We have measured the relationship between image contrast, perceived contrast, and BOLD fMRI activity in human early visual areas, for natural, whitened, pink noise, and white noise images. As root-mean-square contrast increases, BOLD response to natural images is stronger and saturates more rapidly than response to the whitened images. Perceived contrast and BOLD fMRI responses are higher for pink noise than for white noise patterns, by the same ratio as between natural and whitened images. Spatial phase structure has no measurable effect on perceived contrast or BOLD fMRI response. The fMRI and perceived contrast response results can be described by models of spatial frequency response in V1, that match the contrast sensitivity function at low contrasts, and have more uniform spatial frequency response at high contrasts.
Bibliographical noteFunding Information:
The authors would like to thank Essa Yacoub for his assistance in fMRI data collection, and the two reviewers for their suggestions for improving the manuscript. Work on this project was funded by NIH RO1 EY02587, NSF/IGERT DGE 9870633, BTRR P41-RR008079, the Keck Foundation, and the MIND Institute.
- Functional brain imaging
- Models and theory
- Natural images
- Spatial vision