Abstract

We extended earlier results [J. Opt. Soc. Am. A 16, 2625 (1999)] to examine how the responses of the three postreceptoral mechanisms are combined to subserve discrimination of suprathreshold stimuli. Test thresholds were obtained in the presence of suprathreshold pedestals selected in different quadrants of the red–green/luminance and blue–yellow/luminance planes of cardinal color space. We showed that (1) test threshold was directly proportional to pedestal contrast for pedestal contrasts exceeding five times pedestal contrast threshold, and (2) there were exceptions to this proportionality, notably when the test and pedestal directions were fixed in the cardinal directions. Results support a ratio model of suprathreshold color–luminance discrimination, in which discrimination depends on a ratio of outputs of the postreceptoral mechanisms. We also observed that when test threshold was measured as a function of test color-space direction, masking by the achromatic component of the pedestal was less than that by the chromatic component. In addition, masking by a dark (negative luminance component) pedestal was lower than masking by a light (positive luminance) pedestal of a similar contrast. Our results demonstrated that (1) there is no fundamental difference between discrimination in the isoluminant and in the two chromoluminant cardinal planes, (2) there exists the possibility that discrimination in cardinal directions differs from that in noncardinal (intermediate) directions, and (3) suprathreshold discrimination of luminance differences may be more sensitive than that of chromatic differences for a given suprathreshold pedestal.

© 2002 Optical Society of America

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References

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  1. M. Gur, V. Akri, “Isoluminant stimuli may not expose the full contribution of color to visual functioning: spatial contrast sensitivity measurements indicate interaction between color and luminance processing,” Vision Res. 32, 1253–1262 (1993).
    [CrossRef]
  2. K. T. Mullen, M. J. Sankeralli, “Evidence for the stochastic independence of the blue–yellow, red–green and luminance detection mechanisms revealed by subthreshold summation,” Vision Res. 39, 733–743 (1999).
    [CrossRef] [PubMed]
  3. E. Switkes, A. Bradley, K. K. Devalois, “Contrast dependence and mechanisms of masking interactions among chromatic and luminance gratings,” J. Opt. Soc. Am. A 5, 1149–1162 (1988).
    [CrossRef] [PubMed]
  4. G. R. Cole, T. J. Hine, W. H. MacIlhagga, “Estimation of linear detection mechanisms for stimuli of medium spatial frequency,” Vision Res. 34, 1267–1278 (1994).
    [CrossRef] [PubMed]
  5. C.-C. Chen, J. M. Foley, D. H. Brainard, “Detection of chromoluminance patterns on chromoluminance pedestals I: threshold measurements,” Vision Res. 40, 773–788 (2000).
    [CrossRef] [PubMed]
  6. C.-C. Chen, J. M. Foley, D. H. Brainard, “Detection of chromoluminance patterns on chromoluminance pedestals I: model,” Vision Res. 40, 789–803 (2000).
    [CrossRef]
  7. B. A. Wandell, “Colour measurement and discrimination,” J. Opt. Soc. Am. A 2, 62–71 (1989).
    [CrossRef]
  8. M. J. Sankeralli, K. T. Mullen, “Ratio model for suprathreshold hue-increment detection,” J. Opt. Soc. Am. A 16, 2625–2637 (1999).
    [CrossRef]
  9. J. Krauskopf, D. R. Williams, D. W. Heeley, “Cardinal directions of colour space,” Vision Res. 22, 1123–1131 (1982).
    [CrossRef]
  10. A. M. Derrington, J. Krauskopf, P. Lennie, “Chromatic mechanisms in lateral geniculate nucleus of macaque,” J. Physiol. (London) 357, 241–265 (1984).
  11. W. H. Press, S. A. Teukolsky, W. T. Vetterling, B. P. Flannery, Numerical Recipes in C: The Art of Scientific Computing, 2nd ed. (Cambridge U. Press, Cambridge, UK, 1992).
  12. The elliptical model presupposes that in the presence of a suprathreshold noncardinal pedestal, test threshold is determined by two distinct mechanisms: one detecting the test component parallel to the pedestal color direction (a contrast increment), the other detecting the component perpendicular to this direction (a hue increment). Our previous results supported this separation, at least for the isoluminant plane.8
  13. In the single-variable, two-treatment analysis of variance (ttest), the difference of the means of the two treatments was compared with the 95% acceptability level of the tparameter given the number of trials involved per treatment.
  14. To determine whether the parameter Δ is constant within each plane, we performed a chi-squared test of the error across treatments (MST) compared with the error within each measurement (MSE). This analysis did not include data from the cardinal pedestal directions. MST is given by the standard error of the fitted mean Δ’s; MSE is calculated from the width W of the 95% confidence interval for each fitted Δ: MSE=mean{(W/2)/tα/2}, where tα/2 is the t statistic at α/2=0.025. The quantity χ2=MST/MSE was used to compute a Q value—the probability that the variability in Δ could be accounted for by a random measurement variability. As in the main test, the variation in Δ was accepted as random (as opposed to a systematic departure from uniformity) if Q>0.1.
  15. It is possible that this pedestal direction lies near the actual “blue” cardinal pole for this subject.
  16. M. J. Sankeralli, K. T. Mullen, “Assumptions concerning orthogonality in threshold-scaled versus cone-contrast colour spaces,” Vision Res. 41, 53–55 (2001).
    [CrossRef] [PubMed]
  17. G. R. Cole, C. F. Stromeyer, R. E. Kronauer, “Visual interactions with luminance and chromatic stimuli,” J. Opt. Soc. Am. A 7, 128–140 (1990).
    [CrossRef] [PubMed]
  18. K. T. Mullen, M. A. Losada, “Evidence for separate pathways for color and luminance detection mechanisms,” J. Opt. Soc. Am. A 11, 3136–3151 (1994).
    [CrossRef]
  19. M. J. Sankeralli, K. T. Mullen, “Postreceptoral chromatic detection mechanisms revealed by noise masking in three-dimensional cone contrast space,” J. Opt. Soc. Am. A 14, 2633–2646 (1997).
    [CrossRef]
  20. J. Krauskopf, D. R. Williams, M. B. Mandler, A. M. Brown, “Higher order colour mechanisms,” Vision Res. 26, 23–32 (1986).
    [CrossRef]
  21. M. D. Zmura, “Color in visual search,” Vision Res. 31, 951–966 (1991).
    [CrossRef] [PubMed]
  22. J. Krauskopf, H.-J. Wu, B. Farrell, “Coherence, cardinal directions and higher-order mechanisms,” Vision Res. 36, 1235–1245 (1996).
    [CrossRef] [PubMed]

