fMRI BOLD Signal as a Function of Neuronal Activity
Quantifying the relationship between neuronal activity and the fMRI blood oxygenation level dependent (BOLD) signal is hard because “neuronal activity” is a multivariate quantity, and precisely measuring neuronal activity required invasive techniques. Attempts to infer this relationship from stimulus-evoked BOLD response have been frustrated by the complex nonlinearity between stimulus and neuronal activity. Here we describe a unique in-vivo model for non-invasively determining the relationship between neuronal and BOLD activities. We demonstrated in the low-level visual cortex of a human subject who was born without the optic chiasm that there are two nearly identical populations of non-interacting but co-locating neurons, with non-overlapping receptive fields. By presenting identical stimuli to both of these receptive fields instead of just one, we can double the local neuronal activity, regardless of the definition of “neuronal activity”. Using this in-vivo model, we found that BOLD response amplitude is proportional to approximately the square root of the sum-total of the local neuronal activity.
