Stromeyer, C.F. Ill, Kronauer, R.E., Eskew, R.T., Jr. (1992). 

Relative temporal phase of L vs M cone signals within the 
luminance motion mechanism. Annual Meeting of Research in 
Vision and Ophthalmology, Sarasota, FL. Invest. Ophthal. & 
Visual Sci . . 33/4 , 756.