2001

M. J. Sankeralli, K. T. Mullen, “Assumptions concerning orthogonality in threshold-scaled versus cone-contrast colour spaces,” Vision Res. 41, 53–55 (2001).
[CrossRef] [PubMed]

2000

C.-C. Chen, J. M. Foley, D. H. Brainard, “Detection of chromoluminance patterns on chromoluminance pedestals I: threshold measurements,” Vision Res. 40, 773–788 (2000).
[CrossRef] [PubMed]

C.-C. Chen, J. M. Foley, D. H. Brainard, “Detection of chromoluminance patterns on chromoluminance pedestals I: model,” Vision Res. 40, 789–803 (2000).
[CrossRef]

1999

K. T. Mullen, M. J. Sankeralli, “Evidence for the stochastic independence of the blue–yellow, red–green and luminance detection mechanisms revealed by subthreshold summation,” Vision Res. 39, 733–743 (1999).
[CrossRef] [PubMed]

M. J. Sankeralli, K. T. Mullen, “Ratio model for suprathreshold hue-increment detection,” J. Opt. Soc. Am. A 16, 2625–2637 (1999).
[CrossRef]

1997

1996

J. Krauskopf, H.-J. Wu, B. Farrell, “Coherence, cardinal directions and higher-order mechanisms,” Vision Res. 36, 1235–1245 (1996).
[CrossRef] [PubMed]

1994

K. T. Mullen, M. A. Losada, “Evidence for separate pathways for color and luminance detection mechanisms,” J. Opt. Soc. Am. A 11, 3136–3151 (1994).
[CrossRef]

G. R. Cole, T. J. Hine, W. H. MacIlhagga, “Estimation of linear detection mechanisms for stimuli of medium spatial frequency,” Vision Res. 34, 1267–1278 (1994).
[CrossRef] [PubMed]

1993

M. Gur, V. Akri, “Isoluminant stimuli may not expose the full contribution of color to visual functioning: spatial contrast sensitivity measurements indicate interaction between color and luminance processing,” Vision Res. 32, 1253–1262 (1993).
[CrossRef]

1991

M. D. Zmura, “Color in visual search,” Vision Res. 31, 951–966 (1991).
[CrossRef] [PubMed]

1990

1989

1988

1986

J. Krauskopf, D. R. Williams, M. B. Mandler, A. M. Brown, “Higher order colour mechanisms,” Vision Res. 26, 23–32 (1986).
[CrossRef]

1984

A. M. Derrington, J. Krauskopf, P. Lennie, “Chromatic mechanisms in lateral geniculate nucleus of macaque,” J. Physiol. (London) 357, 241–265 (1984).

