Abstract
Purpose
To investigate the relationship between signal strength and latency of the blood oxygenation level‐dependent (BOLD) signal.
Materials and Methods
Several correlation analyses were performed on data obtained in a functional magnetic resonance imaging (fMRI) experiment, where subjects were presented with a simple visual stimulus. The BOLD signal strength was correlated with both the phase shift of the spectral density matrix and time‐to‐peak calculated from trial‐averaged time courses. Correlation coefficients were calculated for visual stimuli of 2, 6, and 15 seconds in duration.
Results
Analyzing all functional runs for the same subject separately, i.e., including for each run all significantly activated voxels, we observed that correlations between phase shift and signal strength, as well as between time‐to‐peak and signal strength, decreased with increasing stimulus length. However, when analyses were restricted to voxels found activated in all functional runs, we observed similar correlations between BOLD signal strength and latency in all runs, independent of the length of stimulation. This result was again obtained for both latency measures: the spectral density phase shift and time‐to‐peak.
Conclusion
For both latency measures, phase shift and time‐to‐peak, a high correlation between BOLD signal strength and latency was observed. We have shown that this correlation is independent of the length of visual stimulation. Thus, the correlation between BOLD signal strength and latency seems to be an inherent property of the BOLD response that is independent of the length of stimulation and can be observed using different methods for determining signal latency.
