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Area summation in human vision at and above detection threshold.

Meese TS, Summers RJ - Proc. Biol. Sci. (2007)

Bottom Line: To overcome this confound, a novel stimulus class is designed where: (i) the observer operates on a constant retinal area, (ii) the target area is controlled within this summation field, and (iii) the pedestal is fixed in size.Using this arrangement, substantial summation is found along the entire masking function, including the region of facilitation.Our analysis shows that PS and uncertainty cannot account for the results, and that suprathreshold summation of contrast extends over at least seven target cycles of grating.

View Article: PubMed Central - PubMed

Affiliation: School of Life and Health Sciences, Aston University, Birmingham B47ET, UK. t.s.meese@aston.ac.uk

ABSTRACT
The initial image-processing stages of visual cortex are well suited to a local (patchwise) analysis of the viewed scene. But the world's structures extend over space as textures and surfaces, suggesting the need for spatial integration. Most models of contrast vision fall shy of this process because (i) the weak area summation at detection threshold is attributed to probability summation (PS) and (ii) there is little or no advantage of area well above threshold. Both of these views are challenged here. First, it is shown that results at threshold are consistent with linear summation of contrast following retinal inhomogeneity, spatial filtering, nonlinear contrast transduction and multiple sources of additive Gaussian noise. We suggest that the suprathreshold loss of the area advantage in previous studies is due to a concomitant increase in suppression from the pedestal. To overcome this confound, a novel stimulus class is designed where: (i) the observer operates on a constant retinal area, (ii) the target area is controlled within this summation field, and (iii) the pedestal is fixed in size. Using this arrangement, substantial summation is found along the entire masking function, including the region of facilitation. Our analysis shows that PS and uncertainty cannot account for the results, and that suprathreshold summation of contrast extends over at least seven target cycles of grating.

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Related in: MedlinePlus

Contrast-masking (dipper) functions. (a) Results from experiment 2 averaged across two observers (L. M. and L. W.; approx. 800 or 1600 trials per point). The average standard error was 1.28 dB for L. M. and 0.92 dB for L. W. (b) Behaviour of the main model (using equation (3.1)). Two parameters (z and k; see appendix B) control the ‘dip’ and were adjusted to match the data by eye. (c) Matched model, the same model as in (b) except that the region of summation on the numerator of equation (3.1) is restricted to the high-contrast parts of the target in the checks-on-full condition. Data and models are normalized by the sensitivity to the full stimulus with 0% pedestal contrast (left-most points).
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fig3: Contrast-masking (dipper) functions. (a) Results from experiment 2 averaged across two observers (L. M. and L. W.; approx. 800 or 1600 trials per point). The average standard error was 1.28 dB for L. M. and 0.92 dB for L. W. (b) Behaviour of the main model (using equation (3.1)). Two parameters (z and k; see appendix B) control the ‘dip’ and were adjusted to match the data by eye. (c) Matched model, the same model as in (b) except that the region of summation on the numerator of equation (3.1) is restricted to the high-contrast parts of the target in the checks-on-full condition. Data and models are normalized by the sensitivity to the full stimulus with 0% pedestal contrast (left-most points).

Mentions: In experiment 2 we replicated the key result from experiment 1 (comparison across check and full stimuli) for seven other observers and extended the study above threshold. The results are shown in figure 3a and averaged across the two observers who performed all of the conditions (L. M. and L. W.). The filled circles are for when the full stimulus (figure 1a) was used as both pedestal and target and have a classic ‘dipper’ shape. The crossed squares are for when the ‘white’ checks stimulus was used as both the pedestal and the target. Although the two stimulus types have the same diameters (figure 1), the sum of contrast over area for the full stimulus is twice that of the check stimuli. Hence, we refer to the full stimulus as having a greater (signal) area than the check stimulus of corresponding size. A comparison of these two conditions replicates the classic area summation result of Legge & Foley (1980): at low pedestal contrasts there is a distinct advantage for the full stimulus, which has the greater area, but at higher pedestal contrasts the two masking functions converge. The half-filled squares are for when the target was one of the check stimuli (figure 1b,c), but the pedestal was the full stimulus. (The results were almost identical for ‘black’ and ‘white’ checks—as confirmed in figure 4a below—and have been averaged together.) A comparison of this with the full-on-full condition (compare circles and half-filled squares) shows the effect of fixing the pedestal area and increasing only the target area. In this case, the area advantage at detection threshold extends across the entire dipper function, providing strong evidence for a spatial summation process that remains intact across a wide range of contrasts.


