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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

Summation levels as functions of the diameter of a circular summation aperture. Different symbols are for different models (see text for details).
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fig6: Summation levels as functions of the diameter of a circular summation aperture. Different symbols are for different models (see text for details).

Mentions: For simplicity, contrast was summed over the entire stimulus region in the modelling in figures 3b and 4b, but the question arises, what is the smallest region over which summation is required? Figure 6 shows the results of rerunning the model on the full-on-full stimulus and the ‘white’ checks-on-full stimulus for a pedestal contrast of 32% (30 dB), and varying the diameter of a circular (hard-edged) summation aperture at their centres. The figure plots the ratio of target increment thresholds for these two stimuli (i.e. summation). The main model (equation (3.1), filled diamonds) must sum over at least seven carrier cycles (vertical solid line; a diameter of two checks) before it reaches the level of summation shown by the six observers in figure 3 (horizontal solid line).


Area summation in human vision at and above detection threshold.

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

Summation levels as functions of the diameter of a circular summation aperture. Different symbols are for different models (see text for details).
© Copyright Policy - open-access
Related In: Results  -  Collection

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

fig6: Summation levels as functions of the diameter of a circular summation aperture. Different symbols are for different models (see text for details).
Mentions: For simplicity, contrast was summed over the entire stimulus region in the modelling in figures 3b and 4b, but the question arises, what is the smallest region over which summation is required? Figure 6 shows the results of rerunning the model on the full-on-full stimulus and the ‘white’ checks-on-full stimulus for a pedestal contrast of 32% (30 dB), and varying the diameter of a circular (hard-edged) summation aperture at their centres. The figure plots the ratio of target increment thresholds for these two stimuli (i.e. summation). The main model (equation (3.1), filled diamonds) must sum over at least seven carrier cycles (vertical solid line; a diameter of two checks) before it reaches the level of summation shown by the six observers in figure 3 (horizontal solid line).

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