Many properties of contrast detection in human vision may be described with reference to a set of tuned spatial frequency channels. The spatial sensitivity of the channel with optimal sensitivity at 3.0 c/deg was studied by measuring threshold as a function of the width of truncated 3.0 c/deg sine-wave gratings that ranged from 2.3′ to 4.6°. Three strategies were used to isolate the threshold response of the channel: (1) The channel at 3.0 c/deg was chosen because of its position at the peak of the contrast sensitivity function. (2) A discrimination paradigm was used in which test stimuli were superimposed on a low contrast grating which was shown to selectively facilitate their detection. (3) The detecting channel was more sensitive to the sine-wave configuration of the test stimuli than to more conventional spatial summation stimuli, such as rectangular bars. Results of the main experiment showed that threshold contrasts for the truncated sine-wave stimuli declined in two stages. From 2.3′ to 40′, the threshold decline was steep, with a plateau at 10′. From 40′ to 4.6°, threshold declined as a power function of stimulus width with an exponent of -0.35. The data of the main experiment were used to derive the spatial receptive field sensitivity for the channel at 3.0 c/deg. The data were accounted for by spatial summation within a receptive field, and probability summation in space across receptive fields.
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dcknowledgements-I wish to thank Dr R. J. W. Mansfield. Professor David M. Green, and Dr C. F. StromeyerI II for their very helpful comments.s uggestionsa nd criticisms.I also wish to thank Amy Aldrich. Janice Kilpatrick and Wendy Willson Legge.I am gratefult o Scott Bradner, who designeda nd constructedt he circuitry necessaryt o producet he displays.I thankt he National ResearchC oun-cil of Canada for a postgraduates cholarship.a nd the Medical ResearchC ouncil of Canada for a postdoctoral fellowship. The work was supported in part by NSF ResearchG rant BNS75-08437 to Dr Mansfield.T he study reportedh ere formed part of a doctoral dissertations ub-mitted to Harvard University.T he paper was completed at The PhysiologicalL aboratory,U niversityo f Cambridge. Cambridge.E ngland.
- contrast sensitivity function
- probability summation
- sine-wave gratings
- spatial frequency
- spatial summation