1982

J. Krauskopf, D. R. Williams, D. W. Heeley, “Cardinal directions of colour space,” Vision Res. 22, 1123–1131 (1982).
[CrossRef]

Akri, V.

M. Gur, V. Akri, “Isoluminant stimuli may not expose the full contribution of color to visual functioning: spatial contrast sensitivity measurements indicate interaction between color and luminance processing,” Vision Res. 32, 1253–1262 (1993).
[CrossRef]

Bradley, A.

Brainard, D. H.

C.-C. Chen, J. M. Foley, D. H. Brainard, “Detection of chromoluminance patterns on chromoluminance pedestals I: model,” Vision Res. 40, 789–803 (2000).
[CrossRef]

C.-C. Chen, J. M. Foley, D. H. Brainard, “Detection of chromoluminance patterns on chromoluminance pedestals I: threshold measurements,” Vision Res. 40, 773–788 (2000).
[CrossRef] [PubMed]

Brown, A. M.

J. Krauskopf, D. R. Williams, M. B. Mandler, A. M. Brown, “Higher order colour mechanisms,” Vision Res. 26, 23–32 (1986).
[CrossRef]

Chen, C.-C.

C.-C. Chen, J. M. Foley, D. H. Brainard, “Detection of chromoluminance patterns on chromoluminance pedestals I: threshold measurements,” Vision Res. 40, 773–788 (2000).
[CrossRef] [PubMed]

C.-C. Chen, J. M. Foley, D. H. Brainard, “Detection of chromoluminance patterns on chromoluminance pedestals I: model,” Vision Res. 40, 789–803 (2000).
[CrossRef]

Cole, G. R.

G. R. Cole, T. J. Hine, W. H. MacIlhagga, “Estimation of linear detection mechanisms for stimuli of medium spatial frequency,” Vision Res. 34, 1267–1278 (1994).
[CrossRef] [PubMed]

G. R. Cole, C. F. Stromeyer, R. E. Kronauer, “Visual interactions with luminance and chromatic stimuli,” J. Opt. Soc. Am. A 7, 128–140 (1990).
[CrossRef] [PubMed]

Derrington, A. M.

A. M. Derrington, J. Krauskopf, P. Lennie, “Chromatic mechanisms in lateral geniculate nucleus of macaque,” J. Physiol. (London) 357, 241–265 (1984).

Devalois, K. K.

Farrell, B.

J. Krauskopf, H.-J. Wu, B. Farrell, “Coherence, cardinal directions and higher-order mechanisms,” Vision Res. 36, 1235–1245 (1996).
[CrossRef] [PubMed]

Flannery, B. P.

W. H. Press, S. A. Teukolsky, W. T. Vetterling, B. P. Flannery, Numerical Recipes in C: The Art of Scientific Computing, 2nd ed. (Cambridge U. Press, Cambridge, UK, 1992).

Foley, J. M.

C.-C. Chen, J. M. Foley, D. H. Brainard, “Detection of chromoluminance patterns on chromoluminance pedestals I: model,” Vision Res. 40, 789–803 (2000).
[CrossRef]

C.-C. Chen, J. M. Foley, D. H. Brainard, “Detection of chromoluminance patterns on chromoluminance pedestals I: threshold measurements,” Vision Res. 40, 773–788 (2000).
[CrossRef] [PubMed]

Gur, M.

M. Gur, V. Akri, “Isoluminant stimuli may not expose the full contribution of color to visual functioning: spatial contrast sensitivity measurements indicate interaction between color and luminance processing,” Vision Res. 32, 1253–1262 (1993).
[CrossRef]

Heeley, D. W.

J. Krauskopf, D. R. Williams, D. W. Heeley, “Cardinal directions of colour space,” Vision Res. 22, 1123–1131 (1982).
[CrossRef]

Hine, T. J.

G. R. Cole, T. J. Hine, W. H. MacIlhagga, “Estimation of linear detection mechanisms for stimuli of medium spatial frequency,” Vision Res. 34, 1267–1278 (1994).
[CrossRef] [PubMed]

Krauskopf, J.

