Abstract
Sustained positive BOLD (blood oxygen level-dependent) activity is employed extensively in functional magnetic resonance imaging (fMRI) studies as evidence for task or stimulus-specific neural responses. However, the presence of sustained negative BOLD activity (i.e., sustained responses that are lower than the fixation baseline) has remained more difficult to interpret. Some studies suggest that it results from local “blood stealing” wherein blood is diverted to neurally active regions without a concomitant change of neural activity in the negative BOLD regions. However, other evidence suggests that negative BOLD is a result of local neural suppression. In both cases, regions of negative BOLD response are usually interpreted as carrying relatively little, if any, stimulus-specific information (hence the predominant reliance on positive BOLD activity in fMRI). Here we show that the negative BOLD response resulting from visual stimulation can carry high information content that is stimulus-specific. Using a general linear model (GLM), we contrasted standard flickering stimuli to a fixation baseline and found regions of the visual cortex that displayed a sustained negative BOLD response, consistent with several previous studies. Within these negative BOLD regions, we compared patterns of fMRI activity generated by flickering Gabors that were systematically shifted in position. As the Gabors were shifted further from each other, the correlation in the spatial pattern of activity across a population of voxels (such as the population of V1 voxels that displayed a negative BOLD response) decreased significantly. Despite the fact that the BOLD signal was significantly negative (lower than fixation baseline), these regions were able to discriminate objects separated by less than 0.5 deg (at ∼10 deg eccentricity). The results suggest that meaningful, stimulus-specific processing occurs even in regions that display a strong negative BOLD response.
Highlights
In functional MRI experiments, the neural response to visual stimulation is inferred from the change in the BOLD signal
The results of the experiment here demonstrate that the negative BOLD response in visual areas V1 through V4 carries surprisingly precise information about object position
We found that as the positions of flickering Gabor stimuli were incrementally shifted, the spatial correlation in the pattern of activity decreased. The slope of this decrease was an indicator of the selectivity for object position. This position selectivity was nearly as precise in the regions that displayed a negative BOLD response as in those that displayed a positive BOLD response; regions that display either a positive or a negative BOLD response discriminated objects separated by 0.19 deg
Summary
In functional MRI (fMRI) experiments, the neural response to visual stimulation is inferred from the change in the BOLD (blood oxygenation level-dependent) signal. Subjects view a stimulus in the test condition and a blank screen in the control condition. When these two conditions are contrasted in data analysis, researchers attribute perceptual and cognitive functions to regions that display a positive BOLD response compared to baseline, since positive BOLD is closely coupled with neuronal activity [1,2]. To understand the neural correlates of visual perception, fMRI studies should consider the negative BOLD signal as informative
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
