Limits...
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.

Show MeSH

Related in: MedlinePlus

Detection and identification of ‘white’ checks and full targets for (a(i)(ii)) T.S.M. and (b(i)(ii)) R.J.S. for pedestal contrasts of (a(i),b(i)) 0% and (a(ii),b(ii)) 20%. The insets report spatial summation (the difference between the upper and lower contrast axes dB) and the lateral shift of the identification threshold relative to the detection threshold of the full stimulus (ID offset). The average slopes of the psychometric functions at detection threshold were  and  for detection and identification, respectively. For the 20% pedestal, they were  and .
© Copyright Policy - open-access
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC2211515&req=5

fig5: Detection and identification of ‘white’ checks and full targets for (a(i)(ii)) T.S.M. and (b(i)(ii)) R.J.S. for pedestal contrasts of (a(i),b(i)) 0% and (a(ii),b(ii)) 20%. The insets report spatial summation (the difference between the upper and lower contrast axes dB) and the lateral shift of the identification threshold relative to the detection threshold of the full stimulus (ID offset). The average slopes of the psychometric functions at detection threshold were and for detection and identification, respectively. For the 20% pedestal, they were and .

Mentions: Our main proposal is that the full and check stimuli are detected by a common pooling process (e.g. equation (3.1)). Subjective reports of our observers (who were questioned during the experiment) are consistent with this view: when the checks-on-full stimulus was close to threshold, the target increment appeared to be applied to the entire pedestal. If so, it should be difficult for the observers to identify a check target in a 2IFC experiment, where equally detectable check and full increments are placed in the two intervals. An alternative hypothesis is that the different increments are detected by different mechanisms. For example, the check stimulus might be detected by a second-order mechanism sensitive to contrast modulation (Georgeson & Schofield 2002). If these involve labelled lines (Watson & Robson 1981), then observers should be able to identify the different increment types close to their thresholds (Georgeson & Schofield 2002). Figure 5 shows that this does not happen for pedestal contrasts of either 0% (figure 5a(i),b(i)) or 20% (figure 5a(ii),b(ii)). On these normalized axes, the psychometric functions for detecting the two different increment types (squares and circles) superimpose (i.e. the results for the checks condition were slid laterally). In the identification task (crosses), equally detectable full and ‘white’ check contrast increments were made in the two test intervals and observers had to identify the checks. But the contrast increment needed to do this successfully was much higher than for detection. Although this experiment does not identify the form of pooling (PS or signal combination), it does suggest that a common pooling process was used to detect the two different targets.


Area summation in human vision at and above detection threshold.

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

Detection and identification of ‘white’ checks and full targets for (a(i)(ii)) T.S.M. and (b(i)(ii)) R.J.S. for pedestal contrasts of (a(i),b(i)) 0% and (a(ii),b(ii)) 20%. The insets report spatial summation (the difference between the upper and lower contrast axes dB) and the lateral shift of the identification threshold relative to the detection threshold of the full stimulus (ID offset). The average slopes of the psychometric functions at detection threshold were  and  for detection and identification, respectively. For the 20% pedestal, they were  and .
© Copyright Policy - open-access
Related In: Results  -  Collection

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

fig5: Detection and identification of ‘white’ checks and full targets for (a(i)(ii)) T.S.M. and (b(i)(ii)) R.J.S. for pedestal contrasts of (a(i),b(i)) 0% and (a(ii),b(ii)) 20%. The insets report spatial summation (the difference between the upper and lower contrast axes dB) and the lateral shift of the identification threshold relative to the detection threshold of the full stimulus (ID offset). The average slopes of the psychometric functions at detection threshold were and for detection and identification, respectively. For the 20% pedestal, they were and .
Mentions: Our main proposal is that the full and check stimuli are detected by a common pooling process (e.g. equation (3.1)). Subjective reports of our observers (who were questioned during the experiment) are consistent with this view: when the checks-on-full stimulus was close to threshold, the target increment appeared to be applied to the entire pedestal. If so, it should be difficult for the observers to identify a check target in a 2IFC experiment, where equally detectable check and full increments are placed in the two intervals. An alternative hypothesis is that the different increments are detected by different mechanisms. For example, the check stimulus might be detected by a second-order mechanism sensitive to contrast modulation (Georgeson & Schofield 2002). If these involve labelled lines (Watson & Robson 1981), then observers should be able to identify the different increment types close to their thresholds (Georgeson & Schofield 2002). Figure 5 shows that this does not happen for pedestal contrasts of either 0% (figure 5a(i),b(i)) or 20% (figure 5a(ii),b(ii)). On these normalized axes, the psychometric functions for detecting the two different increment types (squares and circles) superimpose (i.e. the results for the checks condition were slid laterally). In the identification task (crosses), equally detectable full and ‘white’ check contrast increments were made in the two test intervals and observers had to identify the checks. But the contrast increment needed to do this successfully was much higher than for detection. Although this experiment does not identify the form of pooling (PS or signal combination), it does suggest that a common pooling process was used to detect the two different targets.

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