J. Krauskopf, H.-J. Wu, B. Farrell, “Coherence, cardinal directions and higher-order mechanisms,” Vision Res. 36, 1235–1245 (1996).
[CrossRef] [PubMed]

J. Krauskopf, D. R. Williams, M. B. Mandler, A. M. Brown, “Higher order colour mechanisms,” Vision Res. 26, 23–32 (1986).
[CrossRef]

A. M. Derrington, J. Krauskopf, P. Lennie, “Chromatic mechanisms in lateral geniculate nucleus of macaque,” J. Physiol. (London) 357, 241–265 (1984).

J. Krauskopf, D. R. Williams, D. W. Heeley, “Cardinal directions of colour space,” Vision Res. 22, 1123–1131 (1982).
[CrossRef]

Kronauer, R. E.

Lennie, P.

A. M. Derrington, J. Krauskopf, P. Lennie, “Chromatic mechanisms in lateral geniculate nucleus of macaque,” J. Physiol. (London) 357, 241–265 (1984).

Losada, M. A.

MacIlhagga, W. H.

G. R. Cole, T. J. Hine, W. H. MacIlhagga, “Estimation of linear detection mechanisms for stimuli of medium spatial frequency,” Vision Res. 34, 1267–1278 (1994).
[CrossRef] [PubMed]

Mandler, M. B.

J. Krauskopf, D. R. Williams, M. B. Mandler, A. M. Brown, “Higher order colour mechanisms,” Vision Res. 26, 23–32 (1986).
[CrossRef]

Mullen, K. T.

M. J. Sankeralli, K. T. Mullen, “Assumptions concerning orthogonality in threshold-scaled versus cone-contrast colour spaces,” Vision Res. 41, 53–55 (2001).
[CrossRef] [PubMed]

M. J. Sankeralli, K. T. Mullen, “Ratio model for suprathreshold hue-increment detection,” J. Opt. Soc. Am. A 16, 2625–2637 (1999).
[CrossRef]

K. T. Mullen, M. J. Sankeralli, “Evidence for the stochastic independence of the blue–yellow, red–green and luminance detection mechanisms revealed by subthreshold summation,” Vision Res. 39, 733–743 (1999).
[CrossRef] [PubMed]

M. J. Sankeralli, K. T. Mullen, “Postreceptoral chromatic detection mechanisms revealed by noise masking in three-dimensional cone contrast space,” J. Opt. Soc. Am. A 14, 2633–2646 (1997).
[CrossRef]

K. T. Mullen, M. A. Losada, “Evidence for separate pathways for color and luminance detection mechanisms,” J. Opt. Soc. Am. A 11, 3136–3151 (1994).
[CrossRef]

Press, W. H.

W. H. Press, S. A. Teukolsky, W. T. Vetterling, B. P. Flannery, Numerical Recipes in C: The Art of Scientific Computing, 2nd ed. (Cambridge U. Press, Cambridge, UK, 1992).

Sankeralli, M. J.

M. J. Sankeralli, K. T. Mullen, “Assumptions concerning orthogonality in threshold-scaled versus cone-contrast colour spaces,” Vision Res. 41, 53–55 (2001).
[CrossRef] [PubMed]

M. J. Sankeralli, K. T. Mullen, “Ratio model for suprathreshold hue-increment detection,” J. Opt. Soc. Am. A 16, 2625–2637 (1999).
[CrossRef]

K. T. Mullen, M. J. Sankeralli, “Evidence for the stochastic independence of the blue–yellow, red–green and luminance detection mechanisms revealed by subthreshold summation,” Vision Res. 39, 733–743 (1999).
[CrossRef] [PubMed]

M. J. Sankeralli, K. T. Mullen, “Postreceptoral chromatic detection mechanisms revealed by noise masking in three-dimensional cone contrast space,” J. Opt. Soc. Am. A 14, 2633–2646 (1997).
[CrossRef]

Stromeyer, C. F.

Switkes, E.

Teukolsky, S. A.

W. H. Press, S. A. Teukolsky, W. T. Vetterling, B. P. Flannery, Numerical Recipes in C: The Art of Scientific Computing, 2nd ed. (Cambridge U. Press, Cambridge, UK, 1992).

Vetterling, W. T.

W. H. Press, S. A. Teukolsky, W. T. Vetterling, B. P. Flannery, Numerical Recipes in C: The Art of Scientific Computing, 2nd ed. (Cambridge U. Press, Cambridge, UK, 1992).

