Abstract
Blood oxygen level-dependent (BOLD) responses were measured in parts of primary visual cortex that represented unstimulated visual field regions at different distances from a stimulated central target location. The composition of the visual scene varied by the presence or absence of additional peripheral distracter stimuli. Bottom-up effects were assessed by comparing peripheral activity during central stimulation vs. no stimulation. Top-down effects were assessed by comparing active vs. passive conditions. In passive conditions subjects simply watched the central letter stimuli and in active conditions they had to report occurrence of pre-defined targets in a rapid serial letter stream. Onset of the central letter stream enhanced activity in V1 representations of the stimulated region. Within representations of the periphery activation decreased and finally turned into deactivation with increasing distance from the stimulated location. This pattern was most pronounced in the active conditions and during the presence of peripheral stimuli. Active search for a target did not lead to additional enhancement at areas representing the attentional focus but to a stronger deactivation in the vicinity. Suppressed neuronal activity was also found in the non distracter condition suggesting a top-down attention driven effect. Our observations suggest that BOLD signal decreases in primary visual cortex are modulated by bottom-up sensory-driven factors such as the presence of distracters in the visual field as well as by top-down attentional processes.
Highlights
Presenting visual stimuli at a certain location in the visual field increases neural firing in corresponding portions of primary visual cortex
With increasing distance from the center, Blood oxygen level-dependent (BOLD) signal in the respective Regions of Interest (ROIs) progressively decreased and eventually fell under the level of the baseline condition where subjects fixated the central cross (Fig. 1)
BOLD signal was lower in the distracter condition than in the non distracter version
Summary
Presenting visual stimuli at a certain location in the visual field increases neural firing in corresponding portions of primary visual cortex. Often enough the positive BOLD response is accompanied by negative signal changes (i.e., a reduction compared to baseline) in other areas of the visual system [5,6,7] Until recently, such effects have been routinely discounted either because their origin was thought to be vascular and unrelated to neural firing rates (blood stealing) or because they were regarded as meaningless, non-specific and largely stimulus independent. This is surprising given the fact that experiments in laboratory animals have shown a long time ago that the presence of a visual stimulus does change the firing rate in its own cortical representation and in adjacent visual cortex. There are good reasons to believe that BOLD changes in visual cortex that is not directly stimulated reflect some meaningful aspect in visual processing
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