Dynamics of positive and negative BOLD responses in visual cortex

Introduction Previous studies have shown that besides the commonly observed positive BOLD response (PBR), a negative BOLD response (NBR) also exists [1] which was suggested to reflect decreased neuronal activity [2] and oxygenation [3]. However, the underlying physiology of NBR is still not clear and the dynamics of NBR has not been yet extensively explored. Our aim in this study was to investigate the dynamics of NBR and compare these with the dyamics of PBR. To this end, we used two different visual stimulus designs in which the baseline conditions were the same. During stimulation, however, the luminance level was either increased or decreased. We showed that mirroring the stimulus design does not simply mirrors the BOLD response and that the dynamics of NBR differs from PBR. Methods fMRI data were collected at 3T Siemens Tim-Trio scanner using GRE-EPI sequence (TR/TE=1000/35ms, voxel-size=3.1×3.1×3.5mm). Full field, black-white checkerboard stimuli rotating at 8Hz were presented to subjects (10 subjects) with four different Michelson contrast level (100, 75, 25, 12.5, gamma corrected) whereas the baseline condition was set to 50 contrast (40s baseline, 5s stimulus; 12 trials). On functional activation maps, 100 most activated voxels in the visual cortex were chosen and time courses of these voxels averaged among trials. Time courses were resampled at 0.1s using data interpolation. Standard deviation (STD) of baseline was calculated for the time points −5 to 0. Time-to-onset of BOLD response was defined as the time point deviating 2 STD from the baseline mean value. Results Positive contrast stimuli (100, 75) elicited positive activation in the early visual areas (red in Fig.1), whereas negative contrast stimuli (25, 12.5) evoked negative (green in Fig.1) as well as positive activation (blue in Fig.1) in posterior and anterior part of the visual cortex, respectively. Average time courses in negative activation region as well as in lateral geniculate nucleus (LGN) are depicted in figure 2-3. Time-to-onset values of negative contrast responses were not significantly different from positive contrast responses (p>0.05), whereas in time-to-peak values negative contrast responses slightly preceded their positive counterparts (Table 1). Although, the absolute peak amplitudes of responses were similar (p>0.05), their widths were not (p