Functional magnetic resonance imaging of the visual cortex:: Relation between stimulus intensity and bold response

Introduction. Visual system is a high interest topic in neuroscience research. The new neuroimaging techniques, such as functional magnetic resonance imaging (fMRI), allow us to quickly improve our knowledge on the visualsystem using non-invasive methods. This work examines the effect of small changes in the intensity of a visual stimulus over the BOLD response in the visual cortex. Aims. To perform a detailed analysis of the visual cortex reaction to different intensities of a light source and to verify, the ties between the intensity of the visual stimulus and the cortical response. Subjects and methods. Using fMRI (3 T), we registered BOLD response (area and intensity of the signal change) in 20 photophobic patients and 20 controls while viewing different stimulus intensities from a light source. Results. We found a direct relation between stimulus intensity and occipital response. We show that cortical reactivity is higher in patients with photophobia than normal controls, specially for the lower and medium intensities. Conclusions.,fMRI is a valid and robust technique to register consistent and reproducible responses in different groups of subjects. It is useful for the study of normal cortex functioning as well as for clinical use.