Abstract
Psychophysical evidence indicates that, in the human retina, the size of the spatial-summation area decreases as illuminance increases. Such a relationship would be beneficial for the detection of spatial contrast in the presence of photon noise. We analyze an image-processing mechanism in which the area of a strictly positive point-spread function varies inversely with local illuminance while its volume remains constant. In addition to its expected effect of improving spatial resolution as illuminance increases, this mechanism also yields center-surround antagonism and all other manifestations of bandpass filtering and accounts for Ricco's law and Weber's law--including the failures of both laws as a function of test conditions. The relationship between this mechanism and lateral inhibition is analyzed.
Highlights
Many psychophysical and physiological experiments can be interpreted as showing that light falling upon any one point ofthe retina creates an excitatory effectat neighboringpoints and that this lateral excitation combines additively with the direct excitation produced by light itself.' Psychophysical evidence indicates that the extent of lateral excitation-the size of the spatial-summation area-increases as retinal illuminance decreases2. ' 3
We show here that the intensity-dependent spatial summation (IDS) model implies that Ricco's law holds for spots of all sizes on a background field of zero intensity-in the sense that the peak value of the output to such an input is the same for all spots ofthe same shape that have the same product of area times intensity
Intensity-Dependent Spatial Summation as a Psychophysical Model For a model based on a single assumption, the IDS model gives a surprisingly complete first-approximation description of *human spatial vision for retinal illuminances ranging from absolute threshold up to around 10 Td
Summary
Many psychophysical and physiological experiments can be interpreted as showing that light falling upon any one point ofthe retina creates an excitatory effectat neighboringpoints and that this lateral excitation combines additively with the direct excitation produced by light itself.' Psychophysical evidence indicates that the extent of lateral excitation-the size of the spatial-summation area-increases as retinal illuminance decreases2. ' 3One obviousand undesirable consequenceof spatial summation is, in effect, to blur the neural image, and so it is natural to look for compensatory benefits of the process. A plausible suggestion is that intensity-dependent spatial summation is an adaptive response to the intrinsic noisiness of light. If the effective flux density in an image is I (absorbed photons/unit time)/unit area, both the mean and the variance of the actual quantum catch per unit time over an area A equal IA. This statistical relationship imposes a fundamental constraint on spatial contrast detection. In crement threshold as a function of test spot area for background fields of various intensities. In log-log coordinates, the value of D required to produce a peak response of 1.15 as A increases over eight log units. The diagonal straight line represents Ricco's law; each curve follows this line up to some area value and departs from it as shown
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callMore From: Journal of the Optical Society of America. A, Optics and image science
Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
