Correlation of fMRI responses to absolute luminance changes in visual cortex

Abstract

Purpose: Even though luminance information is important, there are few papers that focus on brain activity elicited by pure luminance changes without form information (Ganzfeld). We previously reported the responses of visual cortex to Ganzfeld stimuli using functional magnetic resonance imaging (fMRI). We found that the cortical responses varied as a function of eccentricity in spite of using achromatic borderless stimuli. We proposed that this eccentricity dependence might be caused by a different proportion of sustained versus transient response. Here, we address this hypothesis again using a stimulus paradigm optimized for this question.

Methods: We controlled subjects' pupils with mydriatics and placed semi-transparent, hemisphere-shaped covers on their eyes in order to achieve large uniform visual fields and eliminate form information. With this Ganzfeld stimulus, subjects viewed only spatially uniform brightness changes in the following: The illumination increased gradually from the lowest luminance level to the highest in 24 seconds, and decreased gradually to the lowest level in 24 seconds (sustained change). Then it switched to the highest level instantaneously (transient change). Next, it decreased gradually to the lowest level in 24 seconds, and increased gradually to the highest level, and then it switched to the lowest. In summary, one cycle was 96 seconds and was repeated. We used two regressors; one for the transient signal changes and the other for sustained signal changes.

Results and discussion:The responses in a relatively anterior region of the calcarine sulcus correlated more strongly with transient signal changes. In contrast, posterior regions of the calcarine sulcus correlated more strongly with sustained signal changes. In conclusion, we confirmed transient responses in peripheral representations of visual cortex and sustained responses in central representations, respectively.