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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

Stimuli used in experiments 2 and 3: (a) full stimulus (b) ‘white’ checks (c) ‘black’ checks. All three stimulus types served as pedestal (mask) and target in various combinations. They had a diameter of 9° displayed on a uniform square grey region with a width of 20.5° in the centre of the monitor. Closely related stimuli were used in experiment 1.
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fig1: Stimuli used in experiments 2 and 3: (a) full stimulus (b) ‘white’ checks (c) ‘black’ checks. All three stimulus types served as pedestal (mask) and target in various combinations. They had a diameter of 9° displayed on a uniform square grey region with a width of 20.5° in the centre of the monitor. Closely related stimuli were used in experiment 1.

Mentions: The three different types of stimuli used in experiments 2 and 3 are shown in figure 1. The full stimulus (figure 1a) was a horizontal sine-wave grating in sine-phase with the centre of the display, and had a spatial frequency of 2.5 cycles per degree. It was modulated by a circular raised cosine function with a central plateau of 8° and a blurred boundary of 1°, giving a full-width at half-height of 9°. The check stimuli (figure 1b,c) were identical to the full stimulus, except that they were modulated by a ‘raised-plaid’ envelope. The plaid was the sum of two sine-wave grating components with orientations of ±45° and a spatial frequency of 0.5 cycles per degree, each with contrasts of 0.5. This gave minima and maxima of −1 and 1, respectively. The envelope was then ‘raised’ by adding 1 to each point and dividing by 2 to give minima and maxima of 0 and 1. With this arrangement, there are 7.07 cycles of carrier grating for every two checks (i.e. one cycle of a vertical cross-section through the envelope).


Area summation in human vision at and above detection threshold.

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

Stimuli used in experiments 2 and 3: (a) full stimulus (b) ‘white’ checks (c) ‘black’ checks. All three stimulus types served as pedestal (mask) and target in various combinations. They had a diameter of 9° displayed on a uniform square grey region with a width of 20.5° in the centre of the monitor. Closely related stimuli were used in experiment 1.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

fig1: Stimuli used in experiments 2 and 3: (a) full stimulus (b) ‘white’ checks (c) ‘black’ checks. All three stimulus types served as pedestal (mask) and target in various combinations. They had a diameter of 9° displayed on a uniform square grey region with a width of 20.5° in the centre of the monitor. Closely related stimuli were used in experiment 1.
Mentions: The three different types of stimuli used in experiments 2 and 3 are shown in figure 1. The full stimulus (figure 1a) was a horizontal sine-wave grating in sine-phase with the centre of the display, and had a spatial frequency of 2.5 cycles per degree. It was modulated by a circular raised cosine function with a central plateau of 8° and a blurred boundary of 1°, giving a full-width at half-height of 9°. The check stimuli (figure 1b,c) were identical to the full stimulus, except that they were modulated by a ‘raised-plaid’ envelope. The plaid was the sum of two sine-wave grating components with orientations of ±45° and a spatial frequency of 0.5 cycles per degree, each with contrasts of 0.5. This gave minima and maxima of −1 and 1, respectively. The envelope was then ‘raised’ by adding 1 to each point and dividing by 2 to give minima and maxima of 0 and 1. With this arrangement, there are 7.07 cycles of carrier grating for every two checks (i.e. one cycle of a vertical cross-section through the envelope).

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