Brain Topography
Here, we investigate the precision of visuospatial coding in the human brain.
2007
Precise discrimination of object position in the human pulvinar.
Very little is known about the human pulvinar; suggestions for its function include relaying input from cortical areas, allocating visual attention, supporting feature binding, and other integrative processes. The diversity of hypotheses about pulvinar function highlights our lack of understanding of its basic role. A conspicuously missing piece of information is whether the human pulvinar encodes visual information topographically. The answer to this question is crucial, as it dramatically constrains the sorts of computational and cognitive processes that the pulvinar might carry out. Here we used fMRI to test for position-sensitive encoding in the human pulvinar. Subjects passively viewed flickering Gabor stimuli, and as the spatial separation between Gabors increased, the correlation between patterns of activity across voxels within the right pulvinar decreased significantly. The results demonstrate the existence of precise topographic coding in the human pulvinar lateralized to the right hemisphere, and provide a means of functionally localizing this topographic region.
Negative BOLD fMRI Response in the Visual Cortex Carries Precise Stimulus-Specific Information.
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 approximately 10 deg eccentricity). The results suggest that meaningful, stimulus-specific processing occurs even in regions that display a strong negative BOLD response.