The negative BOLD response and its behavioral correlates
Introduction
The negative BOLD response (NBR) describes a phenomenon seen in functional magnetic resonance imaging in which a stimulated cortical region that responds with a positive blood oxygenation-level dependent (BOLD) signal is flanked by an unstimulated region of cortex exhibiting a negative response (Shmuel, Yacoub et al. 2002). It is most easily seen in visual cortex where well-ordered retinotopic maps make it easy to identify stimulated and unstimulated locations.
The NBR is primarily a neural effect: reductions in the BOLD signal correlate with reductions in both LFP activity and firing rates (Shmuel, Augath et al. 2006). Using event-related fMRI, we measured the amplitude of the NBR as a function of ongoing background activity in order to test three potential models of response control: response gain control, contrast gain control and a purely subtractive mechanism. We also measured human psychophysical performance using a stimulus very similar to that in our fMRI experiments in order to identify behavioral correlates of the signal changes that we observe.
Methods General
Our stimulus consisted of a central disk D (diameter 2 degrees) containing a grating of contrast Cd and a surrounding annulus A (diameter 5 degrees) containing a grating of contrast Ca. These components were presented on a uniform mean gray field with a small gap between them. Cd and Ca could be set independently. In our event-related fMRI experiments, the difference between conditions Cd=0% (no center) and Cd=90% (high-contrast center) gave the magnitude of the NBR. We measured the NBR as a function of Ca (0%, 5%, 20% and 45% contrast). In our psychophysical experiments, we used a staircase procedure to measure threshold-versus-contrast curves for both D and A in the presence and absence of high-contrast, spatially-remote maskers (A and D respectively).
fMRI methods
N subs=5, B0=3T, EPI TR=2s, resolution 2x2x2mm. All data from independently-localized regions in V1 defined on flattened, retinotopically-mapped cortex.
Behavioral methods
N subs=5, thresholds estimated using QUEST 2 interval forced choice procedure.
Results
Our fMRI results show that the NBR is best modeled as a multiplicative gain control mechanism with the strongest effects occurring at low background contrast. In addition, the NBR exhibits a strong spatial asymmetry: We measure a strong NBR in the periphery but not in the fovea. Our psychophysical data are in agreement with these results: We measure only weak suppressive effects of a high-contrast annulus on the foveal target but a significant amount of suppression of the surround annulus due to the high-contrast center.
Conclusion
The NBR is a manifestation of a multiplicative gain control mechanism that acts to suppress neural activity in the periphery when a high-contrast stimulus is present in the fovea.
References
1. Shmuel, A., M. Augath, et al. (2006). "Negative functional MRI response correlates with decreases in neuronal activity in monkey visual area V1." Nat Neurosci 9(4): 569-77.
2. Shmuel, A., E. Yacoub, et al. (2002). "Sustained negative BOLD, blood flow and oxygen consumption response and its coupling to the positive response in the human brain." Neuron 36(6): 1195-210.
Conference:
Computational and systems
neuroscience 2009, Salt Lake City, UT, United States, 26 Feb - 3 Mar, 2009.
Presentation Type:
Poster Presentation
Topic:
Poster Presentations
Citation:
(2009). The negative BOLD response and its behavioral correlates.
Front. Syst. Neurosci.
Conference Abstract:
Computational and systems
neuroscience 2009.
doi: 10.3389/conf.neuro.06.2009.03.282
Copyright:
The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers.
They are made available through the Frontiers publishing platform as a service to conference organizers and presenters.
The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated.
Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed.
For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions.
Received:
04 Feb 2009;
Published Online:
04 Feb 2009.