Wandell, B. A.

Williams, D. R.

J. Krauskopf, D. R. Williams, M. B. Mandler, A. M. Brown, “Higher order colour mechanisms,” Vision Res. 26, 23–32 (1986).
[CrossRef]

J. Krauskopf, D. R. Williams, D. W. Heeley, “Cardinal directions of colour space,” Vision Res. 22, 1123–1131 (1982).
[CrossRef]

Wu, H.-J.

J. Krauskopf, H.-J. Wu, B. Farrell, “Coherence, cardinal directions and higher-order mechanisms,” Vision Res. 36, 1235–1245 (1996).
[CrossRef] [PubMed]

Zmura, M. D.

M. D. Zmura, “Color in visual search,” Vision Res. 31, 951–966 (1991).
[CrossRef] [PubMed]

J. Opt. Soc. Am. A

J. Physiol. (London)

A. M. Derrington, J. Krauskopf, P. Lennie, “Chromatic mechanisms in lateral geniculate nucleus of macaque,” J. Physiol. (London) 357, 241–265 (1984).

Vision Res.

M. J. Sankeralli, K. T. Mullen, “Assumptions concerning orthogonality in threshold-scaled versus cone-contrast colour spaces,” Vision Res. 41, 53–55 (2001).
[CrossRef] [PubMed]

G. R. Cole, T. J. Hine, W. H. MacIlhagga, “Estimation of linear detection mechanisms for stimuli of medium spatial frequency,” Vision Res. 34, 1267–1278 (1994).
[CrossRef] [PubMed]

C.-C. Chen, J. M. Foley, D. H. Brainard, “Detection of chromoluminance patterns on chromoluminance pedestals I: threshold measurements,” Vision Res. 40, 773–788 (2000).
[CrossRef] [PubMed]

C.-C. Chen, J. M. Foley, D. H. Brainard, “Detection of chromoluminance patterns on chromoluminance pedestals I: model,” Vision Res. 40, 789–803 (2000).
[CrossRef]

M. Gur, V. Akri, “Isoluminant stimuli may not expose the full contribution of color to visual functioning: spatial contrast sensitivity measurements indicate interaction between color and luminance processing,” Vision Res. 32, 1253–1262 (1993).
[CrossRef]

K. T. Mullen, M. J. Sankeralli, “Evidence for the stochastic independence of the blue–yellow, red–green and luminance detection mechanisms revealed by subthreshold summation,” Vision Res. 39, 733–743 (1999).
[CrossRef] [PubMed]

J. Krauskopf, D. R. Williams, M. B. Mandler, A. M. Brown, “Higher order colour mechanisms,” Vision Res. 26, 23–32 (1986).
[CrossRef]

M. D. Zmura, “Color in visual search,” Vision Res. 31, 951–966 (1991).
[CrossRef] [PubMed]

J. Krauskopf, H.-J. Wu, B. Farrell, “Coherence, cardinal directions and higher-order mechanisms,” Vision Res. 36, 1235–1245 (1996).
[CrossRef] [PubMed]

J. Krauskopf, D. R. Williams, D. W. Heeley, “Cardinal directions of colour space,” Vision Res. 22, 1123–1131 (1982).
[CrossRef]

Other

W. H. Press, S. A. Teukolsky, W. T. Vetterling, B. P. Flannery, Numerical Recipes in C: The Art of Scientific Computing, 2nd ed. (Cambridge U. Press, Cambridge, UK, 1992).

The elliptical model presupposes that in the presence of a suprathreshold noncardinal pedestal, test threshold is determined by two distinct mechanisms: one detecting the test component parallel to the pedestal color direction (a contrast increment), the other detecting the component perpendicular to this direction (a hue increment). Our previous results supported this separation, at least for the isoluminant plane.8

In the single-variable, two-treatment analysis of variance (ttest), the difference of the means of the two treatments was compared with the 95% acceptability level of the tparameter given the number of trials involved per treatment.

To determine whether the parameter Δ is constant within each plane, we performed a chi-squared test of the error across treatments (MST) compared with the error within each measurement (MSE). This analysis did not include data from the cardinal pedestal directions. MST is given by the standard error of the fitted mean Δ’s; MSE is calculated from the width W of the 95% confidence interval for each fitted Δ: MSE=mean{(W/2)/tα/2}, where tα/2 is the t statistic at α/2=0.025. The quantity χ2=MST/MSE was used to compute a Q value—the probability that the variability in Δ could be accounted for by a random measurement variability. As in the main test, the variation in Δ was accepted as random (as opposed to a systematic departure from uniformity) if Q>0.1.