Area summation in human vision at and above detection threshold.

Meese TS, Summers RJ - Proc. Biol. Sci. (2007)

Contrast-masking (dipper) functions. (a) Results from experiment 2 averaged across two observers (L. M. and L. W.; approx. 800 or 1600 trials per point). The average standard error was 1.28 dB for L. M. and 0.92 dB for L. W. (b) Behaviour of the main model (using equation (3.1)). Two parameters (z and k; see appendix B) control the ‘dip’ and were adjusted to match the data by eye. (c) Matched model, the same model as in (b) except that the region of summation on the numerator of equation (3.1) is restricted to the high-contrast parts of the target in the checks-on-full condition. Data and models are normalized by the sensitivity to the full stimulus with 0% pedestal contrast (left-most points).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2211515&req=5

fig3: Contrast-masking (dipper) functions. (a) Results from experiment 2 averaged across two observers (L. M. and L. W.; approx. 800 or 1600 trials per point). The average standard error was 1.28 dB for L. M. and 0.92 dB for L. W. (b) Behaviour of the main model (using equation (3.1)). Two parameters (z and k; see appendix B) control the ‘dip’ and were adjusted to match the data by eye. (c) Matched model, the same model as in (b) except that the region of summation on the numerator of equation (3.1) is restricted to the high-contrast parts of the target in the checks-on-full condition. Data and models are normalized by the sensitivity to the full stimulus with 0% pedestal contrast (left-most points).
Mentions: In experiment 2 we replicated the key result from experiment 1 (comparison across check and full stimuli) for seven other observers and extended the study above threshold. The results are shown in figure 3a and averaged across the two observers who performed all of the conditions (L. M. and L. W.). The filled circles are for when the full stimulus (figure 1a) was used as both pedestal and target and have a classic ‘dipper’ shape. The crossed squares are for when the ‘white’ checks stimulus was used as both the pedestal and the target. Although the two stimulus types have the same diameters (figure 1), the sum of contrast over area for the full stimulus is twice that of the check stimuli. Hence, we refer to the full stimulus as having a greater (signal) area than the check stimulus of corresponding size. A comparison of these two conditions replicates the classic area summation result of Legge & Foley (1980): at low pedestal contrasts there is a distinct advantage for the full stimulus, which has the greater area, but at higher pedestal contrasts the two masking functions converge. The half-filled squares are for when the target was one of the check stimuli (figure 1b,c), but the pedestal was the full stimulus. (The results were almost identical for ‘black’ and ‘white’ checks—as confirmed in figure 4a below—and have been averaged together.) A comparison of this with the full-on-full condition (compare circles and half-filled squares) shows the effect of fixing the pedestal area and increasing only the target area. In this case, the area advantage at detection threshold extends across the entire dipper function, providing strong evidence for a spatial summation process that remains intact across a wide range of contrasts.

Bottom Line: To overcome this confound, a novel stimulus class is designed where: (i) the observer operates on a constant retinal area, (ii) the target area is controlled within this summation field, and (iii) the pedestal is fixed in size.Using this arrangement, substantial summation is found along the entire masking function, including the region of facilitation.Our analysis shows that PS and uncertainty cannot account for the results, and that suprathreshold summation of contrast extends over at least seven target cycles of grating.

View Article: PubMed Central - PubMed

Affiliation: School of Life and Health Sciences, Aston University, Birmingham B47ET, UK. t.s.meese@aston.ac.uk

ABSTRACT
The initial image-processing stages of visual cortex are well suited to a local (patchwise) analysis of the viewed scene. But the world's structures extend over space as textures and surfaces, suggesting the need for spatial integration. Most models of contrast vision fall shy of this process because (i) the weak area summation at detection threshold is attributed to probability summation (PS) and (ii) there is little or no advantage of area well above threshold. Both of these views are challenged here. First, it is shown that results at threshold are consistent with linear summation of contrast following retinal inhomogeneity, spatial filtering, nonlinear contrast transduction and multiple sources of additive Gaussian noise. We suggest that the suprathreshold loss of the area advantage in previous studies is due to a concomitant increase in suppression from the pedestal. To overcome this confound, a novel stimulus class is designed where: (i) the observer operates on a constant retinal area, (ii) the target area is controlled within this summation field, and (iii) the pedestal is fixed in size. Using this arrangement, substantial summation is found along the entire masking function, including the region of facilitation. Our analysis shows that PS and uncertainty cannot account for the results, and that suprathreshold summation of contrast extends over at least seven target cycles of grating.

Show MeSH
Related in: MedlinePlus