Fahle M, Rosik L, Repnow M, Terwey B, 2001, "Contrast independence of fMRI signals induced by luminance-defined figure - ground segregation" Perception 30 ECVP Abstract Supplement
Contrast independence of fMRI signals induced by luminance-defined figure - ground segregation
M Fahle, L Rosik, M Repnow, B Terwey
Presentation of visual stimuli increases the blood flow especially at the posterior pole of the brain. This increase of blood flow can be visualised via the increased blood oxygen concentration by means of functional magnetic resonance imaging (fMRI). We used this method to investigate the dependence of cortical activity on stimulus contrast in a figure - ground segregation task, in order to assess the relative impact of suprathreshold contrast levels on the intensity of cortical processing. Cortical activation was tested in a block design for checkerboard stimuli as compared to homogeneous stimuli. Checkerboards were defined by luminance at contrasts between 1.25% and 97%, flickering at 4 Hz. Six observers participated in the experiments using a Siemens Vision 1.5 T tomograph. We used BrainVoyager® software to evaluate and display the data. Stimulus contrasts of 1.25% yielded a significant increase of the fMRI signal in some observers only, while those between 2.5% and 97% significantly increased the BOLD response of all observers especially in the retinotopically organised cortical areas. Activity increased steeply for contrasts up to 10% and then saturated slowly for higher contrasts. Hence, we find the fMRI response to be relatively independent of contrast for these stimuli.
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ECVP 2001 Abstract Supplement (complete) size: 1517 Kb