It is possible that this pedestal direction lies near the actual “blue” cardinal pole for this subject.

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Figures (8)

Fig. 1
Fig. 1

Experimental paradigms. (a) In the Fixed-Test-Direction Paradigm, the direction of the pedestal (solid line) is fixed in a given plane (rg–lum or by–lum), while that of the test is fixed in a direction orthogonal to the pedestal (arrow). The test threshold is measured as a function of pedestal contrast. (b) In the Variable-Test-Direction Paradigm, the direction and magnitude of the pedestal are fixed (spot on solid line). The test threshold is measured as a function of test direction.

Fig. 2
Fig. 2

Hue-increment detection thresholds in the rg–lum plane for subject MJS. Each panel represents one pedestal direction (shown at the top of the panel). The pedestal angle (deg) is relative to the red axis. The diamonds (with standard error bars) represent measures of test threshold with pedestal contrast. The solid curves represent the ratio model fit. The two asymptotes of the fit are the line T=T0 (horizontal dashed lines), where T0 is the measured value of test threshold at P=0, and the line T=P/Δ (sloping dashed lines), where Δ is a fitted parameter. Note that the sloping asymptote is confined to pass through the origin.

Fig. 3
Fig. 3

Hue-increment detection thresholds in the rg–lum plane for subject KTM. See caption for Fig. 2.

Fig. 4
Fig. 4

Hue-increment detection thresholds in the by–lum plane for subject MJS. The pedestal angle (deg) is relative to the blue axis. See caption for Fig. 2.

Fig. 5
Fig. 5

Hue-increment detection thresholds in the by–lum plane for subject TJH. The pedestal angle (deg) is relative to the blue axis. See caption for Fig. 2.

Fig. 6
Fig. 6

Results of variable-test-direction paradigm. Each panel represents one plane for one subject. The pedestal was fixed in one of four positions (X) in the plane (the coordinates of the pedestal have been scaled by 0.5 for the purposes of the graph). Circles represent the test threshold as a function of test direction for each of the four pedestal values in each plane. Triangles encircling the origin represent the test thresholds in the absence of a pedestal. Triangles encircling each of the four pedestal values are a translation of these test thresholds. Comparison of the circles and the triangles around each pedestal value yields an elongation of the test-threshold contour in the presence of the pedestal. The elliptical fit to this contour (solid lines) shows that the maximum elongation occurs in a direction less than 45 deg from the horizontal (rg or by) axis. This may suggest that the elongation is a result of masking due to the pedestal and a small facilitation along the direction of the luminance axis.

Fig. 7
Fig. 7

Orientations of the major axes of the fitted ellipses. The two panels represent the two planes (rg–lum and by–lum). The horizontal axis represents the pedestal direction in each plane (a 0–360 deg representation has been used for convenience). 0 deg represents the red and blue axes in left and right figures, respectively. 90 deg represents the light axis. The vertical axis represents the absolute value of the angle between the major axis of the fitted test-threshold contour at each pedestal value and the chromatic (rg or by) cardinal axis. Symbols show the fitted orientations for each pedestal for each subject. Dashed curves represent the average orientation over the three subjects tested. The figure shows that the mean elongation is maximum at an angle less than 45 deg from the chromatic axis. This demonstrates that luminance discrimination is more sensitive than chromatic (isoluminant) discrimination with these pedestals and may indicate a direct contribution of the luminance mechanism in discrimination.

Fig. 8
Fig. 8

Zone plots for discrimination (fixed-test-direction paradigm). Asterisks (*) represent the measurements of test threshold for each pedestal direction and contrast (obtained from Fig. 25). Shaded zones represent the region of discriminability for each pedestal direction (the bisector of the zone), as predicted by the ratio model (the width of the zones and the offset of the test threshold points from the pedestal axis have been halved for graphical clarity). The figure shows that, apart from along the cardinal axes and the 112-deg direction in the by–lum plane for subject TJH, the widths of the zones are uniform within each plane for each subject.

Tables (1)

Tables Icon

Table 1 Fitted Parameters for the Ratio Model in the Three Cardinal Planesa

Equations (5)

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T=k1T0+k2(P/Δ),
k1=π4P4+π4;k2=1-k1,
Ri=mipj=1N(aijmjq)+Z,
CR=m1pa11m1p+a12m2p+Z.
m1/m2=CR(1),

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