Summation of grating patches indicates many types of detector at one retinal location.

@article{Watson1982SummationOG,
  title={Summation of grating patches indicates many types of detector at one retinal location.},
  author={A. B. Watson},
  journal={Vision research},
  year={1982},
  volume={22 1},
  pages={17-25}
}
Summation between small, superimposed, fixed patches of sinusoidal grating of various spatial frequencies is explained by detectors with bandwidths of less than one octave. The smoothness of the local contrast sensitivity function indicates that detectors cannot be separated in frequency by more than one bandwidth. Together, these results suggest that the fovea is served by at least seven different frequency-selective detectors. 
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